Quantification and Valuation of Nitrogen Removal Services Provided by Commercial Shellfish Aquaculture at the Subwatershed Scale.

Eutrophication is a global environmental challenge, and diverse watershed nitrogen sources require multifaceted management approaches. Shellfish aquaculture removes nitrogen, but the extent and value of this ecosystem service have not been well-characterized at the local scale. A novel approach was employed to quantify and value nitrogen reduction services provided by the shellfish aquaculture industry to a municipality. Cultivated hard clam and eastern oyster nitrogen removal in Greenwich Bay, Connecticut, was valued using the replacement cost methodology and allocated by municipal nitrogen source. Using the preferred analysis allocating replacement costs by nitrogen source, aquaculture-based removal of 14 006 kg nitrogen was valued at $2.3-5.8 (2.3-6.4€) million year-1. This nitrogen removal represents 9% of the total annual Greenwich-specific nitrogen load, 16% of the combined nonpoint sources, 38% of the fertilizer sources, 51% of the septic sources, 98% of the atmospheric deposition to the watershed, or 184% of the atmospheric deposition to the embayments that discharge to Greenwich Bay. Our approach is transferable to other coastal watersheds pursuing nitrogen reduction goals, both with and without established shellfish aquaculture. It provides context for decisions related to watershed nitrogen management expenditures and suggests a strategy to comprehensively evaluate mechanisms to achieve nitrogen reduction targets.