ECOLOGICAL CHARACTERISTICS OF THE ISKAR RIVER CATCHMENT

An environmental assessment of 16 sampling sites in the Iskar River catchment has been fulfilled accomplished for the period April 2004 - August 2006. The following basic structural parameters of macrozoobenthic communities have been determined: dominance (c), evenness (e), species diversity (H), species richness (d) and the saprobic index after Pantle & Buck. The larger part of the upper river stream was in the best ecological state. The saprobic index varied between xenosaproby and beta-mesosaproby, and the macrozoobenthic communities were stable and ecologically sufficient. Local worsening of the saprobic condition was observed only below the town of Samokov, where the saprobic index was between beta-or alpha-mesosaproby. The worst environmental conditions were observed in the middle part of the Iskar River. The macrozoobenthic communities at Novi Iskar sampling site (after the city of Sofia) were highly disrupted and the saprobic index showed polysaproby as a result of high organic loading. At the sampling site near to Eliseina the disruption of the community structure was also observed during some seasons, but the probable reason for that was the heavy metal pollution. A slow improvement of the ecological state as a result of active self-purification was reported at the stations located in the lower river stream. In the section between Koinare and Oryahovitsa the state was stable beta-mesosaproby. The specific conditions near the village of Gigen right before the infusion into the Danube (slow stream velocity, unsuitable sandy substrate, and large variations in depth due to the direct influence of the Danube River) did not allow the development of stable makrozoobenthic communities.