Phytoplankton dynamics within the subsurface chlorophyll maximum of Lake Michigan

The Lake Michigan deep chlorophyll maximum during July 1977was a broad band of phytoplankton –15 m thick rather thanthe thin layers observed in smaller lakes. The maximum concentrationof chlorophyll was typically four times surface values. ¹⁴Cuptake incubations showed that the deep plankton layer was aviable component of the pelagic ecosystem accounting for upto 60% of primary production on an areal (m²) basis. The LakeMichigan chlorophyll maximum was composed primarily of largediatoms, filaxnentous green algae, and flagellates. Althoughlarge scale physical processes such as seiches had some effecton the chlorophyll maximum, the basic integrity of the layerwas maintained by biological processes. In particular, epilinmeticnutrient depletion was apparently a major factor affecting thevertical distribution of phytoplankton. Application of a diffusionmodel to the chemical and chlorophyll data indicated that nutrientuptake and primary production by phytoplankton were more importantin determining chemical and chlorophyll vertical profiles thandiffusivity. ¹Contribution No. 389 from the Great Lakes Research Division,University of Michigan.