Lake Michigan provides an ideal location for comparing episodic physical forcing events (storms) on phytoplankton processes and the more persistent seasonal variability of phytoplankton communities. This is due to the duration and extent of the highly turbid, recurrent coastal plume (RCP) in the lake during the winter/ early spring. Although the RCP can coincide with the initiation of the basin wide spring diatom bloom, linkages between the duration and intensity of the plume and the prominent role of light availability in regulating Lake Michigan phytoplankton growth during the spring isothermal period have been postulated, but not verified. The concurrent physical and biological events provide a novel opportunity to examine phenomena associated with the RCP affecting the distribution and abundance of species in Lake Michigan's planktonic diatom flora. In this study, planktonic diatom assemblages from pre, post and active spring plume events were examined from stations along Lake Michigan’s southern shoreline. Species abundance changed rapidly during storm events. Sediment resuspension via storm activity created a sub-optimal growth environment. Post-storm event diatom phytoplankton communities were floristically distinct from pre-storm event communities, with resting cell-forming taxa playing a significant role in these community-restructuring periods. Laboratory simulations of resuspension events using Lake Michigan sediments were conducted under a variety of environmental conditions. Parameters varied included day length, temperature, and silica. The resulting assemblages were quantitatively counted. A statistically significant relationship was identified between day length and the vegetative growth of many resting cell-forming diatom species. When day length was calculated for post-storm event field data, it revealed a high correlation between post-storm event communities and those predicted by the laboratory simulations. Timing of storm events and latitudinal position of the aquatic system are then important elements for consideration when predicting diatom phytoplankton community structure due to the relationship these factors have with day length.

Key words: coastal plume, diatoms, resting cells, sediment resuspension