Again I am not an expert in this area, but this is what I hear experts say about it. The answer is no. Although water clarity has improved because of the zebra mussels, you need to understand the quagga mussels are quickly “out-muscling”, if you will, the zebra mussels. I'm sorry, that wasn't intended.
They are processing many nutrients through their systems, and in some instances they are actually magnifying the effects of the nutrients once they process themselves through the system.
The other thing is they are having a negative effect on some of the other critical biota, in particular, diaporia. Especially in Lake Michigan and I believe Lake Huron, those levels have gone down dramatically. It's felt that is the responsibility of the zebra and quagga mussels. That's the base of the food chain for the sport and commercial fishery.
So I don't think anyone thinks they've had a positive effect on water quality, although some people say, “Oh, I can see deeper. It must be better.” Not so.