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

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170 (a) M-V Rienegraben Greifswalder Bodden Horst I Groß Petershagen Ryck Jarmshagen II Wackerow Wieck Bachgraben Boltenhagen III Heilgeisthof Eldena (b) 5 km Greifswald Figure 1. Maps showing the study area in northeastern Germany (a) and the sampling stations (b). M–V ¼ Federal State of Mecklenburg-Vorpommern; I, II, III ¼ sampling stations. (e.g. O¨ kland, 1963; Tudorancea, 1972; Lewandowski & Stanczykowska, 1975; Arter, 1989; Lewandowski, 1991; Krzyzanek, 1994; Mu¨ ller & Patzner, 1996). Data on populations living in running waters are infrequent (Negus, 1966; Hickley, 1983; Lewandowski, 1990). Thus, the aim of this work is to redress the balance and to provide further information on unionids in running waters. Moreover, the results should disclose possible environmental factors, which may affect the mussels. Data on the structure of populations are required to take measures for their preservation and to provide base-line data for further investigations to reveal changes in the studied river. Study area The river Ryck is a small coastal river located in the northeast of Mecklenburg-Vorpommern (northeastern Germany). Its catchment basin is of an area of 230.7 km 2 (Amt fu¨ r Wasserwirtschaft, 1970) and is predominantly used agriculturally. After a flowing distance of approximately 24.5 km (VE Meliorationskombinat Rostock, 1968, 1970), the river Ryck discharges into the Greifswalder Bodden near Greifswald, i.e. into a bay of the Baltic Sea. While the Ryck upriver of Greifswald is of freshwater, the lower stretches carry brackish water from the Bodden. In irregular intervals the flowing direction of the river changes due to the Table 1. Width and depth of the river Ryck at the sampling stations I, II, and III Station Width (m) Depth (m) I 9.0 0.9 II 9.5 1.3 III 19.5 1.3 water level of the Greifswalder Bodden and the wind conditions. Those changes are occasionally connected with an influx of brackish water into the stretch of the Ryck upriver of Greifswald. The upper course of the river tables a slope of only 0.1& (VE Meliorationskombinat Rostock, 1968). In the upper course, three sampling stations were set up (Fig. 1). Depth and width of the Ryck at these stations can be seen in Table 1. The shores are fringed with reeds from Phalaris arundinacea (station I) or Phragmites australis (stations II and III). Among the hydrophytes in the river Nuphar lutea, Potamogeton lucens, Potamogeton perfoliatus, Ceratophyllum demersum, and Elodea canadensis dominate. Materials and methods Environmental characteristics In order to examine water properties and to identify factors which could influence the mussels,
171 Table 2. Analyzed water properties and the used measuring instruments and methods Variable NO 3 -N, NO 2 -N NH 4 -N, PO 4 -P O 2 , temperature Conductivity Current velocity Seston Instruments and methods Ion chromatograph (Sykam Chromatographie Vertriebs GmbH) Photometer Nanocolor 200D (Macherey- Nagel GmbH) Oximeter Oxi 96 (WTW) Conductometer LF 95 (WTW), temperature: 20 °C Drift body method Water samples (3 · 1 l) were filtered with washed (Aqua dest.), dried and weighed GF/ C filters (Whatman), after that the filters were dried (50 °C) and re-weighed water samples were analyzed monthly between April till October in the years 1996–1999. During these months, the mussels reach their maximum activity (e.g. growth, reproduction, metabolism, translocation) and react therefore stronger to environmental influences. The concentration of oxygen, temperature, and current velocity were measured at the sampling stations. The other variables were analyzed in the laboratory (Table 2). To describe the sediment composition at each station, 15 samples were taken across the width of the river using a corer (inner diameter: 55 mm). In this study, the upper 50 mm of the cores were investigated. The organic content of the sediments was determined by loss on ignition after heating the samples for 2 h at 550 °C in a muffle furnace. The percentages of sand, silt, and clay were measured applying a laser particle sizer (Fritsch analysette 22). Survey of the mussel populations The samples were taken between 1996 and 1998. In order to secure well-founded data on the abundance of unionids, the entire sediment of defined areas was sieved completely (Haukioja & Hakala, 1974; Hanson et al., 1988; Miller & Payne, 1988, 1993; Amyot & Downing, 1991; Richardson & Yokley, 1996). Thus, a 0.25 m 2 plastic frame (quadratic, height: 0.25 m) was pushed into the substratum. The upper sediment layers within the frame were removed with a net until more compact layers, consisting of marl, sand, or peat, were reached. The removed material was sieved (4 mm), and the mussels were collected and placed in labeled bags. With the intention to investigate different ranges of the river bed, six of those samples from the bank and further six from the middle of the river were taken for every sample series. In the middle of the river at station III, samples could not be taken as described because of too soft sediments. However, areas of 0.25 m 2 were investigated using a dredge closable by a lid in a distance of 5.5–6 m off the bank. The dredged material was sieved, and the mussels were picked out at the sampling station, too. There were seven sample series taken at station I, 3 at station II, and 4 at station III. Within one sampling site, only different areas were investigated by the several sampling series applying a preestablished pattern. In order to ascertain the total size of the mussel populations at every station, four transects were installed additionally across the river. In every transect, one sample per meter was taken. The sampling was carried out as described using frame and net or the dredge, respectively. The collected mussels were brought to the laboratory for ageing and kept there in a large vessel filled with river water. The age was identified by means of dark annual rings observed on the shells (e.g. Haskin, 1954; Crowley, 1957; Negus, 1966; Haukioja & Hakala, 1978; Neves & Moyer, 1988). Difficulties in counting these rings could appear if mussels stopped their growing after disturbances and formed pseudannuli. Those were not easily to distinguish from true annual rings. Furthermore, the age determination could become difficult if several rings were very close. Such ring concentrations occurred mainly in the fringe ranges of shells of older mussels. Mistakes in the age determination were avoided by means of repeated controls and comparisons of shells. After the investigations, the mussels were released at the location of their origin. The similarity of species compositions and age structures ascertained at the sampling stations was determined using three indices. First, for comparing the presence or absence of age classes at the stations, the set quotient was calculated (Mu¨ ller, 1978, 1987):
Page 1: Hydrobiologia (2005) 537: 169-183
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
Page 11 and 12: 179 There can also be assumed conne
Page 13 and 14: 181 Appendix 1. Fish species in the
Page 15: 183 Niemeyer, B., 1993. Vergleichen

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