THE MORPHOMETRIC VARIATION OF ACTINOMMA-BOREALE (RADIOLARIA) IN ATLANTIC BOREAL WATERS

A morphometric study on the polycystine radiolarian species Actinomma boreale (Cleve) from ten trigger weight core-tops from the Norwegian-I celand Seas, three piston cores taken offshore western Norway and thre e surface sediment samples from Lygrepollen, Sogndalsfjord and Hoyangs fjord (western Norwegian fjords) shows a variation in morphology that groups A boreale into three distinct clusters, interpreted to be relat ed to different oceanographic settings. Tile largest specimens of A. b oreale are found in the western Norwegian fjords, the smallest in the Iceland Sea, giving an apparent positive correlation with temperature. Down core studies in piston cores from the Norwegian Sea demonstrate a considerable size variation of the cortical shell of A. boreale. In the eastern Norwegian Sea, the climatically cold Younger Dryas had a p opulation of A. boreale that was characterized by large cortical shell s, while the climatically warm Holocene population was dominated by sm all sized cortical shells, showing a negative correlation with tempera ture. We suggest that the large sized cortical shell population of A. boreale in the Younger Dryas is not reflecting precisely the sea-surfa ce water temperature. Another factor must play the dominant role here, probably nutrients.