Zebra mussels are filter feeders and that have dramatically cleared the water of lots of stuff, including the silt that was responsible for such terrible visibility in the Great Lakes for so many decades. They've also filtered out a lot of pretty disgusting toxins. This is good.
Additionally, the reality is that there is more algae in the water now than before the mussels arrived. As they filter out the crud, light is better able to penetrate and the algae is able to photosynthesize and grow. Ten years ago algae blooms were a very rare occurence in Lake Michigan, now they are an annual event. As with the algae, so too the seaweed - harbors that used to be mostly devoid of seaweed have been mowing these past few years to keep the stuff down and we're seeing increasing amounts of it outside the harbors. This is also good.
All-in-all, the zebra mussel is hard on man-made things like gloves and wrecks and boat hulls and water intakes. This is bad. Overall, however, they seem to be having a salutory effect on the lake ecology. The water is clearer, cleaner and the algae and seaweed are starting to come back. A couple of years ago the perch population crashed, largely due to loss of cover for the fry as the water cleared. Now that the algae and seaweed are back, the fry are able to hide again and the population is recovering pretty quickly.
It's popular to cry doom and gloom about the little suckers (and they do stink) but the visibility is better than it has been in several generations and the lakes seem to be coming back to life. Now if the round head gobies (that displaced the native sculpin) will eat enough of the mussels, things should find some kind of equilibrium. Around here, the density of the mussel encrustation seems to have been dwindling over the past couple of years, so there may be cause for some hope that they won't continue to be so insanely omnipresent.
