Quantification and potential role of ocean nutrient loading to Boston Harbor, Massachusetts, USA

While tidal flushing helps export most of the nitrogen added to Boston Harbor (MA, USA) from land sources (>8000 mmol N m^-2 yr^-1; 90% in sewage effluent) to the offshore waters of Massachusetts Bay, the tidal inflow also brings material into the Harbor. For Boston Harbor and many other coastal embayments, tidal inputs must be quantified if we are to develop complete nutrient budgets. This study quantifies tidal input of nutrients and suspended solids (i.e. 'ocean loading') and predicts the future role of ocean loading after sewage effluent discharge is diverted away from the Harbor to a location about 15 km into the Bay. Ocean loading is determined by simple box modeling using data sets available for the 1994 annual cycle. Critical data for modeling include a series of surveys on which high-resolution data for salinity and turbidity were collected using in situ sensors housed in a towed instrument package (i.e. a 'towfish'); surveys covered 2 transects in and out of the 2 Harbor inlets which regulate tidal exchange. Study results show that ocean loading dominates the input-output budgets of nutrients and suspended solids, generally providing more than twice the loading from present land sources. Results further suggest that, although the absolute values of ocean loading will decrease after effluent diversion, the relative contribution of the ocean to the Harbor budget will increase. Predictive modeling suggests that total nitrogen concentrations will decrease about 20% and dissolved inorganic concentrations will decrease about 50% from present levels; these predicted decreases are smaller than one would calculate if the ocean loading term of budgets were neglected. Ocean loading thus will have a role in the nature of Harbor recovery from the planned sewage diversion.