Dinoflagellate cysts as indicators of pollution and past climate in a Norwegian fjord

Abstract

Two sediment cores from a semi enclosed eutrophic fjord and one from the open adjoining fjord revealed a sequence of dinoflagellate cyst assemblage changes during the past 1000 years. Higher percentages of the warmer water cyst Lingulodinium machaerophorum (Deflandre and Cookson) Wall 1967 (10-41%) within the restricted fjord correspond with the Medieval Warm Period, and is succeeded by decreased percent ages (5%) from about AD 1380 corresponding with the onset of the more adverse climate of the 'Little Ice Age'. Two peaks of Operculodinium centrocarpum (Deflandre and Cookson) Wall 1967, at about AD 1700 followed by decreased total cyst concentrations in the enclosed fjord and increased total cyst concentrations in the open fjord, is used to correlate cores from the two fjords, and suggests climatic change. Increased pollution from sewage discharge as the human population of the city of Bergen expanded is suggested by decreased total cyst concentrations (cyst/g dry sediment) accompanied by peak concentrations of O. centrocar pum at about 1850 and a peak concentration of cysts of the heterotrophic species Protoperidinium conicum (Gran) Balech 1974 at about 1930. A smaller peak of O. centrocarpum together with slightly increased total cyst concentrations towards the top of the cored sediments may indicate improved water conditions after sewage discharge into the inner fjord was reduced in the 1980s.