Nitrogen fixation and nitrogen isotope abundances in zooplankton of the oligotrophic North Atlantic

Abstract

Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean.15 N : 14 N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15 N content relative to average deep-water nitrate. Herein we show that the low ␦ 15 N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N 2 fixation. These results provide direct, integrative evidence that N 2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.Recent biological and geochemical studies have produced greatly increased estimates of the importance of biological N 2 fixation in supporting new production in oligotrophic areas of the ocean (Carpenter and

Spatial variation in N<sub>2</sub>-fixation rate and diazotroph activity in the Tropical Atlantic

“…As a result, we now have a much better understanding of the environmental factors and physiological underpinnings of N 2 -fixation by Trichodesmium than we do for any other marine diazotroph. Compilation of extensive data from multiple cruises also provides substantial information on the spatial distribution of N 2 -fixation by Trichodesmium within the North Atlantic: Trichodesmium biomass and N 2 -fixation rates both tend to be highest in the western part of the basin (Capone et al, 2005), as is the overall contribution of N 2 -fixation to the water column N budget (Capone et al, 2005;Montoya et al, 2002;McClelland et al, 2003) Despite its clear importance, N 2 fixation by Trichodesmium cannot account for the total rate of N 2 fixation implied by geochemical budgets for the oligotrophic ocean in general or the North Atlantic in particular (Gruber and Sarmiento, 1997;Deutsch et al, 2001;Capone et al, 2005). Other diazotrophs must be making a significant contribution to N 2 -fixation on both a basin and a global scale.…”

Modeling and mapping isotopic patterns in the Northwest Atlantic derived from loggerhead sea turtles

“…Loggerheads in the SNWA reside in areas with higher rates of N 2 fixation, with a more depleted isotopic composition (Montoya et al 2002(Montoya et al , 2007, while turtles at higher latitudes are in a region with higher rates of denitrification, leading to enriched phytoplankton d 15 N (Fennel et al 2006).…”

Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

“…N 2 gas has an N isotope ratio (δ 15 N) of 0 ‰ and preferential uptake of 14 N leads to δ 15 N values as low as −2.5 ‰ for diazotrophs (Montoya et al, 2002). By comparison, the average ocean nitrate δ 15 N value is ∼ 5 ‰ (Sigman et al, 1997(Sigman et al, , 1999, leading to higher δ 15 N values for primary producers using nitrate as their nitrogen source.…”

“…Stable isotope analysis has served as a powerful tool for investigating the contribution of new N to pelagic food webs (Carpenter et al, 1999;Hannides et al, 2009;Landrum et al, 2011;Mompean et al, 2013;Montoya et al, 2002). N 2 gas has an N isotope ratio (δ 15 N) of 0 ‰ and preferential uptake of 14 N leads to δ 15 N values as low as −2.5 ‰ for diazotrophs (Montoya et al, 2002).…”