| Summary: | The spatial variability of biomass and stable isotopes in plankton size fractions in the upper 200 m was studied in a high spatial resolution transect along 24°N from the Canary Islands to Florida to determine nitrogen and carbon sources. Vertical advection of waters predominated in lateral zones, while the central Atlantic (30–70°W) was characterized by strong stratification and oligotrophic surface waters. Plankton biomass was low in the central zone and high on both the eastern and the western sides, with most of the variability due to either large (>2000 µm) or small plankton (<500 µm). Carbon isotopes reflected mainly the advection the deep water in lateral zones. Stable nitrogen isotopes showed a nearly symmetrical spatial distribution in all fractions, with the lowest values (δ15N < 1‰) in the central zone, and were inversely correlated with carbon stable isotopes (δ13C) and with the abundance of the nitrogen-fixer Trichodesmium. Diazotrophy was estimated to account for >50% of organic nitrogen in the central zone, and even >30% in the eastern and the western zones. The impact of diazotrophy increased with the size of the organisms, supporting the wide participation of all trophic levels in the processing of recently fixed nitrogen. These results indicate that atmospheric sources of carbon and nitrogen prevail over deep water sources in the subtropical North Atlantic and that the zone influenced by diazotrophy is much larger than reported in previous studies.
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