FOSSIL AKINETES OF APHANIZOMENON AND ANABAENA AS INDICATORS FOR MEDIEVAL PHOSPHATE-EUTROPHICATION OF LAKE GOSCIAZ (CENTRAL POLAND)

Records of fossil akinetes of Aphanizomenon and Anabaena (Cyanobacteri a) in the laminated sediments of Lake Gosciaz are interpreted. Increas ing human impact in the catchment area of the lake (as can be interpre ted from the pollen records of human impact indicators) apparently had its effect on the trophic conditions of the lake water: from ca. 1000 AD on, fossil akinetes of Aphanizomenon and Anabaena are present in e normous quantities in the sediment. The increases of the Cyanobacteria are interpreted as the effect of an intensification of fanning and la nd fertilization in the area around Lake Gosciaz, causing eutrophicati on of the lake. Phosphorus enrichment from effluent and excreta in the catchment area of the lake at limes will have become so high that N-l imited growth conditions occurred. In such conditions Cyanobacteria ca pable of nitrogen fixation (namely Aphanizomenon and Anabaena) could b loom. The deposits of the last ca. two centuries are characterised by the successive appearance of Pediastrum boryanum, Tetraedron minimum, Coelastrum cf. reticulatum, Botryococcus, Scenedesmus, Spirogyra, Gloe otrichia and Staurastrum manfeldtii, whereas Cyanobacteria show a decl ine. This phytoplankton succession could be interpreted in terms of co mpetition for nutrients and light: higher eutrophication levels and hi gher turbidity caused a decline of available light, and as a consequen ce less energy was available for the energy consuming process of nitro gen fu;ation by Aphanizomenon and Anabaena.