While temperate lakes are commonly thought to turnover twice annually, in the fall and the spring, there are several factors that can reduce the probability of turnover. Whether or not a lake turns over has important implications for nutrient dynamics and food webs. In this study, we investigated several small deep lakes in SE Michigan to determine whether spring turnover had occurred. One factor affected by lake turnover is the distribution of oxygen in the lake. Lakes receive oxygen from the atmosphere at their surface and from small plant-like organisms called phytoplankton within the body of the lake. Photosynthesizing phytoplankton are typically more productive in the in the upper water layers because light is extinguished with depth. Oxygen is consumed over winter by bacteria in sediments at the bottom of the lake, which respire as they decompose debris, releasing nutrients. Wind forces and temperature changes in the spring and fall drive the water layers to mix. This process helps maintain a balance by circulating oxygen from the epilimnion (upper water layer) to the hypolimnion (bottom water layer) and nutrients from the hypolimnion to the upper layers. Factors that could affect whether a lake mixes include higher densities (from salinity), depth, temperature, and the shape of the lake in relation to wind direction. If a lake does not mix, we expect to find anoxic (oxygen depleted) conditions in the hypolimnion and lower nutrient concentrations in the epilimnion, resulting in a change in the distribution and productivity of phytoplankton.
Whether spring mixing events are occurring in small deep lakes in SE Michigan was the focus of this study. The results of the research show that complete mixing in spring occurred in only 2 of 5 lakes surveyed. In lakes with incomplete mixing, we found anoxic conditions in the hypolimnion and higher phytoplankton productivity in the metalimnion (middle layer) instead of the epilimnion. While the importance of monitoring lakes is acknowledged by the state of Michigan, annual surveys are rare due to budget constraints. More frequent monitoring would allow us to determine whether lack of turnover varies annually and what combination of factors increases the probability of turnover in small deep lakes.