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HomeMy WebLinkAboutDWQ-2025-004762 Utah Lake Water Quality Study Charge Questions Report Developed by the ULWQS Science Panel Technical support provided by Tetra Tech Draft January 3, 2025 1.0 BACKGROUND AND APPROACH Subgroups of the Utah Lake Water Quality Study (ULWQS) Science Panel (SP) have compiled responses to the ULWQS Charge Questions according to topic areas. Charge questions are listed below, followed by a detailed review of evidence available to answer each question as well as a synthesis response to each question that includes an assessment of confidence. The evaluation of charge questions has proceeded according to the Utah Lake Water Quality Study—Uncertainty Guidance document. 2.0 CHARGE QUESTIONS Question 1: What was the historical condition of Utah Lake with respect to nutrients and ecology pre- settlement and along the historical timeline with consideration of trophic state shifts and significant transitions since settlement? 1.1. What does the diatom community and macrophyte community in the paleo record tell us about the historical trophic state and nutrient regime of the lake? i. Can diatom (benthic and planktonic) and/or macrophyte extent or presence be detected in sediment cores? And if so, what are they? ii. What were the environmental requirements for diatoms and extant and locally extirpated macrophyte species? iii. How have environmental conditions changed over time? 1.2. What were the historic phosphorus, nitrogen, and silicon concentrations as depicted by sediment cores? (add calcium, iron, and potentially N and P isotopes) 1.3. What information do paleo records (eDNA/scales) provide on the population trajectory/growth of carp over time? What information do the paleo records provide on the historical relationship between carp and the trophic state and nutrient regime of the lake? 1.4. What do photopigments and DNA in the paleo record tell us about the historical water quality, trophic state, and nutrient regime of the lake? 2 Question 2: What is the current state of the lake with respect to nutrients and ecology? 2.1. What are the impacts of carp on the biology/ecology and nutrient cycling of the lake and how are those impacts changing with ongoing carp removal efforts? i. What contribution do carp make to the total nutrient budget of the lake via excretion rates and bioturbation? How much nutrient cycling can be attributed to carp? ii. What is the effect of carp removal efforts on macrophytes, nutrients, secchi depth, turbidity, and primary productivity? iii. How much non-algal turbidity and nutrient cycling is due to wind action versus carp foraging? How much does sediment resuspension contribute to light limitation, and does wind resuspension contribute substantially in the absence of carp? 2.2 What are the environmental requirements for submerged macrophytes currently present at Utah Lake? i. What is the role of lake elevation and drawdown in macrophyte recovery? Are certain species more resilient to drawdowns and nutrient related impacts? Can some species establish/adapt more quickly? ii. What is the relationship between carp, wind, and macrophytes on non-algal turbidity and nutrient cycling in the lake? What impact could macrophyte reestablishment have? 2.3. What are the linkages between changes in nutrient regime and Harmful Algal Blooms (HABs)? i. Where do HABs most frequently start/occur? Are there hotspots and do they tend to occur near major nutrient sources? ii. Which nutrients are controlling primary production and HABs and when? iii. If there are linkages between changes in nutrient regime and HABs, what role if any does lake elevation changes play? iv. How do other factors affect HAB formation in Utah Lake (e.g., climate change; temperature; lake stratification; changes in zooplankton and benthic grazers and transparency) v. What is the role of calcite “scavenging” in the phosphorus cycle? vi. What is the relationship between light extinction and other factors (e.g., algae, TSS, turbidity)? 2.4. How do sediments affect nutrient cycling in Utah Lake? i. What are current sediment equilibrium P concentrations (EPC) throughout the lake? What effect will reducing inputs have on water column concentrations? If so, what is the expected lag time for lake recovery after nutrient inputs have been reduced? ii. What is the sediment oxygen demand of, and nutrient releases from, sediments in Utah Lake under current conditions? iii. Does lake stratification [weather patterns] play a result in anoxia and phosphorus release into the water column? Can this be tied to HAB formation? 2.5. For warm water aquatic life, waterfowl, shorebirds, and water-oriented wildlife: i. Where and when in Utah Lake are early life stages of fish present? ii. Which species are most sensitive and need protection from nutrient-related impacts? 3 Question 3: What additional information is needed to define nutrient criteria that support existing beneficial uses? 3.1: For warm water aquatic life, waterfowl, shorebirds, and water-oriented wildlife 3.2: For primary contact recreation 3.3: For agricultural uses, including irrigation of crops and stock watering Question 4: Can the lake be improved given current management constraints? 4.1. What would be the current nutrient regime of Utah Lake assuming no nutrient inputs from human sources? This question may require the identification of primary sources of nutrients. 4.2. Assuming continued carp removal and current water management, would nutrient reductions support a shift to a macrophyte-dominated state within reasonable planning horizons (i.e., 30-50 years)? 4.3. If the lake stays in a phytoplankton-dominated state, to what extent can the magnitude, frequency, and extent of harmful and nuisance algal blooms be reduced through nutrient reductions? 3.0 QUESTION EVALUATION 1.1. What does the diatom community and macrophyte community in the paleo record tell us about the historical trophic state and nutrient regime of the lake? Specifics of this question are addressed as part of sub-questions 1.1.i, 1.1.ii, and 1.1.iii below. Overall, there is a higher degree of eutrophication and nutrient concentrations in Utah Lake at present compared to pre-European settlement times, with associated shifts in the biological community that is preserved in the paleolimnological record. The assessments of confidence around these relationships are detailed as part of the response for each relevant sub-question. 4 1.1.i. Can diatom (benthic and planktonic) and/or macrophyte extent or presence be detected in sediment cores? And if so, what are they? & 1.1.ii. What were the environmental requirements for diatoms and extant and locally extirpated macrophyte species? Evidence evaluation Several studies have analyzed diatom composition in paleolimnological sediment cores in Utah Lake. The most recent diatom analysis effort (Brahney et al. 2024) collected cores in Goshen Bay and in the main basin near Bird Island. Diatom samples were indexed using the Biological Condition Gradient (BCG), which ranks taxa from 1 (specialist/sensitive species) to 5 (tolerant species) (Davies and Jackson 2006). In addition, diatom species were characterized as planktonic or benthic. These are well-accepted methods to evaluate shifts in trophic status. Results from Brahney et al. (2024) are as follows: “In Goshen Bay, distinct shifts in the community composition are frequently observed between 30 and 40 cm and again between 15 and 10 cm depth, which correspond roughly to the late 19th century and between 1960-1975... In general, during the first transition at ~30 cm we see the loss of some species while others for the first time appear in the record. A slower transition away from epiphytic diatoms (Epithemia spp.) is observed during the first transition, which is followed by a more recent reduction in epissamic, or sand dwelling species such Staurosirella martyi, S. construens, S. venter, represented by the ‘small benthic sum’. Large pollution tolerant species begin to increase around ~30-35 cm and continue to increase upcore. At around 30 cm an increase occurs in some benthic species such an Amphora copulata (BCG 4) and Pleurosigma spenceri (BCG 4), Navicula cryptochephala (BCG 4; High pH) at the expense of benthic species that are more sensitive (e.g Pinnularia microstauron (BCG 3). The pollution tolerance index shows a sharp increase at 30 cm and remains high until 2.5 cm where it falls down to lower levels. Pollution tolerant planktonic species such as Aulacoseira granulata, Cyclotella meneghiniana, and Stephanodiscus hantzschii begin to appear above 12 cm. At around 10 cm we see a significant increase in the representation of planktonic species vs benthic species. “In summary, diatom taxa below ~30 cm are represented by more sensitive benthic species (BCG 2, 3) and includes the presence of epiphytic species. At 30 cm we see a shift in the taxonomic representation of many species towards more pollution tolerant benthic species (BCG 5). At ~12 cm there is a sharp increase in pollution tolerant planktonic species and an increase in planktonic representation. The modern community is representative of shallow, alkaline, and eutrophic conditions whereas the pre-disturbance community is representative of shallow, alkaline, mesotrophic conditions with a greater presence of macrophytes. “At Bird Island, distinct shifts in community composition are observed between 40 to 30 cm, 20 cm and 10 cm. The early section of the core (pre 40 cm) is characterized by large sensitive benthic species such as Pinnularia microstauron, small benthic epissamic species such as, Staurosira spp, most of which completely disappear after 11 cm. Epithemia spp. decline sharply around 40 cm and continue to decrease upcore. At 30 cm we see an increase in taxa tolerant of fine sediment disturbance, such as Navicula salinarum and Navicula veneta, as well as more pollution tolerant benthic species mixed with more sensitive species, such as Nitzschia palea (BCG 5), Tryblionella spp (BCG 4-5). Gyrosigma acuminaun (BCG 4), Diploneis elliptica (BCG 3), and Staurosira spp. (BCG 4-5). We also see the emergence of the planktonic Discostella psudostelligera (BCG 4). Above 10 cm, D. stelligera is replaced with pollution tolerant species such as A. granualta (BCG 5), Cyclostephanos dubius (BCG 4), and Cyclotella meneghiniana (BCG 5), Stephanodiscus sp. as well as the benthic Pleurosigma spenceri (BCG 4). The planktonic to benthic ratios rapidly and significantly increase at 10 cm while the pollution tolerance index increases gradually through the core, peaking at 4.5 cm (~1990). As with the Goshen Bay core, many of 5 the species identified are common in brackish, alkaline waters. Towards the surface of the core the representation by eutraphentic species increases. “In summary, the early part of the core is characterized by more sensitive species and includes species that live on aquatic vegetation. This period is followed by an increase in large benthic pollution and turbid tolerant species. The upper 10 cm is in contrast characterized by an increase in pollution tolerant planktonic species at the expense of both benthic species and more sensitive planktonic species. The modern community is representative of shallow, alkaline, and eutrophic conditions whereas the pre- disturbance community is representative of shallow, alkaline, mesotrophic conditions with a greater presence of macrophytes.” Older studies have used several benthic/epiphytic species to indicate benthic diatom presence and highlight Diploneis smithii as an indicator genus. Bolland (1974) found that from 26-176 cm and 180 to ~300 cm in a sediment core from the northern main basin, benthic/epiphytic species dominate the diatom community. Historical dominance of D. smithii was also indicated by Jakuval and Rushforth (1983), from 80 cm deeper in a sediment core collected north of Provo Bay. Together, these sources of evidence indicate that the ratio of planktonic to benthic diatoms has increased through time in Utah Lake, and the species are dominated by more eutraphentic taxa toward present. Several lines of evidence are available to discern the historical macrophyte species present in Utah Lake: historical observations, physical remains in sediment cores, environmental DNA (eDNA), elemental stoichiometry, the hydrogen index, as well as other proxy evidence such as the presence of gastropods. Physical plant remains have been found in cores from Goshen Bay and Provo Bay and gastropod remains in the former as well as the Bird Island core (King 2019, Brahney et al. 2021). Hardstem bulrush (Schoenoplectus acutus; emergent macrophyte) presence was indicated pre-1900 in Goshen Bay, but other sites were not tested. Physical remains of macrophytes were found across sediment core intervals below 30 cm in Goshen Bay and below 22 cm in Provo Bay, but no preserved remains were noted in core intervals approaching present day (Figure 1, Brahney et al. 2024). Similarly, sediment cores (~10 cm long) retrieved by hand from multiple nearshore areas around the lake in 2019 did not contain Characeae (submerged macrophyte) oospores (Brothers et al. 2021), but given the short distance in which these oospores travel and the size of the lake, this may not be surprising and is not evidence of their historical absence. In the Goshen Bay sediment core, eDNA from 21 samples indicated hardstem bulrush was the only native macrophyte found in historic sediments (Figure 2, Brahney et al. 2024). Bulk organic matter content, C:N ratios, and the hydrogen index of sediments also indicate a shift from a macrophyte-dominated state Figure 1. Physical remains of plants, gastropods, and anodonta in sediment cores (icons), along with the percent of sediment characterized as organic (lines). From Brahney et al. 2024. 