Summer community respiration and pelagic metabolism in upper surface Antarctic waters

Microplanktonic community respiration (R) and net community production (NCP) in upper surface waters around the Antarctic Peninsula were studied in the summers of 1993, 1994 and 2000. Data on pelagic community metabolism from upper surface Antarctic waters (N = 27) were collected from the literature and included in the analysis (N = 96). The variability of phytoplankton chl a concentration found was high, ranging from 0.06 to 16.75 mg m^-3 d^-1. R rates varied from 0.19 to 11.03 mmol O₂ m ^-3 d^-1 and were not related to chl a concentration. The variability in daily NCP rates was higher than that found for R, ranging from -6.29 to 35.4 mmol O₂ m^-3 d^-1, and was positively related to chl a concentration (R² = 0.46). NCP rates were often negative (53 % of the observations) in the 1993 survey, indicative of community respiration in excess of primary production. Negative NCP was less frequent during the 1994 and 2000 cruises and in the literature data (18, 7 and 4 % of the observations, respectively). A positive and strong relationship (R² = 0.92) was observed between NCP and gross primary production (GPP), whereas R was independent of GPP. The relationships found imply that changes in GPP play a dominant role in the control of net planktonic metabolism in Antarctic waters, allowing excess carbon to be fixed by phytoplankton during phytoplankton blooms. In contrast, unproductive Antarctic communities (<0.064 g O₂ m^-3 d^-1) tend to be net heterotrophic, thereby representing CO₂ sources, rather than sinks. Since phytoplankton-poor waters extend over most of the Southern Ocean, these results stress the need to evaluate their role in oceanic carbon flow on the basis of a more comprehensive temporal and spatial data set.