UPTAKE OF METHYL MERCURY BY THE FLOATER MUSSEL, PYGANODON GRANDIS (BIVALVIA, UNIONIDAE), CAGED IN A FLOODED WETLAND

A 16.7-ha wetland with a pond at the Experimental Lakes Area, northwes tern Ontario, was experimentally hooded to causes of elevated waterbor ne methyl mercury (MeHg) associated with impoundment. Unionid mussels, Pyganodon grandis (formerly Anodonata grandis grandis) were caged in the experimental pond before and after flooding to determine their abi lity to monitor the elevated waterborne MeHg. Mussels were also caged in a reference wetland pond and in the lake from which they were colle cted (source lake). Background levels of MeHg in source lake mussel bo dy parts were in the order mantle < gill or visceral remains < foot or kidney and ranged from 108 to 618 ng/g dry weight. Caging in the sour ce lake did not alter MeHg concentration in any body part. Mussels tra nsplanted to the ponds for 90 d showed statistically significant incre ases above background MeHg in the mantle and visceral remains in the p reflood experimental pond (waterborne MeHg 0.09 ng/L) and in the mantl e, visceral remains, foot, and kidney in the reference pond (waterborn e MeHg 0.24 ng/L). The visceral remains of mussels in the reference po nd contained higher levels of MeHg than did those in the preflood expe rimental pond. Flooding increased waterborne MeHg from 0.1 to 2.3 ng/L and resulted in an increase in MeHg and total Hg (THg) in the mantle, foot, and visceral remains of mussels in the experimental pond. Inexp licably, mussels caged in a hypoxic environment in the experimental po nd lost MeHg and THg from all body parts. We concluded that not only c an P. grandis monitor elevation in waterborne MeHg with flooding, but the MeHg levels in mussels also reflect small differences in backgroun d levels of natural MeHg in ponds.