During 1989, the North Sea Community Programme made monthly cruises to study physical, chemical and biological conditions at 110 sites in the 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 concentration in the surface mixed layer. At site CS (55°30'N, 0°55'E) the spring bloom was dominated by diatoms and coincided with the onset of seasonal stratification in late April. The fluorometer record showed that the bloom lasted about a week. At the more strongly stirred site AB (52°42'N, 2°25'E), the bloom was dominated by Phaeocystis and occurred in late May, after the termination of the fluorometer record. It was concluded that the greater optical thickness of the surface mixed layer was responsible for the later bloom at AB. Phaeocystis dominance may be explained by the greater depletion of silicate relative to nitrate at AB. It is suggested that the ending of the blooms depended on several factors, including nutrient limitation, sinking and protozoan grazing.
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