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 (Cyanobacteria) in the laminated sediments of Lake Gosciaz are interpreted. Increasing human impact in the catchment area of the lake (as can be interpreted 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 fosil akinetes of Aphinizomenon and Anabaena are present in enormous quantitites in the sediment. The increases of the Cyanobacteria are interpreted as the effect of an intensiication of farming and land fertilization in the area around Lake Gosciaz, causing eutrophication of the lake. Phosphorus enrichment form effluent and excreta in the catchment area of the lake at times will have become so high that N-limited growth conditions occurred. In such conditions Cynoobacteria capable of nitrogen fixation (namely Alphanizomenon and Anabaena) could bloom. The deposits of the last ca. two centuries are characterised by the successive appearance of Pediatrum boryanum, Tetraedron minimum, Coelastrum cf. reticulatum, Botryococcus, Scenedesmus, Spirogyra, Gloeotrichia and Staurastrum manfeldtii, whereas Cyanobacteria show a decline. This phytoplankton succession could be interpreted in terms of competition for nutrients and light: higher eutrophication levels and higher turbidity caused a decline of available light, and as a consequence less enegry was avilable for the energyu consuming process o nitrogen fixation by Aphanizomenon and Anabaena.