Variation in Lake Michigan plankton: Temporal, spatial, and historical trends

Lake Michigan has been impacted by excessive material loading and invasion by exotic species; however, few studies have evaluated the recent basin-wide response of the lake to these changes, particularly given the reduction of phosphorus loads since the 1970s. From 1994-95, quarterly measurements were made of physical-chemical conditions, plankton biomass, and plankton species composition at 18 stations throughout the lake (n = 111). Sampling sites were clustered according to their physical-chemical similarity; these zones corresponded with depositional regions in the lake (Deep water, Shallow water, and Impacted regions). While plankton biomass did not vary among lake-zones, species composition was zone-specific suggesting that several factors (food web structure, nutrient cycling, and physical mixing) may determine the distribution of species throughout the lake. Plankton biomass and gross composition (phyla) were variable intime (seasons), and exhibited predictable succession patterns. Phytoplankton peaked in June corresponding with the upward mixing of nutrients, while zooplankton peaked during mid-stratification (August) when water temperatures were most warm. Finally, the basin-wide estimates for both total phosphorus and phytoplankton biomass were lower compared with historical estimates (measured in 1970s) and significant differences were not observed between near- and offshore regions. Despite this, the data also show that phytoplankton species composition varies widely throughout the lake, and that some nearshore sites do support impacted assemblages.