Long-term Trends of Benthic Habitats Related to Reduction in Wastewater Discharge to Boston Harbor

A combination of methods (infaunal grabs and sediment profile cameras) were used to monitor the response of Boston Harbor benthic habitats to reductions in wastewater associated with movement of the outfalls to the mouth of the harbor and then offshore. From 1992 to 2006, there was strong evidence that benthic habitats within Boston Harbor have shifted from a more anaerobic state to a more aerobic state and that these changes are directly related to changes in carbon loading associated with outfall placement and improvements in wastewater treatment. Over the period of 1992 to 2000, when the ocean outfall started to operate, there was > 90% reduction in organic loadings to Boston Harbor from 11,400 to 1,200 t C per year. There were also corresponding decreases in primary production due to reduced nutrient loadings. The most apparent change in harbor benthos was the widespread increase in 1992 and subsequent decline by 2005 in Ampelisca spp. tube mats. The long-term increase in thickness of the apparent color redox potential discontinuity layer was consistent with reductions in organic loading and increases in bioturbation. The optimal organic loading for maintaining large areas of amphipod tube mats and high bioturbation rates was around 500 g C per square meter per year. Above and below this level, the area of tube mats in Boston Harbor declined.