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

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c
Sq ½%Š ¼100
a þ b þ c ;
where Sq is the set quotient; a, the number of age
classes only existing within stock A; b, the number
of age classes only existing within stock B; c the
number of age classes existing within both stocks.
Secondly, to calculate the similarity of the
dominance values of species or age classes, Renkonen‘s
coefficient was used (Mu¨ hlenberg, 1989):
Rc ½%Š ¼ Xs
i¼1
D min
i ;
where Rc is the Renkonen’s coefficient; D i min , the
smallest of the two dominance values of a species
or age class.
At last, set quotient and Renkonen’s coefficient
were combined to calculate Wainstein’s index,
which considers the presence of species or age
classes within different stocks and their dominance,
too (Mu¨ ller, 1987; Mu¨ hlenberg, 1989):
Sq Rc
Wi ½%Š ¼ ;
100
where Wi is the Wainstein’s index; Sq, the set
quotient; Rc, the Renkonen’s coefficient.
Results
Environmental factors
The results of water analyses are shown in Figure
2. Of all variables average values, highest and
lowest ones of the investigation period are presented.
The factors followed characteristic seasonal
trends during the years. Amongst nitrogen
ions the nitrate ions predominated (Fig. 2a–c).
The concentration of nitrate was 5–10-fold larger
than the ammonium one and represented the 25-
fold of the nitrite concentration. The concentration
minima were measured in summer. The
phosphate concentration was between 0.002 and
0.424 mg/l (Fig. 2d). Especially high concentrations
appeared at times of low oxygen saturation.
The concentrations of nitrogen and phosphate
ions slightly decreased downwards the river or were
lowest at station II. The gained values of the four
ions mirror a b-mesosaprobic to b-a-mesosaprobic
situation in the river (determination according to
LAWA, 1998). The concentration of oxygen declined
downwards the river (Fig. 2e). During the
year, concentration minima occurred in summer.
The lowest value of 1 mg/l (saturation: 11%) was
measured at station II. The temperature of the
water correlated with the air temperature and appeared
highest in summer. Down the river, the
mean water temperature rose (Fig. 2f). The conductivity
showed only little deviations (Fig. 2g).
The high value of 2.09 mS/cm at station III could
be explained by an influx of brackish water into
the upper river. The current was fastest at station
II (Fig. 2h). Its highest value was 0.4 m/s. The
concentration of seston was ascertained to quantify
the nutrient resources for mussels. The average
concentration at station III was nearly double-fold
as high as at the other two stations (Fig. 2i). No
significant differences between the sampling stations
were found regarding the water properties (ttest:
p > 0.05).
The sediment consisted of silty sand (stations I
and II) or silty loamy sand (station III). The
average content of organic matter varied between
3.6% (station II) and 17.8% (station III). The
sediment composition showed a close relationship
to the current velocity, as with an increase of current
velocity the percentage of sand in the sediment
increased, too. The content of organic material,
however, got reduced (Fig. 3). The percentages of
sand and organic matter in the sediment at station
III differed significantly from those found at the
stations I and II (t-test (sand): p I/II ¼ 0.375, p I/
III ¼ 0.049, p II/III ¼ 0.013; t-test (organic matter):
p I/II ¼ 0.308, p I/III < 0.001, p II/III < 0.001).
Species composition and population size
At all stations, mussels of the species Anodonta
anatina (L.), Anodonta cygnea (L.), and Unio tumidus
Philipsson were found. The abundance of
mussels, which got ascertained from particular
samples and sample series, showed considerable
deviations. In Figure 4 the mean values for the
sampling stations as well as the values of series
with highest and lowest abundances are given.
The highest population densities of the unionids
were observed in the bank regions of the
stations I (maximum: 138.7 indiv./m 2 ; April 1996)
and II (max.: 110.0 indiv./m 2 ; May 1997). In