Trophic Relationships and Status of Reservoirs with and without Occurrence of Dreissena ssp. (Mollusca, Bivalvia) Built on Bulgarian Danube River Tributaries

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.