Eelgrass meadows in Casco Bay have retreated rapidly over the past four years, reducing the amount of the critical marine habitat by more than half and accelerating a trend blamed partly on warming waters, according to a new report.

“This is very alarming,” said Ivy Frignoca, Casco Baykeeper for the Friends of Casco Bay environmental group. “We were aware of the decline in eelgrass, but we thought we had some time to think about this. But we lost 54% of eelgrass in the last four years. The time to act was yesterday.”

Angela Brewer, Marine Unit leader for the Maine Department of Environmental Protection, said while the continued decline in eelgrass is not surprising, “it’s a big deal to lose that much resource in a four-year time span.”

The latest study was submitted to the DEP in January and released this month. Because of cost and the time it takes to conduct the surveys, the agency typically hires a contractor to measure eel grass beds every four to five years.

In 2018, eelgrass meadows covered 5,012 acres in Casco Bay, which stretches from Cape Elizabeth to Phippsburg. By 2022, the eelgrass beds had dropped to 2,286 acres, according to the report. The most pronounced declines were observed near South Freeport, Maquoit Bay and northern Harpswell Sound.

Casco Bay’s eelgrass habitat expanded from 3,650 acres in 2013 to 5,012 acres in 2018, but it has declined over the long term. Eelgrass meadows covered more than 7,000 acres in 1993-94, and more than 8,000 acres in 2001-02.

Eelgrass is a ribbon-like seagrass and the meadows support the base of the marine food web. The grass provides habitat and food to organisms ranging from invertebrates to fish to waterfowl. Eelgrass also shelters adolescent marine life such as lobsters, shrimp and certain types of fish from predators, so its loss can make it difficult for some species to survive.

Eelgrass meadows also help keep the bay healthy by absorbing nutrients, stabilizing sediments and reducing erosion. And the meadows are valuable for their ability to store carbon dioxide.

Climate change is affecting eelgrass in multiple ways.

Warming waters in Casco Bay can lead to algae blooms, which cloud the water and make it difficult for sunlight to penetrate and sustain eelgrass plants, Brewer said.

Because Casco Bay is part of one of the fastest-warming water bodies on Earth, water temperatures may be reaching the point where eelgrass can no longer survive, Frignoca said.

“You can think of it like how plants in your garden or in the nearby forest exist in certain growing zones,” Frignoca said. “Once the temps get too high, certain plants don’t do well.”

Last year was the second warmest on record for the Gulf of Maine, which includes Casco Bay, with the average surface temperature falling just short of the record high set in 2021. The Gulf of Maine’s average annual sea surface temperature was 53.66 degrees Fahrenheit that year, 3.72 degrees above the long-term average, according to the Gulf of Maine Research Institute.

Invasive green crabs in Casco Bay – which have been thriving as waters get warmer – are another reason eelgrass beds are declining.

“The green crabs are not dying back during the winters because the waters are warm enough, and instead they are reproducing at prolific rates,” Frignoca said.

Brewer said young green crabs eat eelgrass and the adult crabs disrupt the roots while digging for prey, including soft-shell clams. While their population explosion has been linked to climate change, Brewer said green crab populations also may wax and wane depending on the amount of prey available. Populations may decline if there’s not enough shellfish to eat, but when shellfish populations rebound, green crab populations also increase.

Brewer said some have discussed trying to knock back green crab populations through commercial fishing, but so far there’s been little demand for the crabs.

Another potential solution to declining eelgrass beds might be “assisted migration” of eelgrass varieties that can survive in warmer waters, such as those from the mid-Atlantic region. But Brewer said there are some concerns about assisted migration because it could introduce diseases from other areas to Maine waters.

Brewer said the DEP has so far only studied eelgrass coverage in Casco Bay, but hopes at some point to have enough funding to research it along the entire Maine coast.

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