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

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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.