FACTORS REGULATING PLANKTON COMMUNITY RESPIRATION IN CHESAPEAKE BAY

Rates of plankton community respiration were measured in surface and bottom waters during spring, summer and autumn in the mesohaline region of Chesapeake Bay, USA. Seasonal patterns of plankton respiration generally followed the annual temperature cycle, with peak rates in July and August of 40 to 70 mug O2 l-1 h-1, which are among the highest values reported in the literature. In addition, strong diel variations in surface water community respiration, with mid-day maxima, were coherent with cycles of irradiance and photosynthesis, suggesting the dependence of community respiration on recently produced organic matter. The range of diel variations in respiration during summer was almost half the annual range in mean rates. Size-fractionated incubations demonstrated the importance of picoplankton (< 3 mum) respiration compared to total rates. On average, picoplankton accounted for most of community respiration in surface waters (56 %) and dominated total rates in the summer pycnocline (89 %). This size fraction was less important in the bottom waters during spring (23 %). Temperature per se as a control on plankton respiration was investigated by comparing rates at in situ temperatures against those measured in temperature-manipulated experiments in spring and summer. Strong relations between respiration and temperature were observed in all cases; least-squares regressions (exponential and linear) for experimental data were not significantly different between seasons, nor between the experiments and the seasonal rates measured at in situ temperatures. These results suggest the absence of significant physiological adaptation and/or selection for temperature optima over the annual cycle. Respiration by bacterioplankton (< 1 mum) exhibited no oxygen dependence down to concentrations of 0.16 mg O2 l-1; however, rates for whole water (unfiltered) declined with decreasing oxygen below 0.8 mg O2 l-1. Evidently, diffusion limited oxygen consumption by proto- and metazoans as well as bacteria associated with detrital particles and organic flocs.