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From Australia to the Arctic, kelp forests are disappearing as the ocean warms. But in some places, such as in the cold waters of the south-west coast of England, the algae ecosystem is not collapsing at all. Instead, it is being replaced. For the past 80 years, the previously abundant forests of the local cold-water alga Laminaria hyperborea have been slowly overtaken by their warm-water cousin L. ochroleuca. Scientists are still unsure of how the newcomer is changing the ecosystem, but marine ecologist Daniel Smale is diving in to find out.
Smale grew up snorkeling in these waters. He has been observing the slow change of algae forests for almost three decades, first as a teenager freediving to see crabs, then as a scientist studying the ecosystem. Some places that were once full of life now feel different, he says. Not lifeless, but less lively, a little less vibrant.
As a key species of kelp forests, kelp provides the structure and habitat that underpins the ecosystem. The kelp species affects what other organisms live there and how nutrients are used. Although much research has been done on what happens when an algae forest disappears, little has been done about what happens when one species of kelp is replaced by another. “These subtle changes can go unnoticed or overlooked,” says Smale, who now works at the UK Marine Biology Association.
To discern how an ecosystem responds when a new kelp arrives in the city, Smale, in a new study, compared kelp forests in four places in the south-west of England to two in Scotland, where water is more fresh and hot water kelp has not yet done so. The work shows that the replacement of L. hyperborea by L. ochroleuca can fundamentally change the organisms that live in the ecosystem and reduce the abundance of life in algae forests.
The range of the cold-water alga Laminaria hyperborea (first photo) extends historically from Portugal to the northern tip of Norway. Laminaria ochroleuca (second photo), on the other hand, thrives normally in the warmer southern waters of the Mediterranean and has only in recent decades been dragged as far as Ireland and the United Kingdom. Photos courtesy of Daniel Smale
Although his study focused on two species of algae and a few study sites, Smale says the overall process, of a new species of algae invading the historically dominant, is likely to take place. surreptitiously along many coasts.
The sharp declines can be explained by the fact that L. hyperborea is an excellent host. A relatively unusual feature for kelp, it supports robust growth of red algae on its brown rubber surfaces. These red algae, in turn, support a thriving community of worms and gastropods, mollusks, and starfish that feed on fish and other hungry animals. In contrast, warm-water kelp L. ochroleuca had much less life growing around and around it.
In laboratory work examining algae collected from their study sites, Smale and colleagues showed how replacing one algae with another makes a big difference in the biodiversity of the ecosystem. They found that cold-water algae can support more than 50 grams of red algae each. The hot water kelp, on the other hand, had almost none. This difference translated into the food chain. The cold water kelp supported up to 375 invertebrates each, while a hot water kelp of similar size reached a maximum of 25.
This difference was also clear in the field. Scottish control sites, dominated by L. hyperborea, had approximately five times more invertebrates than English sites with L. ochroleuca. Because these invertebrates are food for fish and other species, the potential consequences are devastating.
“The idea that switching from one species to a relatively similar one can have such profound effects is fascinating,” says Jarrett Byrnes, a marine biologist at the University of Massachusetts Boston who did not participate in the research. “It’s a subtle change that leads to profound changes that could be amplified as you go up the food chain.”
Because kelp species exchanges are studied much less than kelp forest loss, however, the long-term impacts on everything from coastal fishing to carbon sequestration remain unknown.
“It opens up a lot of research that needs to be done while we observe the change on our planet,” Byrnes says.