Contrasting water quality trends are occurring within and across North America, with waterbodies experiencing increasing phytoplankton blooms, increasing dissolved organic matter, or both, while others are becoming clearer and bluer; dramatically changing water color. To assess the spatial and temporal variability in water color, we quantified trends in satellite-derived dominant wavelength (λd) from 1984 to 2020 for 484 reservoirs across the state of Missouri using the LimnoSat-US dataset. Currently, the vast majority of Missouri reservoirs are classified as green and within a range (538–555 nm) that lies closer to the brown color endmember. Nearly one-third of reservoirs (n = 159) experienced significant temporal shifts in water color, with more (n = 91) negative (e.g., bluer) than positive (n = 68) λd trends. Linear mixed-effect models indicate that periods of extreme wetness and drought are associated with browner and bluer waters, respectively, and boosted regression trees further reveal that waterbody and watershed characteristics are important predictors for water color trends. We also analyzed trends in summer water quality (WQ) parameters from two long-term monitoring programs to evaluate independent and synchronous changes with λd. We provide analyses showing that particulate inorganic matter and Secchi depth most strongly correlated with λd, and total nitrogen and total phosphorus concentrations that are not typically associated with satellite-derived data have greater co-variance with λd than chlorophyll a. Although bluer waters often reflect reductions in inorganic turbidity, our findings show that they can at times coincide with increases in chlorophyll a. We further demonstrate that while λd trends broadly align with changes in water quality, co-occurring water quality and color trends in Missouri reservoirs at times defy a simplistic canonical interpretation, particularly in eutrophic waterbodies where changes in nutrient concentrations, chlorophyll a and water color can occur independent of each other. Our results help explain some of the previously observed heterogeneous controls on water color and emphasize the importance of integrating water quality data alongside commonly used landscape and morphological features.

The Reservoir Observer Student Scientists (ROSS) program engages high school students in year-round water quality monitoring to detect and understand cyanobacterial harmful algal blooms (cyanoHABs). This place-based participatory science program combines hands-on sampling, data analysis, and reflective learning to foster scientific literacy and contribute to environmental research. A mid-year data quality and analysis activity was introduced to improve students’ understanding of sampling rigor and enhance the reliability of their contributions. Additional points of reflection at the end of the school year improved students’ understanding of limnology and the process of doing science. Results show that students not only developed procedural knowledge and data literacy but also produced scientifically valid data, comparable to professional standards. The program demonstrates how structured training and inquiry-based activities can simultaneously advance educational outcomes and support long-term ecological monitoring.

Cyanobacterial blooms present substantial challenges to managers and threaten ecological and public health. Although the majority of cyanobacterial bloom research and management focuses on factors that control bloom initiation, duration, toxicity, and geographical extent, relatively little research focuses on the role of loss processes in blooms and how these processes are regulated. Here, we define a loss process in terms of population dynamics as any process that removes cells from a population, thereby decelerating or reducing the development and extent of blooms. We review abiotic (e.g., hydraulic flushing and oxidative stress/UV light) and biotic factors (e.g., allelopathic compounds, infections, grazing, and resting cells/programmed cell death) known to govern bloom loss. We found that the dominant loss processes depend on several system specific factors including cyanobacterial genera-specific traits, in situ physicochemical conditions, and the microbial, phytoplankton, and consumer community composition. We also address loss processes in the context of bloom management and discuss perspectives and challenges in predicting how a changing climate may directly and indirectly affect loss processes on blooms. A deeper understanding of bloom loss processes and their underlying mechanisms may help to mitigate the negative consequences of cyanobacterial blooms and improve current management strategies.

