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

More documents

Recommendations

Info

178 Table 5. Abundances of unionid mussels in European running waters Author River Abundance (N/m 2 ) Sediment type Franke (1993) Main (banks) and its 2–4 Gravel, sand, mud tributaries (Germany) Hickley (1983) Eden (England) 73 Gravel and silt Lewandowski (1990) Szeszupa (lower stretches; 2–344 Sandy–muddy, muddy Poland) Libois & Hallet-Libois (1987) Meuse (Belgium) 18.1 Mud, sand, fine gravel Negus (1966) Thames (England) 22.3 Different types (e.g. silt, sand, muddy sand) Zettler (1998) Peene and its tributaries 7.6–75.5 Sandy–muddy (Germany) This study Ryck (Germany) Sandy–muddy Station I 33.6 (max.: 138.7) Station II 34.9 (max.: 110.0) Station III 10.9 (max.: 33.3) composed of representatives of two age classes. Because of the low age of one of these age classes and the relatively high population density (ca. 9.9 indiv./m 2 ), U. tumidus was less endangered at this station and had the best chance of reproduction there in the entire river. However, with respect to the age structure, this part of the population has also to be assessed as very vulnerable. At sampling station III, the stock was considerably overaged and will possibly disappear during the next years, if no more juvenile mussels grow up there. The Anodonta species are not so threatened, though even they are also impaired. The age structures prove the absence of some age classes and an overaging of few stocks. The reproductive success changes more or less annually and appears unpredictable. However, because of the low percentage of juveniles in the population, a reduction of the number of individuals of all species is to expect within the next years. This decline can have certain effects on the future self-purification capacity of the river. Furthermore, the population structures found at the three stations, their peculiarities and similarities, are going to be discussed. In the river Ryck, extensive works for river maintenance are carried out annually. During these works a lot of mussels is injured or killed by machines or left on the bank, respectively. At station II in 1997, no 7-year, 8-year, and 9-yearold clams were found. The absence of some successive age classes could lead to the assumption that within this river section an annihilation of the unionids occurred and later on, a resettlement. Because of the fact that plenty individuals of older age classes were collected, this possibility can be excluded. Furthermore, factors which influence the survival of animals at different developmental stages have to be considered. The glochidia of unionid mussels are attached on host fish as ectoparasites for a certain period of time. Species lists of the ichthyofauna prove the existence of a lot of fish species in the river Ryck from which several species are potential suitable hosts for every mussel species (see Appendix 1). There are no hints for a complete collapse of the fish population during the last years, which could have been the reason for the absence of different age classes. Kornmilch (pers. comm.) observed the ichthyofauna in this river for several years and found frequently shoals of juvenile fish. If the state of the ichthyofauna was a reason of the described situation of unionids, the same age classes of mussels should be absent at all or neighboring stations. But a comparison of the age structures showed a higher similarity between the stations I and III as well as a special position of the amidst located sampling station . Therefore, a certain influence of the fish population on unionid mussels and their reproductive success could exist, but it does not seem to be the most decisive one.
179 There can also be assumed connections between the mussels and the water quality or the sediment composition. Unio tumidus is considered to be highly sensitive to phenomena of eutrophication whilst the Anodonta species are less pretentious (Agrell, 1949; Scholz, 1991, 1992; Jueg, 1993; Mouthon, 1996). Bahr (1994) showed a big influence of oxygen concentration on U. tumidus. Hereby, a concentration of 6.2 mg O 2 /l was given as the limiting value below that the mussels were impaired by oxygen deficiency. In the river Ryck, oxygen concentrations lower than the abovementioned limiting value got frequently measured (Fig. 2e). Such shortage situations existed from time to time for several weeks. The Anodonta species suffer from low oxygen concentrations less than U. tumidus. The different tolerance of the species with regard to environmental factors is closely connected with the type of habitat the mussels live in. In this context e.g. the stability of environmental conditions in waters is a very important criterion. U. tumidus prefers larger waters (e.g. rivers and lakes) with sandy sediments. The Anodonta species inhabit additionally little waters like ponds and peat turbaries with a frequently muddy substratum. There they have to endure extreme conditions which result from the small water body and the accumulation of nutrients from surrounding areas (e.g. extreme temperatures, oxygen over- or undersaturation of the water). Thus, at falling oxygen partial pressure A. cygnea can keep the respiration rate constant or change to an anaerobic metabolism (Holwerda & Veenhof, 1984; Massabuau et al., 1991). Therefore, this species will react less sensitive to a shortage of oxygen. Sheldon & Walker (1989) described similar observations comparing two Australian naiad species. Velesunio ambiguus can be found in billabongs and creeks. When the oxygen concentration is low, these mussels are able to stabilize their respiration rate or they close the valves until the oxygen content in the water rises again. In contrast, the respiration rate of the species Alathyria jacksoni which inhabits large rivers decreases with a declining oxygen concentration. This species depends on a relatively stable environment and is not able to regulate the respiration rate under unfavorable conditions (Sheldon & Walker, 1989). Bahr (1994) declared furthermore that an increase of nitrate, nitrite, and phosphate concentrations can reduce the periods of activity of U. tumidus to 82–38%. The ion concentrations used in these experiments were repeatedly traced in the river Ryck (see Fig. 2a, b, d): NO ) 3 : 0.2 mmol/l (2.801 mg/l NO 3 -N, author’s rem.); NO ) 2 : 0.002 mmol/l, 0.01 mmol/l (0.028 mg/l NO 2 -N, 0.140 mg/l NO 2 -N, author’s rem.); PO/ 3) 4 : 0.005 mmol/l (0.155 mg/l PO 4 -P, author’s rem.). Perhaps U. tumidus in the Ryck is better adapted to unfavorable conditions than the test animals of Bahr (1994). But nevertheless, there seems to be the possibility that the mussels of the Ryck are being damaged or influenced at times by low oxygen or higher ion concentrations. This can enlarge the sensitivity of mussels towards further stress-factors and in that way possibly cause a higher mortality. Thus, a certain influence of the properties of the water body on the stability of the mussel populations can be expected. But as these factors are relatively stabile or form gradients in the course of the river they cannot explain the similarity between the stations I and III as well the peculiarities of station II. Fleischauer-Ro¨ ssing (1990) and Engel (1990) investigated influences on the survival of juveniles of the Unio species. Because juveniles live within the upper stratum of the sediment, sediment composition and interstitial water were analyzed by the mentioned authors. Both investigators explained that ammonium and oxygen concentrations and the content of organic and fine-grained material determine the rate of mortality. Bauer (1988) declared as well, considering the freshwater pearl mussel that a high percentage of organic material in the sediment influences the development of juvenile mussels negatively. The given factors are closely connected. During the decomposition of organic matter in the sediment, oxygen is consumed and poisonous ammonium is accumulated. If fine-grained or organic material clogs the pores of the sediment the exchange of the interstitial water will become limited and the conditions will aggravate. Furthermore, since the water temperature influences the turnover rate of organic matter and nutrients it can also modify the situation in the sediment seasonally (Fischer, 2000).
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: 177 Figure 7. Similarity of the age
Page 13 and 14: 181 Appendix 1. Fish species in the
Page 15: 183 Niemeyer, B., 1993. Vergleichen

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

Create successful ePaper yourself

Delete template?

Save as template?