Zooplankton of the open Baltic: Extended Atlas - IOW

Measuring errors are to be divided into rough errors and absolute
errors. The first category is not evaluated and therefore not taken into account.
The second one splits into regular ε r and irregular errors ε ir . Irregular errors
will be determined by calculation of confidence limits. Examples are the
results of sample splitting or of organisms counting in sub-samples. The
regular errors include constant errors and systematic errors. A balance, which
shows always a constant difference to the real weight, would be an example
for a constant error, while an increasing weight of a dried sample due to an
uptake of moisture with time is an example for a systematic error. Both are to
be quantified and consequently provide part of a correction mode. The
terminology bases on Junge (1981). According to the author, a measured
value a differs from the real value x by ε r and the scatter of ε ir , i.e.
x = (a +/- ε r ) ± ε ir
Finally, methods of data analysis may influence the outcome. For
example, it is of importance to choose appropriate software settings, e.g.
interpolation modes, when performing contouring mapping.
These and other aspects need to be considered when elaborating the
appropriate sampling strategies, planning the sampling surveys, or evaluating
the outcomes.
3.2. Sampling of meso- and macrozooplankton
For measuring the total amount of plankton, a set of equipment is
necessary. Protozoan sampling demands water bottles (see below); however,
larger organisms are included only occasionally and in a non-representative
manner in such samples. They will be caught by plankton nets with different
mesh sizes and geometries. Mesozooplankton in the sea is sampled best by
the already mentioned WP-2 UNESCO Standard net (UNESCO, 1968). It is a
closing net suitable for vertical tows and stratified sampling. Considering the
smaller mesozooplankton in the Baltic Sea, this net (Figure 3.2.1a) is
recommended for the HELCOM Monitoring and Assessment programme
with a mesh size of 100 µm (HELCOM, 1988, 2005). In shallow areas, the
use of horizontally or oblique towed instruments of a similar shape is suitable,
like Bongo or Multiple nets (Figures 3.2.1b, c).
Collecting macrozooplankton demands nets with larger opening areas
and mesh sizes (Fig. 3.2.1 d). For details regarding the different net
characteristics see Wiebe and Benfield (2003) and references therein.
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