6 to a planktonic-dominated state, with marked increases in organic matter content and decreases in C:N ratios at 30 cm depth (Error! Reference source not found., Williams et al. 2023, Brahney et al. 2024). Finally, Brahney et al. (2024) provide additional proxy evidence (Figure 1): “In addition to the physical remains of plant material, the presence of gastropods is indicative of benthic production as their habitat generally requires decaying vegetation and/or benthic algae and gastropod concentrations are known to increase in concert with macrophyte density (Diehl and Kornijow in Scheffer Figure 2. Environmental DNA results for Hardstem bulrush and Cyanobacteria from the Goshen Bay core. Vegetation icons indicate presence of physical remains in the sample. From Brahney et al. (2024). Figure 3. C:N ratios from three sediment cores in Utah Lake. Algal organic matter is typically indicated at low C:N ratios and vascular plants at C:N ratios of >20. A decrease in C:N ratios over time suggests a shift from macrophytes to phytoplankton as the dominant primary producers. From Williams et al. 2023. 7 and Jeppesen 1998). Gastropod remains were found in Goshen Bay below 30 cm only. In Provo Bay, above 22 cm gastropod remains were found in less than 40% of samples. In contrast, below 22 cm, gastropod remains were found in more than 65% of the samples, many containing more than 10 individuals/fragments. Anodonta sp. shells were found in Bird Island, Provo Bay, and the North Core below 30 cm (Figure 3). As noted above, the presence of diatoms that are epiphytic are found in much greater abundance historically in both cores analyzed (Bird Island and Goshen Bay).” The submerged macrophyte stonewort (Characeae) and reeds are considered to have been historically present in Utah Lake (Figure 4) and are experimentally shown to be a preferred foraging food source for invasive common carp in this lake (Miller and Crowl 2006; Miller and Provenza, 2007). Although stonewort meadows no longer occur in Utah Lake, they are elsewhere considered to indicate clear-water conditions in lakes (Lambert-Servien et al. 2006) and may improve water clarity by stabilizing sediments, precipitating calcite, and storing nutrients in their recalcitrant tissues (Kufel and Kufel, 2002). King et al. (2024) indicated: “Altogether, the presence of intact macrophyte and gastropod remains, organic matter contents, stable sediments, and natural δ15N values indicates that the lake featured a clearwater state with an abundant macrophyte community, specifically within Goshen Bay and Provo Bay, and was largely undisturbed by anthropogenic activities.” Figure 4. Original survey of the Great Salt Lake Basin from 1852. The map indicates “reeds” in Goshen Bay and cartography lines indicating vegetation in Provo Bay (Stansbury et al. 1852). 8 Synthesis Diatom analysis in sediment cores has been conducted as a part of three separate research efforts and is a well- accepted method because diatoms are well-preserved in the fossil record. All three studies showed a historical dominance of benthic and epiphytic species in the diatom community, which has shifted toward a dominance of planktonic species approaching present day. Diatom species have also shifted from more sensitive taxa to more nutrient-tolerant taxa over time. These lines of evidence support a shift from benthic-dominated primary production to more nutrient-rich, planktonic primary production. The Science Panel has high confidence in this evaluation. Several lines of evidence have confirmed the historical presence of submerged and emergent macrophytes in the bays and nearshore areas of Utah Lake. The lines of evidence each vary in their strengths and weaknesses, but as a total body of evidence support a similar conclusion. The Science Panel has high confidence in the historical presence of macrophytes in bays and nearshore areas and medium confidence about the geographic specificity of macrophyte presence in other areas of the lake. 9 1.1.iii. How have environmental conditions changed over time? Questions 1.1.i, 1.1.ii, 1.2, and 1.4 all address the changing environmental conditions for specific topics. A high- level summary of the chronology of the lake is described here, and readers are referred to the questions noted above for additional detail. 1. Utah Lake has experienced major phase shifts, including European settlement and carp introduction in the late 1800s and early 1900s, a shift to cyanobacteria dominance in the phytoplankton community in the 1950s, increases in population in the catchment, and changes in wastewater loads associated with population growth in the catchment as well as treatment technologies. 2. The trophic state of Utah Lake has increased to more eutrophic conditions from pre-European settlement to present day, as supported by multiple indicators. 3. Climate change has impacted temperature and precipitation in the basin, resulting in changes in the hydrologic and thermal regime of Utah Lake. 10 1.2. What were the historic phosphorus, nitrogen, and silicon concentrations as depicted by sediment cores? (add calcium, iron, and potentially N and P isotopes) Evidence evaluation Sediment cores provide extensive information about historical concentrations and forms of elements. Several independent sediment core analyses have been conducted in Utah Lake, each providing information on elemental concentrations and forms through time. Early studies noted issues in accurately obtaining isotope dating of sediments (Brimhall 1972, Bolland 1974), whereas more recent studies have been more successful in attributing specific dates to sediment core depths (Williams et al. 2023, Brahney et al. 2024). Phosphorus (P) P concentrations have increased over time in Utah Lake sediments. P concentrations increased over time from approximately 20 cm depth to present day in the older core studies (Brimhall 1972, Bolland 1974). Williams et al. (2023) also demonstrated increases in sediment P over time, with the most marked increases in Provo Bay starting around 1940 (Figure 5). It is important to note that TP concentrations in sediments cannot be attributed directly to historical concentrations and loading given the mobility of P in sediments. Brahney et al. (2024) also noted P increases over time, particularly at depths of 30 cm and 10 cm, which were associated with the timing of carp introduction and loss of macrophyte vegetation (Phase Transition 2 at 30 cm) and increased trophic status and cyanobacterial presence (Phase Transition 3 at 10 cm) (Figure 6). Associated with these phase transition in most cores was an increase in exchangeable and organic P, which could indicate an increase in trophic status but is also complicated by organic matter degradation and mobilization of sediment P in porewater, processes that are active in surface sediments. The HCl fraction of sequentially extracted P is associated with calcite minerals, and this fraction can also integrate a water column P concentration. The Goshen Bay sediment core contained an abrupt increase in the HCl P fraction near Phase Transition 3, and the Bird Island and Provo Bay cores contained increases in HCl P around Phase Transition 2. As part of size-separated CaCO3-P analysis, Brahney et al. (2024) found calcite-bound P (and other P extracted by sodium acetate) doubled from baseline concentrations to present day (Figure 7). The increase began in the late 1800s, coincident with the introduction of common carp and shortly after European settlement. A second significant increase was observed post-1970. Figure 5. TP concentrations in three Utah Lake sediment cores: deepwater (black), Goshen Bay (blue), and Provo Bay (green). From Williams et al. 2023. 11 Figure 6. Sequential P extractions for Bird Island, Provo Bay, and Goshen Bay sediment cores. Two additional sediment cores collected in Goshen Bay and the northern main basin are also displayed in Brahney et al. 2024. 12 Brahney et al. (2024) concluded: “All cores revealed increases in sediment phosphorus concentrations through time, with distinct shifts occurring around 30 and 10 cm in most cores, which align with Phase Transitions 2 and 3, which temporally align with Carp introduction into the Utah Lake system and the more modern-day hypereutrophic conditions. Provo Bay had the highest modern sediment P concentrations and the North core the lowest, which agrees with conditions present in the water column as determined through DEQ monitoring. The most insightful records come from the targeted CaCO3-P analyses, which revealed strong relationships between water column P and anthropogenic sources of pollution as deduced from strong correlations to the Provo Bay population, metrics of sewage (d15N), and indicators of eutrophic conditions in the lake (water column cyanobacterial pigments). The latter analyses suggest a doubling of water- column phosphorus through the sediment record.” Carbon (C) and Nitrogen (N) In sediment cores analyzed by Williams et al. (2023) and Brahney et al. (2024), the C and N content of sediments increased over time (Figure 8, Figure 9). The sediment core in Williams et al. (2023) showed a steady increase from pre-European settlement to present, whereas the core in Brahney et al. (2024) showed an abrupt increase at that time and fairly steady C and N content since. Concurrently with increases in %C and %N, C:N ratios decreased, consistent with a shift from macrophytes to phytoplankton. Decreases in δ13C over time are also consistent with a shift from vascular plants to phytoplankton, but could also be associated with the Suess effect, a global phenomenon of declining δ13C values in response to fossil fuel combustion. δ15N values have increased over time, shifting from 3‰ to 7‰. Nitrate derived from sewage is isotopically enriched in 15N (10-25‰) compared to natural soils (2-5‰) and atmospheric N (0±3‰), so this increase is consistent with a greater contribution of N from wastewater. There was also a significant positive correlation between societal development and δ15N, as Figure 7. Sediment core collected in Goshen Bay. Panel A: Population of Utah Valley (World Population Review, 2022) with the covered wagon indicating European Settlement in 1849 (Janetski, 1990), the fish indicating Carp introduction in ~1883 (Heckmann et al. 1981), and the spigot indicating the implementation of secondary sewage treatment in ~1950. Panel B: The concentration of phosphorus in physically and chemically separated authigenic calcite grains, Panel C: accumulation rates of P bound in authigenic calcite, and Panel D: SEM-EDS determined mean concentrations of P from individual authigenic CaCO3 grains, error bars represent the standard error. E: Total Phosphorus (TP). Dashed lines indicate significant (p < 0.1) shifts in the mean. From Brahney et al. 2024. 13 well as the timing of wastewater infrastructure in Provo (sewers 1908, treatment plant 1956) and Orem (sewers 1945, treatment plant 1959). While the direction of change for these metrics was consistent across the lake, the timing and magnitude varied among regions of the lake that were differentially impacted by agricultural development and wastewater sources over time. Figure 8. Sediment N and P concentration (blue) and stable isotope values (black) in the deepwater core. C and N analysis in additional sediment cores collected in Goshen Bay and Provo Bay are also displayed in Williams et al. 2023. Figure 9. Sediment core profiles of organic matter (OM), C and N content and stable isotope values in the northern main basin of Utah Lake. Dashed lines indicate significant abrupt changes detected by STARS. C and N analysis in an additional core from Goshen Bay is also displayed in Brahney et al. 2024. 14 Other Elements The iron:manganese (Fe:Mn) ratio decreased at 30 cm depth in the Goshen Bay sediment core, suggesting a shift to more oxic conditions, which could be associated with the introduction of carp to Utah Lake and a subsequent disruption of sediment burial and reducing conditions (Brahney et al. 2024). The Fe:Mn ratio did not shift abruptly in the Bird Island sediment core, but there was a general increase in both Fe and Mn at both 30 cm and 15 cm depth. Note that these analyses were conducted for total elemental concentrations rather than iron and manganese oxides. Williams et al. (2023) noted that increases in lead in the sediment were observed as early as 1920, consistent with the introduction of leaded gasoline and the construction of the Ironton pig-iron plant (active 1920s-1962). The construction of Geneva Steel was hypothesized to be a source of additional heavy metals (1944-2002). Silica was noted by Science Panel members as an element of interest as a potential limiting nutrient for diatom growth. However, there is a dearth of water column Si concentration data in Utah Lake, so the question of Si availability for diatom growth remains unanswered. Synthesis Phase transitions in sediment cores have indicated shifts in lake biogeochemistry around the time of European settlement and carp introduction in the late 1800s as well as the timing of hypereutrophic conditions. Concentrations of P, N, and C have increased over time in Utah Lake sediments, as confirmed by multiple studies. Increasing P concentrations in the calcite fraction of sediments suggests that water column P concentrations may have doubled from pre-European settlement to present day. C and N isotopes support a shift from a macrophyte-dominated state to a phytoplankton-dominated state and an increasing prevalence of wastewater in lake N supply. Additional analysis of metals and other elements have indicated changes in redox conditions over time, but these observations are not consistent across locations. The Science Panel has high confidence in the big picture of these major shifts in the lake, based on multiple independent studies with similar findings. 