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:

• A series of 24-h incubation bottle experiments were conducted at selected stations in southern Lake Michigan in March and June 1999 and March and May 2000 to examine potential nitrogen uptake and regeneration rates by microbial food web organisms.
• N-15 labeled ammonium and amino acids were added to Lake Michigan water, in the presence and absence of added phosphorus, and net fluxes of the added nitrogen compounds were observed in light and dark bottles over 24 h.
• Samples were collected at the same stations for enumeration of microbial food web organisms to compare with N fluxes.

Results:

• Net uptake or regeneration rates for ammonium and amino acids were measurable over 24 h at most stations.
• Fluxes of ammonium ranged from a net production of 20 nM h^-1 to a net uptake of 30 nM h^-1 except for stations at or near the St. Joseph River mouth that showed uptake rates of up to 100 nM h^-1.
• Comparison of net fluxes to total (obtained with the N-15 ammonium tracer) uptake or regeneration rates in June 1999 showed that the two approaches yielded similar uptake patterns among stations. Variation among stations was less for regeneration rates than for uptake rates.
• Light conditions and phosphorus additions had minimal effects on the net fluxes of ammonium and amino acids at most stations, but both light and added phosphorus significantly enhanced ammonium uptake rates in St. Joseph River mouth stations. These results imply that heterotrophic processes dominated at most stations except for those in the region of the St. Joseph River mouth where rates were high and influenced by autotrophic processes.

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).