Elevated 15N/14N in particulate organic matter, zooplankton, and diatom frustule-bound nitrogen in the ice-covered water column of the Bering Sea eastern shelf

Abstract

We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ15NDB), and zooplankton from water column material collected with net tows across the eastern Bering Sea shelf in late winter of 2007 and 2008, to investigate the N dynamics of primary and secondary production in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1–14.7‰; frustule-bound N: 4.9–20.7‰; zooplankton: 6.4–18.0‰), with POM δ15N reaching ~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28‰ and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic–pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.