15 1.3. What information do paleo records (eDNA/scales) provide on the population trajectory/growth of carp over time? What information do the paleo records provide on the historical relationship between carp and the trophic state and nutrient regime of the lake? Evidence evaluation The introduction of common carp to Utah Lake (~1881) aligns well with a rapid transition to eutrophic conditions (fewer macrophytes, higher phytoplankton and cyanobacteria presence) as identified in a multi-proxy analysis of lake sediment cores (transition point dated to 1877 +/- 34 years) (King 2019, Brahney et al. 2023). In addition, sediment elemental concentrations suggest that prior to carp introduction, sediments may have been more stable and were capable of more substantial Fe and Mn reduction (Brahney et al. 2023). eDNA analysis was attempted for fish populations, but data were inconclusive. Synthesis Paleolimnological evidence indicates that the introduction of carp to Utah Lake (known from historical records) was associated with a shift to more eutrophic conditions, as evidenced by a shift from a macrophyte-dominated primary producer community to a phytoplankton-dominated community and a shift to less stable sediments. Given the concurrent timing of carp introduction and other human disturbances in the watershed, it is difficult to parse the specific mechanisms and magnitude of the impact of carp from other drivers in Utah Lake. The Science Panel has medium confidence in this statement, due to a lack of available multi-proxy carp indices. 16 1.4. What do phytopigments and DNA in the paleo record tell us about the historical water quality, trophic state, and nutrient regime of the lake? Evidence evaluation Sediment cores provide information about phytopigments and DNA that serve as indicators for water quality conditions in Utah lake. Phytopigment information indicates a massive lake disturbance with an increase in diatom production around 1890 and greater chlorophyll a degradation rates post-1890 in Goshen Bay and Bird Island (King et al. 2024). The highest chlorophyll a concentrations were observed near the surface of sediment cores in Goshen Bay and Provo Bay, consistent with a transition from diatom production to cyanobacteria and green algae production in recent decades (King et al. 2024). Chlorophyll a concentrations and cyanobacterial pigments showed minor decreases in production around 1950 before increasing to highest concentrations toward present day (King et al. 2024). eDNA records also show an increasing abundance of cyanobacteria post-1900 (King et al. 2024, Brahney et al. 2024). There was greater preservation of pigments historically when the sediments were more stable, with increased degradation toward present with destabilization and increased oxic exposure. King et al. (2024) provided an analysis of algal pigments associated with specific taxa (Figure 10). In general, pigments showed 5- to 10-fold increases from baseline to present day. In Provo Bay, pigments associated with chlorophytes (pheophytin b) increased toward present day. In Goshen Bay, two increases in concentration of cyanobacterial pigments (echinenone) were observed, for a total of a 30-fold increase from baseline to present Figure 10. Concentrations of photosynthetic pigments and the degradation ratio (Chl a:Pheo a) in Utah Lake sediment cores. Dashed lines indicate significant abrupt changes as detected by STARS. From King et al. 2024. 17 day. Diazotrophic (N-fixing) cyanobacteria (via aphanizophyll) were not detected in cores until circa 1967, displaying a sharp increase approaching present day. The pigment associated with diatoms (diatoxanthin) was the only pigment that showed a decrease in concentrations, “suggesting a shift in relative taxa from diatoms towards green algae [i.e., chlorophytes] and cyanobacteria.” Synthesis Photosynthetic pigments have increased from baseline to present day, indicating increasing levels of primary production in Utah Lake. Phase transitions were associated with the late 1800s/early 1900s as well as the mid- 1900s.The concentration of taxa-specific pigments indicates a shift from the dominance of diatoms towards green algae (chlorophytes) and cyanobacteria at present. Pigment analysis is consistent with observations across other sediment proxies of increasing trophic state in Utah Lake from historical baseline toward present day. Degradation rates of photopigments have also increased over time, associated with sediment destabilization and increased exposure to oxygen. The Science Panel has high confidence in this evaluation. 18 2.1. What are the impacts of carp on the biology/ecology and nutrient cycling of the lake and how are those impacts changing with ongoing carp removal efforts? Specifics of this question are addressed as part of sub-questions 2.1.i, 2.1.ii, and 2.1.iii below. Overall, there are several direct and indirect impacts of carp on the ecology and biogeochemistry of Utah Lake, including excretion, sediment resuspension, disturbance of macrophytes, and bioturbation. The assessments of confidence around these relationships are detailed as part of the response for each relevant sub-question. 2.1.i. What contribution do carp make to the total nutrient budget of the lake via excretion rates and bioturbation? How much nutrient cycling can be attributed to carp? Evidence evaluation In response to carp removal efforts, carp biomass in Utah Lake has decreased from a high of ~50 million kg in 2010 to a low of less than 20 million kg in 2018 and 2019. As a compensatory response to lake level increases, carp biomass increased to ~35 million tons in 2021 (Landom et al. 2022, Walsworth et al. 2024; Figure 11). Assuming literature values of 8-12% N and 1.0-4.5 % P (Sterner and Elser 2002), the 2021 Utah Lake carp biomass represents 2,800,000-4,200,000 kg N and 350,000-1,575,000 kg P. Lakewide carp excretion estimates based on individual size, density, and regression (mean and 95 % credible interval) were 71,500 kg TP y-1 (51,100-117,000), 23,400 kg SRP y-1 (16,700-38,500), 694,000 kg TN y-1 (496,000-1,140,000), and 436,000 kg NH4+ y-1 (312,000-717,000) (Tetra Tech 2021). Importantly, carp excretion represents recycling, meaning that they may incorporate rapidly cycling pools of N and P that may pass through carp multiple times in a year rather than a distinct one-way flux of nutrients such as external loading or removal. Carp likely decrease the capacity for sediments to effectively sequester P due to bioturbation (Goel et al. 2020), as evidenced by a study in Minnesota that showed increases in sediment mixing depth led to an increase in P mobility by 55-92% (Huser et al. 2016). Synthesis Given the available information, the Science Panel has medium confidence that carp excrete a substantial amount of N and P in Utah Lake and decrease the capacity for sediments to permanently sequester nutrients due to bioturbation and food web cycling. Thus, carp play a mediating role in determining how much of the nutrients that enter the lake reside within the sediments vs. water column. Carp excretion is not a new discrete source of N and P to Utah Lake, but represents nutrient recycling, so comparisons of excretion rates with external loading are not equivalent. Figure 11. Carp biomass in Utah Lake. From Walsworth et al. 2024. 19 2.1.ii. What is the effect of carp removal efforts on macrophytes, nutrients, secchi depth, turbidity, and primary productivity? Evidence evaluation Experimental studies using exclosures in Utah Lake have shown that carp can have a negative effect on macrophyte growth and abundance (due to bioturbation that disrupts root stabilization, as well as direct herbivory Miller and Provenza 2007), as well as macroinvertebrate abundance and diversity (Miller and Crowl, 2006). Carp impacts on nutrient recycling are summarized in question 2.1.i and may impact the pool of bioavailable nutrients available for phytoplankton. However, interannual variability in recent years (2016-2018) in Utah Lake does not provide strong support that submerged macrophytes are returning with decreased carp populations due to removal efforts (Landom et al. 2019). This may be due to the fact that submerged macrophytes do not necessarily return rapidly on their own following improved water clarity (Jeppesen et al. 2005; Hilt et al. 2006), and successful recovery may require physical planting of macrophytes to re-establish communities (Liu et al. 2018). Also, lake restoration measures that focus solely on internal remediation (i.e., carp removal) without including external nutrient loading reductions may result in unstable intermediate macrophyte recovery states (Hilt et al. 2018), which may be reflective of the unusual patchiness in macrophyte distribution described in Utah Lake in recent years (Landom et al. 2019). The most recent June Sucker Recovery Implementation Program ecosystem reports (Landom et al. 2022, Landom and Walsworth 2024) detailed the following water quality and aquatic community impacts of carp reduction: increases in Secchi depth, increases in green algae abundance (a preferred food source for zooplankton), no change in cyanobacteria abundance, a shift to large-bodied zooplankton taxa, increase in nearshore macrophyte species richness, and increases in some macroinvertebrate taxa including Annelids and Chironomids. Answering this question is also dependent on our understanding of wind-driven sediment resuspension and impacts of changing lake level (see question 2.1.iii, 2.2, and 2.3.vi). Synthesis Given the available information, the Science Panel has high confidence that carp removal efforts relieve negative pressures on macrophyte community growth and reestablishment, reduce nutrient recycling through the carp population, reduce bioturbation that mobilizes sediments and creates more turbid conditions. Macrophyte reestablishment is unlikely to occur spontaneously with carp removal efforts alone and may require active planting efforts and external nutrient loading reductions. Carp removal efforts may have mixed impacts on phytoplankton growth because carp bioturbation and recycling have the capacity to both reduce transparency and also mobilize sediment nutrient pools into the water column. Carp removal efforts have been associated with several shifts across the aquatic food web in Utah Lake, including positive impacts on native fish recovery including a shift to larger-bodied zooplankton, a higher prevalence of green algae, and increases in macroinvertebrate abundance. 20 2.1.iii. How much non-algal turbidity and nutrient cycling is due to wind action versus carp foraging? How much does sediment resuspension contribute to light limitation, and does wind resuspension contribute substantially in the absence of carp? Evidence evaluation No experimental studies have been conducted in Utah Lake to directly quantify the causal impacts of carp on clarity. However, carp exclosure experiments revealed stronger negative outcomes for macrophytes and macroinvertebrates in large exclosures compared to smaller ones, as well as differences between lake sides, indicating that resuspension and wave action are important controls on Utah Lake macrophytes as well (Miller and Crowl 2006). Data across recent years also indicated an important role for lake level as a control of submerged macrophyte abundance (Landom et al. 2019, Landom and Walsworth 2024). Wind conditions in the lake are sometimes, but not usually, sufficient to entrain sediments into the water column (typically wind speeds above 3- 4.5 m/s depending on site; Figure 12). Critical shear was exceeded for 24% of samples at the North site, 7 % of samples at the State Park site, and 15% of samples at the South site (Tetra Tech 2021). Light attenuation from non-algal turbidity made up 74 ± 8% (mean ± standard deviation) of total light attenuation (Tetra Tech 2021). Non- algal turbidity is likely made up of a combination of wind action and bioturbation, but the relative contribution of these sources has not been quantified. Assuming a light compensation point for macrophyte growth of 10 µmol m-2 s-1, 22% of sampled light conditions in Utah Lake are below the compensation point. Time of year had a significant effect as well. The probability of being below the light compensation point was 5% at 1 m depth, 23% at 2 m depth, and 61% at 3 m depth. The depth at which there were equal odds of being above and below the compensation point was 2.73 m (Figure 13; Tetra Tech 2021). Figure 12. Relationship between wind-driven wave shear and turbidity. Critical wave shear (the wave shear necessary to entrain suspended sediments into the water column) it noted as dashed vertical lines, typically associated with wind speeds exceeding 3.0-4.5 m/s depending on location. From Tetra Tech 2021. 21 Synthesis Given the available information, the Science Panel has medium confidence that carp and wind both contribute to increased non-algal turbidity and light limitation of photosynthesis in Utah Lake, with wind being the primary hypothesized driver of increases in non-algal turbidity. However, there is low confidence in the ability to assess the relative impacts of carp and wind, because available studies did not evaluate these impacts concurrently. Figure 13. Relationship between water column depth and probability of light conditions at the bottom being below the light compensation point for macrophyte growth. Measured samples are noted as dots, with a value of 0 indicating the sample was above the light compensation point and a value of 1 indicating the sample was below the light compensation point. The line represents a logistic regression based on observed samples. From Tetra Tech 2021. 