The Ecology of Rockweed

The jungle of Rockweed that drapes the intertidal zone on Kent Island is a fascinating area for scientific study, as I found when I accompanied Christine Walder on her biodiversity surveys. Rockweed is extremely ecologically important to the coasts of Canada and Maine, including Kent Island, which has some of the oldest and longest strands. Rockweed is an algae that lives in a narrow band on the intertidal zone, the area of the coast that the tide floods and drains every twelve hours. Each strand of Rockweed is rooted to a holdfast, which is anchored to rocks on the seafloor, but buoyant, air-filled bladders along the fronds keep the algae afloat at high tide. The holdfasts may be up to 400 years old, and individual strands can be 10-15 years old, though they only grow 4-6 cm per year, making Rockweed the marine equivalent of an old growth forest. The floating fronds form a dense canopy that creates a three dimensional habitat for a diversity of organisms, as well as a source of nutrients for coastal ecosystems. A review of the literature on Rockweed found that Ascophyllum nodosum provides habitat for over 150 species of vertebrates, invertebrates, and algae, including commercially valuable species such as lobster, clams, pollock, herring, flounder and cod (Seeley and Schlesinger, 2012).

Rockweed is also an important resource for Common Eiders, which I love to watch foraging in the Bay of Fundy in giant flotillas of thirty ducklings herded by a handful of mothers. Adult Eiders feed on the invertebrates that live in the Rockweed, which is also a crucial food source for young ducklings. The ducklings can’t yet dive and are dependent on the buffet that the floating Rockweed provides at high tide. The long Rockweed strands also camouflage them from predators such as gulls and eagles, for whom ducklings are easy prey.

Eiders foraging in Rockweed  (image:

Eiders foraging in Rockweed

Click here to watch Eider ducklings foraging in Rockweed at Head Harbor Lighthouse, New Brunswick

Alfred Gross, the first Director of the Bowdoin Scientific station, observed the Eiders closely, and vividly recounts their behavior in his 1938 Eider Ducks of Kent’s Island. One adorable episode illustrates the importance of Rockweed to the Eiders. Gross rescued an abandoned Eider duckling, the weakest and last to hatch, by placing it in his hat and taking it back to camp. He kept it in a box by the stove, and after it had regained its strength, handed it off to Ernest Joy. Gross recounted,

“The warden carried it about in his pocket and when at the shore allowed it to forage among the seaweeds. It discovered that the little crustaceans commonly known as “sand fleas” were good for food. By uncovering sheaves of rock-weed hundreds of these leaping creatures kept it busy dashing first this way then that in futile effort to get every one in sight. In the course of a few minutes the little fellow’s crop was bulging and so heavy that it was forced to sit down to rest from sheer exhaustion.”

Rockweed is full of hidden life, including amphipods, the “sand flies” that the duckling gorges himself on. While not many would decry their love for Rockweed,  it is at the base of a complex food web that includes the more charismatic Eider ducklings, and lobster and herring, which feed the fisheries that local economies depend on.


Alfred A. Gross. “Eider Ducks of Kent’s Island.” The Auk, Vol. 55, No. 3. (Brunswick, Maine: 1938).

Seeley, R., and Schlesinger, W. 2012. Sustainable seaweed cutting? The rockweed (Ascophyllum nodosum) industry of Maine and the Maritime Provinces. Annals of the New York Academy of Sciences, 1249: 84-103.


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