Spatial distribution of microbial biomass and activity (bacterivory and bacterial production) in the northern Weddell Sea during the austral summer (January 1994)

Changes in bacterial production, bacterivory and microbial biomass (autotrophic and heterotrophic) were determined in a complex area characterized by different physico-chemical regimes during the austral summer (January 1994, ECOANTAR-94 cruise). The main goal was to explore the importance of bacterivory as a factor potentially controlling bacterial production in such an extreme environment. The area studied was located in the eastern part of the Bransfield Strait; between 59.5-64degreesS and 58-50degreesW. It covered part of the South Shetland Islands shelf and extended northwards to the Weddell-Scotia Confluence and southwards towards the ice edge. Stations were grouped into 4 zones under the influence of different hydrographic conditions: IEZ, influenced directly by the ice edge zone; WS, presenting typical characteristics of the Weddell Sea; FR, located within the Bransfield Strait frontal system and its continuation in the Weddell-Scotia Confluence; and BSO-WSC, containing stations influenced by waters originating in the Bellingshausen Sea and the Weddell-Scotia Confluence. We determined the spatial distribution of microbial biomass (chlorophyll a, bacteria, heterotrophic and phototrophic nanoflagellates and ciliates), bacterial heterotrophic production, and bacterivory. The highest values of chlorophyll a concentration and primary production as well as the lowest grazing rates (0.3 mug C l(-1) d(-1)) and bacterial production values were found at IEZ, together with the lowest temperatures. In contrast, the highest bacterial losses (4.7 mug Cl-1 d(-1)) coincided with high heterotrophic nanoflagellate biomass and bacterial production at the Weddell-Scotia Confluence and waters of Bellingshausen Sea origin (BSO-WSC). These waters showed the highest temperatures, and low chlorophyll a concentration and primary production values. Taking into account the whole data set, bacteria and heterotrophic nanoflagellate biomass explained 68% of the variability in grazing rates. Our results indicated that bacterial losses due to protists represented a large proportion of bacterial production (average 67% d(-1)). However, the variability detected in the study area was very large (from undetectable to >100%). Therefore, bacterivory does not seem to be the main factor in controlling bacterial production in all the zones studied.