Oysters fuse together as they grow, creating large rocklike underwater reefs that protect shorelines from wave erosion. These reefs provide habitat for other creatures, such as barnacles, anemones and mussels, which in turn serve as food sources for many fish species.

Rising atmospheric CO₂ levels are acidifying oceans worldwide. As our study shows, local events like tropical cyclones can add to global acidification.

The main cause of the unprecedented acidification that occurred after Hurricane Harvey was the excessive amount of rainfall and runoff that entered Galveston Bay. To help manage large-scale flooding in the Houston area, the city released large volumes of water from reservoirs for more than two months after Harvey. These releases extended the time during which stormwater entered Galveston Bay and increased its acidity.

Scientists use the pH scale to measure how acidic or basic (alkaline) water is. A pH value of 7 is neutral; higher values are basic, and lower values are acidic. The pH scale is logarithmic, so a decrease of one full unit – say, from 8 to 7 – represents a tenfold increase in acidity.

Rainwater is more acidic than either river water or seawater, which pick up minerals from soil that are slightly basic and can balance out absorbed carbon dioxide from the atmosphere. Rainwater’s pH is around 5.6, compared with between 6.5 and 8.2 for rivers and about 8.1 for seawater.

Galveston Bay contains a mix of fresh water from rivers and salty seawater from the Gulf of Mexico – oysters’ preferred habitat. We collected water samples in the bay two weeks after Harvey and found that the bay was made up almost entirely of river water and rainwater from the storm.

Since rainwater, river water and seawater all have different chemistries, we were able to calculate that rainwater made up almost 50% of the water in the bay. This means that acidic rainwater from Harvey replaced the basic seawater within the bay after the storm. The average bay water pH had dropped from 8 to 7.6, a 2.5-fold increase in acidity. Some zones had pH even as low as 7.4 – four times more acidic than normal.

This extreme acidification lasted for more than three weeks. Bay waters became corrosive not only to more sensitive larval and juvenile oyster shells, but to adult oyster shells as well. Scientists had predicted that increasing CO₂ could cause this scale of coastal acidification but did not expect to see it until around the year 2100.

The fresh water from Harvey also caused a severe oyster die-off in the bay because oysters need slightly salty water to survive. Harvey struck in the middle of oyster spawning season, and acidification may have slowed reef recovery by making it harder for young oysters to form new shells. Officials at the Texas Parks and Wildlife Department have told us that four years later, in late 2021, some Galveston Bay oyster reefs still showed very low additions of new oysters.

Only a few studies, including ours, have analyzed how tropical cyclones affect coastal acidification. In our view, however, it is highly possible that other storms have caused the kind of extreme acidification that we detected in the wake of Harvey.

We reviewed the 10 wettest tropical cyclones in the U.S. since 1900 and found that nine, including Harvey, caused large amounts of rain and flooding in coastal areas with bay or estuary ecosystems. Other storms didn’t produce as much rainfall as Harvey, but some of the affected bays were much smaller than Galveston Bay, so less rain would have been needed to replace seawater in the bay and cause a similar level of acidification to what Harvey produced.

We think that this likely has already occurred in other places struck by hurricanes but went unrecorded because scientists weren’t able to measure acidification before and after the storms. As climate change continues to make tropical cyclones larger and wetter, we see storm-induced acidification as a significant threat to coastal ecosystems.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. Like this article? Subscribe to our weekly newsletter.

Read more:
What made the rain in Hurricane Harvey so extreme?

As climate change alters the oceans, what will happen to Dungeness crabs?

Tacey Hicks received funding from Texas Sea Grant to support the publication of this study. Tacey Hicks is currently affiliated with the National Oceanic and Atmospheric Administration and Texas Sea Grant as part of the John A Knauss Marine Policy Fellowship.

Kathryn Shamberger receives funding from the National Science Foundation, US Department of Energy, and US Environmental Protection Agency.