The Effects of a Recurrent Coastal Plume on the Dynamics and Structure
of the Lower Food Web in Southern Lake Michigan

W. Gardner and P. Lavrentyev

The Lake Michigan Recurrent Coastal Plume (LMRCP) may fundamentally affect food web dynamics in Lake Michigan by causing major changes, during the winter-spring transition period, that, in turn, define the annual evolution of the biotic community in the lake. These effects can be observed by examining phytoplankton, zooplankton, and microbial food web structure and function in the field in combination with N-cycling studies that serve as an indicator of autrophic and heterotrophic community dynamics. We hypothesize that:
  1. The population growth of phytoplankton and bacteria are enhanced at the edge of the plume, where light levels are higher than inside the plume and nutrients are higher than in surrounding waters. The phytoplankton community structure will shift toward small-sized taxa in the plume relative to offshore waters.
  2. The proportion of phytoplankton production consumed in the pelagic zone will, in turn, increase due to increase in herbivory by nano- and microzooplankton at the edge of the plume.
  3. Increased growth and grazing rates of these consumers will lead to increased nutrient regeneration rates at the plume edge and increased high-quality food supply for crustacean zooplankton.
To examine these hypotheses, we will compare structure and abundances of microbial food web organisms before, during, and after plume formation; measure N cycling rates in relation to community and species-specific grazing rates of microzooplankton within, at the edge, and outside of the LMRCP. Observations, experiments, and results will be coordinated with those from primary production and zooplankton grazing experiments and other studies of nutrient and physical processes to complete a conceptual model and provide data for the proposed coupled biological/physical model.