Something in the water: how kelp is helping Maine's mussels boom

The Guardian

11-06-2025

On a glimmering May morning, Tom Briggs pilots a 45ft aluminium barge through the waters of Casco Bay for one of the final days of the annual kelp harvest. Motoring past Clapboard Island, he points to a floating wooden platform where mussels have been seeded alongside ribbons of edible seaweed.
'This is our most productive mussel site,' says Briggs, the farm manager for Bangs Island Mussels, a Portland sea farm that grows, harvests and sells hundreds of thousands of pounds of shellfish and seaweed each year. 'When we come here, we get the biggest, fastest-growing mussels with the thickest shells and the best quality. To my mind, unscientifically, it's because of the kelp.'
Zoe Benisek, oyster lead at Bangs Island Mussels, harvesting kelp. The seaweed changes water chemistry enough to lower the levels of carbon dioxide to nourish the mussels
A growing body of science supports Briggs's intuition. The Gulf of Maine is uniquely vulnerable to ocean acidification, which can impede shell development in mussels, clams, oysters and lobster, threatening an industry that employs hundreds of people and generates $85m to $100m (£63m to £74m) annually.
Atmospheric carbon dioxide from fossil fuels is the main driver of declining ocean pH, increasing the acidity of the world's oceans by more than 40% since the preindustrial era and by more than 15% since 1985. Add carbon runoff from growing coastal communities, regular inflows of colder, more acidic water from Canada, and intense thermal stress – the Gulf of Maine is warming three times faster than the global average – and you're left with a delicate marine ecosystem and key economic resource under threat.
Enter kelp. The streams of glistening, brownish-green seaweed that Bangs Island seeds on lines under frigid November skies and harvests in late spring are a natural answer to ocean acidification because they devour carbon dioxide. Sensors placed near kelp lines in Casco Bay over the past decade have shown that growing seaweed changes water chemistry enough to lower the levels of carbon dioxide in the immediate vicinity, nourishing nearby molluscs.
'We know that, in general, for shell builders, ocean acidification is bad, and we know that kelp do better in a high-CO2 environment,' says Susie Arnold, the senior ocean scientist at the Island Institute, a non-profit climate and community organisation in Rockland, Maine, and a pioneer of the Bangs Island water experiments.
Working with the Bigelow Laboratory for Ocean Sciences, an independent Maine-based research organisation, Arnold and others began testing the water off Chebeague Island in 2015 'to see if we could detect a difference between water chemistry in the middle of all that kelp and far away from it', she says. 'We planted juvenile mussels inside and outside the kelp, and we were able to show that the mussels inside the kelp had a thicker shell. Now you see Bangs Island growing kelp around their mussels because they can make a profit on kelp and also buffer the mussels.'
The Bangs Island crew harvesting kelp on their boat in the Gulf of Maine
The CEO and co-owner of Bangs Island Mussels, Matt Moretti, studied marine biology in college and grad school, helped raise baby lobsters at the New England Aquarium, and worked on an oyster farm before buying the sea farm with his father in 2010. Within a year, they had started growing kelp alongside the mussels in an approach known as integrated multi-trophic aquaculture.
'Even before we started farming mussels, I was interested in that concept as an environmentally friendly way of farming, and of farming an ecosystem rather than a single species,' Moretti says from his bare-bones upstairs office in Bangs Island's warehouse on the Portland pier.
As the kelp harvest grew, Moretti realised they needed a way to stabilise the seaweed, which didn't last long after it came out of the water. For a while they dried it themselves, hanging it in the warehouse and on the docks. Now, they sell the entire fresh seaweed catch to a local processor, which turns it into fermented foods such as kimchi, among other products.
Gillian Prostko, chief science officer at Bangs Island Mussels. The harvested kelp is sold to a processor and turned into fermented foods such as kimchi
'We always suspected that there was this positive interaction between the mussels and kelp, and we suspected that because kelp photosynthesises, it sucks carbon out of the water, then therefore it must be good for the ocean and good for the mussels,' Moretti says. Bigelow's water testing has proven that 'we're having a positive impact'.
Nichole Price, the director of Bigelow's Centre for Seafood Solutions, collaborated with Arnold on those early experiments and continues to monitor the water around Bangs Island mussel and kelp lines, an effort that has expanded to include water monitoring at seaweed farms from Alaska to Norway. In a paper published this year in the journal Nature Climate Change, Price, Arnold, and a host of co-authors documented yet another way in which seaweed farms can contribute to the health of the world's oceans: by trapping carbon at the bottom of the sea.
'When you harvest, you're not pulling up every last bit of seaweed,' Price says. 'We've been diving under farms during harvest, and you can see the bits and pieces that rain down. Then there's a culling process, the bits and pieces that get tossed over, and that's what this paper has measured: the unusable, unsellable parts of the harvest that end up on the sea floor.'
Matt Moretti, founder of Bangs Island Mussels (left) and farm manager Tom Briggs
Those discarded seaweed scraps can contribute to what is known as passive deposition of carbon. 'Fingers crossed, it gets covered with sediment fast enough that it's taken out of the global carbon cycle,' Price says.
Given the environmental and financial benefits of growing kelp and shellfish together, you might think everyone would be doing it. But co-farming mussels and kelp at scale requires more than just planting and harvesting. With five boats, a plankton monitoring programme, and tanks on the ground floor of the warehouse where baby mussels from a nearby hatchery are carefully seeded on to lines before being placed in the ocean, Bangs Island is part farm, part science lab.
Changes in mussel-spawning and seed-collection cycles in recent years have forced Moretti and his staff to pay much closer attention to the surrounding water and its inhabitants, from barnacles – a nuisance to shellfish farmers because they set on mussels – to the microscopic larvae of tunicates, pestilent sea squirts that seeded on nearly all of the farm's mussel lines several years ago, crowding out the shellfish and almost sinking the business.
'Conceptually, what we do is very simple: we grow mussels, harvest them, sell them,' Moretti said. 'But adding all the pieces together is a really big, complicated puzzle.'
Today, Bangs Island harvests about 600,000lb (270,000kg) of mussels and 100,000lb of seaweed a year; last fall, they began farming oysters. The oysters, along with about half the mussels, grow in proximity to kelp.
'Climate change, ocean acidification, is a global problem. And when you try to think about it, like, what you can do? It's so daunting,' Moretti says. 'But when you think about us farming kelp in the ocean, it's really the only way we've ever been able to figure out to have a local-scale mitigation of this global problem. It's something we can do here that can help the waters around us that actually has a significant impact.'
Kelp ready for harvesting in the Gulf of Maine