Mussels are filters. So if you're looking for a good source of everything gross in the Lake, chow down.
If you wouldn't eat lake trout because of mercury levels, why would you eat a filter that soaks in all manner of septic runoff, animal scat, mercury, petroleum, phosphates, etc., etc., etc.?
(Not that I haven't been tempted many times over the years...and, yes, I have tried the crayfish -- not really comparable to the mudbugs so coveted in Louisiana...) 