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.
- Evaluate nitrogen dynamics in southern Lake Michigan relative to plume and microbial
food web characteristics.
- Determine whether phosphorus limits the microbial turnover of nitrogen compounds.
- 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.
- 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
- 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.
- 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.
- 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
Chen (modeling), Johengen (nutrient fluxes), Cotner (bacterial dynamics), Fahnenstiel
(phytoplankton), Eadie (sediment resuspension).