In lakes and reservoirs, climate change increases surface water temperatures, promotes thermal stability, and decreases hypolimnetic oxygen. Increased anthropogenic land-use and precipitation enhance nutrient and sediment supply. Together, these effects alter the light and nutrient dynamics constraining phytoplankton biomass and productivity. Given that lake and reservoir processes differ, and that globally, reservoir numbers are increasing to meet water demands, reservoir-centric studies remain underrepresented. In the agricultural Midwest (United States), ubiquitous reservoirs experience eutrophy and hypolimnetic anoxia. Here, we explore influences of eutrophication and landuse on the proximate light and nutrient status of phytoplankton communities in 32 Missouri reservoirs. Light and nutrient status indicators include mixed layer irradiance, phosphorus (P) and nitrogen (N) stoichiometry/debts, photosynthetic efficiency, and photosynthetic-irradiance parameters. Contributing to the ongoing P vs. N and P management debate, we evaluate if phytoplankton biomass and productivity are constrained by light, P, N, or a combination thereof, across gradients of trophic status and land-use during two contrasting wet and dry summers. Despite agricultural prevalence, P-deficiency is more prominent than either Nor light-deficiency. In 2018, ∼46% of samples were P-deficient with ∼36% indicating neither light nor nutrient deficiency. Gross primary productivity per unit chlorophyll-a (GPP B) demonstrates negative relationships with nutrients, biomass, and turbidity, and positive relationships with light availability. GPP B is highest in oligotrophic reservoirs where light utilization efficiency is also highest. Overall, phytoplankton biomass and productivity appear constrained by P and light, respectively. If Midwestern reservoirs are precursors of future inland waters affected by climate change and eutrophication, our crystal ball indicates that both P and light will be important regulators of phytoplankton dynamics and subsequent water quality.

Worldwide, nutrient pollution, or eutrophication, is one of the most pervasive environmental issues threatening water quality. Anthropogenic influences, primarily urbanization and agriculture, have drastically increased inputs of bioavailable nutrients to surface water resources, such as lakes and reservoirs. Excessive phosphorus (P) and nitrogen (N) in these waters amplify the growth of suspended algae, or phytoplankton, resulting in unsightly, sometimes odorous, cyanobacterial harmful algal blooms (CyanoHABs) that further degrade water quality and potentially endanger human and animal health. As global climate change increases surface water temperatures, promotes water column stability, and drives down bottom-water oxygen concentrations, environmental conditions are also becoming more favorable for CyanoHABs. With eutrophication prevailing and CyanoHABs increasing in frequency, intensity, and distribution around the globe, I seek to further understand the role of light and nutrients as limiting agents for phytoplankton biomass and primary productivity across Midwestern reservoirs. I utilize numerous lines of evidence to create a robust assessment exploring the influences of climate, eutrophication, and land-use on the proximate light and nutrient status of phytoplankton in 32 Missouri reservoirs. Through observation and experimentation, deficiencies of light, P, and N are evaluated using general indicators of water quality and physiological stress, including mixed layer irradiance, nutrient stoichiometry/debts, photosynthetic efficiency, and the photosynthetic-irradiance (P-E) parameters. Ultimately, I determine if phytoplankton biomass and productivity are constrained by light, P, N, or a combination thereof, across gradients of trophic status and land-use within the context of two contrasting wet and dry summers. As expected, higher proportions of agricultural land-use correspond with higher total in-reservoir nutrient concentrations. Despite agricultural prevalence, however, bioavailable N concentrations in the mixed layer are, overall, relatively low. Yet, P-deficiency is more prominent than either N- or light-deficiency. For the 2018 season, I estimate nearly half of all samples to be P-deficient, with fewer than 20 percent suggesting alternative deficiency or co-deficiency combinations, and approximately one-third indicating sufficiency in both light and nutrients. Primary productivity demonstrates negative relationships with nutrients, biomass, and turbidity, and positive relationships with light. Thus, productivity is highest in clear, low-nutrient reservoirs where light utilization efficiency is also highest. Overall, in Missouri reservoirs, phytoplankton biomass and primary productivity are constrained by P and light, respectively. If current conditions in Missouri reservoirs are at all indicative of those to come as surface waters are further affected by climate change and eutrophication, both P and light will be important regulators of phytoplankton dynamics and subsequent water quality. Contributing to the ongoing P vs NP nutrient management debate, these results both support and challenge aspects of the traditional P-paradigm of limitation on phytoplankton dynamics. It underscores the importance of P control in reservoirs, while offering support for additional consideration of light and N. Having critical implications for watershed management throughout the region, these results are particularly useful within watersheds experiencing high agricultural nutrient loading. Results inform resource managers seeking to employ more effective strategies to control phytoplankton biomass, avoiding harmful regime shifts and CyanoHAB development. Additionally, results may inform lawmakers and regulators developing policies and standards to mitigate nutrient pollution and its effects on water quality at the local, regional, and, potentially, global scales.