P-limitation of respiration in the Sargasso Sea and uncoupling of bacteria from P-regeneration in size-fractionation experiments

Community respiration rates were measured in unfiltered seawater collected in the upper 75 m of the water column along a transect in the Sargasso Sea and at the Bermuda Atlantic Time-series Study (BATS) station (31degrees50'00 N; 64degrees10'00 W) during a cruise in June and July 2001. Community respiration rates in the upper 75 m of the water column averaged 1.1 +/- 0.4 muM O-2 d(-1) and exhibited significant spatial and temporal variability. Concurrent determination of the heterotrophic and autotrophic community revealed no relationship between community respiration and the abundance of any of the major metabolic groups. Addition of inorganic nutrients (NO3 and PO4) and organic carbon (glucose) indicated that community respiration was P-limited in the surface mixed layer. Size-fractionation experiments indicated that the abundance of heterotrophic bacteria in the <0.6 mum fraction was similar to80% of the abundance in unfiltered seawater, but respiration rates in the <0.6 mum size fraction accounted for only 23% of community respiration. Addition of P to the <0.6 mum size fraction increased respiration rates similar to2-fold, indicating bacterial respiration was P-limited. It appears that the uncoupling of bacteria from nutrient regeneration in size fractionation experiments resulted in a reduction of bacterial metabolism in the P-limited surface waters of the Sargasso Sea.