Nitrogen dynamics in the Bellingshausen Sea during the Austral spring of 1992

A series of 15N experiments were conducted in the early summer of 1992 during the course of cruise 198 of R.R.S. Discovery, with a focus on the seasonal ice zone (SIZ) of the Bellingshausen Sea. Nitrogen (NO3, NH4 and urea) uptake was measured over the nominal euphotic zone, and size fractionation experiments of a similar nature were conducted on surface waters. The most productive zone during this study was not associated with stable ice-melt water but with a west-east oriented, haline dominated density front centred around 67.3°S. The pelagic bloom coincident with the area south of the front had a banded structure with chlorophyll a concentrations up to 5 mg m 3. Integrated N uptake rates within the bloom varied between 56 and 34 mmol N m −2 day −, with a decreasing trend from north to south. A similar trend was evident in the |-ratios, where values decreased from 0.6 to 0.3. From size fractionated experiments in the bloom it was found that 82–87% of the chlorophyll a was in the netplankton fraction, which also accounted for 28 to 95% of the N uptake. The bloom was dominated by Porosira glacialis, Coscinodiscus bouvet, Thalassiosira antarctica and Phaeocystis spp. North of the front, integrated N uptake was 25 mmol N m −2 day − (f-ratio 0.28) and 60 to 69% of the uptake in surface waters was in the netplankton fraction. Waters adjacent to the ice-edge exhibited a lower rate of N uptake than the frontal stations (19 mmol N m −2 day −1) with an f-ratio of 0.1. Specific rates on N uptake indicated strong bloom viability in an Antarctic context but a lack of response of VN0P 3 to [N0 3]. The cross-frontal trends may provide an explanation of bloom development and persistence in the SIZ of the Bellingshausen Sea. Rates of new production in mature waters of the SIZ were comparable to those of previous studies in other regions, but rates measured within the bloom were higher. The rates of new production presented here are the first to have been made in the Bellingshausen Sea and as such can be incorporated in regional and global models relating to the sink of carbon.