Macrellis, A. N.. Michigan State University, macrell1@pilot.msu.edu
Ostrom, N. N.. Michigan State University, ostromn@pilot.msu.edu
McCusker, E. M.. Michigan State University, mcgervey@pilot.msu.edu
Ostrom, P. H.. Michigan State University, ostrom@pilot.msu.edu

 
GEOCHEMICAL AND ISOTOPE DYNAMICS OF DISSOLVED INORGANIC NITROGEN IN LAKE MICHIGAN
 
Water column samples and physical characteristics were collected in the western arm of Grand Traverse Bay, Lake Michigan between April and September 1997. The water column was unstratified until mid-June indicating a cold, protracted spring and strong winds. Chlorophyll fluorescence readings were characteristic of a spring diatom bloom followed by a gradual decrease in productivity in July and August. Surface concentrations of nitrate decreased from 19 uM in April to 16 uM in July, while ammonium concentrations were never greater than 2.5 uM. The range in the isotopic composition of nitrate between May and July (2.0 to 7.7 per mil) was higher than that observed in August and September (0.3 to 3.8 per mil). Low isotopic values for nitrate at the base of the fluorescence maximum may be suggestive of nitrification. Isotope values for ammonium (12.1 +/- 2.3 per mil) were higher than those of nitrate at all times. Decreasing concentrations of nitrate occurring concommitantly with declining concentration weighted average isotope values for seston during the season (5.8 to 2.0 per mil) are consistient with increasing dependence of phytoplankton on nitrate over time.
 
Day: Tuesday, Feb. 2
Time: 10:30 - 10:45am
Location: Eldorado Hotel
 
Code: SS32TU1030E