Spatiotemporal variation of ecosystem metabolism in a eutrophic, polymictic reservoir: water column stability begets metabolism stability

Reservoirs are globally important aquatic ecosystems that provide critical anthropogenic functions, including water storage and fisheries, yet their ecosystem metabolism remains understudied. We estimated daily lake metabolism (net ecosystem production [NEP], gross primary production [GPP], ecosystem respiration [R]) at 9 locations throughout Clinton Lake, a temperate, hypereutrophic, polymictic reservoir in Kansas, USA, throughout a growing season. Using dissolved oxygen monitoring and Bayesian hierarchical generalized additive mixed models, we assessed spatiotemporal variation in metabolism and relationships with environmental predictors, including thermal stratification, temperature, wind, and phytoplankton. Clinton Lake exhibited frequent daily alternations between autotrophy and heterotrophy, resulting in the near-zero mean NEP (0.01 mg O₂ L⁻¹ d⁻¹) for the study period. Seasonal patterns were strong, with GPP and R magnitudes increasing throughout the growing season. Temporal variation dominated spatial variation in all metabolism metrics, but GPP and R models showed modest site-level effects. Thermal stratification was associated with metabolic variability. Stronger stratification was associated with higher GPP and R, whereas changes in stratification strength corresponded to directional changes in NEP: stratification promoted autotrophy while destratification promoted heterotrophy. We suggest the coupling between metabolism and stratification is driven by abundant buoyant cyanobacteria controlling oxygen dynamics near the surface. Our results suggest that Great Plains reservoirs can be highly dynamic ecosystems where wind-driven mixing strongly influences metabolic processes. The frequent stratification–destratification cycles characteristic of polymictic systems create spatiotemporally variable ecosystem function. These findings provide a baseline understanding for monitoring changes in reservoir ecosystems and highlight the importance of physical forcing in structuring metabolism in engineered systems.