22 2.2. What are the environmental requirements for submerged macrophytes currently present at Utah Lake? Evidence evaluation Several factors dictate the survival and growth requirements for submerged macrophytes. The response to question 2.2 focuses on light, and additional factors are discussed in questions 2.2i and 2.2ii. General requirements for submerged macrophytes in freshwater systems include light availability, water level, appropriate sediment substrate and nutrient availability, and sheltering from mechanical disturbance including wave action, bioturbation, and ice heaving. Light compensation points for species documented in Utah Lake (Ceratophyllum demersum, Elodea canadensis, Myriophyllum spicatum, Potamogeton pectinatus, Potamogeton praelongus; Brotherson 1981, Miller and Crowl 2006, Landom et al. 2019) range from 3.5-45 µmol m-2 s-1 (Madsen et al. 1991, Sand-Jensen and Madsen 1991, Spencer and Rejmanek 2010). C. demersum, a submerged macrophyte documented as a native species in Utah Lake (Landom et al. 2019, Landom and Walsworth 2024), was found to have a light compensation point of 7.2 µmol m-2 s-1, within the range of compensation points for seven species (6.9 ± 1.9 µmol m-2 s-1; Sand-Jensen and Madsen 1991). Multiple factors impact light level in Utah Lake, including sediment resuspension, carp bioturbation, and phytoplankton shading, and additional factors may impact macrophytes beyond light, including physical disturbance of sediments, carp bioturbation, wind and wave action, and seasonal/rapid/large changes in lake level (see questions 2.2i and 2.2ii). Modeling studies have also indicated that carp and epiphytic algae can act together to eliminate submerged macrophyte communities in lakes (Hidding et al. 2016), providing support for concurrent internal (carp removal) and external (nutrient loading reduction) efforts. Water clarity and benthic primary production models indicate a historical clear-water state, featuring a self-stabilizing submerged macrophyte community would likely require mean phytoplankton chlorophyll a concentrations < 18 µg/L and mean Secchi depths of ~ 1 m (considering 2018 water levels), compared to 2018 mean chlorophyll a concentrations of 40 µg/L and Secchi depths of 0.25 m (King et al. 2023). A consideration that may impact these requirements is whether a given macrophyte species maintains biomass low to the ground, hence requiring light conditions to be maintained throughout the growing season, or if the species grows nearer to the water surface throughout the growing season and thus may only need requisite light conditions to be maintained at the start of the growing season. These analyses assume sufficient nutrient availability in the sediment and/or water column to sustain macrophyte growth, which is likely a safe assumption in Utah Lake. Synthesis The current absence of macrophytes in Utah Lake is partially attributed to reduced clarity from a range of drivers including phytoplankton primary production, carp bioturbation, and sediment resuspension, which reduce the light availability for submerged macrophytes. Increased water clarity is needed to re-establish macrophyte communities. Light requirements for submerged macrophytes in Utah Lake have been analyzed through literature review, analysis of monitoring data, and scenario modeling, and all point to a need for increased water clarity to support macrophyte communities. The Science Panel has high confidence that light is a primary growth limiter for submerged macrophytes in Utah Lake, acknowledging that macrophyte reestablishment relies on several factors other than light such as lake level, carp reduction, and limiting competition from invasive macrophytes, which are discussed in subsequent questions. 23 2.2.i. What is the role of lake elevation and drawdown in macrophyte recovery? Are certain species more resilient to drawdowns and nutrient related impacts? Can some species establish/adapt more quickly? Evidence evaluation Utah Lake experiences substantial inter- and intra-annual lake level fluctuations due to hydrologic variability in the watershed and active management (Figure 14). Landom et al. (2019) found that declining lake levels were associated with a decrease in macrophyte coverage across three sampled locations in Utah Lake. Seasonal changes in species composition were variable, and lake level had less of an association with native emergent species, Alkali bulrush (Bolboschoenus maritimus) and cattail (Typha latifolia). Studies in other lakes have shown that regulating water levels in tandem with natural local hydrological inter-seasonal variability (Zhao et al. 2012), while also maintaining internal concurrent bioremediation efforts (Beklioglu et al. 2017), may improve submerged macrophyte recovery in shallow eutrophic lakes. An updated analysis (Landom et al. 2022) indicated that lower lake levels allow for colonization of emergent-terrestrial taxa including tamarisk trees, cottonwood trees, and alkali bulrush. These same taxa, however, may not be able to tolerate inundated conditions when the lake returns to higher lake levels. Establishment of invasive phragmites has been noted as an issue, leading to treatment which is nonspecific and can also harm native macrophytes. Recolonization by native macrophytes may occur depending on lake level, provided further invasion of phragmites is inhibited. Synthesis In addition to water clarity, fluctuating lake levels and invasive phragmites also represent barriers to macrophyte re-establishment in Utah Lake. Some emergent species such as cattails and bulrush are more resilient than submerged macrophytes to lake level fluctuations. Invasive phragmites is also a resilient species, though establishment of phragmites may impede native community establishment. Given the available evidence, the Science Panel has high confidence that fluctuating water levels negatively impact native macrophyte reestablishment in Utah lake. Figure 14. Utah Lake historical lake levels. Accessed from utahlake.gov/water-levels/. 24 2.2.ii. What is the relationship between carp, wind, and macrophytes on non-algal turbidity and nutrient cycling in the lake? What impact could macrophyte reestablishment have? Evidence evaluation Non-algal turbidity makes up ~3/4 of turbidity in Utah Lake, and increases in turbidity are associated with wind events (Figure 15, Figure 16, Tetra Tech 2021). Thus, wind-driven sediment resuspension is hypothesized as the main driver of non-algal turbidity in Utah Lake. Forthcoming information from the EFDC/WASP application for Utah Lake will allow for an examination of the causal relationship between wind and turbidity. High wind events have the dual impact of limiting submerged vegetation persistence through sediment resuspension and physical disturbance, as demonstrated in eastern areas of Utah Lake, which experience a higher number of high wind and wave events (Miller and Crowl 2006). Carp bioturbation may also increase turbidity and disrupt rooted macrophytes, the latter of which has been demonstrated as part of controlled carp enclosure experiments in Utah Lake (Miller and Crowl, 2006). Paleolimnological sediment core data on chlorophyll degradation products indicates the impacts of carp as well, supporting the idea that macrophytes stabilized sediment prior to carp introduction (Brahney et al. 2021). Macrophytes have the capacity to take up and store nutrients, which in pre-settlement conditions may have been a pathway to maintaining low water column nutrient concentrations. Senescence of macrophytes represents an additional pathway of nutrient release that alongside watershed nutrient loading and carp bioturbation could have supported the shift from a macrophyte-dominated state to a phytoplankton-dominated state. Figure 15. Proportion of light attenuation attributed to non-algal (i.e., suspended sediment) turbidity in the Main Basin (orange) and Provo Bay (purple). 25 James et al. (2004) demonstrated a 46 and 63% reduction in bed shear for two different species of macrophytes at low biomass (<20 g/m2), and Wang et al. (2010) demonstrated a 20-80% reduction in bed shear when macrophytes were present. Assuming that sampling dates and locations are representative of the lake, resuspension events would be nearly eliminated if macrophyte cover reduced bed shear by 60% or more (Tetra Tech 2021), though this analysis did not address duration, frequency, and sheltering effects of bays and nearshore areas. Macrophyte reestablishment would reduce sediment resuspension events, which could create a positive feedback loop to expand macrophyte cover (Landom et al. 2022). However, the question remains as to how particle sinking and epiphyte growth would impact macrophyte establishment. Synthesis Wind and carp both have a positive relationship with turbidity in Utah Lake through sediment resuspension. This evidence is supported by Utah Lake data, which is consistent with literature from other systems. Additional evidence from EFDC/WASP could help to strengthen the causal connection in this system. Both wind-driven waves and carp have a negative impact on macrophytes in Utah Lake through physical disturbance and reduction in light availability. Reduced water clarity hinders macrophyte reestablishment, but macrophytes could have a positive feedback on water clarity in Utah Lake through sediment stabilization. The Science Panel has high confidence that wind and carp have negative impacts on macrophyte recovery and high confidence that macrophytes have a positive impact on sediment stabilization. Figure 16. Relationship between wind speed and turbidity in four locations around Utah Lake. From Tetra Tech (2021). 26 2.3. What are the linkages between changes in nutrient regime and Harmful Algal Blooms (HABs)? Specifics of this question are addressed as part of sub-questions 2.3.i through 2.3.vi below. Overall, there is a positive relationship between nutrients and HABs in Utah Lake, but this relationship is complicated by mediating factors including spatial and temporal variability, bioavailability of nutrient pools, non-algal turbidity, climate, and the balance of internal and external nutrient loading. The assessments of confidence around these relationships are detailed as part of the response for each relevant sub-question. 2.3.i. Where do HABs most frequently start/occur? Are there hotspots and do they tend to occur near major nutrient sources? Evidence evaluation There are hot spots of HAB abundance in Utah Lake. In some cases, HAB hot spots appear to be related to nutrients, but other physical and chemical factors may play a role as well (e.g., vertical and horizontal mixing, residence time, light). The abundance of HAB taxa, including cyanobacteria as well as individual genera (Aphanizomenon, Oscillatoria, Phormidium, Dolichospermum, and Cylindrospermopsin) have highest abundance in Provo Bay and in the northeast portion of the main basin (Figure 17; Tetra Tech 2021, Utah Lake Data Explorer). Nutrient bioassay experiments showed a similar spatial pattern, where the highest concentrations of chlorophyll and phycocyanin were observed in Provo Bay compared to the eastern and western main basin (Aanderuud et al. 2021). This observation appears to be consistent among years, with Tate (2019) noting Provo Figure 17. Cyanobacteria (HAB) abundance across monitored locations in Utah Lake. From Tetra Tech (2021). 27 Bay and the eastern shoreline of the main basin experienced algal blooms in 30 out of 34 years that were analyzed as part of the study. These hot spots correspond with the highly urbanized areas surrounding Utah Lake which contribute watershed nutrient loading to the lake as well as high sediment phosphorus concentrations (Figure 25; Randall et al. 2019). Devey (2022) found that parts of Utah Lake most affected by harmful algal blooms, such as Goshen and Provo Bays, may have experienced phosphorus increases of over 100% in the last two hundred years. Stable isotope and elemental ratio data from sediment cores has also indicated that nitrogen concentrations have increased and are consistent with agricultural and urban land uses (Brahney et al. 2024). HABs in Provo Bay are in close proximity to sub-catchment outflows that contain publicly owned treatment works (i.e., Mill Race with Provo POTW, Spring Creek - Springville with Springville POTW, and Dry Creek - Spanish Fork with Spanish Fork POTW). HABs in the northeast portion of the main basin are located near Lindon Marina, which may have requisite chemical and physical conditions from both nutrient sources and circulation patterns. Additional upcoming mechanistic modeling applications covering both in-lake (EFDC-WASP) and watershed (HSPF) conditions can help to parse the causal relationship between nutrient sources and HABs. Satellite-based observations of primary production and HABs in Utah Lake show hotspots in Provo Bay and Goshen Bay, with Provo Bay and areas on the eastern side of the lake showing a significant increasing trend in chlorophyll concentration over the course of the most recent two decades (Figure 18; Narteh 2011, Cardall et al. 2021). Hansen et al. (2019) found that remote sensing evidence observed a significant increasing trend of average, extreme, and variability in chlorophyll concentration, with a shift of peak bloom occurring earlier in the season. Figure 18. Median chlorophyll values for July from 1999-2015, representing a total of 31 images. Scale legend was not provided in the published study. From Cardall et al. 2021. 