AbstractThe world's 1.4 million lakes (≥10 ha) provide many ecosystem services that are essential for human well‐being; however, only if their health status is good. Here, we reviewed common lake health issues and classified them using a simple human health‐based approach to outline that lakes are living systems that are in need of oxygen, clean water and a balanced energy and nutrient supply. The main reason for adopting some of the human health terminology for the lake health classification is to increase the awareness and understanding of global lake health issues. We show that lakes are exposed to various anthropogenic stressors which can result in many lake health issues, ranging from thermal, circulatory, respiratory, nutritional and metabolic issues to infections and poisoning. Of particular concern for human well‐being is the widespread lake drying, which is a severe circulatory issue with many cascading effects on lake health. We estimated that ∼115,000 lakes evaporate twice as much water as they gain from direct precipitation, making them vulnerable to potential drying if inflowing waters follow the drying trend, putting more than 153 million people at risk who live in close vicinity to those lakes. Where lake health issues remain untreated, essential ecosystem services will decline or even vanish, posing a threat to the well‐being of millions of people. We recommend coordinated multisectoral and multidisciplinary prevention and treatment strategies, which need to include a follow‐up of the progress and an assessment of the resilience of lakes to intensifying threats. Priority should be given to implementing sewage water treatment, mitigating climate change, counteracting introductions of non‐native species to lakes and decreasing uncontrolled anthropogenic releases of chemicals into the hydro‐, bio‐, and atmosphere.

Invasive species are often assumed to have measurable impacts throughout their potential range, but this is not always true. This study evaluated whether zebra mussels (Dreissena polymorpha) were causing an increase in water clarity near the southern edge of their North American expansion using a 40+ year dataset. We looked for abrupt shifts and long-term trends in water clarity and compared these to the estimated zebra mussel invasion date for 7 reservoirs in Missouri, USA. We also looked at water clarity in 26 non-invaded reservoirs to evaluate whether zebra mussel impacts were being masked by variation in environmental factors. Collectively, our analyses provide a weight of evidence based on a lack of consistent shifts in water clarity corresponding with zebra mussel invasions. Mussel establishment did not increase water clarity, likely because densities were too low to result in a measurable impact. The highest zebra mussel density we observed was 65 mussels m−2, an order of magnitude less than in systems where they have had a sustained impact. Low densities could be due to a combination of sublethal environmental conditions such as warm water temperatures, suspended inorganic particulates, fluctuating water levels, and reduced oxygen concentrations, as well as limited hard substrata or predation by fish. While additional study is required to understand which, if any, of these factors may be limiting zebra mussel establishment in Missouri, our study shows that dreissenids are not having an impact as elsewhere throughout their range, suggesting that how an invader alters ecosystem functions is critical to determining its impact.

Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep-water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1-126,909 ha), maximum depth (6-370 m), and morphometry, with a median time-series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll a concentrations, and oxygen demand across the 656-lake dataset. Likewise, we found further support for these relationships by analyzing time-series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake-specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyll a in high-phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.

Invasive, filter feeding zebra mussels (Dreissena polymorpha) typically cause an increase in water clarity shortly after their establishment. The purpose of this study is to evaluate whether this occurred in Midwest reservoirs, near the southern edge of their North American expansion, using a 40 + year dataset. We look for regime shifts and long-term trends in annual water clarity and compare these to the estimated zebra mussel invasion date for 7 invaded reservoirs in Missouri, USA. We also look at water clarity in 26 non-invaded, reference reservoirs to evaluate if zebra mussel impacts are being masked by changes in environmental factors. Collectively, our analyses provide a weight of evidence showing that zebra mussel establishment did not increase water clarity, likely because densities are too low to result in a noticeable impact. The highest zebra mussel density we observe is 65 mussels m − 2 , an order of magnitude less than in systems where they have had a sustained impact. Low densities could be due to a combination of sublethal environmental conditions. We identified common characteristics of invaded reservoirs, including reduced particulate inorganic material and water temperatures.

AbstractCyanobacterial blooms have substantial direct and indirect negative impacts on freshwater ecosystems including releasing toxins, blocking light needed by other organisms, and depleting oxygen. There is growing concern over the potential for climate change to promote cyanobacterial blooms, as the positive effects of increasing lake surface temperature on cyanobacterial growth are well documented in the literature; however, there is increasing evidence that cyanobacterial blooms are also being initiated and persisting in relatively cold‐water temperatures (< 15°C), including ice‐covered conditions. In this work, we provide evidence of freshwater cold‐water cyanobacterial blooms, review abiotic drivers and physiological adaptations leading to these blooms, offer a typology of these lesser‐studied cold‐water cyanobacterial blooms, and discuss their occurrence under changing climate conditions.

