Toxic dinoflagellate ''blooms'' in the Kattegat, North Sea, during the Holocene

Data are presented on the temporal distribution of the toxic dinoflagellate Gymnodinium catenatum in the Kattegat region of the North Sea. Based on dinoflagellate cyst and pollen analyses and a high resolution chronology, a relationship between climatic and oceanographic variations and the blooming of G. catenatum and other dinoflagellates is demonstrated. Gymnodinium catenatum first migrated into the Kattegat about 6200 yr B.P. and had its first occurrence of high numbers at about 4300-4500 yr B.P. At about 4000 yr B.P., in connection with an oceanographic change and climatic deterioration, the species decreased abruptly and subsequently disappeared. It became re-established again at about 2000 yr B.P. and occurred in massive ''blooms'' during the so-called mediaeval warm epoch round about 700-800 yr B.P. (1200-1400 A.D.). At the time of the so-called Little Ice Age, approximately 300 yr B.P. (1500-1600 A.D.), G. catenatum again became extinct in the Kattegat area. This study clearly demonstrates that massive ''blooms'' of toxic dinoflagellates occur in the historical record before any anthropogenic impact on the environment.