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Many researchers are looking toward aquaculture as a way to reduce carbon, while also producing food. Yet, in some cases, it is unclear whether these methods offer a way to reduce carbon or if they're just another source of it. Oyster farming is one such case.

Previous research has indicated that the process oysters use to create their shells releases carbon along with their respiration, making them a carbon source. However, new research indicates that this is not the whole picture.

The new study, in the Proceedings of the National Academy of Sciences, suggests that oyster farming may offer more carbon sequestration than previously thought, largely through filter feeding and adding particulate and dissolved organic carbon into the water.

The research team, based in China, conducted a 120-day experiment to study changes in environmental carbon in large outdoor tanks containing Pacific oysters at different densities. The team measured all carbon species, air–sea CO₂ fluxes, and chlorophyll-a (Chl-a)—an important indicator of phytoplankton abundance, which also indicates primary productivity (PP), or the rate that energy is converted to organic matter. The data was then compared to ponds without oysters.

The results showed that 2.39 times more carbon was sequestered by oyster-driven organic production processes than that stored in shells. The researchers also found that the water in the oyster tanks shifted toward a more autotrophic and alkaline state, increasing atmospheric CO₂ uptake and mitigating acidification caused by CO₂ abundance. The team notes that it's important to view the ecosystem as a whole to assess the effects of all processes.

"There is a spatial heterogeneity in Chl-a distribution and CO₂ fluxes across field oyster farming ecosystem, driven by the trade-offs among three ecophysiological processes: photosynthesis, calcification, and respiration in the oyster farming ecosystem. At the individual scale, oyster biocalcification and respiration dominate the carbon dynamics. However, at the ecosystem scale, photosynthesis is significantly enhanced and emerges as the dominant process," the study authors explain.

The researchers also note the differences between the tanks of different oyster densities. It is apparent that moderate densities maximize carbon sequestration, while overstocking reduces efficiency.

Describing the highest density tank, the authors say, "The 4 ind. m^−2 stocking density case consistently exhibited the lowest Chl-a and PP among all treatments, significantly lower than the control group. This result suggests that high-density stocking effectively suppressed phytoplankton biomass."

They go on to say, "These findings suggest that reasonable stocking densities promote PP in oyster farming ecosystems, while overstocking reduces PP by depleting phytoplankton biomass."

Overall, the study highlights the potential for oyster farming's dual benefit of sustainable seafood production and carbon sequestration. However, more research is needed to fully capture all of the complexities of open coastal systems and to assess for sequestration reduction due to sediment organic carbon being respired.

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