Phosphate availability and the ultimate control of new nitrogen input by nitrogen fixation in the tropical Pacific Ocean

Abstract : Due to the low atmospheric input of phosphate into the open ocean, it is one of the key nutrients that could ultimately control primary production and carbon export into the deep ocean. The observed trend over the last 20 years, has shown a decrease in the dissolved inorganic phosphate (DIP) pool in the North Pacific gyre, which has been correlated to the increase in di-nitrogen (N2) fixation rates. Following a NW-SE transect, in the Southeast Pacific during the early austral summer (BIOSOPE cruise), we present data on DIP, dissolved organic phosphate (DOP), and particulate phosphate (PP) pools and DIP turnover times (TDIP) along with N2 fixation rates. We observed a decrease in DIP concentration from the edges to the centre of the gyre. Nevertheless the DIP concentrations remained above 100 nmol L-1 and TDIP were more than a month in the centre of the gyre: DIP availability remained largely above the level required for phosphate limitation. This contrasts with recent observations in the western Pacific Ocean at the same latitude (DIAPALIS cruises) where lower DIP concentrations (<20 nmol L-1) and TDIP<50 h were measured during the summer season. During the BIOSOPE cruise, N2 fixation rates were higher within the cold water upwelling near the Chilean coast. This observation contrasts with recently obtained model output for N2 fixation distribution in the South Pacific area and emphasises the importance of studying the main factors controlling this process. The South Pacific gyre can be considered a High P Low Chlorophyll (HPLC) oligotrophic area, which could potentially support high N2 fixation rates, and possibly carbon dioxide sequestration, if the primary ecophysiological controls, temperature and/or iron availability, were alleviated.
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T. Moutin, D. M. Karl, S. Duhamel, P. Rimmelin, Patrick Raimbault, et al.. Phosphate availability and the ultimate control of new nitrogen input by nitrogen fixation in the tropical Pacific Ocean. Biogeosciences Discussions, European Geosciences Union, 2007, 4 (4), pp.2407-2440. ⟨hal-00330253⟩

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