Vertical flux of phytoplankton and particulate biogenic matter in the marginal ice zone of the Barents Sea in May 1993

Abstract

Hydrography, nutrients, chlorophyll a (chl a), phaeopigments (phaeo) and phytoplankton abundance as well as the vertical flux of particulate organic carbon and nitrogen (POC and PON) and pigments in the upper 200 m were investigated at 4 stations along a north-south transect in the Barents Sea in May 1993. Two stations were located in Arctic water and 2 in Atlantic water. The northernmost station had a well-developed ice edge bloom with high abundance of Fragilariopsis sp., Thalassiosira spp. and Chaetoceros spp. The phytoplankton community at the ice edge and the northernmost Atlantic station was dominated by Phaeocystis sp. colonies in concert with Chaetoceros spp. and Thalassiosira spp. The phytoplankton community at the southernmost station was dominated by large Chaetoceros spp. cells. Vertical flux of biogenic matter in the upper water column (20-100 m) varied along the transect (2.1-13.3 mg chl a m-2 d-1, 0.6-11 mg phaeo m-2 d-1 and 409-1090 mg POC m-2 d-1), while vertical flux in the deeper water column (100-200 m) was less variable (0.7-2.9 mg chl a m-2 d-1, 1.5-6.5 mg phaeo m-2 d-1 and 200-300 mg POC m-2 d-1). Hydrographic conditions, nutrient status and species specific sinking rates did not have a profound influence on the vertical flux of phytoplankton. High daily loss rates of chl a (4 to 18 % d-1 of standing stock chl a) and microplankton at 2 stations (southernmost and northernmost Atlantic) was probably caused by high production of extracellular carbon, increasing the aggregation potential of the phytoplankton community. Evidence of incomplete dissolution of Phaeocystis sp. colonies was observed to a depth of 200 m. Faecal matter and detritus were the main vehicles for the vertical export of organic matter to deeper water (below 100 m). Protozoan grazing was more important in retaining organic matter and nutrients in the upper water than mesozooplankton grazing.