MacGregor, B. J. Northwestern University, bmacgreg@anton.civil.nwu.edu
Stahl, D. A. Northwestern University, d-stahl@nwu.edu
Van Mooy, B. University of Washington, bvm@ocean.washington.edu
Hollander, D. Northwestern University, david@earth.nwu.edu
Baker, B. University of Wisconsin - Milwaukee, bbaker@csd.uwm.edu
Mellon, M. T. Rensselaer Polytechnical Institute, mellom@rpi.edu
Paerl, H. W. Institute of Marine Sciences, hpaerl@emamil.unc.edu
Zehr, J. P. Rensselaer Polytechnical Institute, zehr@rpi.edu

 
NITROGEN FIXATION IN OFFSHORE LAKE MICHIGAN: STABLE ISOTOPE AND MICROBIOLOGICAL EVIDENCE
 
We are integrating stable isotope and microbiological techniques to relate microbial populations to elemental cycling at an offshore Lake Michigan site. Recent evidence unexpectedly suggests that nitrogen fixation attributable to Nodularia spp. may be a significant source of combined N, although the surface waters of the open lake have been considered phosphate-limited during stratification. Isotopically depleted particulate organic nitrogen was found at the thermocline in July and October 1997, closely reflecting the d15N of atmospheric N2. There was a dissolved N:P minimum at the thermocline, potentially favoring nitrogen-fixing species. EPA data show this ratio decreasing over the past decade. Microscopic examination revealed abundant heterocystous cyanobacteria, identified as Nodularia spp. Anabaena spp. have been seen in Lake Michigan before, but to our knowledge this is the first report of Nodularia. Lipids at the thermocline were dominated by cyanobacterial fatty acids with a characteristic C-isotopic signature. In addition, we have amplified cyanobacterial 16S and nifH (nitrogenase) sequences from Lake Michigan water samples. We speculate that recent changes in the predator population, specifically replacement of Daphnia by Bythotrephes, may have contributed to a change in the microbial population. We are also considering the possibility that Nodularia is an introduced species.
 
Day: Tuesday, Feb. 2
Time: 10:45 - 11:00am
Location: Hilton of Santa Fe
 
Code: SS40TU1045H