Phytoplankton dynamics within the subsurface chlorophyll maximum of Lake Michigan

Abstract

The Lake Michigan deep chlorophyll maximum during July 1977 was a broad band of phytoplankton −15 m thick rather than the thin layers observed in smaller lakes. The maximum concentration of chlorophyll was typically four times surface values. ¹⁴ C uptake incubations showed that the deep plankton layer was a viable component of the pelagic ecosystem accounting for up to 60% of primary production on an areal (m ² ) basis. The Lake Michigan chlorophyll maximum was composed primarily of large diatoms, filaxnentous green algae, and flagellates. Although large scale physical processes such as seiches had some effect on the chlorophyll maximum, the basic integrity of the layer was maintained by biological processes. In particular, epilinmetic nutrient depletion was apparently a major factor affecting the vertical distribution of phytoplankton. Application of a diffusion model to the chemical and chlorophyll data indicated that nutrient uptake and primary production by phytoplankton were more important in determining chemical and chlorophyll vertical profiles than diffusivity.