Layered lake: Ecosystem Dynamics in a Freshwater Estuary During the Growing Season with Respect to Climate Change, Stratification, and Episodic Mixing Events

Hypoxia is being increasingly detected in estuaries and lakes around the world. While a natural phenomenon in some cases, it is new or worsening in others due to climate change and increased nutrient load. Muskegon Lake, Michigan, U.S., is a mesotrophic drowned rivermouth, freshwater estuary on Lake Michigan’s east coast, that experiences hypoxia annually during summer. To address questions regarding the dynamics of hypoxia and how it affects the lake ecosystem, we documented bi-weekly chemical, physical, and biological changes in the lake’s 4 sub-basins from May-October 2015, and analyzed 5 years of time-series buoy data from the water column at the central basin. Dissolved oxygen (DO) profiles revealed that mild hypoxia (DO < 4 mgL^-1) and severe hypoxia (DO < 2 mg^-1) were detected in each basin for over 25% of the year, but they did not experience consistent levels of hypoxia throughout the season with alterations between clinograde and negative heterograde stratification. By reducing fish habitat, bottom water hypoxia seems to have played a role in decreasing fish species diversity and abundance in the hypolimnion from 35 individuals across 5 species in the first pre-hypoxic sampling to zero individuals during peak hypoxia. Nutrients show slightly higher concentrations of soluble reactive phosphorous (SRP) at the bottom compared to the surface in pre-stratification, yet surface SRP decreased to below detection limits as bottom concentrations continued to increase to over 20 ug^-1 during post-stratified, hypoxic periods. Using time-series wind, water temperature, and DO data as a proxy for water movement, we can also visualize the impacts of episodic wind events on stratification and hypoxia, which reveal different wind-induced mixing patterns within the lake depending on the time of year and properties of the wind. In lakes and estuaries, strong mixing events may provide a mechanism for bringing benthic-derived nutrients to the surface to fuel algal blooms, while weak wind-events serve to deepen the epilimnion thus isolating the hypolimnetic waters and worsening hypoxia. Additionally, climatic conditions consistent with predictions of warmer temperatures and episodic weather may promote the formation of more severe hypoxia and episodic algal blooms.