Phytoplankton biomass and primary production in the marginal ice zone of the northwestern Weddell Sea during austral summer

During the austral summer of 1995, distributions of phytoplankton biomass ( as chlorophyll a), primary production, and nutrient concentrations along tw o north-south transects in the marginal ice zone of the northwestern Weddel l Sea were examined as part of the 8th Korean Antarctic Research Program. A n extensive phytoplankton bloom, ranging from 1.6 to 11.2 mg m(-3) in surfa ce chlorophyll a concentration, was encountered along the eastern transect and extended ca. 180 km north of the ice edge. The spatial extent of the bl oom was closely related to the density field induced by the input of meltwa ter from the retreating sea ice. However, the extent (ca. 200 km) of the ph ytoplankton bloom along the western transect exceeded the meltwater-influen ced zone (ca. 18 km). The extensive bloom along the western transect was mo re closely related to local hydrography than to the proximity of the ice ed ge and the resulting meltwater-induced stability of the upper water column. In addition, the marginal ice zone on the western transect was characteriz ed by a deep, high phytoplankton biomass (up to 8 mg Chl a m(-3)) extending to 100-m depth, and the decreased nutrient concentration, which was probab ly caused by passive sinking from the upper euphotic zone and in situ growt h. Despite the low bloom intensity relative to the marginal ice zone in bot h of the transects, mean primary productivity (2.6 g C m(-2) day(-1)) in sh elf waters corresponding to the northern side of the western transect was a s high as in the marginal ice zone (2.1 g C m(-2) day(-1)), and was 4.8 tim es greater than that in open waters, suggesting that shelf waters are as hi ghly productive as the marginal ice zone. A comparison between the historic al productivity data and our data also shows that the most productive regio ns in the Southern Ocean are shelf waters and the marginal ice zone, with e merging evidence of frontal regions as another major productive site.