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While kelp forests persist along northern Maine's rocky coast, kelp abundance has declined by as much as 80% on the southern coast in recent decades. In its stead, carpet-like turf algae have moved in.

A team, led by scientists at Bigelow Laboratory for Ocean Sciences, are examining the broad consequences of this shift. Their research, appearing in , shows that predator-prey interactions and the flow of energy are fundamentally different on turf-dominated reefs compared to the remaining kelp forests.

Using visual dive surveys and cutting-edge stable isotope methods, they found that dominant predatory fish species acquired the majority of their energy from kelp. Meanwhile, the same fish species on turf-dominated reefs have compensated for the loss of kelp by turning to phytoplankton for energy. The turf algae, while abundant, was not a significant source of energy.

The study is the first to quantify the importance of kelp-derived carbon to the food web in the Gulf of Maine, highlighting how kelp forest collapse is reshaping energy dynamics in this rapidly warming ecosystem. It's also the first to trace the flow of carbon in this region using a novel amino acid-based stable isotope technique, which could prove useful for differentiating energy sources in farmed kelp environments and other ecosystems.

"People have studied the importance of kelp forests for habitat and food around the world, but we never knew that providing energy was such a critical function of kelp forests in the Gulf of Maine," said the study's lead author, Dara Yiu, a University of Maine Ph.D. candidate based at Bigelow Laboratory. "So, when we lose kelp forests, it's fundamentally changing the energy sources that are supporting the food web."

As warming waters have along large swaths of the coast, turf algae have proliferated, providing few of the same ecosystem services as kelp.

In terrestrial forest and coral reef environments, scientists have long understood how this sort of "state shift," as this foundational habitat change is called, can alter food web dynamics. Yet, scientists are only beginning to unravel what it means in the Gulf of Maine.

"We have a better handle on how energy is produced and flows through food webs on tropical reefs. There are many knowledge gaps here in the Gulf," said Doug Rasher, a senior research scientist at Bigelow Laboratory and senior author on the study.

"A lot of the work we're doing in kelp forests is foundational, in terms of revealing how this iconic ecosystem is changing, and how these changes reverberate to impact other animals."

After several seasons of intensive dive surveys to assess the condition of kelp forests along the coast, the researchers used stable isotope methods to trace the flow of energy through the food web from primary producers like kelp up to predatory fish like pollock.

They first measured ratios of carbon and nitrogen stable isotopes in whole, or "bulk," tissues of the two most widespread fish species, a popular method for understanding resource use and food web position.

Comparing fish on kelp- versus turf-dominated reefs, they found that the food web was seemingly more complex in the remaining kelp forests, with the two fish species occupying larger ecological niches with less overlap in their diet.

These traditional measurements, however, are not effective for tracing carbon that comes from multiple, distinct sources. To that end, the team deployed an approach that looks at carbon isotope ratios in essential amino acids in fish muscle tissue.

These molecules can't be made or modified by animals as they move up the food web, so each retains a distinct "fingerprint" from the primary producer that created it—whether that's kelp, red macroalgae, or phytoplankton floating in the water column.

Yiu worked with Emma Elliott Smith, a postdoctoral scientist at the University of New Mexico and co-author of the study, to undertake the analysis. The amino acid approach, Yiu said, is more labor-intensive and time-consuming than traditional bulk stable isotope methods, but it enabled them to determine how sources and flows of energy varied along the coast.

"By analyzing individual amino acids, we can zoom in on specific biomarkers to trace energy flow through an ecosystem with much more precision than traditional methods," Elliott Smith said.

"This approach gave us a much clearer picture of how energy is moving through kelp forests, and how kelp contribute energy to the food web in the Gulf of Maine, which we couldn't get from bulk analysis alone."

The analysis revealed that fish residing in kelp forests are deriving more than half of their essential amino acids from kelp. Without kelp, phytoplankton, not turf algae, provide the dominant source of energy to the system, confirming that this large-scale loss of a foundational species has removed a key source and pathway for energy to flow in the food web.

"In most parts of the ocean, especially areas like the Gulf of Maine where phytoplankton productivity is so high, the assumption is that phytoplankton support the food web. So, it was exciting to find that kelp forests here are so productive that fish and the nearshore food webs can rely on them instead," Yiu said.

"And where you don't have robust kelp forests, it changes the underlying structure of the food web, the full consequences of which we still don't know."

These findings provide a springboard for future studies of the reef food web and its resilience to ongoing change, which Rasher and his team are continuing to explore.

They plan to partner with Elliott Smith to apply these new methods next to Cashes Ledge, a seamount 90 miles off the coast of Maine. With its thriving kelp forests and abundant fish populations, Rasher said, it may provide a glimpse into what the Maine coast once looked like.

"While we found that kelp is a key source of energy for coastal reef fish, we still have much to learn about the impacts of kelp forest loss on commercially and ecologically important fish," Rasher said.

"Our study is just the tip of the iceberg, and shows there is much more research needed on food web resilience in the Gulf."