Green Lake, in Green Lake County, Wisconsin, is a very deep (72 m), dimictic, mesotrophic system. The lake experiences an annual summer minimum in dissolved oxygen (DO) concentrations within its metalimnion. The minimum appears to be a natural feature of the lake, but it appears to be worsening over the past several decades, with concentrations below 1 mg/L occurring in some years. As a result, the lake has been classified as impaired based on low DO with phosphorus identified as the pollutant, yet the linkage between nutrients, productivity, and metalimnetic oxygen depletion is poorly understood. We combined high-frequency in situ observations of temperature and dissolved oxygen with several different modeling approaches to better understand the physical and biological processes behind the minimum. We also explored the potential roles of sediment oxygen demand and entrainment of tributary inputs. Results confirm that the metalimnion is a site of elevated community respiration. It appears that the extent of oxygen depletion in a given year may be more closely linked to springtime productivity than to ongoing epilimnetic productivity during the growing season, suggesting long-term reductions in the phosphorus inventory of the lake will be necessary to lessen the severity of the DO minimum. Physical factors, such as timing and duration of stratification, are also important, and therefore climate change may also exacerbate the problem.

Explaining Seasonal Hypoxia in the Metalimnion of Wisconsin’s Deepest Natural Inland Lake

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[Paper Proceedings](https://doi.org/10.1061/9780784483466)

Conference DOI: https://doi.org/10.26226/m.6329c39b68fb0e00131f77d2