STORIES FROM OLD RESERVOIRS - SEDIMENT HG AND HG METHYLATION IN ONTARIO HYDROELECTRIC DEVELOPMENTS

We sampled several Ontario Hydro reservoirs to determine the changes i n mercury (Hg) and organic profiles of sediment cores with reservoir d evelopment We also examined Hg methylation among reservoirs of differe nt age and water chemistry. In sediment cores from four Ontario reserv oirs, reservoir (watershed) specific differences appeared to supersede general trends, with the differences between headpond and run-of-rive r reservoirs particularly important. In general, the rate of Hg and or ganic accumulation appeared to increase with reservoir impoundment, bu t we were unable to discern consistent changes in concentrations of Hg or loss on ignition (LOI) with reservoir impoundment. We also observe d significant positive correlations of sediment Hg with concentrations of chlorophyll derivatives and bacterial photopigments. Our results a re in agreement with earlier studies which indicated that an increase in Hg supply caused by erosion and release from flooded roils is centr al to the changes in Hg dynamics within reservoirs following impoundme nt. Although Hg methylation activity of sediments from a series of res ervoirs on the Mississagi River was positively correlated with organic content of the sediment, the observed rates were sufficiently variabl e that the potential effects of reservoir age or preparation methods c ould not be resolved. Similarly, in our measurements of net methylmerc ury (MeHg) flux in two older reservoirs, the large range of variation in net MeHg flux observed among replicate samples obscured the potenti al effects of differences in geology and water chemistry. Future studi es should focus on resolving the underlying causes of this variability and in consolidating the 'microscale' measurements obtained using sed iment con: incubation techniques and the 'macroscale' values obtained by whole lake mass balance techniques.