Reservoirs are globally ubiquitous and critical for inland carbon (C) cycling, but the coupled influence of land use and hydro-climatic fluctuations on reservoir C source and composition is relatively unexplored. In this study, we used chromophoric dissolved organic matter (CDOM) absorbance and fluorescence excitation and emission matrices modeled by parallel factor analysis to characterize the source and composition of reservoir CDOM. Water samples were collected from reservoirs along a land use gradient in Missouri, USA during a wet and dry summer, respectively. Dissolved organic C (DOC) and CDOM quantity assessed by absorbance at 350 nm ( a 350 ) were significantly ( r 2 = 0.5; p < 0.001) correlated, with CDOM quantity per unit DOC increasing with percentage agriculture. Stronger correlations were observed in the wet summer compared to the dry summer. The influence of land use and hydro-climatic variability was further confirmed by redundancy analysis between CDOM compounds, reservoir limnological parameters, watershed characteristics, and precipitation anomaly (Palmer Drought Severity Index). Agricultural reservoirs were shallow and abundant in a 350 , aromatic, humic acid- and fulvic acid-like components (C2), chlorophyll- a , and particulate organic matter. These OM signatures in agricultural reservoirs were more pronounced during wet summer conditions, indicating the greater watershed influence. In comparison, CDOM composition in deeper reservoirs was less influenced by watershed influxes. Further, reservoirs with longer residence times experienced internal CDOM processing and an abundance of low-molecular-weight, microbial (C3), and protein-like (C4) CDOM, particularly during drier conditions. Understanding the sensitivity of reservoir CDOM dynamics to watershed land use and variable hydro-climatic conditions will be useful for effective monitoring and resource management.

Winter, historically a largely un-monitored season, is important and changing. There is evidence of the importance of under-ice phytoplankton in temperate lakes, but it is currently unknown if the often high winter phytoplankton biomass translates to high productivity and what influence it has on year-round lake metabolism. Winters are getting shorter, but our ability to forecast change is hindered by our limited understanding of what happens under the ice. Here, we compare under-ice and open-water rates of areal gross production (AGP) and areal respiration (AR) from 3 Canadian reservoirs and one large lake using oxygen (O2) changes in light-dark bottle experiments, δ18O-O2 models, and fluorometry. During the open-water season, AGP was 81× greater than under-ice rates, with AR rates 8× higher than measured during winter. Open-water samples indicated autotrophy (P:R= 1.10). Consistent with current assumptions, the cold under-ice environment is associated with low primary productivity. Our results challenge the assumption that mean water column irradiance is lowest during the winter in dimictic water bodies; we find similar light conditions during the open-water season. Winter mean light is regulated by snow thickness; upon manual snow removal, we observe a 67 % increase in under-ice mean water column irradiance. The first-ever under-ice application of the δ18O2-method indicated that AGP responded to improvements in light. This study reveals further insights into the importance of under-ice metabolism on year-round processes in a changing climate.

Lakes are classified by thermal mixing regimes, with shallow waterbodies historically categorized as continuously mixing systems. Yet, recent studies demonstrate extended summertime stratification in ponds, underscoring the need to reassess thermal classifications for shallow waterbodies. In this study, we examined the summertime thermal dynamics of 34 ponds and shallow lakes across temperate North America and Europe to categorize and identify the drivers of different mixing regimes. We identified three mixing regimes: rarely ( n = 18), intermittently ( n = 10), and often ( n = 6) mixed, where waterbodies mixed an average of 2%, 26%, and 75% of the study period, respectively. Waterbodies in the often mixed category were larger (≥4.17 ha) and stratification weakened with increased wind shear stress, characteristic of “shallow lakes.” In contrast, smaller waterbodies, or “ponds,” mixed less frequently, and stratification strengthened with increased shortwave radiation. Shallow ponds (<0.74 m) mixed intermittently, with daytime stratification often breaking down overnight due to convective cooling. Ponds ≥0.74 m deep were rarely or never mixed, likely due to limited wind energy relative to the larger density gradients associated with slightly deeper water columns. Precipitation events weakened stratification, even causing short‐term mixing (hours to days) in some sites. By examining a broad set of shallow waterbodies, we show that mixing regimes are highly sensitive to very small differences in size and depth, with potential implications for ecological and biogeochemical processes. Ultimately, we propose a new framework to characterize the variable mixing regimes of ponds and shallow lakes.

