Summary The main focus of the current thesis was to extend the knowledge about the use of fish parasites as bioindicators. On one hand the research emphasis was aimed at the faunistical and ecological aspects of parasite communities of barbel (Barbus barbus) in relation to environment conditions - the use of parasites as effect indicators (Chapter 1). On the other hand the thesis evaluated and reverified the acanthocephalan Pomphorhynchus laevis as accumulation indicator, while covering all critical aspects (as summarized by Sures, 2003) concerning its application - the effects of size and sex composition of its infracommunities (Chapter 2) as well as the seasonality (Chapter 3) on the metal uptake process in the parasite. Finally, the thesis delivered a detailed metal monitoring survey focused mainly on the lower Danube (Chapter 4), which was perforemed using the suggested barbel – P. laevis system. The important results are summarized in the following: Chapter 1: The endohelminth fauna of barbel correlates with water quality of the Danube River in Bulgaria. Infection of barbel with ten species of metazoan parasites including three trematodes, three acanthocephalans and four nematodes was observed in fish collected from three localities in the Bulgarian part of the river Danube between summer 2004 and summer 2007. New host records for three parasitic species – the nematode larvae of genus Eustrongylides sp. and Hysterothylacium sp. as well as the acanthocephalan Leptorhynchoides plagicephalus were recorded for first time for the host Barbus barbus. The most prevalent species was the acanthocephalan Pomphorhynchus laevis, which was also the dominant species of the intestinal component communities at all sampling sites. The second most frequent parasite at all Danube localities was Rhabdochona hellichi, which occurred in significantly higher numbers at the less polluted sites. The composition as well as the diversity characteristics of the parasite communities showed a clear correlation with the composition of the invertebrate fauna and water quality – overall, the diversity of helminth communities increased with decreasing levels of nutrients and pollutants at all sampling sites. Chapter 2: Is metal accumulation in Pomphorhynchus laevis dependent on parasite sex or infrapopulation size? From total twelve analyzed elements (As, Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb, Sn, V, Zn) five (As, Cd, Cu, Pb and Zn) were detected in significantly higher concentrations in the acanthocephalan P. laevis compared to its host tissues (muscle, intestine, liver) According to the calculated mean bioconcentration factors, three more elements (Co, Mn, V) usually were with higher concentrations in P. laevis. Comparisons between high and low infected fish revealed significant differences only for V with higher concentrations for the heavily infected group. Concerning sex specific metal accumulation V and Zn showed significant differences (V, at p < 0.05; Zn, at P = 0.05), with higher levels in females of P. laevis each. Chapter 3: Seasonal differences of metal accumulation in Pomphorhynchus laevis and its definitive host Barbus barbus. The concentrations of As, Cd, Cu, Pb and Zn were significantly higher in the parasite samples presented than in the host’s tissues. These elements showed also a clear seasonal pattern, while the concentrations in the fish tissues remained similar in spring, summer and autumn. Seasonal variation in the mean individual weight of parasite infrapopulations, whereas the infrapopulations in autumn characterized with significantly lower mean individual weight than these in spring and summer - sign for predominant young specimens. Composition pattern of all accumulated elements reflected the pattern of the mean individual weight over the year - the highest concentrations obtained for As, Cd, Cu, Pb and Zn in P. laevis were found in autumn, followed by spring and summer. Concentration of Cd and Pb in P. laevis correlated negatively with the mean worm weight. Significant differences for the metals Cd and Pb were found, when the concentration in worms from summer and autumn were compared. Chapter 4: Application of acanthocephalan Pomphorhynchus laevis from its host barbel (Barbus barbus) as metal indicator in the Danube River. The elements As, Cd, Cu, Pb and Zn were found to be significantly higher accumulated in the parasite compared to its host tissues. The concentrations of Cd, Cu, Pb and Zn in P. laevis exceeded the concentrations reported for the water column at the selected sampling sites. The longitudinal pattern of As, Cd, Cu, Pb and Zn in the parasite samples corresponded to the background available data (for water and suspended particulate matter) along the Danube River in Bulgaria. The comparisons between upper and lower Danube, performed in summer 2007, demonstrated increased concentrations of As, Cd and Pb in the lower Danube, whereas As showed also a peak in the upper