New production in the Tropical and Subtropical Atlantic Ocean
University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science,
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Subtropical gyres represent the single largest oceanic biome and may be very important for carbon cycling on account of their areal extent, yet our understanding of how these regions operate biologically is lacking. To address this issue measurements of NO3-, NH4+ and urea uptake were made using the 15N technique on 2 Atlantic Meridional Transect (AMT) cruises between 50oS and 50oN in May-June 2003 (AMT12) and April-June 2004 (AMT14). Distinct vertical profiles in the uptake of each nutrient were evident with urea uptake primarily confined to tropical and subtropical surface waters, NH4+ uptake to surface and intermediate waters and NO3- uptake peaking at depth leading to the observation of deep maxima in NO3- uptake within the subtropical gyres often coincident with the nutricline and with the deep chlorophyll maximum. Comparisons between the two cruises reveal interannual changes including an increase in the uptake rates of all three nutrients during AMT14, particularly NO3- which is driven by an increase in ambient NO3- concentrations. Several other parameters (e.g. chlorophyll, 14C carbon fixation, other nutrients) were also higher during AMT14 compared to AMT12 suggesting gyre scale variation. Estimates of NO3- based new production, obtained via a stoichiometric (Redfield) ratio, for the two cruises reveal relatively consistent rates within the tropics and subtropics within each cruise but higher rates of new production during AMT14. A greater proportion of new production was observed to occur in the lower 15% of the euphotic zone than in the surface layer reflecting the position and influence of the nutricline. A northward increase in new production in the lower regions of the euphotic zone was also observed which is hypothesized to represent a seasonal signal implying a degree of seasonality in the behaviour of the deep chlorophyll maximum and of deep NO3- uptake. The historical AMT database (AMT cruises 1-14) has been used to further examine the deep chlorophyll maximum. The observations of high NO3- uptake at depth, in excess of 14C based productivity nitrogen requirements and new production estimates exceeding total production rates suggests that nitrogen and carbon uptake may be decoupled at depth or that the measured NO3- uptake may not in fact represent new production. Integrated rates of new production were also occasionally found to locally exceed integrated community gross production and community respiration. Comparisons between new production and 234Th export production during AMT14 reveal that the two measurements are not equal, most likely on account of the different time scales of the two techniques, but comparisons with other parameters (carbon fixation, gross and net production and respiration) reveal that none of the parameters produce exact answers although similar patterns and magnitudes of carbon flux can be obtained. This would suggest that local decoupling of all rate processes is likely.
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