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

Abstract

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 (31° 50' 00 N; 64° 10' 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 µM O2 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 abun- dance 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 µm fraction was ~80% of the abundance in unfiltered seawater, but respiration rates in the <0.6 µm size fraction accounted for only 23% of community respiration. Addition of P to the <0.6 µm size fraction increased respiration rates ~2-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.