Effect of anticyclonic eddy on bacterioplankton in the Black Sea: an experimental study

The effect of the anticyclonic eddy on the morphological structure, abundance (NB) and biomass of bacterioplankton (BB) in the deep phytoplankton maximum (DPM) was experimentally studied on natural phytoplankton in the Black Sea. Characteristic processes were simulated: the death of phytoplankton as a result of deepening DPM below the photic zone and a simultaneous increase in nutrient concentrations. In the experiment, primary production was terminated. Two cases were simulated: Regular and Deep DPM sinking with medium to high daily nutrient additions. Deep and Regular cases showed similar dynamics without significant differences. The NB increased during the first 4 days by 3.5 times to the maximum of 1.12 × 10 6 ml −1 cells. BB increased by 11.8 times to 42.5 mg C m −3 . At the same time, the bacterial morphological structure changed towards the dominance of large rods in the BB (up to 100%), resembling copiotrophic bacteria. The sharp increase in BB cannot be explained by utilizing the organic matter released from dead eukaryotic phytoplankton biomass since it was low (4.0 mg C m −3 ). We assume that the enrichment of the environment with biogenic elements resulting from anticyclonic deepening can promote the development of large, fast-growing copiotrophic forms of bacteria. This growth can be achieved through a more complete use of the already available labile organic matter rather than the new one released from the dying DPM.