As the ocean warms across its temperate regions, kelp forests are collapsing and turf algae species are creeping in. This shift from dense canopies of tall kelp to low-lying mats of turf algae is driving losses in biodiversity, altering the flow of energy and nutrients through reef systems, and fundamentally altering the chemical ecology of coastal ecosystems.

A study published in the journal Science by scientists at Bigelow Laboratory for Ocean Sciences in Maine and Daniel Petras’ research group at the University of California, Riverside, shows for the first time how turf algae release chemicals that can kill young kelp. According to the researchers, a feedback loop is created where more turf means more harmful chemicals, inhibiting recovery and reinforcing kelp forest collapse. This chemically-mediated interaction, which scientists call allelopathy — or what the authors call chemical warfare — reveals an indirect way that climate change is reshaping ocean ecosystems, complicating kelp forest recovery in the area studied along Maine’s rapidly warming coast.

The Bigelow Laboratory-led study also includes researchers from University of Maine, University of Tübingen, Perry Institute for Marine Science, and Harvard University, working together to combine extensive field surveys, advanced chemical analysis, and novel lab experiments.

“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 landscape of kelp forests and turf reefs are fundamentally different, to demonstrating that turf algae and the chemicals they exude prevent kelp recruitment.”

The impacts of kelp forest collapse, and replacement by turf algae, have been well documented in temperate ecosystems around the world.

“This shift from kelp to turf is analogous to a terrestrial forest transitioning into a grassland,” said the study’s lead author, 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.”

Previous work has shown that once turf algae are established, they can inhibit the recovery of kelp by taking up space on the reef or harboring small grazers that eat baby kelp.

In tropical ecosystems, such as rainforests and coral reefs, scientists have previously shown that changes in the chemical environment also play a role in locking ecosystems into a degraded state and preventing recovery of foundational species. But no studies had considered whether that kind of chemical change could be at play in temperate kelp forests.

To answer this question, the researchers completed three years of field surveys across the Gulf of Maine, documenting a pattern of new kelp struggling to survive in the southern reaches of Maine’s coast where forests have collapsed. During those surveys, the team collected water and seaweed samples for chemical analysis.

Rather than focusing on known substances, they teamed up with Petras, an assistant professor of biochemistry, to apply non-targeted metabolomics analysis to understand the full array of chemical changes in the samples. This approach involves analyzing all the small molecules within a system, which enabled the researchers to broadly identify the unique chemical features — in the water, in the seaweeds, and on the reef itself — at both kelp- and turf algae-dominated sites.

To characterize the suite of waterborne chemicals present, these methods rely on separating the molecules and breaking them into fragments, which are then matched against reference libraries, much like identifying a person from a fingerprint.

But, as Farrell pointed out, less than 2% of the chemical features the researchers found in this environment had been previously described. To fill those gaps, the team turned to novel computational tools, which use those fragmentation patterns to predict compound identities, molecular formulas, and even chemical structures. These predictions allowed the researchers to classify unknown compounds into broad chemical families, highlighting just how distinct the chemical environment of a kelp forest is from a turf-dominated reef.

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

In a series of laboratory experiments, the researchers then tested the effects of (1) all the waterborne chemicals around the turf-dominated reefs, and (2) the specific chemicals released by the five most abundant species of turf algae, on gametophytes, an early life stage of kelp. The experiments showed that their survival declined dramatically — up to 500% in some cases — when exposed to chemicals released by turf algae, confirming that the new chemical environment is directly responsible for kelp mortality.

“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.”

Previous work by Rasher’s research group confirmed that ocean warming is the fundamental driver of kelp forest collapse in the Gulf of Maine. But these new findings, showing how turf algae can lock an ecosystem into a degraded state, will make it more challenging to promote kelp forest recovery.

“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.”

This study was supported by NSF Established Program to Stimulate Competitive Research (Grant #OIA-1849227), the Louise H. & David S. Ingalls Foundation, the PADI Foundation, and the German Research Foundation.

This news release is a slightly modified version of a Bigelow Laboratory news release written by Leah Campbell.

Header image shows study authors, Shane Farrell and Dara Yiu, dive off of Allen Island carrying all the necessary gear for a coastal reef survey to document kelp forest loss on the coast of Maine. (Rene Francolini)