What causes metalimnetic hypoxia in Green Lake, Wisconsin, and why have they gotten worse?

In the early 1900’s, Green Lake, the deepest natural lake in Wisconsin, was oligotrophic with hypoxia only occurring in the deepest part of its hypolimnion. Increased nutrient (phosphorus) loading caused the lake to become mesotrophic, with increased algal production and hypoxia also occurring in its metalimnion (referred to as a metalimnetic oxygen minimum - MOM). Statistical and hydrodynamic water-quality modeling with the General Lake Model (GLM-AED) were used to describe the factors affecting MOMs and what could be done to reduce their extent. Model results indicated that worse MOMs occurred in years with increased productivity (higher chlorophyll a), poorer water clarity, and warmer water. Since 1905, MOMs increased in mean severity from ~ 7 mg/L to ~2 mg/L, and in recent years have a range of ~5 mg/L. Empirical relations developed from correlation analyses indicate that the recent variability in MOMs was about equally caused by variations in summer productivity and water temperatures. Climatic change may have caused stratification to develop earlier and the mean MOM to become more severe by ~0.6–2.1 mg/L. The remaining long-term change in mean MOM severity (2.9–4.4 mg/L) may have been caused by increased productivity caused by increased nutrient loading. Hydrodynamic water-quality modeling with incremental changes in phosphorus loading indicated that relative to 2014–18, a 57% decrease in controllable external phosphorus sources was needed to eliminate MOMs <5 mg/L in over 75% of the years, the target set by the Wisconsin Department of Natural Resources.

Primary Presenter: Dale Robertson, U.S. Geological Survey (dzrobert@usgs.gov)

Authors:

Dale Robertson, U.S. Geological Survey  (dzrpbert@usgs.gov)

Benjamin Siebers, U.S. Geological Survey  (bsiebers@usgs.gov)

Paul Reneau, U.S. Geological Survey  (pcreneau@usgs.gov)

Cory McDonald, Michigan Technological University  (mcdonald@hydroconsulting.com)

Robert Ladwig, University of Wisconsin / Aarhus University  (rladwig@ecos.au.dk)

David Hamilton, Griffith University  (david.p.hamilton@griffith.edu.au)