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Planktonic diatoms and their relation to environmental factors at three stations in the Southern Adriatic, Mediterranean Sea.

The seasonal distribution of planktonic diatoms was investigated at three coastal stations in the southern Adriatic, with monthly samplings from February 1988 to March 1989. The nutrient concentrations showed the eutrophic character of the Mali Ston and Gruž Bay stations, as compared to station P-100 which is similar to the open sea stations of the southern Adriatic. A total of 119 diatom species were identified, of which 38 species were found in  ; 10% of the total plankton samples. The most abundant diatoms were small, chain-forming species, mostly Chaetoceros spp., capable of rapid growth as a response to favorable conditions. The most abundant diatom species, with cell numbers exceeding 105 cells l-1, were Chaetoceros compressus and Guinardia striata found in the Mali Ston Bay. Community similarity coefficients between the diatom flora found in the bays of Gruž and Mali Ston, Gruž Bay and station P-100, Mali Ston Bay and station P-100, ranged from 0.45-0.73, 0.60-0.81 and 0.57-0.78, respectively. The total diatom cell numbers depended on particular environmental factors according to the period of mixing and/or stratification of the water column. Temperature and NO3 most directly correlated with the total diatom cell numbers at all three stations. The total diatom cell numbers were not dependent upon the salinity, TIN and PO4 at all stations. A correlation analysis between particular diatom species and physical-chemical parameters showed that the greatest number of diatom species depended on temperature, and the lowest number on PO4. The only species with a correlation to temperature, salinity and density at all three stations was Thalassionema nitzschioides. The greatest number of species, which correlated with NO3 and SiO4, were recorded at the P-100 and Mali Ston Bay stations, respectively. On the basis of our results, we concluded that the ecology of diatoms could be better understood when considering individual species rather than the collective group.

planktonic diatoms, environmental factors, southern Adriatic

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