Studi limnologici sul Lago Alto Flumendosa (Sardegna centrale): stato trofico e fitoplancton

Alto Flumendosa Reservoir (Central Sardinia), was investigated from February 1990 to February 1993, to study either long time period phytoplankton variations in a mesotrophic mediterranean reservoir and to value the influence of a cage rearing pisciculture on its trophic status. The annual mean values of clorophyll a varied from 2.3 mg m^-3 in 1991 to 11.1 mg m^-3 in 1990, annual mean values of biomass from 0.5 mg l^-1 in 1993 to 3.9 mg l^-1 in 1990 and annual mean values of cellular density from 6 x 106 cells l^-1 in 1993 to 11.3 x 106 cells l^-1 in 1990. Despite great variations in the species composition of phytoplankton in each year, a general scheme in the algal succession was found with Cryptophyceae in the colder months, Chlorophyceae in the spring months (sometimes in autumn), Cyanophyceae and Dinophyceae in summer, Bacillariophyceae in autumn, Chrysophyceae between winter and spring and Conjugatophyceae in summer or in winter. The trophic status on the basis of phytoplankton composition, clorophyll a, annual mean values of total phosphorus (from 19 mg P m^-3 in 1993 to 25 mg P m^-3 1990) was mesotrophic but in 1990 the eutrophic probability, on the basis of chlorophyll a, was higher. The cage rearing pisciculture was presumed to be responsible of the strong oxygen decreases in hypolimnetic water at the end of the stratification periods.