Large-scale variability of planktonic net community metabolism in the Atlantic Ocean: importance of temporal changes in oligotrophic subtropical waters

We used the oxygen method (dark and light bottles) to study photic-layer-integrated microbial metabolism along a latitudinal transect in the Atlantic Ocean (50° S to 50° N) during May and October 1997. The temperate region was more productive than the other oceanic environments, showing a net autotrophic balance (gross primary production/total respiration, GPP/R = 1.41 ± 0.34; mean ± SE). The GPP/R ratio in the upwelling regions was close to 1 (GPP/R = 1.32 ± 0.41), whereas in subtropical, oligotrophic environments it tended to be lower than 1 (GPP/R = 0.83 ± 0.27). The south oligotrophic gyre showed a higher organic carbon deficit (–235 ± 167 mmol O2 m–2 d–1) than the north oligotrophic gyre (–77 ± 162 mmol O2 m–2 d–1). The occurrence and extent of net heterotrophic metabolism in the oligotrophic ocean (north and south subtropical gyres) are not constant and can vary as a response to seasonal variations in physical forcing. The degree of nutrient limitation is a critical factor in the control of the microbial community metabolism in the oligotrophic regions of the open ocean. Relatively weak variations in the structure of the water column in the subtropical gyres may have a major impact on the metabolic functioning of the pelagic ecosystem.