Bacterial abundance and production in the western Black Sea

The unusual stability and vertical extent of the oxic–anoxic interface of the central Black Sea water column facilitate studying microbially mediated redox processes. Bacterial abundance, cell volume, 3H-thymidine and -leucine incorporation were measured in the Black Sea during May–June 2001. Sampling was conducted along a shelf–gyre transect, and was focused at the suboxic–anoxic interface at the deep stations. Bacterial variables (abundance, incorporation rates, biomass, production, and specific growth rates) were significantly higher on the northwest shelf compared to the Southwestern Gyre, and were generally intermediate at shelf-break stations. Mean bacterial abundances ranged from 1.4 to 2.9×109 cells l−1 in the mixed layer along the shelf–gyre transect, while isotope incorporation rates ranged from 7 to 74 pM-TdR h−1 and 60 to 209 pM-Leu h−1. Bacterial biomass ranged from 6.7 to 13.5 μg C l−1, bacterial production rates ranged from 2.3 to 7.7 μg C l−1 d−1, and specific growth rates ranged from 0.2 to 1.5 d−1. In the Southwestern Gyre, surface bacterial production was equivalent to about 50% of 14C-primary production. In the oxycline layer, mean bacterial abundance (0.38×109 cells l−1), leucine incorporation (4.1 pM-Leu h−1), and bacterial production (0.15 μg C l−1 d−1) were significantly greater compared to the suboxic and anoxic layers, while cell volume and biomass increased with depth. Large filamentous bacteria (>10 μm in length) were present below the suboxic boundary, and comprised up to 10% of the total bacterial abundance and up to 53% of the total bacterial biomass. Approximately 60% of the surface primary production was consumed above the suboxic–anoxic interface.