Lake Michigan has undergone several ecological changes since the 1972 Great Lakes Water Quality Agreement, including oligotrophication, introduction of invasive Dreissena mussels, and shifts in the zooplankton community. As a result of nutrient depletion and high water clarity, a water column feature called the deep chlorophyll layer (DCL) has become increasingly common. With a decrease in phosphorus loading to the offshore environment and declines in phytoplankton abundance and productivity, the DCL increasingly composes more of the areal primary production. This study investigates the formation and dissipation patterns of the DCL, explores patterns in summer zooplankton community, and quantifies trends in zooplankton grazing in the southern basin of Lake Michigan. In the field, continuous profiles of temperature and in situ fluorescence were recorded, water samples were collected, and stratified zooplankton tows were carried out. In the lab, sample processing of extracted chlorophyll a and gut fluorescence analysis were completed.
The water column profiles showed seasonal changes in Lake Michigan. In May, the lake was isothermal with no visible deep chlorophyll layer. By July, there was thermal stratification and DCLs in the metalimnion to hypolimnion in the western transects and shallower or absent DCLs in the eastern transects. September had the highest surface temperatures and steeper thermoclines with DCLs shifting up the water column. Higher chlorophyll a values for zooplankton gut fluorescence in the eastern transects indicated that grazing mostly occurred in the epilimnion when there was no DCL, while zooplankton in the western transects exploited the DCL as a food source. Calanoid copepods dominated the zooplankton community across all transects, whereas few cladocerans were strictly found in the eastern transects. Information about chlorophyll a as a chemical indicator of phytoplankton biomass and zooplankton as an important source of food for fish, has implications for the greater food web of Lake Michigan as well as the other Great Lakes.