Uptake of methyl mercury by the floater mussel, Pyganodon grandis (bivalvia, unionidae), caged in a flooded wetland

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Abstract
A 6.7‐ha wetland with a pond at the Experimental Lakes Area, northwestern Ontario, was experimentally flooded to determine causes of elevated waterborne 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 ability to monitor the elevated waterborne MeHg. Mussels were also caged in a reference wetland pond and in the lake from which they were collected (source lake). Background levels of MeHg in source lake mussel body 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 source lake did not alter MeHg concentration in any body part. Mussels transplanted to the ponds for 90 d showed statistically significant increases above background MeHg in the mantle and visceral remains in the preflood experimental pond (waterborne MeHg 0.09 ng/L) and in the mantle, visceral remains, foot, and kidney in the reference pond (waterborne MeHg 0.24 ng/L). The visceral remains of mussels in the reference pond contained higher levels of MeHg than did those in the preflood experimental 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. Inexplicably, mussels caged in a hypoxic environment in the experimental pond lost MeHg and THg from all body parts. We concluded that not only can P. grandis monitor elevation in waterborne MeHg with flooding, but the MeHg levels in mussels also reflect small differences in background levels of natural MeHg in ponds.