Carbon, nitrogen, and phosphorus are critical macroelements in freshwater systems. Historically, researchers and managers have focused on inorganic forms, based on the premise that the organic pool was not available for direct uptake by phytoplankton. We now know that phytoplankton can tap the organic nutrient pool through a number of mechanisms including direct uptake, enzymatic hydrolysis, mixotrophy, and through symbiotic relationships with microbial communities. In this review, we explore these mechanisms considering current and projected future anthropogenically-driven changes to freshwater systems. In particular, we focus on how naturally- and anthropogenically- derived organic nutrients can influence phytoplankton community structure. We also synthesize knowledge gaps regarding phytoplankton physiology and the potential challenges of nutrient management in an organically dynamic and anthropogenically modified world. Our review provides a basis for exploring these topics and suggests several avenues for future work on the relation between organic nutrients and eutrophication and their ecological implications in freshwater systems.

Many Pythium spp. are causal agents of diseases of turfgrasses. Pythium spp. disseminate through irrigation systems in agricultural settings, and this study provides evidence that Pythium spp. also disseminate through golf course irrigation. Water samples were collected from irrigation heads and water sources at 10 golf courses in Missouri and Kansas, U.S.A. Samples were collected from 2018 to 2019 in April, July, and October. Phosphorus, nitrogen, and chloride concentrations were measured from irrigation head samples to determine if these parameters influence frequency of Pythium spp. detected. Pythium spp. were detected in samples through baiting and membrane filtration. Cultures were isolated on PARP media, and DNA was extracted from putative Pythium isolates. The internal transcribed spacer region was PCR-amplified and sequenced. Phylogenetic trees were constructed using representative sample sequences, sequences from seven morphologically identified reference isolates of Pythium, and similar GenBank accessions. Detected oomycete species include Lagenidium giganteum, Pythium biforme, Pythium insidiosum, Pythium marsipium, Pythium plurisporium, and Saprolegnia hypogyna. Twenty-one clades lacked species-level resolution, and 14 of these clades were associated with Pythium species. Clades A, C, D, E, I, and M contain Pythium species that cause root and crown rot on creeping bentgrass. Detected Pythium communities were dependent on the detection method used and sampling source. Pythium frequency and diversity were highest in April 2019. Sample temperature, sampling site, and chloride and nutrient concentrations did not influence Pythium frequency in samples. Irrigation systems using surface water sources contained at least three Pythium spp. over the course of 2 years.

Dissolved organic matter (DOM) and particulate organic matter (POM) can influence biogeochemical processes in aquatic systems. An understanding, however, of the source, composition, and processes driving inland reservoir organic matter (OM) cycling at a regional scale over the long term is currently unexplored. Here, we quantify decadal patterns (> 20 yr) of DOM quantity and composition and POM in 40 reservoirs in the midcontinent United States. We built 184 Random Forest models to identify how the relative influence of watershed characteristics and limnological parameters on OM dynamics may vary over time and in synchrony with hydroclimatic anomalies. The reservoir OM quantity and composition varied nonmonotonically through time and in contrast to lake browning observed in the northern hemisphere. Reservoir DOM composition switched from humic and aromatic during wet summers to aliphatic, potentially autochthonous DOM during particularly prolonged dry summers in the mid-2000s. The shift in reservoir DOM quantity and composition could be attributed to the change in time-varying control of watershed and limnological factors mediated by the hydroclimatic conditions. Watershed control (e.g., percent crops) was predominant during wet summers, while the effect of reservoir morphology (e.g., maximum depth) and water quality parameters (e.g., Secchi depth, chlorophyll a) were evident during dry summers. Thus, future predictions of drier conditions may promote "greening" with negative implications for reservoir water quality and treated drinking water. Considering the nonlinear nature of reservoir OM dynamics and its controls will help to better mitigate water quality issues in these constructed systems increasingly impacted by global changes.

