Local production does not control the balance between plankton photosynthesis and respiration in the open Atlantic Ocean

Recent experimental data from the subtropical NE Atlantic appear to support the prevalence of net heterotrophy in unproductive pelagic ecosystems. However, the proximity of these studies to the NW African upwelling does not exclude the possibility that remote areas of the oceanic gyres are in metabolic balance. Here we present measurements of plankton gross photosynthesis (GPP) and community respiration (CR) made during a latitudinal transect (Atlantic Meridional Transect (AMT) cruise 11, 49°N-33°S) that traversed five biogeochemical provinces of the Atlantic Ocean, including both the subtropical NE Atlantic and the central part of the South Atlantic subtropical gyre. In these oligotrophic provinces, the euphotic zone average chlorophyll a concentration (0.15±0.01 and 0.16±0.02 mg m−3, respectively), the relative contribution of picoplankton (%Chla<2 μm 75±1 and 76±1%, respectively), and GPP (0.41±0.12 and 0.47±0.09 mmol O2 m−3 d−1, respectively) were almost identical. However, net heterotrophy prevailed in the euphotic zone of the tropical NE Atlantic (−0.28±0.13 mmol O2 m−3 d−1, n=3), while the net metabolism of the plankton community in the central S Atlantic gyre was autotrophic (0.20±0.02 mmol O2 m−3 d−1, n=5). These results therefore suggest the existence of more than one type of unproductive open-ocean situation, that may be characterised by differences in the relative importance of local vs. allochthonous sources of organic matter.