[ChrisPebble]

Zebra mussels have had a noticeable effect on water clarity in Chicago harbors in the last ten years. When the light is right it's possible to see 20-30 feet down easily when the view was always previously occluded.

Aside from the detrimental effects on local species it also means that algae and weed blooms are common and it's not uncommon for harbors to be choked with weeds by August.

Not sure if all the current water clarity across the lake can be attributed to them but the change inshore has been rapid and remarkable.