Large-scale distribution and production of bacterioplankton in the Adriatic Sea

Two oceanographic cruises encompassing the whole Adriatic Sea were carried out during February and October 2008. Selected stations were sampled at several depths to determine total prokaryotes and picocyanobacteria abundance using epifluorescence microscopy, and to estimate prokaryotic carbon production by 3H-leucine incorporation. Biological data were related to physical parameters including temperature, salinity and fluorescence, and an attempt to associate bacterial dynamics to water mass characteristics was performed. In both seasons prokaryotic distribution and production showed a decreasing latitudinal gradient likely dependent on riverine inputs highlighted by a strong negative correlation with salinity (P < 0.001). A vertical gradient with higher cell numbers at the surface and lower values at the bottom layer was also always detected. In the southern basin in February, however, picocyanobacteria were retrieved also in deep waters, probably linked to higher nutrient loads carried by the Levantine Intermediate Waters and/or the deep water ventilation known to occur in this area. From an oceanographic point of view, we sampled within four different water types, but no relationship between these water types and bacterioplankton abundances was found. The present work contributes to the acquisition of a more holistic overview of prokaryotic distribution and production in the Adriatic Sea, both on a spatial and temporal scale.