Clam chowder, bay scallops, fried oysters. Wampum bracelets. Shellfish are the grand bounty of the soft, sparkling salt ponds that ring the Island shore. We’d be hard pressed to find a local cultural symbol more significant than the water-worn purple and white quahaug shell. Purple — the Island color.

Today the ponds and shellfish are stressed by human impacts — nitrogen from septic systems and road runoff pollution that includes pesticides, lawn fertilizer, oil, plastic, glass and dog feces. Bird droppings add overloads of bacteria that contribute to shellfish closures.

Enter global warming and an entirely new set of problems emerge: higher water temperatures, ocean acidification, sea level rise, more severe storms and increased freshwater runoff from more frequent and intense rainfall.

These changes are causing an increase in invasive species and harmful algae blooms, less salinity in the water, deterioration of shellfish shells, eelgrass stress, increased amounts of sand filling in the ponds during storms, shellfish disease, loss of salt marsh breeding grounds to a rising sea and an increase in that annoying sea creature, the jellyfish.

The oceans are becoming more acidic, a grave threat to shellfish. If the level of acidity rises, “even a tiny, tiny, bit,” says Martha’s Vineyard Shellfish Group director Rick Karney, “it will make it difficult for shellfish to access the calcium needed for their shells, and the shells will dissolve, especially in the young.”

Scott Doney, senior scientist at the Woods Hole Oceanographic Institution, presented testimony to the U.S. Senate on ocean acidification: “Climate change and acidification trends will accelerate over the next several decades unless there is deliberate action to curb greenhouse emissions.” He added that acidification will put further pressure on “living marine resources, such as fisheries . . . that we depend upon for food, tourism and other economic and aesthetic benefits.”

Climate change puts shellfish at risk from invasive species. “We will see more invasive species with milder temperatures; some have colonized farther south and will likely move northward,” said Oak Bluffs shellfish constable David Grunden. What’s here already? “Two species of crabs, one jellyfish and eight sea squirts,” Mr. Grunden said.

Sea squirts, or tunicates, have no known predators, reproduce rapidly and foul shellfish beds. Mr. Grunden is working with the U.S. Environmental Protection Agency and Woods Hole to evaluate their impact on Island eelgrass meadows, which he called “one of the most productive ecosystems in the world.”

Green crabs are a major shellfish predator and will become a bigger problem as winters get milder, according to Mr. Karney.

Hazardous algae blooms are also expected to increase due to climate change. According to Mr. Grunden, “they are likely to be more frequent due to warmer water and more nutrients.” The New England Aquarium reports that “warmer seas could contribute to the increased intensity, duration and extent of harmful algal blooms, which damage habitat and shellfish nurseries and can be toxic to humans.”

Red tide is the term we commonly associate with hazardous algae blooms and it can cause paralytic shellfish poisoning in humans, a condition that can range from numbness, nausea and headaches to paralysis, respiratory failure and death.

The Cape Cod Times reported last year that a 2005 red tide bloom “shut down shellfishing from Maine to Martha’s Vineyard and led to $20 million in losses for the Massachusetts shellfish industry.” According to a report by the Woods Hole institution, the outlook this year is for a “moderate regional bloom of a toxic algae that can cause red tides.”

Warmer water temperatures will stress the phytoplankton that shellfish feed on. The Conservation Law Foundation reports that “the availability of phytoplankton, one of the most important nutrient sources for marine life, could be reduced. These microscopic organisms are the nutrient base for the entire marine food web.”

The oyster disease Dermo, first found in more southern waters, has migrated north as water temperatures rise.

Mr. Grunden said jellyfish are “a sign of low oxygen, that’s why there are so many in the far end of the Lagoon.” Warmer water allows jellyfish to flourish, to expand their ranges and appear earlier in the year.

Heavier and more frequent rainfall is a concern. The Conservation Law Foundation reports: “Changes in the amount of freshwater runoff are likely to impact the salinity levels of estuaries. Many organisms may not be able to adapt to changing salinity levels, especially those that are relatively sedentary such as clams, snails, and fish eggs.” Mr. Karney added that “basic changes in salinity will impact many aspects of shellfish ecology.”

The good news is that bivalves — clams, oysters, mussels and scallops — help improve pond water quality. They feed by filtering, or cleaning, the water. They don’t need clean water to grow in, which is why the towns stock the ponds with contaminated shellfish from elsewhere. They are filter feeders, considered a keystone species, explained by Dr. Paul R. Epstein and Dan Ferber in their book, Changing Planet, Changing Health. “Oysters perform a crucial function in estuarine systems. A single oyster filters 30 to 50 gallons of water each day, removing excess nutrients, chemical pollutants and algae. This essential filtering role makes oysters what ecologists call a keystone species. A keystone is a wedge-shaped rock placed at the top of a stone arch; when the keystone is removed, the arch collapses,” the authors write.

Can shellfish survive in the changing climate? Mr. Karney notes that shellfish “have a lot of genetic diversity in their population so possibly, as the water gets warmer and more acidic, they will have enough genetic variability to handle the changes.” He said flexibility is in their genes. And so many millions reproduce each year that they are quicker to adapt than some other species, like the polar bear.

Shellfish exemplify the Island way of life. They are the original “eat local” food source. They color our culture from family clamming traditions to favorite recipes. We wear beautiful, locally-made wampum jewelry and stroll the beach for that perfect shell. The local economy is shellfish-charged, from the hardworking commercial shellfishermen to fish markets, restaurants, clam shacks, the artisans’ fair and jewelry shops.

Each of us can help protect the ponds and shellfish. Here is a short to-do list:

• Support your local shellfish department and the Martha’s Vineyard Shellfish Group.

• Use locally-tolerant plants that don’t require fertilization or a lot of watering.

• Do not use pesticides, fertilizers or herbicides.

• Think about what goes down your drain and gets washed off your driveway (and into the ponds).

• Replace impervious surfaces on your property, like paved driveways, with pervious stones or shells.

• Buy a shellfish license and discover the joy of digging your own clams for supper.

• Get involved — join your local pond association.

If you live directly on a coastal pond, plant a buffer strip of local vegetation between the pond and (unfertilized) lawn to filter and absorb pollutants (and help keep the geese at bay).

Conserve energy, support renewable energy and reduce your consumption of fossil fuels.

Climate change is going to be a big deal on this finite little piece of land. As a community we need to understand the impacts so we can respond wisely, with the benefit of scientific backing, local insight, and community support.

Liz Durkee is the conservation agent for the town of Oak Bluffs. This is the fifth part in an occasional series she is writing for the Gazette Commentary Page about climate change and what it means for the Vineyard.