CHANGES IN C-13 CONTENT OF THE ORGANIC-COMPONENT OF LAKE-SEDIMENTS DURING THE LAST 500 YEARS IN CRAWFORD LAKE, SOUTH ONTARIO, CANADA

The major cyanobacteria in Crawford Lake are benthic mat forming Lyngb ya and Oscillatoria and not phytoplankton. The eutrophication of the l ake has resulted in a decline in the mat forming cyanobacteria as infe rred from palaeopigment analyses of the core from this lake. In previo us palaeolimnological studies the concentrations of oxillaxanthin and myxoxanthophyll have been used as correlates with lake trophic levels. High concentration of osciIlaxanthin and myxoxanthophyll were interpr eted as indicating eutrophic conditions prevailed. Our results indicat ed that when Crawford Lake was most eutrophic little oscillaxanthin an d myxoxanthophyll was produced. High values of delta(13)C at the depth of 34-48 cm (1500-1760 A. D.) were related to a dense population of b enthic Oscillatoria and Lyngbya living on the bottom of the lake durin g that period. The Oscillatoria and Lyngbya utilize bicarbonate as a s ource of inorganic carbon. Carbonate has a high delta(13)C value. Very low delta(13)C values were found at 0-7 cm (1955- 1992 A. D.) in the Crawford sediment core at the time when phyto- plankton dominated the core. Phytoplankton are enriched in C-12 by photosynthetic assimilatio n of CO2.