As the ocean warms across its temperate regions, kelp forests are collapsing and turf algae species are taking over. This shift from dense canopies of tall kelp to low-lying mats of turf algae is driving biodiversity loss and altering the flow of energy and nutrients through reef ecosystems.
It's also fundamentally altering the chemical ecology of coastal ecosystems.
New research in Science , led by researchers at Bigelow Laboratory for Ocean Sciences, has shown for the first time how turf algae release chemicals that can kill young kelp. That creates a feedback loop where more turf algae means more harmful chemicals, which further inhibits recovery and reinforces kelp forest collapse. This chemically-mediated interaction, which scientists call allelopathy — or what the authors more bluntly call chemical warfare — reveals an indirect way that climate change is reshaping ocean ecosystems, complicating kelp forest recovery along Maine's rapidly warming coast.
The study also includes researchers from University of Maine, University of California Riverside, 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 Bigelow Laboratory Senior Research Scientist Doug Rasher, 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 pinpointing 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 candidate 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 kelp recovery 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 Daniel Petras's research group at the University of California, Riverside, employing non-targeted metabolomics analysis to understand the diverse chemistry 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 in those gaps, the team turned to novel computational tools, which use chemical 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 both all the waterborne chemicals around the turf-dominated reefs, and 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 gametophyte 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 primary driver of kelp forest decline 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 the NSF Established Program to Stimulate Competitive Research (Grant #OIA-1849227), the Louise H. & David S. Ingalls Foundation, the PADI Foundation, the Essex Avenue Foundation, and the German Research Foundation.
