Zooplankton of the open Baltic: Extended Atlas - IOW

organisms are suspended in filtered tap water or distilled water for analysis.
The procedure should be carried out under a fume-hood. The dilution of the
total sample will be chosen according to experience to reach an appropriate
concentration for the analysis. The sample is often so densely concentrated
that it demands sub-sampling into aliquots. Therefore, 1/32 ± 1/8 of the
sample were analysed, for example, within the monitoring programme of the
Leibniz Institute of Baltic Sea Research in 2005 (Wasmund et al., 2006).
Technically, the volume of the total sample, measured in a graduated cylinder,
is noted as the reference amount. The sample is then poured into a beaker to
allow a thorough mixing until the organisms are distributed randomly before
taking an aliquot. Repeated sub-sampling by the Stempel pipette (Hensen,
1887) produces a coefficient of variation of 7-9%, applying a bulb pipette – of
14-15%, and a Folsom splitter – of 5-18%. The variability between total
counts amounts to 0.3–2.5% (Guelpen et al., 1982). The use of pipettes is 5 to
8 times faster than the splitter technique. Its limitation lies in the size of
zooplankters in case they are larger than the pipette’s diameter. The Kott
splitter (Kott, 1953) is more convenient in comparison to the Folsom splitter
(Sell & Evans, 1982; Griffiths et al., 1984). The Kott splitter produces 8 subsamples
at the same time, while the Folsom device splits samples into halves
and increases the error from one step to another (Behrends & Korshenko,
pers. comm.).
For routine sorting of larger zooplankton, a dissecting stereomicroscope
will be used. It makes manipulation of the specimen during the
identification procedure possible. For the smaller mesozooplankton of the
Baltic Sea, such as rotifers, cladocerans, copepods and their developmental
stages, an inverted microscope accomplishes the same. It allows routine
survey with the 50x magnification and the analysis of details with a
magnification factor of 80x to 125x as well. For more specific investigations,
like the examination of the fifth leg of copepods, a compound microscope
with achromatic condenser and 10x to 70x objectives is the preferred
instrument. For details of rotifer morphology, 100x oil immersion objective is
necessary.
An inverted microscope needs an open counting chamber with high
transparency like the Mini-Bogorov chamber (modified after Arndt, 1985).
Closed types of counting chambers are preferably used in microzooplankton
studies. The trays are provided with sections to allow a better orientation and
to avoid a repeated counting of the same organism. One counting strip is fully
covered with the 50x magnification. The Mini-Bogorov chamber (Fig. 3.4.1)
is easy to produce in a workshop. It has the following dimensions: the length,
width, and height are 40, 70, and 8 mm, respectively. The counting paths are
6 mm deep, their width amounts to 3 mm, the section walls are 1 mm wide,
and their height is 4.5 mm. The sides and walls are tapered sloping at top. The
tray is made of clear plastic and needs to be polished to high quality (Postel et
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