Population size and structure of three mussel species (Bivalvia ...

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180 The above-described bearings seem to be transferable to the river Ryck. At the sampling sites I and II, where the content of organic material in the sediments is lower, there were juvenile U. tumidus found. Within the river stretch at station II characterized by the lowest losses on ignition of the sediments, U. tumidus reached its highest abundance. At station III, where the highest losses on ignition were measured juvenile U. tumidus did not exist, and the stock was obviously overaged. In contrast to the results regarding U. tumidus, juveniles of the Anodonta species were found at all stations. The absence of some age classes can probably be traced back to the same causes as for U. tumidus, even if the clams of the genus Anodonta are less sensitive to their environment. Therefore, at station II the clearly higher current velocity causes a diverging composition of the sediment and creates living conditions for mussels which are different from those at the other two stations. The living conditions influence particularly the survival of the juvenile unionids during the first months after leaving the host fish which is reflected in abundance, dominance, and age structure of the species. As mentioned above, brackish water flows occasionally into the river stretch at station III. A determinant influence of this water on the unionid mussels there is not to be expected because of two reasons. First, living A. anatina and A. cygnea were found further down the river, too. Secondly, the similarity of the mussel stock of station III to that of station I which was not influenced by brackish water rejects this thesis. Summary and conclusions The results of this survey showed that unionids attained very high densities in the river Ryck. Species composition and abundance corresponded with this type of running waters, i.e. small and slowly flowing rivers. But changes of stocks can be predicted. The number of individuals is going to decrease, and U. tumidus may disappear in several river sections. The data suggest that the dominance values of the species as well as the peculiarities of their age structures are influenced first of all by current velocity and sediment composition, which change seasonally and along the river. Especially the juvenile unionids inhabiting the upper stratum of sediment are dependent on the conditions in the interstitium. A strong current ensures a better ventilation of the sediment, prevents an enrichment of poisonous substances, and reduces the sedimentation of further fine material that clogs the interstitium. In contrast, the concentrations of ions and oxygen in the water body appear to have only a low influence on the mussel populations. Thus, the decline of unionid populations in the Ryck seems to result from the eutrophication of this river. The sediments that are largely enriched with organic material will – for the longer term – maintain the internal eutrophication of the river. This situation will continue even if the influx of matter into the river decreases. Therefore, there is no rapid improvement in the living conditions of the water organisms to be expected. The presented results suggest to register not only the presence of unionid species, but also the age structure of their populations in monitoring programs to characterize waters. Age structures present important hints for the actual state of the populations (e.g. present ability for reproduction), their future stability and development (undisturbed, impaired, or overaged populations). Therefore, age structures appear as sensitive indicators for the situation in the waters and as a necessary supplement for a solely recording of the species and their abundances. Acknowledgements The study was carried out at the Zoological Institute and Museum of the Ernst Moritz Arndt University of Greifswald and supported by grants of the foundations ‘Studienstiftung des deutschen Volkes’ and ‘Fazit-Stiftung’. Thanks to Mrs M. Sandhop for her encouragement and help in improving the manuscript. Mr J.-C. Kornmilch kindly provided data on the ichthyofauna of the river Ryck. I also wish to thank Dr C. Weber for the translation of the manuscript into English and Dr A. J. Anderson (Daberkow) as well as S. Schubert (Greifswald) for linguistic advice. I am grateful to an anonymous reviewer for important comments on an earlier version of this manuscript.
