Nitrogen Fuelling of the Pelagic Food Web of the Tropical AtlanticReport as inadecuate

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* Corresponding author 1 GEOMAR - GEOMAR - Helmholtz Centre for Ocean Research Kiel 2 LOV - Laboratoire d-océanographie de Villefranche

Abstract : We estimated the relative contribution of atmosphere ic Nitrogen N input wet and dry deposition and N fixation to the epipelagic food web by measuring N isotopes of different functional groups of epipelagic zooplankton along 23°W 17°N-4°S and 18°N 20-24°W in the Eastern Tropical Atlantic. Results were related to water column observations of nutrient distribution and vertical diffusive flux as well as colony abundance of Trichodesmium obtained with an Underwater Vision Profiler UVP5. The thickness and depth of the nitracline and phosphocline proved to be significant predictors of zooplankton stable N isotope values. Atmospheric N input was highest 61% of total N in the strongly stratified and oligotrophic region between 3 and 7°N, which featured very high depth-integrated Trichodesmium abundance up to 9.4×104 colonies m-2, strong thermohaline stratification and low zooplankton δ15N ~2‰. Relative atmospheric N input was lowest south of the equatorial upwelling between 3 and 5°S 27%. Values in the Guinea Dome region and north of Cape Verde ranged between 45 and 50%, respectively. The microstructure-derived estimate of the vertical diffusive N flux in the equatorial region was about one order of magnitude higher than in any other area approximately 8 mmol m-2 d 1. At the same time, this region received considerable atmospheric N input 35% of total. In general, zooplankton δ15N and Trichodesmium abundance were closely correlated, indicating that N fixation is the major source of atmospheric N input. Although Trichodesmium is not the only N fixing organism, its abundance can be used with high confidence to estimate the relative atmospheric N input in the tropical Atlantic r2 = 0.95. Estimates of absolute N fixation rates are two- to tenfold higher than incubation-derived rates reported for the same regions. Our approach integrates over large spatial and temporal scales and also quantifies fixed N released as dissolved inorganic and organic N. In a global analysis, it may thus help to close the gap in oceanic N budgets.

Author: Vera Sandel - Rainer Kiko - Peter Brandt - Marcus Dengler - Lars Stemmann - Pieter Vandromme - Ulrich Sommer - Helena Hauss -



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