Environmental influence on the diatom and silicoflagellate assemblages in Koljö Fjord (Sweden) over the last two centuries

Abstract

Throughout the last century, the Swedish coasts, have been exposed to climatic variation and human influence, and this has altered the biota in some areas. Diatoms and silicoflagellates in a periodically-laminated sediment core from Koljo Fjord (Swedish west coast) were examined to assess recent changes in the microalgal community. The most notable changes in the plankton occurred at the beginning of a long period of water column stratification in the fjord that extended from 1930 to 1980. The planktonic flora changed from a community dominated byBacterosira bathyomphala andThalassionema nitzschioides during the 1800s to a community dominated byDetonula confervacea, T. nitzschioides, andThalassiosira spp. after 1930. Silicoflagellates, were more abundant after 1940. Planktonic variations corresponded to oscillations in climate, hydrography, and weather, which determine water column stability in the fjord. The tychopelagic species,Paralia sulcata, was more abundant in unlaminated sections of the core, indicating preference for a vertically mixed water column. No direct effects of increased nutrient loading on the plankton could be established. Epiphytic diatoms show a period of decline from the 1950s to 1990s. This trend probably follows a shift in the macroalgal community to less-suitable host species. To what extent this pattern has been influenced or reinforced by humans cannot be determined at the present time. The results from Koljo Fjord, in particular the exploration of meteorological and physical oceanographic influences on algal dynamics, emphasize the importance of distinguishing between natural and humaninduced changes in the environment.