Dynamics of Phaeocystis pouchetii blooms in the Wadden Sea of Sylt (German Bight, North Sea)

Abstract

The colony-forming alga Phaeocystis pouchetii (Haptophyceae) forms regular blooms in spring and early summer in the German Wadden Sea. This paper gives a summary of observations of Phaeocystis blooms in 1975, 1976, and 1981 in the Wadden Sea of Sylt (German Bight, North Sea). The onset of Phaeocystis pouchetii blooms started after the decline of diatoms at low inorganic nutrient concentrations, when particularly inorganic phosphate had dropped to minimal concentrations. Phaeocystis blooms did not start unless water temperatures were above 7°C in the investigation area, and the admixture of organic substances seemed to be an important factor in controlling the onset and the size of Phaeocystis blooms. As diatom populations increased again at the end of the Phaeocystis blooms, a causal relationship between the successive development of both algal groups, possibly in combination with other abiotic factors, cannot be excluded.