A time-series study of the spring bloom at the Bering Sea ice edge I. Physical processes, chlorophyll and nutrient chemistry

An intense but short-lived phytoplankton bloom develops in the low-salinity melt waters at the edge of the Bering Sea ice as the ice melts and retreats each spring. In spring 1988 we followed the development of this bloom by sampling every 3 h while following a freely drifting drogue in the marginal ice-edge zone for two four-day periods. The first period (29 April–3 May) was at an early stage of the bloom while the second period (10–13 May) was at the peak of the bloom. Early in the bloom, the phytoplankton consumed all the nitrate (∼400 mmoles m−2) initially present in the surface water producing large accumulations of particulate carbon (>1000 mmoles C m−2). By the time of peak chlorophyll concentrations (∼35 mg M−3), nitrate concentrations had been depleted so that the sustained high productivity depended on either recycled or imported nutrients. After this point, there was little net additional accumulation of biomass. From these data plus cruise data from previous years, we find that the Bering Sea ice-edge bloom typically begins in the last week of April and appears to precede blooms in the adjacent ice-free waters by days to weeks. The variability in bloom onset observed over several years is not linked very closely to the large scale climatic variations found in this region, but rather appears to be related to local weather during the end of April and the first part of May, with calm, sunny weather being required to initiate the blooms.