Rockweed as Habitat

Life Within the Rockweed
(Published in the Quoddy Tides, September 22, 2000)

When the tide retreats from Cobscook Bay's shores, the seaweeds left behind in tangled masses on exposed ledges, rocks, and mud appear limp and lifeless. Yet, entire communities of marine life can be found living within this vegetation. The most abundant seaweeds found here are knotted wrack and bladder wrack, more commonly known as rockweed. Like the trees in a forest, the rockweeds help to define what forms of life can persist in the harsh intertidal environment.

Trescott resident Julie Hodgkins' entire life has been involved in fisheries. Through many hours spent clamming, periwinkling, and fishing along the shores of Cobscook Bay, she has observed how rockweed is connected to the other fisheries important to her community.

"I find all different types of things in the rockweed - periwinkles, crabs, eels, limpets, sand fleas - each piece of seaweed is home to something. The fish come in to feed in the rockweed when the tide is high, and I see eagles and loons and other birds follow them in," she comments. "When I'm clamming, I find more clams beneath rockweed beds. And there are animal tracks all over the beach in the morning. Many of these animals feed on the crabs and snails living in the rockweed - it's all connected."

What Julie has observed is beginning to be documented through scientific studies of rockweed in Cobscook Bay and elsewhere along the coast. Several research projects in Maine and in Canada have provided clues as to the importance of rockweed as habitat for intertidal marine life.

Studies completed by Professor Bob Vadas and doctoral student Jill Fegley, both of the University of Maine, have revealed that, at sites along the Maine coast, close to sixty different marine animals and plants use rockweed as habitat during low tide. Beneath the dense blankets of rockweed, a variety of shore and bottom dwelling sea animals find food, refuge, and protection from the drying sun. Smooth periwinkles rely on rockweed as their primary source of food, while other mobile creatures like sea stars, dogwhelks, and crabs prey on shellfish and other organisms hidden within the seaweed.

Attached to the rockweeds heavily branched fronds, smaller seaweeds and bacteria provide an additional source of food for grazers like the periwinkles, tiny shrimp-like creatures called amphipods, and sea urchins. The sea oak hydroid, a filter feeding animal that grows in colonies attached to plant and rock surfaces, and the polychaete worms, which create the white spiral tubes often found on seaweeds, are among those invertebrates that need the cool, moist conditions found in rockweed beds. They in turn provide food for other animals.

The role that rockweed plays as habitat for other marine life shifts with the tide. When the tide comes in, tiny air bladders along the length of the rockweed stems and branches allow them to rise and sway with the current. This tall undersea 'forest' serves as a nursery and feeding area for many species of fish. Larval and small fish take advantage of the cover this provides, and feed on small crustaceans that also find food and a place to hide there. Larger fish prey upon the smaller and younger fish, and seabirds forage for snails and crabs as well as the smaller vegetation that grows on rockweed.

Research presented two years ago at a conference in Eastport highlights the importance of rockweed as fish habitat. Researcher Bob Rangeley, of the Department of Fisheries and Oceans Biological Station in St. Andrews, New Brunswick, reported that during high tide, 31 fish species were found in the intertidal zone where rockweed is abundant. Seventeen species used the rockweed habitat as juveniles, including herring, pollock, and winter flounder. His work supported other studies that show juvenile fish use the rockweed as a refuge from predators and also feed on the crustaceans that live in the seaweed.

Many species of seabirds commonly found in Cobscook Bay waters move inshore during high tide to forage in the rockweed canopy. It provides important habitat for eider ducks in particular. According to Brad Allen, a biologist for the Department of Inland Fisheries & Wildlife, it is well-documented that common eider ducks require rockweed as brood rearing habitat for ducklings. He states, "Where rockweed abounds, that is where the adult eiders take ducklings to forage for the amphipods that live amongst the rockweed. The periwinkles found in this habitat are also a very important food item for eiders and other common waterfowl feeding in intertidal areas."

The many functions of rockweed do not end if individual plants are torn from their place on the rocks and are left to drift. Floating mats of rockweed often contain living plant material, and continue to provide small marine animals with a place to feed and to hide from predators. Migrating seabirds rely on these floating mats to forage for their final sources of food before flying south.

Even after the masses of seaweed are left on shore, beach dwelling insects, bacteria, and fungi are attracted to the decaying piles and decompose the plant matter into nutrients that stimulate the growth of other marine plants and enrich habitat for clams.

Because rockweed provides habitat for such a variety of marine animals found in the intertidal area, and holds high value as a source of food and refuge for fish and waterfowl, it is an essential part of the community of life in Cobscook Bay.

This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.

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