Abstract
Eutrophication is a challenge to coastal waters around the globe. In many places, nutrient reductions from land-based sources have not been sufficient to achieve desired water quality improvements. Bivalve shellfish have shown promise as an in-water strategy to complement land-based nutrient management. A local-scale production model was used to estimate oyster (Crassostrea virginica) harvest and bioextraction of nitrogen (N) in Great Bay Piscataqua River Estuary (GBP), New Hampshire, USA, because a system-scale ecological model was not available. Farm-scale N removal results (0.072 metric tons acre−1 year−1) were up-scaled to provide a system-wide removal estimate for current (0.61 metric tons year−1), and potential removal (2.35 metric tons year−1) at maximum possible expansion of licensed aquaculture areas. Restored reef N removal was included to provide a more complete picture. Nitrogen removal through reef sequestration was ~ 3 times that of aquaculture. Estimated reef-associated denitrification, based on previously reported rates, removed 0.19 metric tons N year−1. When all oyster processes (aquaculture and reefs) were included, N removal was 0.33% and 0.54% of incoming N for current and expanded acres, respectively. An avoided cost approach, with wastewater treatment as the alternative management measure, was used to estimate the value of the N removed. The maximum economic value for aquaculture-based removal was $105,000 and $405,000 for current and expanded oyster areas, respectively. Combined aquaculture and reef restoration is suggested to maximize N reduction capacity while limiting use conflicts. Comparison of removal based on per oyster N content suggests much lower removal rates than model results, but model harvest estimates are similar to reported harvest. Though results are specific to GBP, the approach is transferable to estuaries that support bivalve aquaculture but do not have complex system-scale hydrodynamic or ecological models.
Highlights
Nutrient load reductions have been mandated in the USA (US; Clean Water Act) and in the European Union
The objectives of this study were to: 1) determine the mass of N removed through oyster cultivation at current and expanded aquaculture production and by restored reefs; 2) assess how significant the removal is in relation to the total N loading under current and expanded production scenarios; 3) estimate the economic value of the ecosystem service of N removal being provided by oyster aquaculture and restored reefs
Farm-scale results were scaled up to evaluate current and potential system scale N removal using; 1) current acres of oyster licensed area (25.5 acres in 2014–8.5 acres used per oyster year class) and 2) estimates of maximum expanded cultivation area (98 acres–33 acres used per oyster year class)
Summary
Nutrient load reductions have been mandated in the USA (US; Clean Water Act) and in the European Union Management has primarily targeted land-based sources of nutrients including maximizing efficiency of nutrient removal from wastewater treatment plants (WWTP; e.g., Kessler 2010), implementing best management practices (BMPs; e.g., Evans 2008) to reduce nutrient runoff from agriculture and stormwater, and regulation of US electrical generating units to reduce atmospheric inputs (e.g., Linker et al 2013; Eshleman et al 2013). In some cases, these measures have been successful and water quality improvements were observed. In Long Island Sound, a 40% reduction in wastewater nitrogen (N) inputs from 1995–2013 resulted in increased bottom-water
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