reaches. Their concentration pattern reflected the pattern of suspended particular matter obtained during the 2nd Joint Danube Survey performed also in summer 2007. The long term monitoring at sampling site Kozloduy (685 km) showed progressive decrease in the concentrations of Cd, Cu, Pb and Zn for the period summer 2004 to summer 2007, while the As contents remained similar. Conclusions and future prospects The collective data suggests that the fish parasites can be successfully used to characterize the ecosystems health and integrity. Following the alternations in composition and diversity of their communities, it was possible to detect differences in the environmental conditions between investigated sampling sites. Therefore fish parasites can be efficiently applied as effect indicators in the aquatic monitoring. The last published data on parasite fauna of barbel for the Bulgarian part of Danube River was from 1960 and 70-ties (Kakacheva-Avramova, 1962, 1977; Margaritov, 1959, 1966). The results obtained from the present investigation showed that the fauna composition completely differed from the one reported 40-50 years ago. The disrepancy can be associated with the changed/disturbed ecological conditions in the investigated river stretch. Further investigations aimed on other river stretches in middle and upper part of the river and on other aquatic habitats as well should be performed for the sake of the future implementation of fish parasites as effect indicators in the Danube River and in the hydrobiological praxis as well. The faunistical data from the different parts of the Danube should be compared and subsequently correlated with the local abiotic and biotic data, in order to confirm profoundly the relationship between parasites and environmental conditions. Furthermore, different host-parasite systems should be also be included in such surveys and studied from a bioindication perspective. A major goal of the current thesis was filling in on the lack of knowledge regarding the application of fish acanthocephalans as accumulation indicatiors. The obtained results suggested that the size and the sex composition of acanthocephalan’s infrapopulations play no considerable role in the metal uptake process. Therefore, in metal monitoring surveys, especially in those aimed on toxic elements such as As, Cd and Pb, these aspects should not be taken into account. Worth noticing is that the results regarded the acanthocephalan P. laevis. If other acanthocephalan species are taken as accumulation indicators, these aspects should be studied in order to confirm the tendencies obtained in the thesis. On the other hand the results revealed a seasonal pattern in the metal uptake, which was found to be dependent on the stage of acanthocephalan’s development in the final host. Thus, the seasonality of transmission of P. laevis under the local climate conditions should be considered in order to make our monitoring surveys more precise. In some geographical regions, where the seasonality of transmission is not clearly pronounced, the seasonal factor can be suppressed. Of course, the sentinel features of fish acanthocephalans should also be investigated under different climate conditions, in order to select the proper sampling periods for metal monitoring surveys. The same should be done with other acanthocephalan species, if they are taken as metal indicators. With the help of the background metal monitoring data delivered by the International Commission for Protection of Danube River (ICPDR), it was confirmed that the levels of the elements accumulated in P. laevis corresponded to these in the environment. The pollution profile in the Danube River basin, obtained during the both Joint Danube Surveys (in 2001 and 2007), was additionally confirmed by the concentrations measured in the parasites. Desipite the mobility of the fish host, the results of this thesis suggest the fish-parasite system is a perfect model in the field of ecological monitoring. However, future detailed analysis and correlations between the raw data from the JDS2 and obtained parasite data are required. Unfortunately, this data concerning the element concentrations in water, SPM, sediment and biota is still not available. Regarding the practical use of a fish-parasite system as sentinel, the first step was made during the second Joint Danube Survey in 2007, where fish muscle tissue was analyzed. During the thesis I had the opportunity to contribute to the survey with metal analysis carried out on barbel – P. laevis system. The combinded results suggest that the additional use of fish acanthocephalans as sentinels represents a more powerful approach in heavy metal monitoring surveys, due to the higher accumulation capacity of acanthocephalans compared to fish muscle. Consequently, in future monitoring programs the fish parasites should be accessorily implemented as sentinels, especially in large and complex lotic systems like Danube River.