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It’s low tide in Bodega Bay, north of San Francisco, California, and Hannah Hensel is wading through thick mud in search of clams. Articulated molluscs are everywhere, buried in sediment, filtering seawater to feed on plankton. But Hensel isn’t looking for live bivalves, he’s looking through the mud for the shells of dead clams.
“I lost a boot or two,” he recalls. “It can go pretty deep.”
Hensel, a doctoral candidate at the University of California, Davis, is studying the shells, which are composed of acid-buffering calcium carbonate, as a tool that could one day help shelled species survive in shrinking oceans. they acidify quickly.
The inspiration for Hensel’s research came from indigenous marine gardening practices. On beaches from Alaska to Washington state, First Nations and tribal communities built rock-walled terraces in the intertidal zone to bolster shellfish and other invertebrate populations. Although such sea gardens have not been documented further south, clams were also a vital livelihood in central California. The Coast Miwok and Southern Pomo people harvested clams for food and shaped the shells into money, says Tsim Schneider, an archaeologist at the University of California, Santa Cruz, and a member of the Federated Indians of Graton Rancheria. “So taking care of your clam beds was kind of protecting your vault, your bank,” says Schneider.
In Pacific Northwest marine gardens, caretakers crushed the shells of harvested clams and mixed the fragments on the beach. Recent research has shown multiple positive effects of this broken-shell “hash,” from opening up spaces in the sediment so young clams can burrow and grow more easily, to releasing chemical signals that encourage larvae to settle in. close by
This age-old practice may be the key to addressing a new crisis. As humans burn fossil fuels, the oceans are absorbing carbon dioxide from the atmosphere, making seawater more acidic. At lower pH levels, clams and other shellfish struggle to build shells. As their protective structures weaken and dissolve, animals become vulnerable to damage and predation. But studies suggest that adding shell fragments to clam beds could release carbonate into the water, potentially neutralizing the acidity caused by the greenhouse gas.
To find out if shell hash might help California clams survive increasingly acidic conditions, Hensel brought shells from the tidal flat into the lab, where he crushed them with a mortar and pestle and mixed the fragments together. in four plastic buckets of sand. Hensel filled these buckets, and four others that contained only sand, with local seawater and added the offspring of Pacific clams collected in Bodega Bay. He bubbled carbon dioxide through the seawater in half of the buckets to increase acidity. With their delicate shells, young clams are thought to be particularly vulnerable to acidification.
In the lab, Hannah Hensel bubbles carbon dioxide through seawater in experimental clam beds to test whether mixing crushed shells in the sediment can protect young Pacific clams from acidic conditions. Photos courtesy of Hannah Hensel
After 90 days, Hensel dug up all the clams. Comparing buckets containing more acidic seawater, he observed that bivalves burrowed in shell hash had grown more than clams in sand alone. Interestingly, though, the larger clams weren’t heavier, and Hensel plans to cut the shells open to assess whether the new growth was thinner or less dense.
The results inform researchers that shell hash has a dampening effect under certain conditions, says Leah Bendell, a marine ecologist at Simon Fraser University in British Columbia, who was not involved in the study. “It was a well-done lab experiment.”
Bendell also studies the damping power of the shell hash. Working with the Tsleil-Waututh Nation, Bendell and graduate student Bridget Doyle added shell fragments to clam beds in Burrard Inlet near Vancouver, British Columbia. In this study, the hash reduced the pH fluctuations of seawater seeping through the sediment, which can vary markedly with the ebb and flow of tides. Although the reduction was limited to areas with coarse sediments and the hash didn’t lower the overall pH, Bendell sees the results as a sign of something promising. Given a longer time frame, shell hash could have a greater effect on pH in certain clam beds, he says.
Shell hash may not be a panacea for ocean acidification everywhere, but Bendell and Hensel are putting together how carbonate can help individual beaches in caustic conditions. Next summer, when Hensel begins adding shell hash to Bodega Bay’s clam beds, he’ll incorporate another element of traditional marine gardening. Indigenous caretakers regularly tilled the clam beds, loosening the sediment and mixing in shell fragments. This repeated digging could bring oxygen to the excavated clams, open up more space for sediments and alter the chemistry of the seawater, Hensel says, and he plans to measure how the physical process affects both the chemistry of the seawater and the clam growth.
Schneider is hopeful that Hensel’s work will improve the health of the clam beds in his community, and the two researchers are discussing ways to involve indigenous communities around Bodega Bay. “I think it would be very rewarding to see community members of my tribe have opportunities to go back into the landscape to interact with traditional resources the way our ancestors did,” says Schneider.