Nutrient Bioassimilation with Oysters

The health of the Chesapeake Bay has declined as a result of anthropogenic eutrophication has occurred along with the reduction in oyster beds across the Bay to less than 1% of pre-19th-century populations. A major focus for improving water quality is the reduction of nutrients at the source. However, nutrient source reductions currently meet only 47% of N and 63% of P targeted watershed goals. Alongside specific concerns about N and P over-enrichment, the environmental impacts of anthropogenic ocean acidification and climate change have become additional water quality issues in the Bay.

New strategies to expand nutrient sinks, including in situ nutrient removal by oyster aquaculture, are being considered. Permanent removal of nutrients from the Bay can be achieved by harvesting oysters grown through intensive aquaculture. Oysters assimilate nutrients from phytoplankton biomass into tissue and shell, and when the oysters are harvested, the nutrients are permanently removed from the ecosystem.

Sustainable oyster aquaculture may reintroduce some of the ecosystem services that were lost with the decline of the wild population and has been suggested as a nutrient mitigation tool by several researchers. Nutrient removal through assimilation by aquacultured bivalves with the goal of offsetting terrestrial nutrient sources has been proposed, modeled, or piloted in numerous settings and locations around the world. However, quantification of nutrients sequestered in aquacultured native oyster biomass and the relative magnitudes of nutrients removed through aquaculture are unknown for the Chesapeake Bay.

Laboratory studies report that at seston concentrations of 5 to 20 mg L⁻¹, an Eastern oyster assimilates approximately 50% of filtered particulate organic N and excretes the remainder.

However, similarly sized aquacultured oysters have thinner shells than wild oysters, and, given the variable growth rates of oysters under the wide variety of cultivation conditions in Chesapeake Bay, the quantity of nutrients sequestered in cultivated tissue and shell may differ from previous estimates.

Implementation of oyster aquaculture as a nutrient capture and removal system may be a viable auxiliary approach to bridge the gap when best management practice (BMP) implementation fails to meet targeted source reduction milestones.

So the the researchers examined the nutrient content of oysters reared and harvested from two commercial-scale floating raft aquaculture oyster production sites to quantify the total nutrient content (total nitrogen (TN), TC, total carbon (TC), and total phosphorus (TP)) of aquacultured oysters. Direct measures of TN, TP, and TC in aquacultured oyster biomass were used for regression models that related nutrient content to oyster shell total length (TL) and to provide Bay managers with a reliable nutrient removal quantification tool based on the oyster aquaculture market characteristic, TL at harvest. Using these models, the researchers extrapolated the potential nutrient removal benefits to Chesapeake Bay.

Based on model estimates, harvesting 7.7 × 10⁶ cultivated 76 mm oysters removes 1 t of N from the Chesapeake Bay, a small percentage of the total nutrient reductions needed to achieve Bay water quality goals.

Oyster aquaculture offsets can be meaningful, however, in terms of policy initiatives at the basin level where nutrient allocations are issued and reduction milestones are set. In some basins, production of 200 × 10⁶ cultivated oysters yr⁻¹ (76 mm TL) could offset approximately 10 to 15% of the excess TN load. On a per-area basis, oyster aquaculture removes a relatively large quantity of nutrients from ambient waters compared with nonpoint source controls.

In contrast to many nonpoint source reductions (e.g., agricultural BMPs), the nutrient content of cultivated oysters can be accurately quantified with high precision using the common aquaculture market measurement of shell TL. The use of oyster aquaculture as a water quality management tool would require verification procedures to distinguish oyster aquaculture production from wild oyster harvests and modification of existing public cost-share programs or inclusion in economic nutrient trading programs.

The researchers state that the implementation of a feasible system of ecosystem service payments may have the additional benefit of enhancing public awareness of water quality issues, shifting attitudes toward stewardship, and stimulating local economies.

Material adapted from:

Nutrient Bioassimilation Capacity of Aquacultured Oysters: Quantification of an Ecosystem Service

Colleen B. Higgins, Kurt Stephenson, and Bonnie L. Brown
doi:10.2134/jeq2010.0203; Published online 8 Dec. 2010
https://www.soils.org/files/publications/jeq/abstracts/40-1/q10-0203-abstract.pdf

Photo courtesy of the authors.

For more information, see Cleaning the Bay