Seasonal variation in ratios of community respiration to gross photosynthesis determined by stable isotopes and concentrations of dissolved oxygen in Grand Traverse Bay, Lake Michigan

Dissolved oxygen concentrations (O2) were used in conjunction with stable oxygen isotopes (delta18O-O2) to determine the ratio of community respiration to gross photosynthesis (R:P ratios) in 2000 and 2001 in Grand Traverse Bay (GTB), Lake Michigan. Average R:P ratios in 2000 (1.2 +/- 0.03 SE) and 2001 (1.1 +/- 0.04 SE) indicate that GTB is net heterotrophic on an annual basis and requires an allochthonous source or non-contemporaneous autochthonous source of organic carbon to support excess heterotrophic activity. On a seasonal basis, the system fluctuated between periods of net heterotrophy (R:P > 1) in early spring and late fall and net autotrophy (R:P < 1) in late spring and early summer. Periods of net autotrophy coincided with the onset of stratification. As stratification progressed, R:P ratios near unity were observed in the epilimnion indicating that heterotrophic activity in GTB is strongly dependent upon autochthonous inputs of organic carbon during periods of stratification. We suggest that the temporal discontinuity between the introduction and utilization of organic carbon drives metabolism towards net heterotrophy.