We analysed data from three Bulgarian reservoirs, with trophic status from meso to eutrophic . Two kinds of relations (non-allometric and allometric) of chlorophyll with phytoplankton density, biovolume and surface area were investigated . The non-allometric relationships compare these phytoplankton variables with chlorophyll-a concentration, while the allometric ones include comparisons of average individual volume (AIV) of phytoplankters with chlorophyll-a content per individual (Chl .N -1 ), per unit biovolume (Chl .BV -1 ) and per unit surface area (Chl .SA-1 ) .Maximum values of the rank correlation of the three non-allometric relationships were found in the eutrophic reservoir. The highest allometric correlation coefficients were observed in the mesotrophic reservoir. Two nonallometric relationships, Chl with N and Chl ith BV calculated on unified data from all three reservoirs showed lower significance and a non-linear character . Chl-SA relationship was always statistically significant and varied from linear to slightly non-linear when calculated on weighed values . Two allometric relationships, AIV with Chl .BV -1 and AIV with Chl .N -1 seem to be linear. A third AIV and Chl .SA -1 was described by a polinomial of the second degree, indicating that the smallest and largest phytoplankton individuals have a similar chlorophyll content per unit surface area . At the highest trophy, the Chl .SA-1 seemed lowest .The frequently investigated relationship between chlorophyll-a and biovolume ranged from statistically not significant in the mesotrophic to highly significant in the eutrophic reservoir . This tendency was generalized by obtaining a statistically significant rank correlation between the levels of significance of chlorophyll-biovolume correlations and the corresponding chlorophyll-a concentrations . The non-linear character of the chlorophyllbiovolume relationship over a wide range of trophy was probably caused by changes in surface area-biovolume ratio .

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The trophic status and relationships between Secchi depth transparency (SD) chlorophyll-a (CHL) and total phosphorus (TP) concentrations from nine non-infested and five infested areas with invasive alien species, Dreissena ssp. reservoirs, situated on the Bulgarian Danube River tributaries were studied. The trophic status index (TSI) values after Carlson (1977), and showed statistically significant differences for all three variables between infested and non-infested reservoirs. The three linear regression equations between SD × CHL, CHL × TP and SD × TP were statistically significant for the group of non-infested reservoirs, while in the infested reservoirs only the SDxCHL regression was statistically significant for P < 0.05. Our results showed that the Dreissena invasion destroyed the linear relationship between CHL × TP and SD × TP and seems to affects the accurate application of Carlson TSI.