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

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Sand, org. matter [%]
90
60
30
0
Sediment
II I III
Station
0,15 0.15
0,10 0.10
0,05 0.05
0,00 0.00
Current velocity [m/s]
sand org.matter current velocity
Figure 3. Average percentages of sand and organic matter in
the sediments as well as the average current velocity measured
in the water column. I, II, III ¼ sampling stations.
A comparison of the dominance structures
observed at the three stations by means of
Renkonen’s coefficient underlined the special position
of station II (Table 3).
The mean abundances of the unionid mussels
are given in Table 4. However, the study showed
that the mussels were unequally distributed within
the river cross-section (Fig. 4). Thus, the determined
abundances only enabled to draw rough
conclusions about the total size of the unionid
populations. Therefore, the numbers of clams per
river meter (rm ¼ stretch of the river of 1 m
length) were calculated (Table 4). With 350.5 indiv./rm
the largest number of mussels per river
meter was found at station II. It even exceeded the
number of individuals at station III, though the
river Ryck was twofold wider there (comp.
Table 1).
Age structure
The age structures of A. anatina and A. cygnea
appeared very unbalanced within the period of
Station I (1996)
Station II (1997)
Abundance [N/m 2 ]
160
140
120
100
80
60
40
20
0
66.0
44.6
bank
20.7
0.7
middle
36.7 27.6
9.1
0.0
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Abundance [N/m 2 ]
140
120
100
80
60
40
20
0
61.1
38.6
bank
13.8
8.7
42.2
middle
17.1 20.4
4.7
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Abundance [N/m 2 ]
40
35
30
25
20
15
10
5
0
7.5
Station III (1997)
4.5
bank 18.5 5.5m
2.3 0.7
12.3
4.0 2.2
Un Aa Ac Ut Un Aa Ac Ut
Taxon
Figure 4. Abundance of the Unionidae at the sampling stations. Column and given number ¼ average value; vertical lines ¼ presentation
of maximal and minimal values; Un ¼ Unionidae; Aa ¼ A. anatina; Ac¼ A. cygnea; Ut¼ U. tumidus; 96, 97 ¼ years of
surveys (1996 and 1997); bank ¼ sample series taken within the bank region; middle ¼ sample series taken from the middle of the river;
5.5 m ¼ sample series taken in a distance of 5.5–6 m off the bank.