From Australia to the Arctic, kelp forests are disappearing as the ocean warms. But in some places, such as the frigid waters off England’s southwest coast, the struggling algal ecosystem isn’t collapsing completely. Instead, it is replaced. Over the past 80 years, once abundant forests of the local cold-water kelp Laminaria hyperborea have been slowly overtaken by its warm-water cousin L. ochroleuca. Scientists aren’t yet sure 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’s been observing the slow change of kelp forests for nearly three decades—first as a teenager freediving to track spider crabs, then as a scientist researching the ecosystem. Some spots that were once bustling with life now feel different, he says. Not devoid of life, but less lively – a little less alive. As a foundational species in kelp forests, kelp provides the structure and habitat underlying the ecosystem. The type of algae affects the other organisms that live there and how nutrients are used. However, while much research has been conducted on what happens when a kelp forest disappears, little has been done on 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’s Marine Biological Association. To discern how an ecosystem reacts when a new kelp comes to town, Smale, in a new study, compared kelp forests at four sites around southwest England with two in Scotland, where the water is cooler and the kelp with warm water has not yet The work shows that replacing L. hyperborea with L. ochroleuca can fundamentally change the organisms living in the ecosystem and reduce the abundance of life in kelp forests. The range of the cold water algae Laminaria hyperborea (first photo) historically extends from Portugal to the northern parts of Norway. Laminaria ochroleuca (second photo), on the other hand, normally thrives in warmer southern waters in the Mediterranean and has only reached Ireland and the UK in recent decades. Photos courtesy of Daniel Smale While his study focused on two species of algae and a few study sites, Smale says the general process — of a new species of algae encroaching on the historically dominant species — is likely going on secretly on many coasts. The sharp declines can be explained by the fact that L. hyperborea is an excellent host. A relatively unusual feature for algae, it supports a robust growth of red algae on its brown rubbery surfaces. These red algae, in turn, support a thriving community of worms and gastropods, molluscs and starfish that feed fish and other hungry animals. In contrast, the warm water alga L. ochroleuca had much less life growing on and around it. In laboratory work looking at algae collected from their study sites, Smale and his colleagues showed how replacing one algae with another led to a huge difference in ecosystem biodiversity. Cold-water algae, they found, can support up to 50 grams of red algae each. In contrast, warm water algae had almost none. This difference translated into the food chain. The cold-water kelp supported up to 375 invertebrates each, while a similarly sized warm-water kelp supported 25. This difference was also clear on the pitch. Scottish control sites dominated by L. hyperborea had about 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 changing from one species to another relatively similar one can have such profound effects is just fascinating,” says Jarrett Burns, a marine biologist at the University of Massachusetts Boston who was not involved in the research. “It’s a subtle shift that leads to profound changes that could be amplified as you move up the food chain.” Because kelp species trade-offs are much less studied than kelp forest loss, however, the full long-term effects on everything from coastal fisheries to carbon sequestration remain unknown. “It opens up a wealth of research to be done as we watch our planet change,” says Byrnes.
