INTERPRETATION OF FLUOROMETRIC CHLOROPHYLL REGISTRATIONS WITH ALGAL PIGMENT ANALYSIS ALONG A FERRY TRANSECT IN THE SOUTHERN NORTH-SEA

In 1991 chlorophyll a was measured continuously by fluorometry on the ferry 'Norstar', between Zeebrugge and Hull. In order to assess the us efulness of this platform for monitoring of long-term changes in phyto plankton distribution in the southern North Sea, calibration cruises w ere carried out in January, April, June and August with RV 'Holland' t o relate algal pigment fingerprints to the fluorescence record. Chloro phyll a concentrations, determined after HPLC separation, allowed a qu antification of the continuous fluorescence signal. The relation betwe en fluorescence and chlorophyll a content was found to be rather const ant in June and August and allowed a prediction of the chlorophyll a c oncentration from fluorescence measurements with 21%. In January and A pril, however, this relation was different for separate hydrographic r egions. In April, a major chlorophyll derivative, an allomer indicativ e of degradation of algal matter, was abundant near the Belgian coast. During the decay of the Phaeocystis bloom, this high concentration of allomerized chlorophyll a increased the 'background' fluorescence sig nal. In January, enhancement of fluorescence caused by light scatterin g on non-algal suspended matter could be observed most clearly near th e English coast, where the algal concentration was low (0.5 mug.dm-3) while suspended (inorganic) matter concentration was high (30 mg.dm-3) . By applying linear relations, calculated for every hydrographic regi on, prediction of the chlorphyll a concentration could be improved to an accuracy of 38%. An analysis of taxon-specific chlorophylls and car otenoids reveals that the distribution of individual pigments and of p igment ratios was indicative of taxonomic composition of the phytoplan kton. Patterns in pigment concentrations along the section suggest the same phytoplankton abundance gradients and patchiness as revealed by the continuous fluorescence measurement. A succession in the phytoplan kton population was apparent: from diatoms and traces of green algae i n January, to a spring bloom of Phaeocystis in April close to the Belg ian coast, to diatom dominance in June, to a mixed population of green algae, diatoms and Prymnesiophyceae (probably Coccolithophorids) in A ugust, at the end of the vegetative season. The presence, even occasio nal abundance, of green algae (carrying chlorphyll b) in the southern North Sea has not been reported earlier. The allomerized chlorophyll a concentration was most abundant near the coastal ends of the section during all cruises. Apparently, the phytoplankton is an important sour ce of detritus in the North Sea.