This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration
Lush beds of seaweed sustain coastal food webs, cycle essential nutrients, clarify water, foster fisheries and shelter sea life. But in recent years, it is seaweed’s ability to soak up carbon dioxide, and the potential it could play in reining in planet-heating emissions, that has nabbed headlines.
Recent estimates suggest that when seaweeds die and sink to the seafloor, they bring serious amounts of carbon with them to be locked away in deep-sea sediments. The amount is not nearly as much as mangroves, seagrasses or trees sequester each year, but it’s enough to make a dent in greenhouse gas emissions. Spurred by this, companies are exploring seaweed’s potential in the bourgeoning “blue carbon” offset market.
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There is a snag, though. New research led by marine scientist John Barry Gallagher at the University of Tasmania in Australia, a self-described “blue carbon and wetland iconoclast,” suggests that when you consider the entire ecosystem the seaweed supports — not just the seaweed itself — some of these environments release more carbon than they store. The finding has caused a stir among seaweed scientists.
What tips the balance toward seaweed ecosystems being carbon sources rather than sinks are the contributions of sea squirts, shellfish and other phytoplankton-eating filter feeders. Gallagher uses data drawn from 18 studies of tropical, temperate and polar seaweed habitats to argue that, collectively, these inhabitants breathe out more carbon dioxide than the seaweed absorbs.
Gallagher’s conclusions could have ramifications for the nascent seaweed carbon offset industry, which he says is based on a premature understanding of seaweed carbon storage. If estimates of seaweed ecosystems’ net carbon storage are overblown, companies buying carbon credits could unwittingly be increasing emissions rather than zeroing them out, he says.
But Gallagher’s findings have been met with resistance from a group of pioneering blue carbon researchers who have penned a formal rebuttal to his study. They say Gallagher’s study is deeply problematic