INCREASES IN FLUXES OF GREENHOUSE GASES AND METHYL MERCURY FOLLOWING FLOODING OF AN EXPERIMENTAL RESERVOIR

Experimental flooding of a boreal forest wetland caused the wetland to change from being a small, natural carbon sink, with respect to the a tmosphere, of -6.6 g of C m(-2) yr(-1) to a large source of +130 g of C m(-2) yr(-1). This change was caused by the death of the vegetation, which eliminated the photosynthetic CO2 sink and stimulated the micro bial production of CO2 and CH4 from decomposition of plant tissues and peat. Another type of microbial activity that increased was the methy lation of inorganic mercury to the much more toxic methyl mercury (MeH g) form. The wetland was a source of MeHg prior to flooding and became an even larger source (39 fold) after flooding. MeHg concentrations i n the water sometimes exceeded 2 ng L-1, with the average being 0.9 ng L-1 in the first 2 years after flooding. MeHg also increased in the f looded vegetation and peat, in lower food chain organisms, and in fish . Two recommendations, which should minimize both greenhouse gas produ ction and MeHg production in reservoirs, can be made: (1) minimize the total area of land flooded (i.e., avoid flooding areas of low relief) and (2) minimize the flooding of wetlands, which contain larger quant ities of organic carbon than uplands and are sites of intense producti on of MeHg.