Though freshwater harmful algal blooms have been described and studied for decades, several important dynamics remain uncertain, including the relationships among nutrient concentrations, phytoplankton growth, and cyanotoxin production. To identify when and where nutrients limit phytoplankton, cyanobacteria, and cyanotoxins, we conducted in situ bioassay studies. We added nitrogen (N), phosphorus (P), or N + P across various seasons in water collected from three locations across Utah Lake, one of the largest freshwater lakes in the western U.S. This shallow, hypereutrophic lake provides a powerful testbed for quantifying nutrient-growth-toxin interactions. We assessed a range of parameters over time, including photopigment concentrations, phytoplankton abundance (cell counts), cyanotoxins, and nutrient concentrations. Despite high background nutrient concentrations in lake water, phytoplankton abundance and composition were strongly affected by nutrient addition. Phosphorus limitation was more common in the spring, with N limitation and N + P limitation becoming more common in the fall. Nutrient additions were positively associated with cyanobacteria (Microcystis, Aphanocapsa, Dolichospermum, Merismopedia, Aphanizomenon spp.), eukaryotes (Aulacoseira, Desmodesmus spp.), and two taxonomical categories of phytoplankton (i.e., unicellular and colonial green algae). When detected, anatoxin-a was positively associated with Aphanizomenon and negatively associated with Microcystis spp. However, overall cyanotoxin concentrations were not associated with cyanobacterial cell density but varied seasonally. These findings highlight the importance of considering seasonal nutrient availability dynamics and provide insights into specific nutrient targets, species, and cyanotoxins that play a significant role in the health and management of similar eutrophic lake environments around the world.

We quantified effects of future climate warming on temperature and stability in a variably stratified, hypereutrophic reservoir with large fluctuations in water level by calibrating a 2-D model (CE-QUAL-W2, version 3.7.1, Portland State University, Portland, USA) of reservoir hydrodynamics using a time series (1992 to 2011) of inflow and air and water temperature. The model was then forced with increased air temperature projected by an ensemble of climate models that accounted for complex local topography and seasonality, with greater warming in summer. Warming increased annual evaporation rates by 2.6 to 7.9%. Water temperature increased by 0.44 (whole-reservoir; p < 0.05), 0.47 (epilimnion; p < 0.01), and 0.30 °C (hypolimnion; p < 0.05) per 1 °C increase in air temperature. Thickness of the epilimnion and hypolimnion diminished, with expansion of the metalimnion. Schmidt stability correlated with mean water depth over a wide range of depths (3.9 to 8.1 m; Adj. R² = 0.91 to 0.93; p < 0.001). Increased air temperature increased mean annual stability by 6.1 to 23.6 J m⁻² when depth was large and the reservoir stratified, but when depth was low (due to combined low inflow and, in preceding years, high withdrawals), inhibiting stratification, then water temperatures increased evenly (and more) throughout the vertical profile so change in mean annual stability was near zero (−0.1 to 1.1 J m⁻²). Combined effects of reservoir management (volume, timing, and elevation of water withdrawal) and climate warming (temperature of air and benthic sediment) can impact the hydrodynamic regime differently under variably stratified conditions with implications for release of phosphorus from sediment and vertical transport of phosphorus to the euphotic zone.

Subsurface flow dominates water movement from hillslopes to streams in most forested headwater catchments. Hewlett and Hibbert (1963,https://doi.org/10.1029/JZ068i004p01081) constructed an idealized hillslope model (0.91 × 0.91 × 15.0 m; 21.8°) using reconstituted C horizon soil to investigate importance of interflow, a type of subsurface flow. They saturated the model, covered it to prevent evaporation, and allowed free drainage for 145 days. The resulting recession drainage curve suggested two phases: fast drainage of saturated soil in the first 1.5 days and then slow drainage of unsaturated soil. Hydrologists interpreted the latter as evidence interflow could sustain baseflow, even during extended drought. Since that experiment, typical forest vegetation grew in the model, providing root and litter inputs for 55 years. We removed all aboveground live biomass and repeated the experiment physically and numerically (HYDRUS‐2D), hypothesizing that pedogenesis would change the drainage curve and further elucidate the role of unsaturated flow from hillslopes. Contrary to this hypothesis, drainage curves in our twice‐repeated physical experiments and numerical simulation were unchanged for the first ~10 days, indicating pedogenesis and biological processes had not largely altered bulk hydraulic conductivities or soil moisture release characteristics. However, drainage unexpectedly ceased after about 2 weeks (14.3 ± 2.5 days), an order of magnitude sooner than in the original experiment, due to an apparent leak in the hillslope analogous to commonly observed bedrock fractures in natural systems. Thus, our results are a more natural recession behavior that highlight how incorporation of alternative hydrologic outputs can reduce drainage duration and volume from soils to baseflow.

Human modification of water and nutrient flows has resulted in widespread degradation of aquatic ecosystems. The resulting global water crisis causes millions of deaths and trillions of USD in economic damages annually. Semiarid regions have been disproportionately affected because of high relative water demand and pollution. Many proven water management strategies are not fully implemented, partially because of a lack of public engagement with freshwater ecosystems. In this context, we organized a large citizen science initiative to quantify nutrient status and cultivate connection in the semiarid watershed of Utah Lake (USA). Working with community members, we collected samples from ~200 locations throughout the 7,640 km²watershed on a single day in the spring, summer, and fall of 2018. We calculated ecohydrological metrics for nutrients, major ions, and carbon. For most solutes, concentration and leverage (influence on flux) were highest in lowland reaches draining directly to the lake, coincident with urban and agricultural sources. Solute sources were relatively persistent through time for most parameters despite substantial hydrological variation. Carbon, nitrogen, and phosphorus species showed critical source area behavior, with 10–17% of the sites accounting for most of the flux. Unlike temperate watersheds, where spatial variability often decreases with watershed size, longitudinal variability showed an hourglass shape: high variability among headwaters, low variability in mid-order reaches, and high variability in tailwaters. This unexpected pattern was attributable to the distribution of human activity and hydrological complexity associated with return flows, losing river reaches, and diversions in the tailwaters. We conclude that participatory science has great potential to reveal ecohydrological patterns and rehabilitate individual and community relationships with local ecosystems. In this way, such projects represent an opportunity to both understand and improve water quality in diverse socioecological contexts.