Standard CZCS Case 1 algorithms in Danish coastal waters

The purpose of the current work was to test the performance of several published CZCS (Coastal Zone Color Scanner) Case 1 algorithms for retrieving pigment concentration and vertical attenuation coefficients in Danish coastal waters. This involves in situ measurements and water samples from the North Sea, the Kattegat, the inner Danish Waters and the Baltic Sea. The entire record of CZCS acquisitions of Danish waters was compared with actual cruise ship data of the Danish National Environmental Research Institute (NERI)-14 days matched exactly but, mainly due to cloud cover, a total of only 38 trustworthy data points were found on seven of these. Vertical attenuation coefficients were estimated from profiles of PAR intensity. Pigment concentrations were determined from spectrophotometric measurements on water samples (chlorophyll-a plus phaeopigment). Correlations between CZCS and in situ estimates were found, but the biases were strong, particularly in the case of pigment concentration. This was as expected because of the extensive amounts of yellow substances present in Danish waters. Therefore, a correction procedure for excluding the yellow substance contribution in pigment concentration retrieval was tested. The results seem promising, but the number of observations is small, and further studies are needed before firm conclusions can be drawn. Nevertheless, it can be stated that trustworthy algorithms for Danish waters for new ocean colour instruments (OCTS, SeaWiFS and MERIS) clearly need to take the absorption by yellow substances into consideration.