This article is made possible, in part, by funds from Maine Sea Grant and the Oak Foundation.

In the last several years, Acadian Seaplants Limited (Nova Scotia), has expanded their rockweed harvesting into Maine, stimulating new regulations from Maine’s Department of Marine Resources and raising old and new questions about potential impacts of commercial-scale harvesting.

In February, the Department of Marine Resources, the Maine Seaweed Council, and Maine Sea Grant hosted a symposium at the University of Maine to share the latest scientific knowledge and to develop research priorities for rockweed.

What most of us call seaweed are organisms that scientists call marine macroalgae. The seaweed known as rockweed, Ascophyllum nodosum, grows throughout the Northern Hemisphere on exposed and sheltered shores with clean water. It is the most abundant intertidal alga in the Gulf of Maine.

In their monami, or waving motion, seaweeds are very effective at accumulating nutrients and minerals from the surrounding seawater, and this is what makes them such a valuable resource as nutritional supplements, food, and inexpensive, highly effective fertilizers.

According to Raul Ugarte, resource scientist with Acadian Seaplants, more acreage is needed to supply the market as the popularity of all things natural and organic has increased demand for seaweed and seaweed-based products. This expansion has reignited concerns about the impact of harvesting on the marine ecosystem.

A primary role of rockweed in the ecosystem is physical, as habitat. According to Thomas Trott, a marine biologist at Suffolk University in Boston, rockweed provides shelter and foraging opportunities for numerous species of invertebrates (amphipods, periwinkle and lacuna snails, dog whelks), fish (at least 22 species), and birds. The eel-like rock gunnel is the only fish that spends its entire life in the rockweed; other fish are visitors, especially juvenile pollock.

The irony of the intertidal zone is how tough and sensitive it is at the same time.åÊ Intertidal plants and animals are adapted to extremes of wet and dry, yet slight changes in water quality (such as excess nutrients) can dramatically alter species composition. Other potential threats include introduced predators and competitors, and warmer water temperatures. Storm waves and ice rip rockweed from shorelines; ice scour can remove as much as 50 percent of rockweed biomass.

According to University of Maine marine algal ecologist Robert Vadas, who has been studying rockweed for more than 30 years, dense plant cover is necessary for the rockweed’s reproductive structures (zygotes) to settle and begin establishing new plants. Because this process isn’t always successful, populations of rockweed are maintained by re-growth from the tips; shoots have been estimated as old as 18 years, and holdfasts can be much older. If the holdfast-the “stem” of the plant attached to rock or other substrate-is damaged, regrowth is reduced.

Cut areas recover within two to five years; areas where rockweed is completely removed may take decades to recover, if at all. Vadas has shown that repeated annual harvesting in the same location causes declining yields, which is why harvesters rotate sites in order to allow beds to recover, why they are only supposed to take 17 percent of any one section (a number based on recovery time and considered a sustainable rate in New Brunswick), and why the DMR directs harvesters to leave 16 inches of the plant, including the holdfast, behind. A recent study of bycatch in harvested rockweed found that 7.78 percent of the harvested plants contained holdfast tissue; 12 percent is the holdfast bycatch limit in New Brunswick.

As with any vegetation that is pruned, rockweed puts out new shoots after being cut, and plants from harvested sites grow back “bushier,” like a balsam fir that has been tipped for wreaths.

What isn’t known is whether or how animal populations, including commercially important species, are negatively affected in the long-term by these changes.

In an intensive study of Maine rockweed, Jill Fegley, now a marine biologist for the North Carolina Coastal Reserve, found a temporary decrease in some species (and an increase in dog whelk) immediately after experimental harvest, with all species recovering to their original abundances within two years. Tom Trott, working with the Bigelow Laboratory’s Peter Larsen, also did not find any changes in assemblages of species or the abundance of periwinkle snails.

Common eiders, black ducks, and mallards feed in rockweed beds; a study by Diana Hamilton at the University of Guelph in Ontario found that, while rockweed harvest had no overall effect on the invertebrate community, it did reduce the effectiveness of ducklings feeding at the top of the rockweed canopy.

The fact that none of these studies focused on community-scale effects or long-term impacts has some calling for the establishment of long-term monitoring at harvested and protected sites in Maine, as has been done in New Brunswick.

Participants in the February symposium identified dozens of research questions, but prioritized a handful, including development of an improved, non-destructive method of measuring biomass.

Rockweed grows in patches and clumps, making biomass (weight/volume) difficult to quantify. In Cobscook Bay alone, periodic estimates over the years have ranged from 105,000 metric tons to 46,000 metric tons, depending on the methods used, with the smaller number being Acadian Seaplant’s most recent count. “We only focused on dense, commercially harvestable beds, and so that number is a conservative estimate,” explained Raul Ugarte. The estimates vary because of different ways of measuring biomass, but they all overlap. According to marine biologist Jill Fegley, scientists don’t even have a standard definition of a rockweed bed.

Future research, funded in part by the industry, will be directed towards those questions prioritized by stakeholders at the February meeting.

For more information, go to:

This article is made possible, in part, by funds from Maine Sea Grant and the Oak Foundation. Heather Deese, Ph.D in oceanography, is the Island Institute’s director of marine programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.


Rockweed: A Keystone Species?

Heightened attention to rockweed has prompted claims that rockweed is a “keystone species” in the Gulf of Maine. While used casually as a synonym for “important,” the term has a scientific meaning, defined by University of Washington zoologist Robert Paine in 1969: A keystone species is one whose impacts on its community or ecosystem are large and greater than would be expected from its relative abundance or total biomass. In other words, keystones are only those species having a large, disproportionate effect, with respect to their biomass or abundance, on their community. UMaine’s Dr. Robert Vadas, who worked with Paine while he was developing the concept of keystone species, thinks the term is overused. “The critical distinction is that a keystone species has a disproportionate effect relative to its size or abundance. In the case of Ascophyllum nodosum, the effect is proportional: it is the most abundant organism and as such has a naturally large influence as food. Jill Fegley agrees. “I personally would not call rockweed a keystone species. I would consider rockweed to be an ecosystem engineer because of its three-dimensional structure,” she said.