Zooplankton of the open Baltic: Extended Atlas - IOW

3. METHODS OF COLLECTING AND ANALYSING
ZOOPLANKTON IN THE BALTIC SEA
3.1. Sampling: general aspects
Miscellaneous processes, like seasonality, daily vertical migration,
swarming, etc. produce typical zooplankton distribution patterns (Haury et al.,
1978). In reality, we observe a result of the combination of those processes,
and skilful observation strategies need to be applied in order to distinguish
them by choosing proper sampling duration (length) and measuring intervals
(Sameoto et al., 2000). Appropriate measuring intervals are to be chosen
considering the so called Nyquist sampling theorem (Nyquist, 1928).
Following it, a signal must be measured in equal distances of more than two
times within one period (or wave length) of the specific signal. Otherwise one
produces aliased results as Figure 3.1 illustrates.
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Measurements of a 24 h periodicity by different
sampling intervals
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Abitrary units
100
80
60
40
24 h periodicity
8 h sampling interval
13 h sampling interval
28 h sampling interval
20
0
0 24 48 72 96 120 144
Time [h]
Figure 3.1.1. Examples of sampling intervals producing accurate (by
8 h) and biased (by 13 and 28 h) results of a 24 h periodicity.
Nearly accurate results will be obtained if a certain process will be
measured at least three times within its periodicity or a wave length. For
example, the twenty four hour periodicity of dial vertical migration demands
an equidistant measuring interval of at least eight hours (Fig. 3.1.1). Longer
distances, like thirteen or twenty-eight hours, produce apparent results of a
thirty-six hour or even a seven day periodicity, for example. Continuous
measurements would produce the most accurate outcome in both phase and
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