Processing of Nitrogen by Autotrophic and Heterotrophic Organisms in Southern Lake Michigan

Wayne S. Gardner and Mark McCarthy, The University of Texas; Joann Cavaletto, NOAA-GLERL; Peter Lavrentyev, University of Akron.

Objectives:

  1. Evaluate nitrogen dynamics in southern Lake Michigan relative to plume and microbial food web characteristics.
  2. Determine whether phosphorus limits the microbial turnover of nitrogen compounds.

Approach:

Results:

Conclusions:

  1. Nutrient cycling rates (uptake and regeneration) were significantly faster at stations at the St. Joseph River mouth than at other stations in southern Lake Michigan during winter-spring of 1999 and 2000.
  2. Observations on nitrogen dynamics suggest that heterotrophic activity dominated nutrient turnover and that rates were not phosphorus limited at stations outside of the St. Joseph River influence.
  3. In contrast, autotrophic activity dominated nutrient dynamics at the St. Joseph River mouth as reflected by nitrogen flux increases in response to light and/or added phosphorus.

Collaborators:

Chen (modeling), Johengen (nutrient fluxes), Cotner (bacterial dynamics), Fahnenstiel (phytoplankton), Eadie (sediment resuspension).