28 Synthesis HABs typically occur most strongly in Provo Bay and Goshen Bay as well as eastern portions of the main basin of Utah lake. This observation has been demonstrated over the course of many years through well-accepted phytoplankton analysis methods, including direct measurements of cell count, biomass, and chlorophyll concentration as well as satellite-based observations. This spatial pattern has been demonstrated. HABs are generally associated with areas of the lake that are near watershed loading, and sediment records indicate a consistent enrichment of nutrients in these areas. It is well-known that nutrients are necessary to stimulate HABs, and HABs generally occur in areas of the lake exposed to high watershed nutrient loading. However, parsing the attribution of HABs to the proximity to nutrients alongside the hydrodynamic circulation of the system is complex and subject to variability. The Science Panel has high confidence in this statement given the length of record, established methods, known sources of nutrients, and causal spatial linkages. 29 2.3.ii. Which nutrients are controlling primary production and HABs and when? Evidence evaluation Utah Lake Data Explorer, sourced from the UDWQ monitoring program, indicates a seasonal progression of relative abundance of various phytoplankton taxa typical of eutrophic lakes, with relative abundance shifting from a dominance of diatoms (Bacillariophyta) in the early spring, green algae (Chlorophyta) in the late spring, cyanobacteria (Cyanophyta, which includes HAB taxa) in the summer and early fall, and green algae again in late fall (Figure 19). These successional patterns are suggestive of nutrient limitation and competition among taxa with different growth and nutrient uptake strategies and are commonly observed in large eutrophic lakes (Chaffin et al. 2013, Kramer et al. 2018, Barnard et al. 2021, Xu et al. 2022, Baer et al. 2023). This observation is consistent with the hypothesis that lakes tend toward P limitation in the spring and early summer and co-limitation of N and P and N limitation in the late summer and fall. A subset of cyanobacteria are able to fix N2 from the atmosphere, thus cyanobacteria may at least partly offset N limitation through N2 fixation and outcompete other taxa during periods of N limitation. Aanderuud et al. (2021) demonstrated active N2 fixation in Provo Bay and the eastern main basin under a range of nutrient bioassay conditions, with rates in Provo Bay outpacing the main basin by a factor of four (as measured by the acetylene reduction method). N2 fixation was not observed in the western main basin. The controlled experimental bioassay study by Aanderud et al. (2021) directly assessed the effects of N and P additions on phytoplankton, including cyanobacterial responses (i.e., phycocyanin and microcystin concentrations). The total phytoplankton pool was most commonly co-limited by N and P across sites and seasons, with more variable responses to nutrients in the early part of the growing season (Table 1). Cyanobacteria displayed different nutrient limitation patterns across sites, with Provo Bay exhibiting P limitation or co-limitation through spring and summer, and main basin sites exhibiting no limitation, P limitation, N limitation, and co-limitation. There was sufficient N and P in the late summer and fall to support cyanobacterial growth, potentially reflecting adequate sources of external watershed loading and sediment release. Microcystin production was not nutrient limited and displayed highest concentrations in the early and mid-summer in the bioassay experiments. Micro- and mesocosm bioassay experiments are useful in their capability to address causal mechanisms of nutrient limitation, but they lack the capacity to reproduce all aspects of lake hydrodynamics and biogeochemistry and are best used in concert with whole-ecosystem observations and mechanistic modeling (i.e., the EFDC-WASP model). Figure 19. Seasonal succession of phytoplankton taxa in Utah Lake. From Utah Lake Data Explorer. 30 Table 1. Nutrient limitation of phytoplankton taxa in Utah Lake. From Aanderud et al. (2021). Variable and Location spring early summer summer late summer fall Cyanobacteria nutrient limitation East No limitation No limitation P No limitation No limitation West N+P No limitation N No limitation No limitation Provo Bay P N+P P No limitation No limitation Total phytoplankton nutrient limitation East No limitation N+P N+P N+P N+P West P No limitation N+P N+P N+P Provo Bay N+P N N N+P N The ULWQS Analysis Report (Tetra Tech 2021) evaluated the relationship between phytoplankton metrics (phytoplankton and cyanobacteria cell count and biovolume) and nutrients as a part of a quantile regression and a hierarchical multiple regression. The quantile regression showed a positive relationship between phytoplankton and total P (TP) across all quantiles, which is expected when phytoplankton make up a substantial proportion of particulate nutrients. A modest positive relationship was found between phytoplankton abundance and total N (TN) at the median quantile, but the relationship was not consistently positive across all quantiles. This stressor- response analysis established correlation, not causation, but demonstrates another line of evidence alongside bioassays and mechanistic modeling to establish the relationship between nutrients and HABs in Utah Lake. Upcoming work with empirical stressor-response modeling and the EFDC-WASP model will help to further evaluate the relationships between phytoplankton and nutrients, including scenarios for lower nutrient conditions and their potential impacts on bloom reduction in Utah Lake. Finally, supporting evidence from the literature may help to shed light on patterns of nutrient limitation in Utah Lake, including studies from comparable systems including Lakes Taihu, Okeechobee, and Erie. Synthesis Sources of evidence agree that phytoplankton growth limitation by both N and P exists in Utah Lake, but the occurrence of single nutrient limitation, dual nutrient limitation, and non-limited conditions is variable over time, space, and by phytoplankton taxa. Light, temperature, and hydrodynamics play additional factors in the limitation of phytoplankton growth in Utah Lake. The lines of evidence for this statement span short-term experimental bioassays, mechanistic modeling, stressor-response modeling of observed conditions, and comparisons to other lake systems in the literature. The Science Panel has high confidence in this statement given the extensive amount of evidence, observations over multiple seasons and years, and the robust lines of evidence 31 2.3.iii. If there are linkages between changes in nutrient regime and HABs, what role if any does lake elevation changes play? Evidence evaluation Utah Lake water level varies substantially (Figure 14) and is driven by a complex set of interactive factors, including variable precipitation-driven water inputs, water management, extensive evaporation, and active pumping of lake water for downstream use at the lake outlet. Phytoplankton metrics (cell count and biovolume of cyanobacteria and all phytoplankton) in Utah Lake are negatively correlated with lake elevation, as indicated by a hierarchical multiple linear regression (Tetra Tech 2021). This regression also took into account nutrient concentrations, turbidity, and seasonality separate from the influence of lake elevation. Tate (2019) also found that algal abundance was higher with declining water level, noting that lake level it not necessarily a direct driver but rather an integration of several possible drivers including temperature, nutrient loading, evaporation-driven nutrient enrichment, water clarity, hydraulic residence time, and sediment-water interactions. Upcoming work with the EFDC-WASP model will allow for scenario testing that incorporates changing lake level, which may help to parse the specific mechanistic drivers of lake level-associated impacts on HABs. In the June Sucker Recovery Program ecosystem monitoring report, Landom et al. (2022) demonstrated through multiple regressions that lake level tended to have a stronger statistical effect than carp population biomass, with a similar directionality as other analyses which showed lower lake levels associated with higher nutrients and primary productivity. Synthesis Lower lake elevations are associated with larger HABs, as demonstrated by multiple applications of a statistical regression technique on multiple years of monitoring data. Lake elevation appears to have a smaller impact on HABs than nutrients, and lake elevation encompasses several possible drivers which may interact and co-occur in Utah Lake. The Science Panel has high confidence that HABs are negatively correlated with lake elevation, though the specific causal mechanisms underlying this relationship have not been fully parsed out. 32 2.3.iv. How do other factors affect HAB formation in Utah Lake (e.g., climate change; temperature; lake stratification; changes in zooplankton and benthic grazers and transparency) Evidence evaluation Climate change, including more intense wet and dry cycles, has the capacity to modulate HABs and impact their management (Paerl et al. 2019, 2020). In Utah Lake, a hierarchical multiple regression indicated that all assessed phytoplankton parameters (phytoplankton and cyanobacteria cell count and biovolume) were negatively correlated with antecedent precipitation and evaporation, and cyanobacterial parameters were positively correlated with antecedent air temperature. In context, periods of higher phytoplankton abundance are likely to occur when TP and temperature are high and when precipitation and evaporation are low (Tetra Tech 2021). Tate (2019) demonstrated a positive temperature trend in Utah Lake over time, suggesting that climate change is anticipated to exacerbate phytoplankton growth. While transient stratification has been observed in Utah Lake (Tetra Tech 2021), frequent wind events prevent seasonal stratification and subsequent oxygen depletion and nutrient accumulation in the hypolimnion. Thus, Utah Lake does not behave like deeper dimictic or monomictic lakes that experience redox-driven mobilization of hypolimnetic nutrients that may fuel blooms. As a polymictic system, Utah Lake is so frequently mixed that sediment interactions become highly important over water column stratification as a driver of nutrient dynamics. Sediment-water column interactions and their potential impact on HABs are discussed in question 2.4.iii. Most analyses on zooplankton and macroinvertebrate communities in Utah Lake (e.g., Landom and Walsworth 2020) have focused on their relationships with carp rather than the phytoplankton community. Zooplankton communities in Utah Lake have experienced a shift from small-bodied taxa to large-bodied taxa as a result of carp removal efforts. Concurrently with carp reduction, green algae (the preferred food source of large-bodied zooplankton taxa) have increased in biomass, but there has been no change in cyanobacteria biomass (Landom et al. 2022). However, there is some evidence for the direct top-down impacts of zooplankton on the phytoplankton community in Utah Lake. Aanderuud et al. (2021) evaluated grazing effects explicitly in the nutrient bioassay study, though the microcosms did not contain large-bodied taxa such as copepods or Cladocera and thus represented only microzooplankton such as rotifers. Nonetheless, they observed reductions in both chlorophyll and phycocyanin levels, suggesting that the zooplankton present in the Cubitainers grazed green algae and cyanobacteria. Interestingly, chlorophyll concentrations in experimental microcosms in Provo Bay showed an increase in chlorophyll in response to grazing pressure. It should be emphasized that this study did not evaluate phytoplankton biomass but rather phytopigments and did not include the entire zooplankton community in the microcosms. In a previous study, it was noted that Monogononta, a rotifer species was associated with a decline in cyanobacteria biomass (Collins et al. 2019). Additional analyses are available to evaluate more implicitly the trophic relationships between primary producers and grazers in Utah Lake. The zooplankton biomass:phytoplankton biomass ratio (Z:P) can be used to interpret the relative efficiency of trophic transfer within a system, with lower values associated with more eutrophic systems. Z:P ratios ranged from 0.018- 0.175 in Utah Lake, with the lowest values occurring in the summer months associated with enhanced primary production (Figure 20). This observation suggests that zooplankton grazing is not able to exert top-down pressure to substantially inhibit summer primary production levels in Utah Lake. Similarly, EPA’s chlorophyll-zooplankton model (https://nsteps.epa.gov/apps/chl-zooplankton/; EPA 2021) assessed the relationship between phytoplankton and zooplankton across lakes nationwide and demonstrated a decoupling of zooplankton and phytoplankton at high chlorophyll concentrations due to the inability of zooplankton to maintain grazing pressure on the phytoplankton community. Transparency is addressed in question 2.3.vi. 33 Synthesis Various factors may impact HAB formation in Utah Lake, some of which have been analyzed in detail with long- term observational datasets in the lake, and others which are hypothesized based on relationships in the literature. Climate-related factors (temperature and precipitation) have a demonstrated relationship with phytoplankton in Utah Lake, and these drivers are anticipated to worsen over time with climate change. Consistent seasonal stratification and subsequent hypolimnetic nutrient accumulation is not observed in Utah Lake, though transient stratification may contribute to sediment nutrient release which can fuel HABs. There is some evidence to suggest grazing has the capacity to apply top-down pressure to phytoplankton communities in Utah Lake, but evidence in the literature as well as some initial studies in Utah Lake suggest that zooplankton grazing does not substantially inhibit HABs in the summer. The Science Panel has high confidence in its evaluation of the impacts of climate change, temperature, and thermal stratification on HABs and medium confidence in its evaluation of the impact of zooplankton grazing on HABs. Figure 20. Z:P ratios in Utah Lake. From Richards 2022. 