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The Seaweed Subsidy

August 24, 2015

Seaweed, the much-maligned tangler of swimmers’ feet, stinker-upper of otherwise pristine beaches, and thrower-atter of unsuspecting sisters’ backs, may be finally getting the credit it deserves.

It doesn’t look like much, but washed-up seaweed could represent a subsidy to land ecosystems that could change the way we think about island ecology.

“The ocean here is super productive,” says Sara Wickham, a graduate student at the University of Victoria’s Environmental Studies program and Hakai scholar. “Other studies [of island biogeography] have been done in the Gulf of California or Western Australia. We have richer oceans here — we have more nutrients.”

The established understanding of island biogeography comes from a seminal 1967 paper by ecologists Robert MacArthur and E.O. Wilson, which said that the smaller the island, and the farther from the mainland, the lower the number of species. Since then the theory has been reinforced by studies all around the world.

The islands of B.C.’s Central Coast, however, may stretch that principle, with ecosystems that are more species-rich than expected given their size and distance to the mainland.

The seaweed could very well be playing an important part in that added variety.

“We thought there might be a direct transfer of seaweed to the forest,” says Wickham.

The reality appears to be a bit more complex, though. When the material, made up of seaweeds, seagrasses and other organic matter collectively known as “sea wrack” washes up on the beach, it is eaten by birds, insects, and small mammals. Those animals then return to the forest where the sea wrack’s nutrients become part of the ecosystem.

Wickham and her research assistant, Beatrice Proudfoot, have spent months combing the beaches of the Goose Island Group and elsewhere along the coast, measuring the species composition and amount of seaweed that has washed up, using square-meter plots and a kitchen scale.

The two estimate that over the summer they have surveyed over 7,000m2 of beach, one square meter at a time.

Now, at the end of the season, the pair is working at the Calvert Island field station to start developing an understanding of how seaweed input here changes throughout the seasons. All of this information together will help the 100 Islands project contribute new insights into a classic, 50-year-old theory.

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