MIXING, METABOLISM, AND CLIMATE CHANGE: A MODELING CASE STUDY OF A SHALLOW HYPEREUTROPHIC, POLYMICTIC LAKE
Date of Award
Open Access Master's Thesis
Master of Science in Environmental Engineering (MS)
Administrative Home Department
Department of Civil, Environmental, and Geospatial Engineering
Committee Member 1
Committee Member 2
Globally, lakes are sites of significant carbon cycling, respiring an estimated 0.07 to 0.15 Pg as CO2 and sequestering 0.03 to 0.07 Pg C in sediments annually. These processes can be affected by nutrient availability, with seasonal mixing regulating nutrient transport in monomictic and dimictic systems. However, the effect of intermittent mixing on ecosystem production in polymictic systems has been much less studied. The timing and frequency of lake mixing are expected to be altered by climate change, which has the potential to impact nutrient transport. The first chapter of this thesis introduces lake mixing dynamics and indices of mixing. In chapter two, the relationship between intermittent mixing and changes in productivity in polymictic systems is examined, under the hypothesis that productivity will increase in response to lake mixing. Ecosystem productivity was calculated via the diel oxygen technique for Goose Lake, Marquette Co., MI, over the 2019 field season. The diel changes in Net Ecosystem Production (NEP), Gross Primary Production (GPP), and Respiration (R) were cross-correlated with the diel change in Lake Number (LN), an index of stratification. One day after mixing, dNEP dt-1 and dGPP dt-1 were negatively correlated with dLN dt-1 with coefficients of -0.342 and - 0.209, respectively, at a cross correlation significance threshold of ±0.1859. This corresponds to an increase in NEP and GPP as LN decreases. These correlations suggest that GPP and NEP increase in response to mixing. In chapter three, climate-driven changes in stratification extent and mixing frequency are modeled for the early 2080’s relative to 2019. The one-dimensional General Lake Model (GLM) was autocalibrated for 2019 conditions using simulated annealing. The cost function consisted of the sum of temperature and Lake Number Normalized Root Mean Squared Error (NRMSE) to improve vertical heat distribution. Six Coupled Model Intercomparison Project 5 (CMIP5) climate models for the early 2080’s were input into the GLM model to determine changes in hydrodynamics. In all future scenarios, stratification extent and water temperatures increased relative to 2019. However, mixing frequency also increased and the lake remained polymictic. This increase in stratification is likely due to both increased air temperatures and lower wind speeds. Increased stratification and temperatures will likely exacerbate existing water quality problems by stimulating DO drawdowns and internal loading of phosphorus. These conditions will increase the probability of cyanobacteria blooms. Higher temperatures will likely shift the system further towards net heterotrophy due to the greater temperature dependence of respiration than photosynthesis. While the number of mixing events increased, this was due to significantly increased stratification which would be expected to decrease productivity. Therefore, it cannot be conclusively determined if productivity will increase in Goose Lake in response to climate change.
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Reuss, Benjamin C., "MIXING, METABOLISM, AND CLIMATE CHANGE: A MODELING CASE STUDY OF A SHALLOW HYPEREUTROPHIC, POLYMICTIC LAKE", Open Access Master's Thesis, Michigan Technological University, 2023.