Date of Award

2020

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Environmental Engineering (MS)

Administrative Home Department

Department of Civil and Environmental Engineering

Advisor 1

Cory McDonald

Committee Member 1

Pengfei Xue

Committee Member 2

Dale Robertson

Abstract

Green Lake, located in Green Lake County, Wisconsin, has been experiencing summer Metalimnetic Oxygen Minima (MOM). The severity of the MOM has increased in recent years and the lake was listed as impaired in 2014. To investigate the MOM, the lake was monitored during 2017 and 2018 at two sites using moored temperature and oxygen sensors recording at 1-hour intervals. Using these data, a hydrodynamic model (Simstrat) was configured for the lake. A new model was then developed to simulate the oxygen changes in the lake. Productivity, respiration, sediment oxygen demand (SOD) and fluxes were included, and hydrodynamics were driven by Simstrat output (diffusivity). The model provided a good fit and predicted oxygen concentrations and MOM intensity well. However, the modeled MOM occurred at shallower depths than were observed, potentially due to underestimation of mixing within the metalimnion by Simstrat. The model suggests that productivity affects the oxygen production, and therefore, concentrations in the epilimnion, but does not significantly affect the intensity and location of MOM formation. However, respiration terms affect the overall model behavior, MOM location, and MOM intensity. The availability of oxidizable materials or biomass appears to be the most important factor in explaining the MOM in this lake. By reducing the initial concentration of biomass to 2/3 of the initial value, the MOM occurrence can be managed and the minimum concentration of oxygen in the metalimnion can be kept above the critical value of 5 mg/l.

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