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

Abstract In 1991 chlorophyll a was measured continously by fluorometry on the ferry ‘Norstar’, between Zeebrugge and Hull. In order to assess the usefulness of this platform for monitoring of long-term changes in phytoplankton distribution in the southern North Sea, calibration cruises were carried out in January, April, June and August with RV ‘Holland’ to relate algal pigment fingerprints to the fluorescence record. Chlorophyll a concentrations, determined after HPLC separation, allowed a quantification of the continuous fluorescence signal. The relation between fluorescence and chlorophyll a content was found to be rather constant in June and August and allowed a prediction of the chlorophyll a concentration from fluorescence measurements within 21%. In Janaury and April, however, this relation was different for separate hydrographic regions. In April, a major chlorophyll derivative, an allomer indicative 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 signal. In January, enhancement of fluorescence caused by light scattering of non-algal suspended matter could be observed most clearly near the English coast, where the algal concentration was low (0.5 μg·dm −3 ) while suspended (inorganic) matter concentration was high (30 mg·dm −3 ). By applying linear relations, calculated for every hydrographic region, prediction of the chlorophyll a concentration could be improved to an accuracy of 38%. An analysis of taxon-specific chlorophylls and carotenoids reveals that the distribution of individual pigments and of pigment ratios was indicative of taxonomic composition of the phytoplankton. 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 phytoplankton population was apparent: from diatoms and traces of green algae in January, to a spring bloom of Phaeocystis in April close to the Belgian coast, to diatom dominance in June, to a mixed population of green algae, diatoms and Prymnesiophyceae (probably Coccolithophorids) in August, at the end of the vegetative season. The presence, even occasional abundance, of green algae (carrying chlorophyll 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 source of detritus in the North Sea.