Creeping bentgrass putting greens require frequent irrigation during summer months to meet evapotranspiration requirements. Golf courses that utilize local water bodies for irrigation may potentially spread phytopathogenic waterborne microbes, or toxins they produce, to putting greens. Cyanobacteria associated with yellow spot and black layer have been detected in water bodies and are known to produce cyanotoxins that may hinder plant growth at high concentrations. Water samples were collected from irrigation heads, irrigation intakes, and the epilimnion of exposed water sources. Anatoxin, cylindrospermopsin, microcystin, and saxitoxin concentrations were analyzed using ELISA kits. Microcystin and saxitoxin were detected in irrigation systems that utilize surface water sources. Anatoxin was detected in every irrigation system, regardless of water source. Cylindrospermopsin was only detected in one system that utilized a local water body as a water source. Only two samples from one site contained cyanotoxin (microcystin) concentrations that exceeded the recommended threshold for recreational water use.

Cyanobacterial blooms sometimes create secondary metabolites that can be transferred between trophic levels and accumulate in fish, but little is known about what time of year fish are most susceptible. Here, we examine microcystin in the muscle, liver, and kidney of bluegill and largemouth bass from an agricultural reservoir over 12 months. We identify which fish characteristics and water parameters best explain microcystin accumulation in fish tissues. Microcystin in bluegill was significantly higher than largemouth bass. In both species, microcystin was highest in livers (bluegill mean = 57.6 ng g -1, largemouth bass mean = 71.8 ng g -1 wet weight [ww]), then kidneys (bluegill mean = 27.1, largemouth bass mean = 22.7 ng g -1 ww), followed by muscles (bluegill mean = 7.6, largemouth bass mean = 5.7 ng g -1 ww). Adult bluegill feed on benthic macroinvertebrates and zooplankton, which may explain their higher microcystin concentrations compared to largemouth bass, which are primarily piscivorous. Harvest date emerged as the best predictor of microcystin in muscles and kidneys, with the highest concentrations occurring in April. Microcystin in water also emerged as a significant predictor, albeit much lower than harvest date, suggesting that low but persistent microcystin concentrations in water may result in accumulation of this cyanotoxin in fish. This study is the first to examine microcystin in fish from the North American Great Plains and one of only 5 studies that investigate microcystin in bluegill and largemouth bass. Additional investigation into the relationship between cyanobacteria and fish health is warranted, especially during spring when fish microcystin concentrations were highest.

Landscape-level analyses based on land cover, morphology, and hydrology account for most of the cross-system variation in pelagic nutrients and suspended solids in Missouri reservoirs. They are based on geometric means, which reduce the influence of extreme temporal variation measured in individual reservoirs. This analysis of 3 conservation reservoirs, managed to benefit recreational fisheries, details how internal processes can alter nutrients, chlorophyll, mineral turbidity, and transparency in long-term (21-42 year) datasets, which contribute to temporal variation. Management practices include the addition of grass carp and herbicides to control nuisance macrophytes and shoreline stabilization with rock and water willow. Among these reservoirs, there is strong evidence that macrophyte removal can increase pelagic nutrients by >90%, resulting in a switch to plankton-dominated conditions (alternative states). In one case, eradication of aquatic vegetation increased mineral turbidity by >60%, which was reversed by reestablishing macrophytes and stabilizing the shoreline. This temporal series supports the modifications of phytoplankton-nutrient relations by mineral turbidity shown in state-wide analyses. Collectively, the long-term data show a significant increase in cyanobacteria biovolume and cyanotoxins, with maximum microcystin concentrations increasing as much as 20 times. Actively flipping lakes to plankton-dominated systems via fisheries management and shoreline stabilization practices has negative impacts on overall water quality, with implications for human and wildlife health.

