Chlorophyll a: 
During the survey, water samples from 5 m water depth were filtered for chlorophyll a 
analyses ashore. 
Appendix 3, figure 3.3 shows a comparison between these in-situ measurements and the 
optical measurements made with the CTD for the surface layer. 
Figure 3.3 (top, left) shows the distribution of chlorophyll a in the surface layer determined 
according to Aminot and Rey (2001) after the survey (see also “Equipment and methods”, 
Dage 7). The concentration ranges from more than 11 ug/L close to the coast in the 
southeast North Sea to almost 0 ug/L in the central part. High values mark the areas of high 
biological productivity at shallow waters and close to the coast. 
Figure 3.3 (top, right) shows the same distribution measured by the CTD. 
Differences between the two methods (Figure 3.3, middle, left) were especially high in areas 
of high chlorophyll a concentrations. Figure 3.3 (bottom) show a linear (left) and a quadratic 
(right) fit of CTD chlorophyll to in-situ chlorophyll. These fits are only meant as a first hint, 
that we are far away from seeing a one-to-one relationship between these two kinds of 
measurements. 
Possibly, differences between CTD and in-situ measurements at high concentrations can be 
axplained by effects of turbidity on the optical sensor, as turbidity is also high close to the 
zoast and in shallow waters. Further analyses will be carried out on this topic. 
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