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

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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):