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.

富营养化（Eutrophication）是全球性环境挑战，多样的流域氮源亟需多维度的治理方案。贝类养殖（shellfish aquaculture）可实现氮移除，但该生态系统服务（ecosystem service）的作用规模与价值在局域尺度上尚未得到充分表征。本研究采用一种创新方法，对某市镇范围内贝类养殖业提供的氮削减服务进行量化与价值评估。以康涅狄格州格林威治湾的养殖硬壳蛤（cultivated hard clam）与东部牡蛎（eastern oyster）为例，通过重置成本法（replacement cost methodology）核算其氮移除价值，并按市镇范围内的各类氮源进行价值分摊。基于按氮源分摊重置成本的优选分析方案，该养殖系统年均移除氮14006千克，价值达230万至580万美元（230万至640万欧元）。该氮移除量占格林威治区域年度总氮负荷的9%、非点源（nonpoint sources）总氮负荷的16%、化肥源氮负荷的38%、化粪池排污源（septic sources）氮负荷的51%、流域大气沉降（atmospheric deposition）氮负荷的98%，或是排入格林威治湾的港湾（embayments）大气沉降氮负荷的184%。本研究方法可推广至其他已建立或尚未开展贝类养殖、且旨在实现氮削减目标的近岸流域，可为流域氮治理经费相关决策提供科学依据，并提出了一套可全面评估氮削减目标实现路径的研究策略。