181 Appendix 1. Fish species in the upper course of the river Ryck and their suitability for serving as host. Investigators of the ichthyofauna: a ¼ Hille (1997); b = Kornmilch (pers. comm.); c ¼ Meyer (1962); d ¼ Wanke (1996). Fish species Investigator Suitability of the fish species as host Aa Ac Ut Abramis brama (L.) Carp bream b, c +6 )5 Alburnus alburnus (L.) Bleak b, c Anguilla anguilla (L.) Eel c, d )7 )7 Blicca bjo¨rkna (L.) White bream b, c, d +6 +6 Carassius carassius (L.) Crucian carp b, c, d )3, )7 )1, )7 )5 Cobitis taenia L. Spined loach a, b, c, d Esox lucius L. Pike a, b, c, d 4, +7 +7, +6 Gasterosteus aculeatus L. Three-spined stickleback a, b, c, d +7 +7 +2 Gymnocephalus cernuus (L.) Ruffe c 4, )6 +1, )6 ±5 Leucaspius delineatus (HECKEL) Belica a, b, c +3 Leuciscus idus (L.) Ide d Perca fluviatilis L. Perch a, b, c, d +3, 4, +6, +7 +1, +7 +2, +5 Pungitius pungitius (L.) Ninespine stickleback a, b, c, d Rhodeus sericeus (BLOCH) Bitterling a, b, c )3, )6, )7 )1, )6, )7 Rutilus rutilus (L.) Roach a, b, c, d +3, 4 )2, +5 Scardinius erythrophthalmus (L.) Rudd b, c, d +7 +1, +6, +7 )2, +5 Tinca tinca (L.) Tench a, b, c, d +3 )1 +5 Vimba vimba (L.) Baltic vimba c Suitability of these fish species for serving as host: 1 = Claes (1987); 2 = Fleischauer-Ro¨ ssing (1990); 3 = Franke (1993); 4 = Jokela et al. (1991; finding of infected fish – no check, whether glochidia developed successfully); 5 = Maaß (1987); 6 = Nagel (1985); 7 = Niemeyer (1993). + = fish species that is suitable as host (metamorphosis of glochidia into juvenile mussels); – = unsuitable species (glochidia were shed); Aa = A. anatina; Ac=A. cygnea; Ut=U. tumidus. References Agrell, I., 1949. The shell morphology of some Swedish unionides as affected by ecological conditions. Arkiv fo¨ r zoologi 41A: 1–30. Amt fu¨ r Wasserwirtschaft, 1970. Fla¨ chenverzeichnis der Flußgebieteder DDR-Ku¨ ste. Verlag fu¨ r Bauwesen, Berlin,153 pp. Amyot, J.-P. & J. A. Downing, 1991. Endo- and epibenthic distribution of the unionid mollusc Elliptio complanata. Journal of the North American Benthological Society 10: 280–285. Arter, H. E., 1989. Effect of eutrophication on species composition and growth of freshwater mussels (Mollusca, Unionidae) in Lake Hallwil (Aargau, Switzerland). Aquatic Sciences 51: 87–99. Bahr, K., 1994. Untersuchungen zu Schalenbewegung und Sauerstoffverbrauch einheimischer Großmuscheln unter verschiedenen experimentellen Bedingungen. PhD thesis, Tierärztliche Hochschule Hannover. Bauer, G., 1988. Threats to the freshwater pearl mussel Margaritifera margaritifera L. in Central Europe. Biological Conservation 45: 239–253. Byrne, M., 1998. Reproduction of river and lake populations of Hyridella depressa (Unionacea: Hyriidae) in New South Wales: implications for their conservation. Hydrobiologia 389: 29–43. Claes, M., 1987. Untersuchungen zur Entwicklungsbiologie der Teichmuschel Anodonta cygnea. Ms thesis, Tierärztliche Hochschule Hannover. Crowley, T. E., 1957. Age determination in Anodonta. Journal of Conchology 24: 201–207. Downing, J. A., Y. Rochon, M. Perusse & H. Harvey, 1993. Spatial aggregation, body size, and reproductive success in the freshwater mussel Elliptio complanata. Journal of the North American Benthological Society 12: 148–156. Engel, H., 1990. Untersuchungen zur Auto¨ kologie von Unio crassus (PHILIPSSON) in Norddeutschland. PhD thesis, Universität Hannover. Falkner, G., 1990. Binnenmollusken. In Fechter, R. & G. Falkner, Weichtiere-Europäische Meeres- und Binnenmollusken. Mosaik Verlag, Mu¨ nchen: 112–273. Fischer, H., 2000. The abundance and production of bacteria in river sediments, and their relation to the biochemical composition of organic matter. PhD thesis, Universita¨ t Freiburg.
Page 1 and 2: Hydrobiologia (2005) 537: 169-183
Page 3 and 4: 171 Table 2. Analyzed water propert
Page 5 and 6: 173 (a) NO 3 -N (b) NO 2 -N (c) NH
Page 7 and 8: 175 Dominance [%] 80 70 60 50 40 30
Page 9 and 10: 177 Figure 7. Similarity of the age
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Page 15: 183 Niemeyer, B., 1993. Vergleichen

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