11. Relationships in the system Barbus barbus – pomphorhynchus tereticollis – unio tumidus in connection with the circulation of lead in the freshwater ecosystem of the Danube river, Bulgaria

During the summer of 2015, 13 specimens of barbel (Barbus barbus (Linnaeus, 1758)) were examined by standard techniques for parasites. All the examined barbels (100%) were infected with the acantocephalan Pomphorhynchus tereticollis.The study of the intestinal content and fishfood fixed the presence mainly of Unionidae molluscs.The swollen river mussel (Unio tumidus Philipsson, 1788; Mollusca, Unionidae) is one of the most common Unionidae in the studied habitat (Biotope Vetren, the DanubeRiver). The aim of the research wass to study the lead content in samples of sediments, Unio tumidus, Pomphorhynchus tereticollis, skin, muscles and liver of B. barbus from the Danube River.
From the tissues and organs of the studied fish specimens Barbus barbus, the lowest concentrations of lead were found in the muscles. The content of lead in the samples of liver is higher than in the skin and muscles. The alteration was in the following order: Pb_liver>Pb_skin>Pb_muscles. P. tereticolliss howed significantly higher content of lead than its host tissues and organs. The content of lead in Unio tumidus was higher than the content of lead in the tissues and organs of B. barbus. A highly significant correlation (p<0.01) was fixed for the relationships between the Pb content in the acanthocephalan and mollusks (C_{P. tereticollisPb}-C _U.tumidusPb) and between that in the acanthocephalan and the skin of the fish (C_{P. tereticollisPb}-C_skinPb).

Keywords: lead, Unio tumidus, Barbus barbus, Pomphorhynchus tereticollis, River Danube