The relationship between suspended particulate material, phytoplankton and zooplankton during the retreat of the marginal ice zone in the Bellingshausen Sea

The distribution, abundance and composition of suspended particulates, phytoplankton and zooplankton biomass were investigated for the marginal ice zone in the Bellingshausen Sea during the Austral spring of 1992. Marked changes were observed between the amount and composition of particulates under the sea-ice and those in open waters. Measures of phytoplankton abundance (chlorophyll) ranged from ∼0.05 μg l−1 under the ice to 3 μg l−1 in the open waters to the north. The high nutrient concentrations and low level of phytoplankton under the ice suggest that this region is typical of over-wintering conditions and provides a suitable background comparison to the development of more productive, recently ice-free waters, further north. Nearly all basic measurements of particulate material showed a gradient from south to north. However, the biochemistry of particulates (lipid in particular), a more sensitive measure of environmental growth conditions, showed the area as a whole to be broadly split into two; under ice (light limited) and open water (no light limitation). Total particulate carbon was almost entirely composed of inorganic carbon under the ice; waters away from the ice edge also contained significant levels of inorganic carbon. Hence, C:chlorophyll was estimated from POC or phytoplankton carbon. There was no evidence of nutrient limitation at any of the stations investigated. Integrated phytoplankton carbon for the upper 100 m ranged from 0.1 gC m−2 under the ice to 11.5 gC m 2 in the diatom dominated bloom in open waters. The mesozooplankton biomass in the same depth interval increased from 0.02 gC m−2 under the ice to 0.21 gC m−2 in open waters. However, zooplankton biomass in the upper 600 m changed little from under the ice out to open waters north of the ice edge; although the northernmost station, in the region of highest chlorophyll, had approximately three times more zooplankton carbon (0.67 gC m−2 than the other stations. There is some evidence, however, that the northern station may be associated with a frontal feature, and that conditions observed may not be solely related to previous ice edge processes.