Zooplankton of the open Baltic: Extended Atlas - IOW

amplitude. On the other hand, for exclusion of the dominant twenty four hour
periodicity, for example due to daily vertical migration, one needs to measure
the chosen parameter only during one phase of the process (at day or at night)
or one needs to choose a twenty-four hour measuring distance. Vertically
integrated sampling would also provide acceptable results.
For statistical reasons, to get the unbiased results, a period should be
examined at least three times. That means a twenty-four hour periodicity
demands a measuring length of three days.
Further, the characteristics of the equipment, i.e. the selectivity of a
plankton sampler (type of net, mesh size, etc.) or the sensitivity of a sensor
(e.g. depth recorder) influence the results accordingly. For example, the
UNESCO Standard net WP-2 quantitatively selects organisms between 0.2
and 10 mm size (UNESCO, 1968). The limits are set by the mesh size of 200
µm on one hand and by the net opening area of 0.25 m² in combination with
the towing speed of 45 m/min on the other. To obtain comparable results, one
needs to use standardized techniques in this respect, too.
The “filter mechanisms” modifying “real patterns” into “observed
patterns” determine mainly the accuracy but also the precision of the results
(Fig. 3.1.2).
y
Real patterns
x
z
Modified by observation filters
•Measuring length
•Measuring interval (spatial and temporal)
•characteristics of equipment
(selectivity, sensitivity)
•Measuring errors
•Data analysis
Observed patterns
Time 2
Time 1
Figure 3.1.2. Factors modifying “real patterns” to “observed patterns”
according to Postel (1983).
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