Rockweed, a kind of seaweed that thrives all along Harpswell’s rocky shoreline, is often referred to as a foundation species. That’s because many other creatures in the intertidal zone depend on it.
Elliot Johnston, a coastal ecologist with the Maine Natural History Observatory in Goldsboro, knows Harpswell’s rockweed beds well. He was one of the lead researchers in a 2023 University of Maine study of rockweed harvesting that looked at 38 sites along the Maine coast, including Harpswell.
He knows the intertidal zone is a harsh environment, but rockweed makes it much more habitable for a wide range of species.
“It’s inherently a very stressful place,” he observed.
Because of the tides, inhabitants of the intertidal zone must be able to cope with a world that goes from wet to dry and back again about every six hours. Sharp changes in temperature during the summer add to the challenges. Cooling water covers the flats at high tide, but at low tide, the mud flats and exposed ledges are left baking in the sun.
Rockweed helps to mitigate those extremes by providing a cool, wet refuge for creatures that live in or under its thick, moist layers that blanket the rocky shorelines and ledges.
“Rockweed helps even that out,” Johnston said of the rapidly shifting conditions. “Invertebrates wouldn’t fare very well if that rock were depleted of rockweed.”
In addition to mitigating environmental extremes in the intertidal zone, rockweed provides another life-or-death service: protection from predators. Hidden among strands of rockweed, small creatures are less visible to those who would dine on them.
The scientific name for rockweed is Ascophyllum nodosum. “Nodosum” refers to the characteristic nodes that make each strand buoyant. As the tide rises, the rockweed strands, anchored to the rock at their base, rise toward the surface to form a sort of undulating underwater forest — a perfect place for small fish and other mobile marine creatures to hide from predators. And at low tide, rockweed forms mats overlying the rocks that help to conceal less mobile creatures and reduce heat stress.
“They can take refuge there,” Johnston said of creatures that otherwise would have difficulty surviving on an open shoreline.
So rockweed makes the intertidal zone a more pleasant place to live by moderating environmental extremes, while making it safer by providing protection from predators. Those two contributions alone would seem to qualify rockweed as a foundation species. But there is another important one — nourishment, even though almost no animals eat rockweed itself.
Hannah Webber is the marine ecology director at Schoodic Institute at Acadia National Park. She worked with Johnston on the study of seaweed harvesting along the Maine coast.
“Very, very few animals eat the vegetation of mature rockweed,” Webber said. That’s because rockweed produces chemicals that most animals find unappetizing. Voicing rockweed’s strategy, she said, “I’m going to put a lot of energy into (making these chemicals) so you don’t want to eat me.”
Yet rockweed still manages to be an important source of food for the inhabitants of the intertidal zone. Rockweed, Webber said, serves as “an incredible landing zone for microorganisms.”
Three amphipods are seen in an oyster shell in Cundy’s Harbor on April 1. The photographer collected the shrimp-like crustaceans from the rockweed and placed them on the shell for scale and visibility before releasing them alive. (John Gormley photo)
Rockweed provides a home for bacteria and other microorganisms, such as diatoms, a kind of microalgae. These combine to form a biofilm that many creatures eagerly consume. The rockweed itself acts like “a platter of an all-you-can-eat buffet,” Webber said.
The diners at this buffet include amphipods (tiny, shrimp-like crustaceans), isopods (crustaceans that are related to the roly poly bugs found under rotting logs), and copepods (a kind of crustacean that is barely visible to the naked eye).
The seaweed is covered with epidermis cells, which form a sort of skin that it sloughs off periodically. The biofilm and the epidermis cells enter the water, adding another menu item to the mix. “The same suite of animals will eat this skin,” Webber said.
In this way, the seaweed creates the bottom link in the food chain. “The bigger beasts will eat the little beasts,” she said.
Webber cited pollock as an example of a commercial species of fish that benefits. Copepods residing in the seaweed are “one of the favorite foods of baby pollock,” she said.
This being Maine, you might wonder if lobsters benefit from the abundance of rockweed along our coast. The answer is somewhere between a skeptical maybe and probably not.
Lobsters can often be found near ledges covered with rockweed, but the attraction may not be the seaweed. Lobsters, Webber explained, “need to find rocks and crevices where they can find shelter.”
So when you find them near seaweed-covered ledges, they may be “seeking out cobbles, not rockweed,” she said.
To answer this question, of whether lobsters are attracted to cobbles or seaweed, would necessitate a scientific study. “To the best of my knowledge, that hasn’t been studied,” Webber said.
The benefits rockweed brings to the intertidal zone do not end when the seaweed dies. Mats of seaweed regularly break off from their rocky homes and fetch up on beaches, where the seaweed begins to decay. Here it becomes a source of food for shorebirds. This is especially gratifying to Johnston, whose special interest is birds.
Once again, it is not the decaying seaweed itself the diners eat. It is the small creatures residing in the decaying biomass known as wrack.
Not surprisingly, the menu here includes many of the same items that were on offer when the seaweed was still attached to a ledge or shoreline, as well as some items that were not, such as seaweed flies.
The birds “dig through the wrack for kelp flies,” Johnston said. “It’s a pretty productive area where birds can find food.”
So just how important is rockweed to the residents of the intertidal zone?
“If rockweed were not there, those ecosystems would not function well,” Johnston said.
John Gormley, of Cundy’s Harbor, worked as a reporter and editor at The Baltimore Sun, the Portland Press Herald and Professional Mariner, among other publications. He writes for the Harpswell Anchor about nature, particularly the marine environment.
