Relating low δ15N values of zooplankton to N2-fixation in the tropical North Atlantic: insights provided by stable isotope ratios of amino acids

Recent evaluations of the global nitrogen budget include greatly increased estimates of N2-fixation in oceanic waters. Low stable N isotope ratios in planktonic food webs of tropical and subtropical oceans have been used as one indication of the importance of N2-fixation. Interpretation of bulk stable N isotope ratios can, however, be confounded when the source and process information that they contain cannot be separated clearly. In this paper, we use stable N isotope ratios of amino acids to help separate source and trophic effects associated with changes in bulk stable N isotope ratios of zooplankton across the tropical North Atlantic. Patterns in stable N isotope ratios of amino acids along a transect from the Cape Verde Islands to Barbados identify a change in N source supporting zooplankton production, and virtually no change in the trophic position of zooplankton size classes from the eastern to the western side of the tropical North Atlantic. Furthermore, comparison of stable N isotope ratios of amino acids in zooplankton with those in Trichodesmium suggests that diazotrophs are the source of the low stable N isotope ratios at the western end of the transect. The evidence provided by stable N isotope ratios of amino acids supports the interpretation of large-scale patterns in bulk stable N isotope ratios that N2-fixation indeed makes a major contribution to the global N budget.