Largemouth bass (LMB, 265–475 mm) were collected to document whether changes in fish condition and reproductive status influenced the concentration of total mercury (Hg) and selenium (Se) in axial muscle by season and sex. The fatty acid (FA) composition of fish was also examined to describe seasonal and sexual differences and identify whether arachidonic acid (ARA) could be used as a biomarker of Hg toxicity. There was a trend for females to have lower (p < 0.062) Se concentrations than males. The concentration of Se for females during spring (mean ± SD, 686 ± 51 ng/g dw) was 15% lower than males (806 ± 67 ng/g dw). Lower Se concentrations in females than males continued through summer and fall. Concentration of Hg for females during spring (152 ± 39 ng/g ww) was also 59% lower than males (373 ± 303 ng/g ww), but the difference was not significant (p > 0.2). The percent of lipids was greatest in fall and winter (3%) and comprised primarily of omega-3 fatty acids (35 g/100 g lipid). Fish condition as measured by percent lipids and relative weight was negatively (p < 0.02) related to Hg concentration for females and males. Lipid content for both sexes was also positively (p < 0.05) related to the Se:Hg ratio. Relative weight was positively related to the Se:Hg ratio for females during all seasons (p = 0.014), but only during spring and summer for males (p < 0.007). A low Se:Hg value was associated with an elevation in ARA for both sexes and a reduced hepatosomatic index in males. Data suggested that females transferred muscle stores of Se and Hg to developing oocytes during spring. This study generates hypotheses regarding the physiological drivers of seasonal and sexual variability in Hg, Se, and FA in LMB that may be applicable to other species and have implications for fisheries health and management.

Freshwater cyanobacterial blooms have become ubiquitous, posing major threats to ecological and public health. Decades of research have focused on understanding drivers of these blooms with a primary focus on eutrophic systems; however, cyanobacterial blooms also occur in oligotrophic systems, but have received far less attention, resulting in a gap in our understanding of cyanobacterial blooms overall. In this review, we explore evidence of cyanobacterial blooms in oligotrophic freshwater systems and provide explanations for those occurrences. We show that through their unique physiological adaptations, cyanobacteria are able to thrive under a wide range of environmental conditions, including low-nutrient waterbodies. We contend that to fully understand cyanobacterial blooms, and thereby mitigate and manage them, we must expand our inquiries to consider systems along the trophic gradient, and not solely focus on eutrophic systems, thus shifting the high-nutrient paradigm to a trophic-gradient paradigm.

Among its many impacts, climate warming is leading to increasing winter air temperatures, decreasing ice cover extent, and changing winter precipitation patterns over the Laurentian Great Lakes and their watershed. Understanding and predicting the consequences of these changes is impeded by a shortage of winter‐period studies on most aspects of Great Lake limnology. In this review, we summarize what is known about the Great Lakes during their 3–6 months of winter and identify key open questions about the physics, chemistry, and biology of the Laurentian Great Lakes and other large, seasonally frozen lakes. Existing studies show that winter conditions have important effects on physical, biogeochemical, and biological processes, not only during winter but in subsequent seasons as well. Ice cover, the extent of which fluctuates dramatically among years and the five lakes, emerges as a key variable that controls many aspects of the functioning of the Great Lakes ecosystem. Studies on the properties and formation of Great Lakes ice, its effect on vertical and horizontal mixing, light conditions, and biota, along with winter measurements of fundamental state and rate parameters in the lakes and their watersheds are needed to close the winter knowledge gap. Overcoming the formidable logistical challenges of winter research on these large and dynamic ecosystems may require investment in new, specialized research infrastructure. Perhaps more importantly, it will demand broader recognition of the value of such work and collaboration between physicists, geochemists, and biologists working on the world's seasonally freezing lakes and seas.