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

We conducted a survey of the natural abundance ¹⁵N/¹⁴N ratio (δ¹⁵N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ¹⁵N_DB), 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.