Temperature explains the formation of a metalimnetic oxygen minimum in a deep mesotrophic lake

ABSTRACT Green Lake, a deep mesotrophic lake located in a primarily agricultural watershed in central Wisconsin, USA, has experienced annual metalimnetic oxygen minima since the early 20th century. However, the severity of the phenomenon has increased over time, and late-summer dissolved oxygen (DO) concentrations have typically been <3 mg L−1 in recent years. In situ, high-frequency observations of oxygen depletion at multiple depths reveal that while DO consumption during stratification occurs most rapidly in the metalimnion, there is synchrony between DO time series extending into the hypolimnion. A biochemical oxygen demand-based modeling approach suggests that much of the relationship between water depth and respiration rates can be explained by differences in water temperature. The amount of labile organic matter present throughout the water column at the onset of stratification seems to be a primary determinant of the severity of the annual metalimnetic DO minimum in late summer. Productivity has increased in the lake as a result of increased nutrient loading and is the likely driver of the decrease in minimum DO concentrations. In addition, the onset and duration of stratification is an important factor in determining the severity of the DO minimum.