Relationships between heterotrophic bacteria and cyanobacteria in the northern Adriatic in relation to the mucilage phenomenon

High variability of heterotrophic bacterial (HB; 0.1d 109–6d 109 cells L 1), nanoflagellates (HNF; 0.02d 106–2.4d 106 cells L 1) and cyanobacterial (CB; 106–700d 106 cells L 1) abundances were observed during approximately monthly measurements at six stations along the transect Po Delta–Rovinj from March 1999 to August 2002. Substantially higher values were observed during the stratification period (June–September) in the surface layer of the western stations that were under more direct influence of Po River discharges. Changes of the HB abundance were significantly correlated with temperature in the entire water column and chlorophyll a (Chla) concentration in the surface layer. The nutrients did not look to have directly influenced the HB growth, except orthophosphate in the eastern, more oligotrophic part of transect, where probably HB efficiently competed with phytoplankton for this nutrient. Temperature was also important for CB growth that, however, appeared to occur more intensively in waters with low nutrient concentrations. Probably, in these conditions CB can still develop, while larger autotrophs are strongly nutrient limited. In the upper water column of reduced salinity (35–37), the contributions of CB carbon biomass to the total picoplankton biomass (CB+HB) were mostly larger than 30% (up to 80%) in years with mucilage events (1991, 2000–2002) than in other years. For example, in 1999 only a few values were higher than 30% (up to 50%). In abundant presence of mucilaginous aggregates (e.g. in June 2000 and late June 2002) the chlorophyll a ascribed to CB accounted for much larger portions of total measured Chla. From these results it was concluded that cyanobacteria during mucilage events may play a substantially increased role both as primary producers and prey within the microbial loop of the northern Adriatic.