engleski

Disturbances of the microbial loop structure during "mucilage events" in the northern Adriatic Sea

Bacterial abundance and production were measured over a three-year period (1990- 1992) at six stations in the northern Adriatic along the profile Rovinj (western Istria) - Po Delta. Simultaneously, pico-cyanobacteria and heterotrophic nanoflagellates abundances were measured. Bacterial abundance varied from 4 to 56.4 108 cells l-1, while bacterial production ranged from 0.05 to 3.23 ľg C l-1 h-1. The number of cyanobacteria ranged from 5 105 to 8.5 108 l-1 and heterotrophic nanoflagellates from 1.5 105 to 5.8 107 l-1, respectively. Bacterial abundance was significantly correlated with cyanobacterial (r=0.25, p<0.05) and heterotrophic nanoflagellates abundance (r=0.25, p<0.01). However, variances of nanoflagellates abundance could be wider explained rather by cyanobacteria (27%) than bacteria abundance variation (6%). Concerning the remarkable variability of the microbial biomass within the microbial loop during the stratification of the water column, pico- and nanoplankton relations were analysed for separate years as well as for the surface layer, where the influence of the Po river was pronounced. During the spring-summer of 1991, when large quantities of mucilaginous material appeared in the upper water column, a marked discrepancy from expected biomass relation between bacteria, cyanobacteria and nanoflagellates (č20:5:5) was noticed in the surface layer of the profile. The total picoplankton biomass was one order of magnitude higher in June-July 1991 compared to the same period in 1990 and 1992, but cyanobacteria contributed with 60-95% to the total biomass. Meanwhile, a contemporaneous heterotrophic pico- and nanoplankton biomass increase was related to nanoflagellates biomass (50-95%). Although during the summer of 1991 the bacterial growth rate and production was low in the upper water column of the entire area, bacterial abundance was at same level as abundance during the summer of 1990 and 1992, respectively. It is supposed that the increase of nanoflagellates abundance in 1991 was mostly supported by grazing of cyanobacterial population. Partly, the low grazing pressure of ciliates (absence of ciliates during June-August 1991), and/or other microzooplankton would contribute to the increase of nanoflagellates population. Due to the reduced bacterial activity the exudated primary production was wasted for higher trophic levels of the food web and colloidal mucilaginous material accumulated and aggregated in the entire area during the spring-summer of 1991.

Adriatic Sea; microbial loop; marine snow

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