The lack of dissolved oxygen in these rivers is attributed to agricultural practices that allow manure and fertilizers to run off into streams. Additional tributaries fail to meet standards for dissolved oxygen due to loadings from urban runoff. Preliminary results of Friends of Casco Bay volunteer water quality monitoring in Casco Bay itself show that there were seven violations out of 320 samples for dissolved oxygen in SC waters. While Casco Bay does not appear to have major nutrient enrichment problems at present, the potential for problems will increase as population and development grows. The value of Casco Bay is evident to anyone living in its watershed, but hard to capture by any single measure.
Economic figures reveal only one dimension of the bay's diverse array of values.
While the costs of environmental protection are traditionally seen as detracting from economic gains, it is now clear that Casco Bay will provide economic returns only so long as its ecological health is sustained. As the third largest oilhandling port on the East Coast, Portland ships and stores 80 percent of Maine's petroleum. The port also handles other cargo products, including lumber and coal. While most of the fish landed in Portland come from the Gulf of Maine, Casco Bay is thought to be an important nursery for many juvenile finfish. Crabs, mussels, scallops, and marine worms are also harvested commercially in Casco Bay.