Oysters that grow in estuaries and bays face a particular set of challenges with a changing climate, according to a recent study led by the University of California, Davis conducted on Tomales Bay.
Two seasonal phenomena that are believed to intensify and become more frequent with climate change—ocean upwelling during the spring and summer, and freshwater input from streams and rivers during winter storms—significantly influence water chemistry in Tomales Bay to the detriment of both the native Olympia oyster and farmed Pacific oyster, the study found. As a result, the areas closest to both the mouth of the bay and freshwater inlets saw lower growth and survival rates for both species.
The study, funded by the National Oceanic and Atmospheric Administration’s California Sea Grant College Program, sought to provide insight for both oyster restoration projects and commercial oyster farmers. Projections for suitable oyster habitat could help determine where to site projects and farms for the best chance of success.
“The study demonstrates that focusing on ocean acidification alone is misguided,” Dr. Ted Grosholz, the lead professor who this month published the study in the journal Limnology and Oceanography, said in a university release. “Many climate-related stressors contribute to the projected shrinkage of the estuarine zone, where oysters and likely other shellfish would need to flourish.”
Although scientists know that the ocean’s increasing uptake of carbon dioxide makes it difficult for shellfish to form their calcium-based shells, Dr. Grosholz’s study shows that other climate-related factors that affect water chemistry may have a greater impact on oysters in bays and estuaries than on oysters growing in open waters.
At sea, the process of ocean acidification is mostly driven by atmospheric carbon dioxide; as concentrations go up in the atmosphere, more carbon dioxide dissolves into the water, making it more acidic. But bays and estuaries have many other inputs that can influence acidity, including the freshwater coming down from rivers and the abundant plants, macroalgae and phytoplankton. Daily changes in acidity in bays and estuaries can far exceed those in the ocean, which means organisms in those areas have already evolved to tolerate extreme changes.
The new study therefore looked to see what other factors may be of more importance to oysters.
Between 2014 and 2017, Dr. Grosholz and his research partners from several other universities planted juvenile Olympia and Pacific oysters in test beds at nine different locations in Tomales Bay. Each month, they monitored the oysters’ health, measuring their growth and mortality rates. At the same time, they analyzed water chemistry and temperature. The researchers also conducted field experiments with oyster larvae to see how variations in conditions affected survival and “settlement”—the process by which shellfish move from free-floating larvae to settle on a hard surface.
During the winter runoff season, the study found that cold temperatures and low amounts of phytoplankton—which sustain the oysters—corresponded with low oyster growth throughout the bay. However, the lowest rates of growth and survival were in areas most impacted by the low salinity created by storm flows. (Temperature, salinity, acidity and dissolved oxygen levels were among the factors monitored as the primary indicators of stress on the oysters.)
“These extreme run-off events have been linked to atmospheric rivers in nearby San Francisco Bay, and result in mass oyster mortality events,” the study states. “Increased interannual variation in precipitation is also projected in the future.”
The study also found that another seasonal event—ocean upwelling, which takes place during the spring and summer—had a negative impact. The growth rate during the summer season was highest overall, likely due to the combined effects of warmer average temperatures and greater food availability. But the oysters in the portion of the estuary closest to the ocean, which received the most upwelled water, were also found to have lower growth and survival rates.
Increased upwelling can increase the presence of “hypoxic” waters—the cold, oxygen-deficient and more acidic waters that come into Tomales Bay with the tide, the study explained. This generally occurs in the hotter months, when higher water temperatures can cause additional stress in the upper bay.
“Accompanying this increase in wind-driven upwelling is a projected increase in inland temperatures leading to higher inner-bay temperatures and hypersaline conditions,” the study states. “Given the demonstrated stressful nature of both corrosive waters and exceedingly high temperatures, the mid-bay refuge for Olympia and Pacific oysters observed in this study may shrink, with potentially severe ecosystem and economic consequences.”
Terry Sawyer, who co-owns the largest oyster farm on the bay, Hog Island Oyster Company, said the research is extremely important. It is just one of a handful of climate-related studies and collaborations in which Mr. Sawyer’s company is involved.
“The whole point of research like this is that it changes what’s going on not only for current operations but for the next 100 years,” Mr. Sawyer said. “Plus, of course, we have a responsibility for our grandchildren and the next generation.”
Hog Island suffered losses last year—an estimated 30 percent in some beds—during last winter’s heavy storms, Mr. Sawyer said. The company is careful to minimize the number of beds it makes near creek inlets, where sediment alone during storm events can bury the oyster bags that lie on the bay’s floor.
Hog Island also monitors the bay’s temperature and acidity levels, sometimes moving its cultivation into deeper waters in response to weather factors. It also runs its own breeding program at a hatchery in Humboldt Bay so that it can select for resilience to heat and bacteria.
Still, Mr. Sawyer said, “It is slowly getting harder to do this. That’s the trend, that’s the disturbing trend. When you have a business, people depending on you to have a job—that’s disturbing.”