Biogeographic differences in the net ecosystem metabolism of the open ocean

In vitro measurements, predictions, and geochemical estimates of the balance between gross primary production (GPP) and community respiration (CR) in the open ocean do not agree. This has generated an active debate about the carbon balance in unproductive pelagic marine ecosystems. The analysis of generalized GPP:CR relationships that sustains this debate assumes a continuous trophic gradient or the simple partition of the World Ocean into productive and unproductive regimes. We measured euphotic zone GPP and CR along a latitudinal (40° N–30° S) transect across the Atlantic Ocean, which included two open-ocean oligotrophic provinces: the eastern area of the North Atlantic Subtropical Gyre (NAST-E) and the center of the South Atlantic Gyre (SATL). Net heterotrophy prevailed in the euphotic zone of the NAST-E, while the plankton community in the central SATL was net autotrophic. A review of published studies of net plankton metabolism in the open ocean substantiates this geographic pattern, suggesting the existence of different trophic dynamics in similarly unproductive planktonic communities that might be characterized by the relative importance of local vs. allochthonous sources of organic matter. No universal relationship appears to exist between the magnitude of photosynthesis of a pelagic ecosystem and its net metabolism. We tested the published GPP:CR relationships and found that each one was only able to correctly predict GPP:CR balances in either the heterotrophic NAST-E or the autotrophic SATL, thus confirming that the geographic pattern in net community metabolism is related to the functional diversity of unproductive oceanic ecosystems.