Massachusetts Lake Water Quality

Massachusetts lists 829 lakes across 15 counties, 81 of which carry a grade. Most of the input behind those grades flows through monitoring run by the Massachusetts Department of Environmental Protection, which feeds state, federal, and volunteer measurements into the EPA Water Quality Portal. The remainder are here because they are real named lakes, not because anyone has sampled them recently. Roughly 46% of the state's graded lakes carry an A, 37 lakes that earn the top bracket on the combined Secchi, phosphorus and chlorophyll-a rubric. That share is high by national standards and reflects the depth and natural clarity of glacial lakes in the region.

Quabbin Reservoir is the largest lake in this index at about 24,043 acres, one of 28 here that clear 500 acres. The rest run smaller, and small and shallow is where nutrient loading shows up first, because there is less water to dilute it and the bottom sediment stays within reach of the sun. Mesotrophic is the dominant trophic class among the graded lakes, a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. Franklin ranks highest on average county grade (A), and Bristol sits lowest. With only 15 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.