Oysters Clear the Waters, but Do They Muddy the Soil?

Courtesty of American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

If you need to clean up a waterway, plant an oyster farm. Oysters have been shown to improve water quality by removing nitrogen and other nutrients that in excess can cause dead zones and other problems. But even great natural filters like oysters can’t store 100% of what they take in. So, researchers decided to investigate what happens to the seafloor soils beneath oyster farms where copious amounts of poop hit the ground. Their findings, published in the Journal of Environmental Quality (https://doi.org/10.2134/jeq2019.03.0099), were surprisingly positive.

  • Oyster filter-feeding expels N‐ and C‐rich biodeposits onto soils beneath aquaculture racks.
  • Soils and infauna are capable of processing considerable amounts of oyster‐derived N and C inputs.
  • Oyster aquaculture did not appear to have many lasting effects to the soil environment, either positive or negative.

With funding from Rhode Island Sea Grant, researchers Mark Stolt and Jose Amador from the University of Rhode Island, as well as graduate student Chelsea Duball from the University of  Wyoming and graduate student Lauren Salisbury of the University of Saint Andrews (formerly a URI undergrad) studied eight different sites along coastal lagoons in Rhode Island. The sites ranged in age from zero years of aquaculture use (a control site) to 21 years of aquaculture use.

Aquaculture—farming in the water—has grown significantly in the past decade in the U.S. Aquaculture values are up more than 25%, and oyster production has increased 75% in 10 years; across southern New England, those numbers are even higher.

Part of the reason aquaculture has increased is due to oysters’ ability to clean up water. Research has shown, for example, that oyster cultivation in the Potomac River estuary could remove 100% of the nitrogen that is currently entering the river if oyster farms covered just 40% of the riverbed. Removing nitrogen helps with eutrophication, the buildup of nutrients that causes phytoplankton blooms, oxygen depletion, and dead zones.

Generally, oyster farms are made up of thousands of oysters held in mesh bags on top of racks about 8 to 10 cm above the seafloor, though sometimes higher. There, they eat the algae and other nutrients in the water column while the tide washes over them as they grow large enough to be harvested. As they grow, farmers change the mesh bags to grade them out, so the same‐sized oysters are all growing together. Oyster racks are usually barely visible if not invisible at the surface.

An oyster cage submerged and maintained directly on the soil surface of the coastal lagoon. While farmers work hard to periodically clean the cages, algae and other critters are quick to return to the aquaculture gear, using it for viable habitat and shelter. Photo by Annie Ragan.

The lagoons studied were similar: They have significant nitrogen runoff, they are shallow, and they are all on the same sandy soil type that provides a stable substrate for oyster racks to sit on and oyster farmers to walk on. The size of the oyster farms varied, but the average was about 500 oysters per square meter.

At each study site, underwater soil cores up to 20 cm deep below the oyster racks were collected. They measured the physical and chemical properties of the soils—parameters such as nitrogen and carbon content, soil sulfides, and bulk densities—and examined the “benthic infauna,” the insects, microbes, and other animals living in those subaqueous soils.

For the seven preexisting aquaculture sites, the team studied what naturally had been occurring over the years. Because they had such a varied study period—5 to 21 years for the preexisting sites—the team could characterize both shorter‐ and longer‐term effects of aquaculture on the soils.

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Interested in this topic? Check out the original article in the Journal of Environmental Quality at https://doi.org/10.2134/jeq2019.03.0099. Also, our podcast, Field, Lab, Earth , will be releasing an episode about the impacts of oyster aquaculture on subaqueous soils and infauna on 19 June. Listen for free anytime by scanning the QR code below or by visiting https://apple.co/2SpCoGs on Apple devices or https://bit.ly/2Sqf7nM on Android. Subscribe to never miss an episode. CEUs available.

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