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

 
SEASONAL CARBON AND NITROGEN ISOTOPE DYNAMICS OF PARTICULATE AND DISSOLVED ORGANIC MATTER IN GRAND TRAVERSE BAY, LAKE MICHIGAN
 
We describe the seasonal cycling of particulate and dissolved organic N in Grand Traverse Bay, between April and September, 1997. The dominance of autochthonous production was indicated by C/N values of seston. Peak production in May was indicated by elevated integrated water column fluorescence. Seston C isotope values (-31 to -25 per mil) were lowest during the spring phytoplankton bloom and in July, suggesting accumulation of 13-C depleted lipid or uptake of 13-C depleted respiratory CO2. The N isotopic composition of seston ranged from 2.6 to 9.7 per mil. High seston isotope values in the spring are consistent with the utilization of 15-N enriched NH4 (average NH4 = 12.3 per mil) while subsequent decreases suggest assimilation of 15-N depleted NO3 (average NO3 = 2.6 per mil). Large variation in the N isotope values of DON (range = 1.4 to 13.8 per mil) suggests that it is a very dynamic pool of organic matter. In contrast to early spring, DON values increase markedly with depth (ca. 10 per mil in mid-summer) after the development of the thermocline. These enriched values may imply that lower water column DON is refractory and likely reflect isotope effects associated with microbial degradation.
 
Day: Tuesday, Feb. 2
Time: 10:45 - 11:00am
Location: Eldorado Hotel
 
Code: SS32TU1045E