The development and succession of taxa within the diatom genus <i>Fragilaria</i> Lyngbye as a response to basin isolation from the sea

Abstract

Thirteen lake basins isolated during the Late Weichselian and Holocene are studied. The lakes are situated in two study areas, one on the west coast and one on the south coast of Norway. The diatom, genus Fragilaria shows a predominance in sediments deposited around the isolation independent of when the basins were isolated. The highest percentages may occur before, during or after the actual isolation from the sea based on diatom taxa other than Fragilaria spp. Predominance of Fragilaria spp. seems to develop in accordance with the local environment and more or less independently of the regional differences. The large Fragilaria abundances might be caused by the same effect as in recently deglaciated basins situated above marine limit in Late Weichselian, mainly dependent on the nutrient supply. Summary A diatom analytical study has been made of eight lakes in Hordaland, western Norway and five lakes in Telemark, southern Norway, to investigate their changing environment during isolation from the sea. These lakes have been isolated at different ages, spanning from BØlling in Late Weichselian to Subatlantic in Holocene. The aim of the investigation was to determine whether regional differences or differences in time of isolation would affect the general trend in the development of such basins. The rate of the isolations is more rapid in Late Weichselian and Preboreal than in the remaining Holocene. The pH‐values and trophic status indicate a development from nutrient rich (alkaline and eutrophic) to nutrient poor (acid and oligotrophic) conditions, regardless of geographical position. Large abundances of Fragilaria spp. occur around the isolations. Peaks in Fragilaria spp. may occur before, during or after the isolation from the sea. Therefore Fragilaria taxa are not good isolation indicators for detailed stratigraphic investigations. It is evident that a predominance of Fragilaria spp. may occur in any chronozone. In the two areas investigated there is a tendency for F. construens to reach its highest frequency values in basins isolated between 10,000 and 8,000 years B.P. Large abundances of F. pinnata are not restricted to specific chronozones and the species appear to have a wide salinity range. F. virescens var. subsalina is also present from BØlling to the Subatlantic chronozone and indicates a preference for saline water. It seems plausible that the large abundances of Fragilaria spp. around the isolation levels are mainly caused by the same effect as in recently deglaciated basins above the marine limit in Late Weichselian and seems to be dependent on the nutrient supply. The predominance of Fragilaria spp. seems to develop according to the local ecology of the separate basins more or less independently of the regional differences.