Biogeochemical cycling of nutrient in the western Bering Sea as revealed by nitrogen isotopic composition of nitrate and suspended particles

Abstract

The nitrogen isotopic composition of nitrate and suspended particulate nitrogen (PN) in the upper 500 m water column along a transect in the western Bering Sea was analyzed to characterize the nitrogen cycle in summer. The δ15N of nitrate (δ15N–NO3) increases into the surface, in strong correlation with the upward decrease in nitrate concentration, reflecting the effect of nitrate assimilation by phytoplankton. The isotope effect of nitrate assimilation (15εNO3) in the basin and the slope is estimated to be 6.1 ± 2.1‰ and 4.1 ± 0.8‰, respectively. The variation of 15εNO3 in the basin is probably due to the difference in phytoplankton community, while the 15εNO3 in the slope is underestimated by at least 1‰ by nitrification. There are four types of relationship between δ15N of PN (δ15N-PN) and dissolved inorganic nitrogen concentration ([DIN]), i.e., a constant low δ15N-PN with slightly varying [DIN] in the euphotic zone of the basin, a negative correlation in the euphotic zone of the slope, a positive correlation in the depth from the bottom of the euphotic zone to the temperature minimum layer (Tmin layer), and a relatively constant δ15N-PN at varying [DIN] below the Tmin layer. These relationships reflect the effects of utilization of nutrient substrates in surface water and remineralization below the euphotic zone. By combining the spatial variations of δ15N–NO3 with δ15N-PN, we find that primary production in summer is mainly supported by regenerated nutrients, whether in the basin or in the slope. The effect of nitrification is only apparent in the slope due to its high productivity and active remineralization. δ15N-PN is not a good proxy of surface nitrate use, rather, it may reflect regional change of δ15N of ammonium, at least in summer.