Master copy of the article
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 doesn’t collapse completely. Instead, it is replaced. Over the past 80 years, once abundant forests of the native cold-water algae Laminaria hyperborea have slowly been displaced by its warm-water cousin L. ochroleuca. Scientists aren’t yet sure how the newcomer is changing the ecosystem, but marine ecologist Daniel Smale dives in to find out.
Smale grew up snorkeling in these waters. He watched the slow change of kelp forests for nearly three decades—first as a teenager free diving to observe spider crabs, then as a scientist studying the ecosystem. Some places teeming with life now feel different, he says. Not devoid of life, but less lively – a little less vibrant.
As the main species in kelp forests, kelp provides the structure and habitat that supports the ecosystem. The type of algae affects what other organisms live there and how the nutrients are used. Yet while much research has been conducted on what happens when an kelp forest disappears, little has been done on what happens when one type of kelp is replaced by another. “These subtle changes can go unnoticed or overlooked,” says Smale, who now works at the UK Marine Biological Association.
To understand how an ecosystem reacts when a new alga appears in a city, Smale, in a new study, compared kelp forests at four sites around south-west England with two in Scotland, where the water is cooler and warm-water algae are not yet at work. shows that replacing L. hyperborea with L. ochroleuca can fundamentally change what organisms live in the ecosystem and reduce the abundance of life in kelp forests.
The range of the cold water algae Laminaria hyperborea (first picture) historically extended from Portugal to the northern parts of Norway. Laminaria ochroleuca (second photo), on the other hand, normally thrives in the warmer southern waters of the Mediterranean and has only made its way to Ireland and the UK in recent decades. Photos courtesy of Daniel Smale
While his study focused on two types of algae and a few study sites, Smale says the general process — of a new algal species encroaching on the historically dominant one — is likely playing out in secret along many coasts.
The sharp declines can be explained by the fact that L. hyperborea is an excellent host. A relatively unusual trait for algae, it supports heavy 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 algae L. ochroleuca has much less life growing on and around it.
In laboratory work examining algae collected from their study sites, Smale and his colleagues showed how replacing one algae with another made a huge difference in ecosystem biodiversity. They found that cold water algae can support more than 50 grams of red algae. Warm water algae, in contrast, had almost none. This difference translated up the food chain. A cold-water kelp supports up to 375 invertebrates each, while a warm-water kelp of similar size reaches a maximum of 25.
This difference was also clear on the field. Scottish control sites dominated by L. hyperborea had about five times more invertebrates than English sites with L. ochroleuca. Since these invertebrates are food for fish and other species, the potential consequences are enormous.
“The idea that switching from one species to another that is relatively similar can have such a profound effect is just fascinating,” said Jarrett Burns, a marine biologist at the University of Massachusetts Boston, who was not involved in the study. “It’s a subtle change that leads to profound changes that can intensify as you go up the food chain.”
Because the exchange of kelp species is much less studied than the loss of kelp forests, the full long-term impact on everything from coastal fisheries to carbon sequestration remains unknown.
“This opens up a wealth of research that needs to be done as we watch our planet change,” says Burns.
Add Comment