PALEOPIGMENT EVIDENCE OF COMPETITION BETWEEN PHYTOPLANKTON AND A CYANOBACTERIAL ALGAL MAT IN A MEROMICTIC LAKE NEAR TORONTO, ONTARIO CANADA

Crawford Lake, a meromictic lake located near Toronto, Canada, was cor ed to determine if algal pigments preserved in its sediments would mak e it possible to infer past changes in lake productivity over the last five hundred years. From 1500 to 1910 A.D. the sediments display extr emely high levels of oscillaxanthin and myxoxanthophyll while chloroph yll derivatives and total carotenoids were relatively low. As the lake became increasingly more eutrophic in the latter part of the twentiet h century, this relationship reversed itself. Competition for light be tween the deep dwelling cyanobacteria in the algal mat on the lake's b ottom (8-14 m) and phytoplankton in the overlying surface layers of th e water column (5-7 m) was attributed to the observed reduction in osc illaxanthin and myxoxanthophyll as Crawford Lake eutrophied. Because t he major cyanobacteria in Crawford Lake are benthic mat forming Lyngby a and Oscillatoria, and not phytoplankton, competition for light with the overlying phytoplankton is critical in determining the total quant ity of oscillaxanthin and myxoxanthophyll preserved in the lake's prof undal sediments. These findings have major implications for the use of cyanobacterial pigments as indicators of lake trophic status in lakes where benthic algal mats are present.