Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

Abstract

The concentration of oxygen is fundamental tolake water quality and ecosystem functioning through itscontrol over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophiclakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimneticanoxia is determined by interactions between the lake andits external drivers (e.g., catchment characteristics, nutrientloads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). Howthese drivers interact to control the evolution of lake anoxiaover decadal timescales will determine, in part, the futurelake water quality. In this study, we used a verticalone-dimensional hydrodynamic ecological model (GLMAED2) coupled with a calibrated hydrological catchmentmodel (PIHM-Lake) to simulate the thermal and water qualitydynamics of the eutrophic Lake Mendota (USA) over a37 year period. The calibration and validation of the lakemodel consisted of a global sensitivity evaluation as well asthe application of an optimization algorithm to improve thefit between observed and simulated data. We calculated stabilityindices (Schmidt stability, Birgean work, stored internalheat), identified spring mixing and summer stratificationperiods, and quantified the energy required for stratificationand mixing. To qualify which external and internal factorswere most important in driving the interannual variation insummer anoxia, we applied a random-forest classifier andmultiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primaryproduction). Lake Mendota exhibited prolonged hypolimneticanoxia each summer, lasting between 50 60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior tosummer stratification were the most important predictors ofthe spatial and temporal extent of summer anoxia periods inLake Mendota. Interannual variability in anoxia was largelydriven by physical factors: earlier onset of thermal stratificationin combination with a higher vertical stability stronglyaffected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010was unexplained by the GLM-AED2 model. Although thecause remains unknown, possible factors include invasion bythe predacious zooplankton Bythotrephes longimanus. As theheat budget depended primarily on external meteorologicalconditions, the spatial and temporal extent of summer anoxiain Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.