Kelp forests in crisis: How toxic turf algae are taking over

Kelp forests, once a towering and life-filled part of many temperate coastlines, are vanishing fast. In their place, low-growing mats of turf algae are taking over.

This shift isn’t just cosmetic. It’s causing steep losses in biodiversity, disrupting how energy and nutrients flow through reefs, and changing the chemistry of coastal waters in ways scientists are just beginning to understand.

Researchers from Bigelow Laboratory for Ocean Sciences in Maine and the University of California, Riverside have now uncovered a surprising reason why kelp forests struggle to bounce back.

The study shows that turf algae aren’t just crowding out kelp – they’re also releasing chemicals that can kill young kelp sporelings before they have a chance to grow.

Why kelp forests aren’t bouncing back

“That’s why this study is so powerful,” said Doug Rasher, Bigelow Laboratory senior research scientist and the study’s senior author.

“It moves logically from describing a pattern in nature – the lack of recovery of kelp forests – to revealing that the chemical landscapes of kelp forests and turf reefs are fundamentally different, and to demonstrating that turf algae and the chemicals they exude prevent kelp recruitment.”

For years, scientists have documented the transition from kelp to turf in many temperate marine ecosystems. But understanding what prevents kelp from returning has remained elusive.

“This shift from kelp to turf is analogous to a terrestrial forest transitioning into a grassland,” explained Shane Farrell, a UMaine doctoral student based in Rasher’s research group. “With the loss of kelp forests, we see decreases in biodiversity, productivity, and the ecosystem services they provide to humans.”

Tracking chemical clues in the ocean

Earlier studies pointed out that turf algae crowd out space and often shelter small grazers that nibble on baby kelp.

But in tropical rainforests and coral reefs, chemical signals in the environment have also been shown to lock ecosystems in degraded states. No one had tested if something similar might be happening in cold-water kelp forests.

To explore this, the researchers surveyed sites across the Gulf of Maine for three years. In southern Maine, where kelp forests had collapsed, new kelp was struggling to survive. The researchers gathered seaweed and water samples for chemical analysis.

Instead of targeting known compounds, the team worked with biochemistry experts to use a technique called non-targeted metabolomics. This method looks at every small molecule in a sample – even ones that have never been catalogued – to capture the full chemical picture of a site.

Surprising chemistry of turf takeover

Waterborne chemicals were isolated and analyzed by breaking them into fragments and matching them to reference libraries. But that revealed only a fraction of the story.

According to Farrell, over 98% of the chemical signals had never been described before. So the team used computational tools to predict what those molecules might be, grouping them into chemical families.

“It is awesome to see how our non-targeted metabolomics tools can shed new light on the fascinating chemical complexity caused by shifting environments, such as invasive algae,” said Daniel Petras, assistant professor of biochemistry at UC Riverside.

“This becomes especially powerful when we combine our chemical data with functional information, such as kelp survival.”

The result? Kelp forests and turf reefs had completely different chemical fingerprints. And the chemicals found around turf algae looked suspiciously toxic to baby kelp.

Kelp forests cannot survive turf toxins

To test this, researchers ran a series of lab experiments. They exposed kelp gametophytes – the early life stage of kelp – to two things: water from turf-dominated reefs, and chemicals isolated from the five most common turf algae species.

The outcome was dramatic. In some cases, the survival of the young kelp dropped by 500%. The chemicals being released by turf algae weren’t just unhelpful – they were deadly.

“Our study is the first to reveal that chemical warfare can underpin the rebound potential of cold-water kelp forests. And surprisingly, some of the same types of molecules we identified on turf reefs are involved in the recovery dynamics of tropical coral reefs too,” Rasher said.

“It shows we have a lot to learn about chemical warfare on temperate reefs, the organisms and molecules involved, and how this process varies globally.”

Reversing ocean warming is not enough

The Rasher group’s earlier work showed that warming oceans were the main driver of kelp forest collapse in the Gulf of Maine. But this new study explains why reversing warming alone won’t be enough to bring kelp back.

“Once turf algae are established, just curbing global carbon emissions and reversing ocean warming is not going to bring Maine’s kelp forests back,” Farrell said. “Because of these feedback mechanisms, we need local interventions to remove the turf algae before kelp will actually recover.”

In other words, it’s not just about temperature anymore. To restore kelp forests, we’ll need to tackle the chemical battles happening underwater – battles that are invisible, but no less real.

The full study was published in the journal Science.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–