'Blooms' of the toxic dinoflagellate Gymnodinium catenatum as evidence of climatic fluctuations in the late Holocene of southwestern Scandinavia

Abstract

Previous reports have suggested climatic fluctuations as a possible factor controlling 'blooms' of the toxic dinoflagellate Gymnodinium catenatum Graham 1943 that occurred in Scandinavian waters at about 4000 and between 2000 and 300 BP. Evidence of climatic fluctuations associated with the younger 'blooms', which correspond to the Mediaeval Warm Epoch, is presented here from detailed dinoflagellate cyst records in a 860-cm sediment core from the southern Kattegat. Peak concentrations of the cosmopolitan cyst Operculodinium centrocarpum (Deflandre and Cookson) Wall 1967 suggest environmental changes at about 2000 BP associated with the initial phase of the 'blooms', and at about 300 BP corresponding to the waning phase. Since G. catenatum is only recorded in small amounts from the region today, and otherwise is more characteristic of warmer water, its 'blooms' are considered probable evidence of warmer sea-surface temperatures between 2000 and 300 BP. Supporting evidence for this is provided by peak concentrations of G. catenatum dated to about 830 BP. The occurrence of the extremely cold oceanic species Impagidinium pallidum Bujak 1984 and the cold-water species Protoperidinium conicoides (Paulsen) Balech 1974 in two consecutive samples at 400 BP probably indicate increased influence of colder oceanic waters from the Norwegian Sea. This corresponds to the beginning of the 'Little Ice Age' which most likely contributed to the subsequent waning of the 'blooms'.