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Dissolved Oxygen


One of the early perceptions about the effects of pollution in Boston Harbor was that dissolved oxygen (DO) in the water was too low. We now know that, except for a few isolated locations, DO levels in the harbor were high enough to support healthy marine life even before the Boston Harbor Project began. This change in perspective is not only based on more monitoring data, but also on by the U.S. Environmental Protection Agency showing that DO requirements for marine life are actually lower than previously thought.

DO levels in Boston Harbor have benefited from the harbor being so well-mixed and well-flushed. MWRA’s monitoring has revealed little change in DO concentrations in the harbor’s waters since the early '90s.

Map of Dissolved Oxygen in Boston Harbor

This map shows the average pattern of DO in the bottom waters of the harbor during the summer.


A moored instrument near the New England Aquarium shows DO in the water decreases at night, and increases through the day. Note that even in the early morning when DO is lowest, concentrations are greater than EPA's new recommended standard for marine waters (an average of 4.8 mg/l).

  More on the State of Boston Harbor