Relative inputs of upwelled and atmospheric nitrogen to the eastern tropical North Atlantic food web: Spatial distribution of δ15N in mesozooplankton and relation to dissolved nutrient dynamics

The Eastern Tropical North Atlantic (ETNA) is characterised by a strong east to west gradient in the vertical upward flux of dissolved inorganic nitrogen to the photic zone. We measured the stable nitrogen isotope (delta N-15) signatures of various zooplankton taxa covering twelve stations in the ETNA (04 degrees-14 degrees N, 016-030 degrees W) in fall 2009, and observed significant differences in delta N-15 values among stations. These spatial differences in delta N-15 within zooplankton taxa exceeded those between trophic levels and revealed an increasing atmospheric input of nitrogen by N-2 fixation and Aeolian dust in the open ocean as opposed to remineralised NO3- close to the NW African upwelling. In order to investigate the spatial distribution of upwelling-fuelled versus atmospheric-derived nitrogen more closely, we examined the delta N-15 signatures in size-fractionated zooplankton as well as in three widely distributed epipelagic copepod species on a second cruise in fall 2010 in the ETNA (02-17 degrees 35'N, 015-028 degrees W). Copepods were sampled for delta N-15 and RNA/DNA as a proxy for nutritional condition on 25 stations. At the same stations, vertical profiles of chlorophyll-a and dissolved nutrients were obtained. High standing stocks of chl-a were associated with shallow mixed layer depth and thickening of the nutricline. As the nitracline was generally deeper and less thick than the phosphacline, it appears that non-diazotroph primary production was limited by N rather than P throughout the study area, which is in line with enrichment experiments during these cruises. Estimated by the delta N-15 in zooplankton, atmospheric sources of new N contributed less than 20% close to the African coast and in the Guinea Dome area and up to 60% at the offshore stations, depending on the depth of the nitracline. delta N-15 of the three different copepod species investigated strongly correlated with each other, in spite of their distinct feeding ecology, which resulted in different spatial patterns of nutritional condition as indicated by RNA/DNA. (C) 2013 Elsevier Ltd. All rights reserved.