Large-scale meridional and zonal variability in the nitrogen isotopic composition of plankton in the Atlantic Ocean
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The zonal (ca. 15º-40ºW along 26-29ºN) and meridional (ca. 30ºN-30ºS along 28º-29ºW) variability of δ15N of suspended particles and zooplankton (>40 µm) was studied to assess the influence of nitrogen fixation in the isotopic budget of the tropical and subtropical Atlantic ocean. Two cruises were conducted in October-November 2007 and April-May 2008 comprising a zonal and meridional transect each. In the region between 30º-15ºN, the concurrently measured nitrogen fixation was insufficient to explain the consistent patch of suspended particles with δ15N < 2‰ and points to a significant contribution of atmospheric deposition of light nitrogen to the isotopic budget. The equatorial region (15ºN-10ºS) is subject to intense nitrogen fixation, which, according to a two-end-member mixing model, may explain 40-60% of the observed δ15N in suspended particles and 3-30% in zooplankton. In the South region between 10ºS-30ºS, low values (<4‰) were measured in suspended particles and zooplankton during ...
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