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Phytoplankton blooms have attracted attention since times of old, especially the red tides caused by peridinians. Observed in all regions of the world, red tides occur predominantly in the warmer seas (e. g. West Coast of Florida). An opportunity to study a bloom, caused by the non-toxic dinoflagellateCeratium furca, was provided in North Sea coastal waters of an area ranging from Belgium to Sweden. Its development could be followed from July to October 1981 with the aid of samples taken 5 days a week near the island of Helgoland. A decline in the bloom coincided with a drop in water temperature (increasing turbulence). This condition and the presence, right up to the end, of relatively high amounts of nutrients indicate that nutrient deficiency did not terminate the bloom. Similarly, there was no evidence for a dependence on micronutrients by freshwater input or by the preceding diatom bloom. Longterm investigations indicate that climatic changes coincide with an increase in the phytoplankton stock during all seasons (especially noticeable from July to September) and with a decrease in salinity (there was a minimum in 1981). The stratification of the sea water causes the peridinians to move to depths optimal for their growth which may bring about an aggregation at the surface. There are numerous indications in literature of similar changes of environmental conditions in the North Sea. One example from the Western Baltic Sea demonstrates that the presence of a high vertical density gradient in the sea leads to oxygen deficiency in deeper zones, which, however, is not directly correlated to the coincident phytoplanktonic bloom in the upper layers. It is probable that, under appropriate hydrographical conditions, “red tides” can occur repeatedly in the southern North Sea, produced by non-toxic as well as toxic species.

A “red tide” in the southern North Sea and its relationship to the environment