34 2.3.v. What is the role of calcite “scavenging” in the phosphorus cycle? Evidence evaluation In the context of lake phosphorus cycling, the term “calcite scavenging” refers to the process by which phosphorus (P) binds to calcite minerals, transferring it from the dissolved phase to the particulate phase where it may be subject to sedimentation and burial. Calcite scavenging includes both P sorption and P precipitation. Sorption is a general term that may refer to several processes, but in many cases refers to a path by which P adsorbs to compounds and particles, including calcite, sediment particles, and other charged compounds. Precipitation is a process by which P reacts with calcite or other compounds to form a solid mineral. The distinction between sorption and precipitation occurs along a gradient, and the two terms are often used interchangeably in the literature. Previous work indicates that calcite precipitation may be a dominant pathway for P sedimentation in Utah Lake (LeMonte et al. 2021). Given the high alkalinity and pH, typical oxic conditions in the water column, and calcite saturation in Utah Lake, calcite precipitation is a favorable pathway by which P can be sequestered in the sediments (Randall et al. 2019). Calcite-P is non-bioavailable when sorbed (Devey 2022), and the environmental conditions in Utah Lake are such that the calcite sorption reaction is unidirectional, leading to the hypothesis that the sediments contain a high proportion of calcite-P that should remain in that form. Indeed, sequential sediment P extraction showed that the majority of sediment P is bound to Ca and Fe-(oxy)hydroxide minerals (Randall et al. 2019). Specifically, sequential extraction of sediment P revealed that 63% of sediment P was associated with carbonate minerals (61-65% across sites), 14% with redox-sensitive Fe and Mn compounds (12-17% across sites), 13% with non-extractable minerals and refractory organic matter (9-15% across sites), 8% with organic BI GB PB PP PV SS VY Si t e I D NH 4Cl BD NaOH HCl Residual Sequential P-Fraction 0 10 20 30 40 50 60 70 80 90 100 Total P-fraction (%) BI GB PB PP PV SS VY B) A) Figure 21. Sequential extraction of P from Utah Lake sediments (a) before and (b) after sorption isotherm experiments. Sequential P fractions represent P bound to loosely binding compounds (NH4Cl), redox-sensitive Fe- and Mn-compounds (BD), organic matter and OH-exchangeable compounds (NaOH), carbonate minerals (HCl), and non-extractable minerals and refractory organic matter (residual). BI = Bird Island, GB = Goshen Bay, PB = Provo Bay, PP = Pelican Point, PV = west of Provo Marina, SS = Saratoga Springs, VY = Vineyard (see Figure 22). From LeMonte et al. (2023). 35 matter and OH-exchangeable compounds (6-9% across sites), and 3% with loosely bound compounds (2-3% across sites) (Figure 21; LeMonte et al. 2023). These observations are consistent with Hogsett et al. (2019), Devey (2022), and Rivers et al. (2022). LeMonte et al. (2023) conducted further research on P binding in Utah Lake as part of the ULWQS, characterizing the chemical speciation of P in the water and sediment, creating a reaction network of P, quantifying P scavenging under a range of conditions, and calculating P sorption isotherms and partitioning coefficients under a range of conditions. P partitioning coefficients (Kd,f,l) were defined for sediments across several locations in Utah Lake (Figure 22) through Linear, Freundlich, and Langmuir models, respectively (LeMonte et al. 2023). Note that the linear model had the best fit at environmentally relevant concentrations for Utah Lake (<1 mg/L in the water column and <5 mg/L in sediment porewater), but the typical approach for P partitioning analysis is to run the analysis across a wide range of concentrations for completeness. The partitioning coefficient derived from the linear model was recommended for use at “lower, more environmentally relevant” concentrations (Figure 23). Kd,f,l values increased with pH, with total ranges across pH and sampling locations of 11.7-145 (Kd), 56-1,000 (Kf), and 3.89-71.2 (Kl). Figure 22. Sediment sampling and experimental locations in Utah Lake as part of the phosphorus binding study. From LeMonte et al. 2023. 36 Sediment P sorption maxima (Smax) ranged from 2,886-3,176 mg P/kg across sites and pH conditions from 8.0-9.0 (Figure 24). The relationship with pH was variable across sites, but the sorption maxima tended to be lowest at a pH of 9.0 (LeMonte et al. 2023). Measured Smax values were higher than current sediment P concentrations, which ranged from 306-1,894 mg P/kg in Randall et al. (2019) (Figure 25) and 482-1,109 mg P/kg in Carling (2019). This finding indicates that the sediments are capable of retaining more P than they currently do. 0 100 200 300 400 500 600 Cf, Total Dissolved Phosphorus Equilibrium Concentration (mg-P L -1 ) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Q, P h o s p h o r u s S o r p t i o n ( m g - P k g -1 ) S m a x (mg-P kg -1 ) = 1976 Q (mg-P kg -1 ) = 0 Raw Linear Freundlich Langmuir Lines and Points VY (Vineyard) Linear (R 2 = 0.9803) Q = K d*Cf+b d Kd (L kg -1) = 56.7 (52.8, 60.7) bd (mg-P kg -1) = -1.57 (-24.0, 0.00) X, Sorption Origin (mg-P L -1) = 0.03 (0.00, 0.45) Freundlich (R 2 = 0.9820) Q = K f*C f (1 /n )+bf Kf (L kg -1) = 1000.00 (511,1489) n = 5.92 (4.04, 7.81) bf (mg-P kg -1) = -855 (-1346, -364) X, Sorption Origin (mg-P L -1) = 0.39 Langmuir (R 2 = 0.9796) Q = (S m a x *K l*C f)*(1+(K l*Cf))-1+b l Kl (L kg -1)= 0.03 (0.02, 0.04) Sm a x (mg-P kg -1) = 1976 (1873, 2079) bl (mg-P kg -1) = 0.00 X, Sorption Origin (mg-P L -1) = 0.00 pH = 8.6 - 8.2 I = 0.08 T = 25 oC Figure 23. A representative plot of Q (P-sorption; mg-P kg-1) vs. Cf (Total Dissolved Equilibrium P; mg-P L-1) obtained from a sorption isotherm experiment which reacted surface lakebed sediments from the Vineyard site and surface waters. Note that water column TP concentrations in Utah Lake rarely exceed 1 mg/L, indicating that the environmentally relevant portion of this graph is restricted to the lowest x axis values. From LeMonte et al. (2023). 37 Figure 24. Sorption maxima (Smax) sediment concentrations across sites in Utah Lake across pH levels commonly observed in the lake. BI = Bird Island, GB = Goshen Bay, PB = Provo Bay, PP = Pelican Point, SS = Saratoga Springs, VY = Vineyard. From LeMonte et al. 2023. Figure 25. Sediment TP concentrations in Utah Lake. From Randall et al. 2019. 38 A summary of key findings of LeMonte et al. (2023) that pertain to calcite scavenging of P: • “Most phosphorus (63%) in Utah Lake sediments is relatively stable due to its association with carbonate minerals (e.g. calcite)” • “For most sites in the lake, P partitioning coefficients (Kd) increase with increasing pH, suggesting that P partitioning to the sediment is more pronounced at pH > 8.0” • “Phosphorus reacts rapidly with Utah Lake sediments (max sorption by 100 min).” Synthesis Calcite scavenging is a dominant pathway for sediment sequestration of P in Utah Lake, as shown by sequential sediment extraction and batch sorption experiments. 63% of sediment P is stored in calcite minerals, which are not available for biological uptake. Permanent burial of P in the sediments as well as development of complex minerals such as apatite are facilitated by the sequential processes of sorption followed by precipitation. P sorption maxima are higher than observed sediment concentrations, indicating that sediments are capable of retaining more P than they currently hold. There is spatial variability across the lake, with Provo Bay having higher sediment P concentrations and higher capacity to store P than the main basin, with minor differences in P forms. The Science Panel has high confidence that calcite scavenging plays a large role in P sequestration in Utah Lake sediments. 39 2.3.vi. What is the relationship between light extinction and other factors (e.g., algae, TSS, turbidity)? Evidence evaluation Water clarity is influenced by several factors, including lake level, phytoplankton (algal turbidity), suspended sediment (non-algal turbidity), and dissolved organic matter. The ULWQS Analysis Report (Tetra Tech 2021) evaluated water clarity in Utah Lake across several analyses. Trophic state index as computed from Secchi depth was higher than those computed from chlorophyll, indicating non-algal turbidity contributes to reduction in clarity in Utah Lake. The ratio of observed vs. expected Secchi depth, the latter computed from chlorophyll concentrations, was 0.33 ± 0.23 (mean ± standard deviation). Light attenuation from non-algal turbidity made up 74 ± 8% (mean ± standard deviation) of total light attenuation (Figure 26) and was similar in the Main Basin and Provo Bay. There was also a positive correlation between turbidity and total suspended solids (TSS; Figure 27). Light attenuation was positively correlated with turbidity, TSS, chlorophyll, and dissolved organic carbon, consistent with relationships observed in the literature in other systems (Brown 1984, Armengol et al. 2003, Zhang et al. 2007, Devlin et al. 2008). Non-algal turbidity may thus present challenges for phytoplankton as they relate to light availability, although some HAB taxa use buoyancy regulation to alleviate light limitation and establish surface blooms (and access nutrient pools in bottom waters). Figure 26. Proportion of light attenuation attributed to non-algal (i.e., suspended sediment) turbidity. Orange boxplots represent proportions calculated from Secchi depth measurements (“estimated”) and purple boxplots represent proportions calculated directly from light profiles (“measured”). From Tetra Tech (2021). 40 Synthesis Light extinction occurs rapidly with depth in Utah Lake, and the majority of light attenuation is due to non-algal turbidity (i.e., suspended sediment), with approximately ¼ occurring due to phytoplankton. The proportion of light extinction attributed to phytoplankton is likely higher in Provo Bay than in the main basin. Additional factors impact light availability in the lake, including variations in water depth caused by lake level fluctuations. The Science Panel has high confidence in this statement. Figure 27. Relationship between total suspended solids (TSS) and turbidity in Utah Lake. From Tetra Tech (2021). 41 2.4. How do sediments affect nutrient cycling in Utah Lake? Specifics of this question are addressed as part of sub-questions 2.4.i-2.4.iii below. Overall, there is strong interaction between water column and sediments in Utah Lake. The sediments act as a net sink for nutrients but also release bioavailable forms of N and P. The assessments of confidence around these relationships are detailed as part of the response for each relevant sub-question. 2.4.i. What are current sediment equilibrium P concentrations (EPC) throughout the lake? What effect will reducing inputs have on water column concentrations? If so, what is the expected lag time for lake recovery after nutrient inputs have been reduced? Evidence evaluation Equilibrium P concentration (EPC) means the concentration of P in the water column at which there is no net exchange between water column and sediment because the gradients that drive exchange one way or the other are equal (i.e., at equilibrium). EPC was estimated using the equilibrium water column concentrations in sediment core experiments after spiking with a range of P concentrations (Goel et al. 2020). The values for EPC in the water column were calculated as 0.27 mg/L for the Buoy site and 0.86 mg/L for Provo Bay. While these numbers provided a first estimate of EPC, controlled batch sorption experiments are a more direct way to estimate EPC. Work by LeMonte et al. (2023) established that average EPC in sediments across Utah Lake (Figure 22) ranged from 0.30-1.07 mg/L, highest in Provo Bay and lowest in the western sampling sites at Pelican Point and Saratoga Springs (Figure 28). These values and geographic variability were fairly consistent with those estimated in the sediment core experiments, demonstrating agreement among lines of evidence. BI GB PB PP PV SS VY Site ID 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 To t a l D i s s o l v e d P A q (m g - P L -1 ) X (Sorption Origin) 95% C.I. Variable Figure 28. Equilibrium P concentrations as determined from the x-intercept of sorption isotherm models for Utah Lake sediments. Error bars represent 95% confidence intervals. BI = Bird Island, GB = Goshen Bay, PB = Provo Bay, PP = Pelican Point, SS = Saratoga Springs, VY = Vineyard. From LeMonte et al. 2023. 