MACROZOOBENTHOS DIVERSITY OF TWO BULGARIAN RESERVOIRS ACCORDING TO SOME ENVIRONMENTAL PARAMETERS

The reservoirs Ovcharitsa and Byal Kladenets are part of the hydrotechnical complex used to supply and recirculate waters for additional cooling for the coal/steam electric power station Maritsa Iztok II in South-Eastern Bulgaria. The water used as a coolant is withdrawn from the dam part of the Ovcharitsa reservoir and returned to the Byal Kladenets from where it flows back again into the middle sections of the Ovcharitsa reservoir. The average water temperature from Byal Kladenets reservoir is higher than the temperature in the Ovcharitsa reservoir during the whole year. The objective of this study was to represent the difference in the macrozoobenthos communities in these two hydrological connected reservoirs, according to some environmental parameters. The survey was based on samples taken in four occasions between April and August of 2013. Qualitative samples of benthic macroinvertebrates were taken according to the standard methods ISO 9391:1995 and EN 27828:1994. The species composition, monthly distribution pattern and abundance of the macrozoobenthos were studied. A total of 37 bottom invertebrates taxa in the Ovcharitsa reservoir and 31 taxa in the Byal Kladenets reservoir were established, 37 of them identified to a species level. The most abundant was class Oligochaeta, followed by Diptera: Chironomidae and Bivalvia. In nearly the same taxonomic composition, the results showed that the density and biomass in the cooler Ovcharitsa reservoir were several times higher than in the hotter Byal Kladenets reservoir. The highest values were measureаd in April in the Ovcharitsa reservoir, whereas during the same period the abundance and biomass in the Byal Kladenets reservoir were the lowest. This confirms the role of the termal factor for the much more intensive metabolism in the Byal Kladenets reservoir.