WELLFLEET — With a minus-0.6-foot low tide at 5:11 p.m. and the sun setting at 5:26, Bob Wallace and Mattias Christensen have no time to lose. Their oysters have been overwintering in concrete pits on land for the past five weeks. Now, on this late February evening, the shellfishermen are returning them to the flats. When the tide reaches its lowest point, they move on to the work of breaking apart clusters of oysters.

“Shellfishermen are observers,” says Wallace, while confidently using a hatchet to separate a cluster. “You’re just reacting to what Mother Nature throws at you.”

He points to an oyster with a blackened shell, picked out of the muck. The dark color, he explains, signifies hypoxia — this oyster did not get enough oxygen in the substrate. Most of the other oysters appear to be fine — near perfect, in fact — and the two shellfishermen continue their work.

For shellfishermen, reacting to Mother Nature is just one part of the battle. They are now faced with the rising threat of ocean and coastal acidification — a reduced pH environment in which mollusks (like oysters, clams, and scallops), with their calcium carbonate shells, are particularly vulnerable.

Ocean acidification, a global threat, is one of the consequences of carbon dioxide emissions. Coastal acidification is driven locally by nutrient pollution and resulting eutrophication — excessive growth of marine plants and algae. Both are caused by human activities.

Wallace has already witnessed eutrophication in action. “More and more weeds are suffocating the oysters,” says Wallace, who was one of the members of a 2015 working group on climate change impacts on shellfishing in Wellfleet Harbor. Nutrients like nitrogen and phosphorous feed phytoplankton, which feed oysters, Wallace explains, but the rapid growth of aquatic plants can also suffocate the oysters.

While neither Wallace nor Christensen has seen the effects of coastal acidification yet, they know what to look for; they know more acidic conditions make it more challenging for shells to form; they know the shellfish are most vulnerable in their larval stage of growth.

Now, the state is confirming what local shellfishermen have long worried about. According to a report researched by a governor-appointed commission of lawmakers, scientists, and fishermen and released on Feb. 9, the threat of coastal acidification represents a crisis for Massachusetts. “The degree of harm that occurs,” the report states, “depends on the extent of mitigation efforts undertaken now.”

A key finding: land-based nutrient pollution “amplifies localized differences in pH.” That means the eutrophication shellfishermen are already observing is likely a sign local acidification is on the way.

On the upside: many of the land-based sources of the problem, from golf course runoff to septic system discharge, are controllable.

Last month, in response to the report, state Sen. Julian Cyr and state Rep. Sarah Peake, both members of the commission, co-sponsored companion bills in the House and Senate that would tackle acidification in our coastal waters. “If we don’t find a way to mitigate ocean acidification,” Cyr told the Independent, “the marine economy is very much at risk.”

That risk is especially high on the Outer Cape, where in Wellfleet alone “about 450 people make their livings in the shellfishing industry,” said Nancy Civetta, the town’s shellfish constable. Her figure includes wild harvesters as well as farmers and their employees. “That is 15 percent of Wellfleet’s total year-round population and 25 percent of its working-age population,” Civetta added.

“Aquaculture has been a bright spot in the economy in a place that would otherwise be unaffordable to many young people,” said Cyr.

One of the key features of the legislation is the establishment of a permanent council on ocean acidification. The council would commission independent studies to “fill acidification knowledge gaps.” It would also coordinate public and private monitoring efforts, provide monitoring hardware and technical training, and maintain a central repository for acidification data.

Local monitoring efforts are already underway, but the bill would expand their reach and longevity. The Center for Coastal Studies in Provincetown has been monitoring water quality in Wellfleet and Provincetown harbors since 2006, said Amy Costa, director of the center’s Cape Cod Bay Monitoring Program. Last year, two different projects began focusing their attention on measuring pH and the effects of acidification.

Wellfleet is still working on its state Dept. of Environmental Protection watershed permit, which will propose a range of nutrient-mitigating projects to bring the town into compliance with Section 208 of the U.S. Clean Water Act. Some of the proposed waste treatment plans being considered — from salt marsh restoration and shellfish propagation to nitrogen-reducing sewage systems — could also help fight acidification.

For example, Civetta said she and Assistant Constable Johnny Mankevetch are working with the town’s wastewater committee to include a budget for sea clam shells in the watershed permit application. The shells would be used as cultch — material on which oyster spat is grown — but could also potentially add more calcium carbonate to the substrate. Civetta said she has more work to do “to understand just how much shell deployment would be necessary to actually have a significant mitigating effect” on acidification.

Ginny Parker, president of the Wellfleet Shellfishermen’s Association, said that her organization’s goal is to make sure “the voices and expertise of the Outer Cape shellfishermen are heard.” She envisions them as members of the permanent ocean acidification council, “because these decisions will have long-term effects.”