42 Utah Lake sediments are overall a sink for P, which was supported by P sorption experiments. According to LeMonte et al. (2023): “Lakebed sediment is usually a P sink, as indicated by equilibrium phosphorus concentrations (0.3 – 1.07 mg-P L-1) above the average P loading in the water column but near porewater P concentrations, and sorption capacities (Smax) greater than current sediment P concentrations.” Indeed, Figure 30. Total dissolved P (TDP) concentrations in Utah Lake inflow tributaries, measured near the outflow into the lake. From Tetra Tech 2022. Figure 29. Total dissolved P (TDP) concentrations in Utah Lake. From Taggart et al. 2024. 43 EPC P concentrations are lower than most tributary inflow concentrations and the lake retains 90-95% of external P loading (Figure 29; LeMonte et al. 2023, Tetra Tech 2022). The EPC P concentrations for Utah Lake are typically higher than concentrations observed in the water column, meaning that sediments are likely to exhibit desorption behavior (Figure 30; LeMonte et al. 2023, Taggart et al. 2024), but they are similar to or lower than what has been observed in the porewater (Randall et al. 2019; LeMonte et al. 2023). Question 2.4.ii further addresses sediment P release rates, including the interplay between sorption, organic matter decay, and iron reduction LeMonte et al. (2023) demonstrated phosphate porewater concentrations increasing from May through August (Figure 31), which correlated with primary production and algal bloom occurrence timing in Utah Lake. Primary producers may also drive nutrient exchange between the sediment and water column, with uptake in the water column leading to mobilization from the sediments. Figure 31. Porewater phosphate concentrations in Utah Lake sediments. From LeMonte et al. 2023. 44 As part of the sediment core experiments by Goel et al. (2020), reductions in water column P concentrations (i.e., dilutions) caused increased P flux from sediments over the timeframe of hours to several days. This condition is commonly observed in other systems from the literature, due to the shift in gradient between the water column and sediment. Additional factors, including drying-rewetting cycles observed in areas of the lake subjected to variable lake levels, also have the capacity to increase P flux from the sediments (Rivers et al. 2022). Stirred flow experiments showed that rates of P sorption slowed as water column concentrations decreased from higher concentrations approaching the EPC, and equilibrium was reached within 2 hours (LeMonte et al. 2023), consistent with rates of calcite equilibration in the literature (So et al. 2011, Taggart 2021). However, stirred flow experiments represent idealized equilibration times with sediment in suspension with water, and actual rates of equilibration in lake ecosystems are typically observed to last from days to decades depending on the system. Upcoming mass balance analysis by Science Panelist Mike Brett and application of the EFDC-WASP lake model will help to generate estimates of sediment equilibration timeframes under scenarios of reduced external P loading. This analysis will be supported by literature evidence from other calcareous lakes (Søndergaard et al. 2001, Jeppesen et al. 2005) and scenarios considering whether and how equilibrium P reactions between the sediments and water column will change as external nutrient loading decreases. Improvement in nutrient concentrations in Utah Lake could occur in under a decade if levels of internal P release and hydraulic flushing rates are maintained, or in as many as several decades if lake P retention decreases with decreasing external load. Synthesis Equilibrium P concentrations (EPC) from controlled batch sorption experiments ranged from 0.30-1.07 mg/L across sites in Utah Lake, indicating the P concentration at which the sediments switch between P uptake and release. EPCs were generally lower than Utah Lake influent P concentrations but higher than water column P concentrations, in line with the observation that Utah Lake sediments are an overall sink for P but have positive rates of dissolved P release (see question 2.4.ii for additional information). Sediment P retention or release is also dependent on additional processes, including organic matter sinking and decay. Reducing influent P concentrations below the EPC are predicted to cause sediment P release until a new equilibrium is reached. Reducing external P inputs is expected to result in lower water column concentrations, with a potential lag time associated with reaching a new water-sediment equilibrium. Current estimates of lake improvement following external P load reductions are anticipated to take months to decades, and more certainty on these timeframes will be achieved through upcoming mass balance and mechanistic modeling work. 45 2.4.ii. What is the sediment oxygen demand of, and nutrient releases from, sediments in Utah Lake under current conditions? Evidence evaluation Hogsett et al. (2019) conducted chamber deployments to evaluate sediment oxygen demand (SOD) in Utah Lake. SOD was measured as 1-2 g m-2 d-1 in the main basin, and ~4.5 g m-2 d-1 in Provo Bay (Table 2). The water column removed significantly more oxygen as compared to the sediments, suggesting that most activity is occurring in the water column. SOD was correlated with percent volatile solids (%VS), a surrogate for organic matter (Figure 32). Water column oxygen demand was not related to sediment %VS, suggesting the biota and reactions occurring in the water column are driving the ambient dissolved oxygen (DO) conditions in the water column. Figure 32. Table of sediment oxygen demand (SOD) in Utah Lake sediments (left), and correlation between SOD and percent volatile solids (%VS) (right). Site 1 is in Provo Bay, and sites 2-8 are in the main basin. From Hogsett et al. 2019. Table 2. Sediment oxygen demand (SOD) in Utah Lake sediments. Site 1 is in Provo Bay, and sites 2-8 are in the main basin. From Hogsett et al. 2019. 46 Goel et al. (2020) also evaluated SOD as part of sediment core experiments. At the main basin site, SOD was calculated as 2.97 g m-2 d-1. SOD was also modeled by the SedFlux model as part of the C, N, and P study (Tetra Tech 2021). SedFlux-modeled SOD rates were higher than measured SOD by an order of magnitude or more, suggesting that SedFlux may not be accounting for important factors driving SOD in Utah Lake, including the impacts of sediment resuspension and organic matter sinking rates. Note that measured SOD rates are reported from measurements at ambient temperature and are not normalized to 20º C, which contributes to variability across time and space. Key definitions for the consideration of sediment nutrient release: • Internal P loading: P released from the sediment to the water column. The term “internal loading” is a bit of a misnomer, as sediment nutrient release does not represent a novel source of nutrients but rather a recycled source of nutrients from prior deposition. • Net sediment nutrient flux: the rate of sediment release after taking into account the magnitudes of both uptake and release. This value can be negative if sediments are a net sink for nutrients. • Gross sediment nutrient flux: The total rate of sediment release without taking into account sediment nutrient uptake. This value is greater than the net sediment nutrient flux. • Diffusion: The physical process whereby nutrients move down a concentration gradient (into the water column from sediments in the case of sediment nutrient release). Molecular diffusion is a slow process (potentially irrelevant at this scale), but eddy diffusion is orders of magnitude faster and a more ecologically relevant diffusion process. This process can also be biologically driven via phytoplankton vertical migration and luxury nutrient uptake. • Physical exchange pathways: Sediment-water nutrient exchange via sediment resuspension, bioturbation, and other non-diffusive processes. Randall et al. (2019) found that areas with high P concentrations in sediment and pore water also had high concentrations in the water column. The east side of the lake (including Provo Bay) had the highest P concentrations in sediment, pore water, and the water column. This suggests that sediments store P and may act as a diffusive P release to the water column. P concentrations were positively correlated with organic matter, CaO, and Fe2O3 abundances as indicated by x-ray fluorescence, suggesting associations with organic, calcite, and redox-sensitive sediment fractions. The study showed that ~40% of P in sediments could potentially be released to the water column, but the study did not investigate release rates from sediments. Direct observations by Hogsett et al. (2019) and Goel et al. (2020) evaluated sediment nutrient fluxes under ambient redox conditions through field and lab experiments. The sediments were found to be a source of ammonium, nitrate, and orthophosphate associated with decaying organic matter that has settled (Table 3). The positive nitrate fluxes imply that the surface sediments are oxic (Hogsett et al. 2019). Rates of water column nutrient change (negative rates in most cases) indicated that processes such as primary production, calcite sorption, and N cycling were active and were capable of taking up as much or more nutrients than the sediment released. Table 3. Sediment and water column (WC) nutrient fluxes from Utah Lake sediment. Site 1 is in Provo Bay, and sites 2-8 are in the main basin. Note these rates were taken at ambient temperature during the summer months, and extrapolating to other months would require adjusting for temperature. From Hogsett et al. 2019. 47 Sediments were found to be active in terms of P release and uptake depending upon water column P concentrations (Goel et al. 2020). In lab experiments, sediment was a net dissolved P source under ambient or reduced P concentrations in the water column and a net dissolved P sink under elevated water column P concentrations. P release from sediments was highest when water column P concentrations were diluted 0.5x (maximum of 20.40±16.42 mg m-2 d-1). P retention was highest at 4x water column P concentrations (maximum flux of -51.84±8.30 mg m-2 d-1 from the water column to sediments). SedFlux modeling suggested fluxes of bioavailable nutrients (soluble reactive P, ammonium, nitrate) were generally positive, i.e., from sediment to water column and were similar to the range of measured rates (Figure 33). Under scenarios of variable organic matter sinking rates, nutrient fluxes were highly variable (Tetra Tech 2022). The Utah Lake littoral sediment study also quantified sediment nutrient release rates, demonstrating positive nutrient releases from sediments that varied by the incubation time and location (Rivers et al. 2022). TP release rates were -0.001 ± 0.001 g m-2 d-1 in western margin sediments and 0.005 ± 0.003 g m-2 d-1 in eastern margin sediments, and TN release rates were -0.009 ± 0.006 g m-2 d-1 in western sediments and 0.009 ± 0.008 g m-2 d-1. Rates of both TP and TN release increased the rates of nutrient release beyond those reported for the controls, with the dominant forms of these nutrients being dissolved organic P and nitrate. The drying-rewetting experiments suggest that rates of nutrient release from Utah Lake sediments may be highest in areas along the margin that are subjected to seasonal or interannual drying- rewetting cycles but are more poorly studied than permanently inundated areas of the lake. Finally, the EFDC- WASP model will also quantify sediment nutrient flux rates in an upcoming model application. 48 Figure 33. Sediment nutrient fluxes in Utah Lake estimated from the SedFlux model. From Tetra Tech 2022. 49 Synthesis Sediments in Utah Lake consume oxygen (i.e., have positive SOD rates), and rates of SOD are positively correlated with sediment organic matter, consistent with literature from other lake systems. Several sediment core, in situ observation, and modeling studies have demonstrated positive sediment release rates of dissolved nutrients in similar ranges, with variability across time, environmental conditions, and drying-rewetting conditions. Overall, the sediments are a major sink for nutrients, as indicated by 90-95% of P retained by the sediments. The sediments seasonally release bioavailable dissolved forms of N and P due to decaying organic matter and other processes (e.g., Fe reduction, diffusion, phytoplankton uptake, carp bioturbation), and these forms are rapidly taken up by primary producers and chemical reactions such as calcite sorption. The Science Panel has high confidence in this evaluation for SOD and medium-high confidence for nutrient fluxes based on the number of studies available, the variety of methodology supported by the literature, and the limitations of sediment core and chamber approaches. 