Isn't Lake Erie's problem caused by agriculture runoff? The main contributors to the algae growth are sewage and phosphorous from fertilizers and some animal wastes. In the 70's, this same thing was happening, and a lot was invested in sewage treatment plants to cut that back to a liveable level, and algae problems dropped. Phosphorous from fertilizers increased in the 90's and the levels in the lake started gradually rising again. A lot of newer ferilizers are out that cut back on the phosphate content. How to cut back on anumal waste phosphates is more challenging. But argiculture in the Great Lakes region is heavily crop oriented and much less animal oriented.
Not an issue for the OBX water supply; all the open water is salt or highly brackish so the wells are used as noted above. As I understand it, the reverse osmosis water plant in Rodanthe COULD use sound water, but doesn't.
Toledo's water supply intake is in Lake Erie. Lake Erie is generally pretty shallow so any issues show up there pretty readily. It also is ringed with developed areas much moreso than any other of the Great Lakes. So it is the bellweather of such problems, and a lot more susceptible to issues than other large bodies of wataer.