There's a Red Tide bloom in FL this year that is killing Manatees by the droves. It's an algal bloom that can wreak havoc in marine and tidal life.
When Red Tide hits tidal flats you can't pull anything from the flats like clams or mussels...not sure what the algae bloom in the Great Lakes does,but it's a sure bet to block sunlight and oxygen norms.
I remember Lake Erie when it was considered a fairly well dead lake and dead fish lined the shores on a regular basis, and when the Cuyahoga River, in Cleveland, caught fire, it was so polluted. Lake Erie is, I think, one of the big turn around environmental successes in US history.