TOXIC DINOFLAGELLATE BLOOMS IN THE KATTEGAT, NORTH-SEA, DURING THE HOLOCENE

Data are presented on the temporal distribution of the toxic dinoflage llate Gymnodinium catenatum in the Kattegat region of the North Sea. B ased on dinoflagellate cyst and pollen analyses and a high resolution chronology, a relationship between climatic and oceanographic variatio ns and the blooming of G. catenatum and other dinoflagellates is demon strated. Gymnodinium catenatum first migrated into the Kattegat about 6200 yr B.P. and had its first occurrence of high numbers at about 430 0-4500 yr B.P. At about 4000 yr B.P., in connection with an oceanograp hic 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 m ediaeval warm epoch round about 700-800 yr B.P. (1200-1400 A.D.). At t he time of the so-called Little Ice Age, approximately 300 yr B.P. (15 00-1600 A.D.), G. catenatum again became extinct in the Kattegat area. This study clearly demonstrates that massive ''blooms'' of toxic dino flagellates occur in the historical record before any anthropogenic im pact on the environment.