Abundance and production of pelagic bacteria in the southern Bay of Biscay during summer

Abstract

The first studies on bacterial abundance and production in representative areas of the shelf in the southern Bay of Biscay were carried out in coastal, shelf-break and off-shelf zones of the Central Cantabrian Sea in summer 1993 and 1994. In 1993 some coastal stations displayed the effects of an upwelling pulse. Thymidine uptake rates were minimal during upwelling conditions in coastal waters (range 0.4 - 4 pmol l^-1 h^-1). In shelf-break and off-shelf waters, thymidine uptake profiles displayed surface and subsurface peaks (range 0.3 - 15.5 pmol l^-1 h^-1). Vertical distribution of heterobacteria abundance varied from 0.28 to 2.27 x 10⁶ cell ml^-1 in the whole area studied. Cyanobacteria varied from 0.04 x 10⁴ to 1.04 x 10⁵ cell ml^-1. Heterobacteria accounted for 13 to 46% of total particulate carbon. In this study we concluded that during summer, pelagic heterobacteria can enhance high carbon turnover rates when upwelling relaxes and primary production rates increase, allowing the accumulation of phytoplankton. In turn, bacterial production rates decay abruptly during the upwelling process. Off-shore the system is typically oligotrophic, bacterioplankton balance the primary production rates and even bacteria seem to be food-limited. Our results on abundance and production of pelagic bacteria in the Central Cantabrian Sea are within the published range for the Galician upwelling areas.