Climate change threatens seaweed

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Seaweeds are not immune from the effects of global warming, according to a study to be published in a forthcoming issue of the journal Current Biology.

Thomas Wernberg and colleagues came to this conclusion after examining about 22,000 herbarium records of macroalgae (representing 897 species) collected in temperate Australian waters since the 1940s.

The authors found that, on both the Indian and Pacific Ocean coasts of the Australian continent, macroalgal communities in the southern (poleward) part of the tropical-temperate transition progressively came to resemble past macroalgal communities farther north over the years.

These documented changes in communities and geographical distribution limits were found to be consistent with rapid warming over the past five decades.

Interestingly, the differential shifts on the Indian and Pacific Ocean coasts reflected different degrees of warming. In the Pacific Ocean, where the warming was greater, shifts of about 200 km south were recorded, whereas the Indian Ocean coast saw a shift of only about 50 km.

The authors also show that continued warming might drive hundreds of species toward and beyond the edge of the Australian continent where sustained retreat is impossible. In the words of lead author Thomas Wernberg, "…they literally fall off the continent."

They estimate as many as 100–350 species might go extinct over the next 60 years as a result of this phenomenon. This corresponds to about one quarter of the current seaweed flora of southern Australia, of which as much as 25% or more is endemic to the region. The resulting extinction may not be restricted to seaweeds, the authors warn, as these marine habitats maintain equally unique fish and invertebrate communities.

According to coauthor Bayden Russell, "The best way to go ahead in conserving these species is to increase their resilience to increasing temperatures and best way to do that is to reduce other stresses, such as reducing sewage pollution."

Not all seaweed species are moving poleward, though. The study found that a few species (e.g. Caulocystis uvifera) were actually moving northwards (albeit in more modest tens of kilometres). This could be due to the absence of competition brought about by the movement of potential competitors southwards, reason the authors.

For more information, see the paper: Wernberg, T, BD Russell, MS Thomsen, C Frederico, D Gurgel, CJA Bradshaw, ES Poloczanska and SD Connell (2011) Seaweed communities in retreat from ocean warming.  Current Biology doi:10.1016/j.cub.2011.09.028

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