Seasonal and spatial distribution of bacterial production and biomass along a salinity gradient (Northern Adriatic Sea)

Abstract

The Adriatic Sea is a semi-enclosed ecosystem that receives in its shallow part, the northern basin, significant freshwater inputs which markedly increase its productivity with respect to the oligotrophic features of the Mediterranean sea. In this area, especially on the western coast where river plumes diffuse, high physical (density) and chemical (nutrients) gradients occur on a small scale, both horizontal and vertical. Results of bacterial production as ³H-thymidine incorporation, bacterial abundance as DAPI direct count, autotrophic biomass as chlorophyll a and total biomass as ATP from three areas in the Northern Adriatic Sea are reported. The three sites, differently influenced by the river water diffusion, were sampled seasonally over two days, every 24 h, in four surveys from April 1995 to January 1996. Bacterioplankton production, strongly correlated with primary production, was extremely high near the coast in low-salinity, high-nutrient waters, mostly as an indirect consequence of riverine inputs causing an increase in phytoplankton production stimulated by physically driven nutrient inputs. In the warm months bacterial activity was higher than in cold months. While bacteria abundance did not appear related to the salinity gradients, bacterial production (from 0.6 to 372 pM ³H-thymidine h⁻¹ incorporated, corresponding to 0.01–8.2 μ g C 1⁻¹ h⁻¹) and the relative generation times (from 0.2 to 35 days) showed a high range of values, representing a variety of situations, from estuaries to the ocean. The resulting role of the bacterial community in the carbon cycle is very consistent, processing amounts of carbon which have been estimated as high as the 80% and the 260% of those synthesized by autotrophs in summer and winter, respectively.