THE SPRING BLOOM IN THE SOUTH WESTERN NORTH-SEA IN 1989

During 1989, the North Sea Community Programme made monthly cruises to study physical, chemical and biological conditions at 110 sites in th e southern North Sea. This paper describes events during the spring at two sites differing in their tidal stirring characteristics. In situ fluorometers moored at each site gave daily estimates of chlorophyll c oncentration in the surface mixed layer. At site CS (55-degrees-30'N, 0-degree-55'E) the spring bloom was dominated by diatoms and coincided with the onset of seasonal stratification in late April. The fluorome ter record showed that the bloom lasted about a week. At the more stro ngly stirred site AB (52-degrees-42'N, 2-degrees-25'E), the bloom was dominated by Phaeocystis and occurred in late May, after the terminati on of the fluorometer record. It was concluded that the greater optica l thickness of the surface mixed layer was responsible for the later b loom at AB. Phaeocystis dominance may be explained by the greater depl etion of silicate relative to nitrate at AB. It is suggested that the ending of the blooms depended on several factors, including nutrient l imitation, sinking and protozoan grazing.