Want to restore oyster reefs? Find a site where they don’t wash away or become buried under the sand!
Disappearing reefs
Oyster reefs were once abundant in the North Sea and other, so-called marginal seas at the edges of continental shelves. They hosted a significant biodiversity but have disappeared from 97 % of their original locations. Therefore, various restoration initiatives are undertaken, often with limited success.
Deep experiments
In an experimental setting at a depth of 32 m, in the Gemini wind park (85 km north of the Wadden islands), Zhiyuan and colleagues placed oysters on a rack 0,5 m above the seabed, where they were monitored for filtering activity. Also, oysters were placed on the seabed, to see if they were displaced or buried by currents or sediments. Lastly, they placed oysters in an experimental ‘mesocosm’, to monitor how they survived burial.
Keep on gaping
The experiments showed that oysters kept above the seabed survived well, even during storms, as shown by continuous ‘gaping’, showing filtering activity. But oysters on the soft seabed faced a different reality: stronger near-bed hydrodynamics could dislodge them, while rapid sediment accumulation could bury them beyond recovery.
Practical implications
The results of these experiments have several practical implications, Zhiyuan says. “First, it is very important to look at potential short-term physical disturbance, when introducing new oysters to the seabed. Water quality is not the only thing to look at”, he warns. “Oysters may remain physiologically healthy, but still fail, because they are dislodged by strong near-bed hydrodynamics or buried by rapid sediment accretion before a reef can establish.”
Find promising sites
To help select potentially successful sites for reintroduction, the paper describes thresholds at which the risks from hydrodynamic-induced loss and burial-caused mortality become critical. If burial seems likely, only oysters that are placed above the sediment may succeed, while placement in prefab reef structures is an option in areas where dislodgement is likely. “This helps move restoration planning away from trial and error and toward a more risk-informed strategy”, Zhiyuan says. “Our study suggests that the key is not only choosing the right place but also choosing the right method for that place.”
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