50 2.4.iii. Does lake stratification [weather patterns] play a result in anoxia and phosphorus release into the water column? Can this be tied to HAB formation? Evidence evaluation There is evidence of transient thermal stratification in Utah Lake (https://udwq.shinyapps.io/lakedashboard/); the lake is polymictic and does not display consistent seasonal stratification in summer months. Utah Lake thus does not behave in the textbook nature of seasonal stratification and turnover that are associated with accumulation of nutrients and depletion of oxygen in the hypolimnion. Despite a lack of seasonal hypoxia or anoxia in the water column, it is possible that local zones of anoxia in the sediment may contribute to phosphorus because a fraction of P is bound to redox-sensitive Fe-oxyhydroxides. However, given than Fe/Mn oxide dissolution is slow compared to the rate of precipitation when oxygen is present, it is unlikely that stable anoxic conditions persist for long enough at the sediment-water interface or a transient hypolimnion for this process to be relevant. Physical processes also play an important role in P release to the water column; see question 2.4.ii for further discussion. Alternative mechanisms may be algal transfer of P from sediments to the water column via luxury nutrient uptake and vertical migration of phytoplankton (i.e., scavenging of nutrients at the sediment-water interface and harvesting of sunlight near the water surface). P release from sediments potentially contributes to HAB formation (Randall et al. 2019), particularly due to frequent wind-driven resuspension that bring surface sediments into contact with the water column. Synthesis Lake stratification does not occur on a widespread seasonal scale in Utah Lake, though transient stratification has the capacity to alter redox gradients in the water column and at the sediment-water interface. Release of nutrients from sediment porewater from diffusion and physical processes may fuel phytoplankton growth, though release through redox-driven pathways such as iron reduction is less likely given the lack of consistent hypoxic or anoxic conditions at the sediment-water interface. The Science Panel has high confidence that seasonal thermal stratification is not a driver of sediment P release compared to other pathways as described in question 2.4.ii. 51 2.5.i. For warm water aquatic life, waterfowl, shorebirds, and water-oriented wildlife, where and when in Utah Lake are early life stages of fish present? Evidence evaluation PSOMAS and SWCA (2007) evaluated the spawning and rearing habitat for 16 species of fish in Utah Lake as well as the relative percentage of time spawning conditions are met for those species across 14 lake sites (Table 4, Table 5). Exceedances of the 7-day average dissolved oxygen criteria of 6.0 mg/L and 1-day minimum dissolved oxygen criteria of 5.0 mg/L for early life stages have been observed in Provo Bay, and exceedances of the pH criteria of 9.0 have been observed in the Main Basin and Provo Bay (Tetra Tech 2024). Synthesis Given the available information, the Science Panel has medium confidence that spawning and rearing habitat meets the needs for some species in certain in-lake and tributary sites in Utah Lake but does not for other species and sites. The tables above provide more detail on specific species and sites. Further analysis could determine where and when early life stages of fish and birds are present. 2.5.ii. For warm water aquatic life, waterfowl, shorebirds, and water-oriented wildlife, which species are most sensitive and need protection from nutrient-related impacts? Evidence evaluation The June Sucker recovery program focuses on habitat-related recovery efforts but does not focus specifically on nutrient-related impacts. A potential future way to evaluate this question would be to relate the spatial aspects of HABs and the potential toxin-related impacts on aquatic life in those zones and then to the species that are utilizing those zones for spawning and rearing habitat (e.g., Provo Bay, Lincoln Beach, littoral zones). Forthcoming analyses for the technical support document will inform potential exceedances of various aquatic life water quality standards in Utah Lake, their relationships with nutrients, and the sensitivity of each indicator. Synthesis Given the available information, the Science Panel is not prepared to assess which species are in need of protection from nutrient-related impacts. 52 Table 4. Spawning and rearing information for fishes in Utah Lake. From PSOMAS and SWCA 2007. 53 Table 5. Relative percent of time that spawning conditions are met for warm water game fish in Utah Lake. From PSOMAS and SWCA 2007. 54 3. What additional information is needed to define nutrient criteria that support existing beneficial uses? 3.1: For warm water aquatic life, waterfowl, shorebirds, and water- oriented wildlife 3.2: For primary contact recreation 3.3: For agricultural uses including irrigation of crops and stock watering Question 3 pertains to the evaluation of whether there is sufficient information to derive nutrient criteria for the aquatic life (sub-question 3.1), recreation (sub-question 3.2), and agricultural (sub-question 3.3) beneficial uses for Utah Lake, and if not what additional information is needed. For each of the beneficial uses, several lines of evidence have available information to derive nutrient criteria. In the event of missing information, the missing information is not likely to impact our level of confidence in developing numeric nutrient criteria, particularly because the available measures have direct connections to nutrients whereas the missing measures tend to be more indirectly tied to nutrients. Additionally, it is not uncommon to evaluate support of beneficial uses with some sources of information missing. The SP has high confidence that numeric nutrient criteria that protect the beneficial uses of Utah Lake can be developed with available data sources. These questions will be addressed through upcoming development of the Technical Support Document that will support nutrient criteria development. 55 4.1. What would be the current nutrient regime of Utah Lake assuming no nutrient inputs from human sources? This question may require the identification of primary sources of nutrients. Evidence evaluation Synthesis from paleolimnological analysis provides insight into the pre-settlement condition of Utah Lake, which can be used as a point of reference for baseline nutrient concentrations and how they have changed over time. Along with model-based prediction of reference conditions, paleolimnological approaches can be used to infer nutrient conditions with minimal human contributions (USEPA 2000). Williams et al. (2023) concluded: “Total phosphorus, carbon, and nitrogen isotope and abundance data indicate that (a) the C:N ratios of the OM budget of the lake have been dominated by algal contributions, (b) that those algal contributions are, if anything, increasing, (c) that d13C values indicate that there has been a major reorganization of algal communities, and (d) that nutrient loading has variably affected the lake. TP in the deep-water core has increased only modestly, whereas in Goshen and especially Provo Bay, it has increased dramatically. In fact, in Provo Bay, TP has more than doubled since ~1940 and remained fairly stable since ~1960… Although the lake has always been turbid due to endogenic calcite production and algae in the water column, anthropogenically influenced discharge has had a major impact to lake ecology.” On the reference condition of Utah Lake, Brahney et al. (2024) concluded: 1. “Large areas of Utah Lake had substantially more macrophyte cover than present.” 2. “Large areas of Utah Lake were less turbid and allowed for greater light penetration to the sediment environment.” 3. “The present-day hypereutrophic condition combined with toxic cyanobacteria blooms is unprecedented in the history of Utah Lake.” Phase transition shifts observed in Utah Lake were observed concurrent with an ecological shift (introduction of carp, loss of macrophytes) followed by a trophic shift (increased nutrient inputs from the catchment). To return to a macrophyte-dominated system, additional restoration apart from nutrient reduction alone will be necessary. Forthcoming work as part of the EFDC-WASP application for Utah Lake and mass balance modeling by SP member Michael Brett will incorporate a minimal human impact scenario, thus directly addressing this question via a model-based prediction approach. Synthesis The nutrient regime of Utah Lake is expected to be less eutrophic than present day if human nutrient inputs were reduced. Historical P and N concentrations were both lower than present day, potentially as little as half. The historical condition of the lake was also less eutrophic and had fewer cyanobacteria than present day. Paleolimnological reconstruction and model-based predictions can both be used to infer reference conditions, the former of which has been completed in Utah Lake and the latter of which is underway. 56 4.2. Assuming continued carp removal and current water management, would nutrient reductions support a shift to a macrophyte-dominated state within reasonable planning horizons (i.e., 30-50 years)? Evidence evaluation Studies from other lakes have shown that the positive effects of nutrient reduction on water clarity can take 10-15 years to be apparent (Jeppesen et al. 2005), and that concurrent macrophyte recovery is best attained with active planting (Liu et al. 2018). Modeling of benthic primary production in Utah Lake indicated that Secchi depths of 1 m (an increase of 0.8 m) and chlorophyll concentrations of ~20 µg/L could sustain light levels for submerged macrophyte growth in the main basin, and shallow wind-sheltered bays could support macrophyte growth at even more modest Secchi depth improvements of 0.2-0.3 m and chlorophyll concentrations of ~25-28 µg/L (King et al. 2023). Macrophyte recovery would also depend on adequate lake level (depth and variability) and sediment stabilization. Landom et al. (2022) concluded: “Collectively, the combined effects of lake level fluctuations, Phragmites treatment efforts, and carp removal create a dynamic nearshore macrophyte community that has not yet demonstrated a substantial increase in macrophyte coverage, though species richness has increased since the onset of carp removal efforts.” Forthcoming information from the EFDC/WASP application for Utah Lake will evaluate management scenarios, which could provide insights into the impacts of nutrient reduction on water clarity and the potential for macrophyte restoration. Synthesis Evidence from the literature and from preliminary evidence from Utah Lake suggests that macrophyte restoration efforts take time and can require a combination of active management activities, including but not limited to nutrient reductions (i.e., habitat modification, active planting, carp reduction, sediment stabilization). Restoration activities will likely need to occur in specific zones of the lake, starting at small scales. At present, lake level fluctuations present a barrier to macrophyte recovery, which limits confidence in the impact of nutrient reductions in the absence of changing water management strategies. Specific scenario modeling of the timeframes and feasibility for achieving a macrophyte-dominated state in Utah Lake have is not yet available. 57 4.3. If the lake stays in a phytoplankton-dominated state, to what extent can the magnitude, frequency, and extent of harmful and nuisance algal blooms be reduced through nutrient reductions? Evidence evaluation Controlled experimental dilution bioassays by Aanderud et al. (2021) indicated that sufficiently low concentrations of dissolved N and P decrease phytoplankton growth in Utah Lake, but the relationship linking the magnitude of nutrient input reduction to a percent reduction in bloom biomass has not yet been assessed. There is also paleolimnological evidence indicating that lower nutrient concentrations were associated with less eutrophic conditions and fewer HABs (Brahney et al. 2024). Internal nutrient cycling will play a key additional role in determining the responses of phytoplankton to external nutrient loading reductions, particularly with respect to the length of time it may take for sediment nutrient release to equilibrate with reduced water column nutrient concentrations. While internal P loading may lead to a delayed lake reaction to reductions in external nutrient loads up to several decades, Randall et al. (2019) and LeMonte et al. (2024) suggested that the environmental conditions favoring calcite formation in Utah Lake represent a relatively permanent P sink, making sediment P less mobile and subject to re-equilibration with the water column. Reductions in N loading may also have a positive impact on phytoplankton blooms, particularly because sediment and water column N are subject to additional removal from the system via denitrification (Paerl et al. 2016, Scott et al. 2019). Forthcoming mass balance work by Science Panelist Mike Brett will address the length of time it may take Utah Lake to respond to reduced external nutrient loading, providing a potential timeline for water quality increases following management activities. The question of the impact of nutrient reductions on the magnitude, frequency, and extent of HABs will largely be addressed in forthcoming work through: 1. The Technical Support Document, which will demonstrate the levels of nutrients in the water column that are associated with HAB levels that support designated uses through regression analysis that demonstrates a positive correlation; and 2. The implementation planning effort, which will explore nutrient management activities in the watershed to achieve desired levels of water column nutrient concentrations, along with potential timelines for reaching these goals). Synthesis Studies to date have addressed the relationships between nutrients and HABs in Utah Lake through observational linkages as well as bioassay experiments. External nutrient loading and internal nutrient loading from the sediments have been demonstrated to play roles in nutrient supply to the water column, and the impact of external nutrient load reductions may be realized rapidly or over the course of years depending on sediment nutrient release dynamics. 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