Vertical and Seasonal Distribution of Chlorophyll a in Lake Michigan

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Abstract
Vertical and seasonal distribution of chlorophyll a was observed for 2 yr at an offshore station in Lake Michigan. Chlorophyll a concentrations increased uniformly at all depths during spring reaching 3–4 mg/m3 by late May. Thermal stratification was followed by development of a subthermocline chlorophyll peak between 10 and 30 m that reached 8.5 mg/m3 by late July. The major subthermocline peak collapsed in mid-August but was followed by two lesser peaks at depths of 10 and 30 m. Autumn mixing dispersed these peaks in the mixed layer, increasing the chlorophyll content of the epilimnion at a time when integral chlorophyll levels were declining. At fall overturn chlorophyll concentrations were uniformly distributed at approximately 1 mg/m3, where they remained throughout the winter. The presence of a deep chlorophyll maximum in Lake Michigan adds a new dimension to limnological studies of the Great Lakes. The influence of this peak must be considered in future investigations of food chain dynamics and eutrophication processes in the Great Lakes system. Key words: Laurentian Great Lakes, limnology, chlorophyll a, spatial distribution, phytoplankton dynamics