Warming Accelerates Ecological State Shift Along Maine Coast

Warming Accelerates Ecological State Shift Along Maine Coast

The loss of dense kelp forests along the Maine coast - and the northward proliferation of small, carpet-like turf algae in its place - is accelerating as the ocean warms, according to new research by scientists at Bigelow Laboratory for Ocean Sciences.

Published recently in Ecology, the research shows that warming is facilitating the arrival of new species into the Gulf of Maine, and that the transition from kelp forests to turf reefs has progressed rapidly in recent years. The study, which covers some of the hottest years on record in the region, highlights both the direct and indirect impacts of environmental change on temperate reef ecosystems and the vital services they provide.

"The progression of this shift from kelp forests to turf algae played out right before our eyes," said Senior Research Scientist Doug Rasher, the senior author on the paper. "We're digging into what's driving this transition, and what's being gained or lost as a result, which allows us to speak more to the future of this ecosystem."

The new paper builds on previous research published by Rasher's team, including a study published in 2024 that provided a coastwide assessment on the state of Maine's kelp forests up to 2018. That analysis, combined with long-term monitoring data from the Department of Marine Resources, drew a causal link between kelp forest decline and rising ocean temperatures. It also documented a widespread shift to turf algae in the southern reaches of the coast. Subsequently, the team examined several of the consequences of that state shift, including changes to the reef's chemical environment and its food web dynamics.

The current study picks up that thread, providing a breakdown of what species of turf algae are proliferating and what functions they do or don't provide to the larger ecosystem.

Multiple researchers in the Rasher Lab undertook repeated monitoring from 2021-2023 of 11 sites along the coast, analyzing the composition of the algae community at each site, measuring its characteristics, and assessing how it has changed over time.

Their findings suggest the shift from a kelp forest ecosystem to one dominated by turf algae has progressed northward in recent years, all the way to Penobscot Bay, with increases in the coverage of turf algae up to 40 percent per year in some areas.

The study also shows that, while these turf algal carpets may look uniform, they're actually made up of a diverse assemblage of species. That includes natives that have become more common, species typically found further south that are moving north, and invasive species from other ocean basins.

"These turf carpets can be made up of 20 to 30 different species, but they're dominated by invasives like Dasysiphonia japonica, which comes from the Pacific," said Shane Farrell, the study's lead author and a former PhD student in Rasher's group. "Even further north, where kelp forests are still thriving, we are seeing native turf algae in higher abundance than we've ever seen before, which could be an early warning sign of future change."

The paper also highlights how these turf species are physiologically different - in important ways, Farrell says - from the foundational kelp species around which this ecosystem evolved.

Instead, the turf algae display rapid nutrient turnover, have high surface-area-to-volume ratios, and provide smaller interstitial spaces, all of which means they don't provide the same nutrition, stability, or physical habitat as kelp forests. This loss likely has knock-on effects, the authors suggest, for other species that live on these reefs, which could reshape predator-prey dynamics and the movement of energy through the ecosystem.

The researchers also modeled how higher temperatures, wave action, and prior kelp loss, drive the proliferation of turf algae. That finding underscores the importance of the direct and indirect impacts of ocean warming, with higher temperatures both killing off kelp directly and encouraging the arrival of new species that accelerate the replacement of kelp. It also raises concerns about the future of those northerly areas of the coast where kelp forests remain, but turf species are becoming more common.

"The silver lining is that we've untangled what's driving change in our coastal reefs, so we can now forecast how these changes will progress up the coast," Rasher said. "If we can understand and predict the shift, we can inform management and conservation strategies to combat it."

Photo: Turf algae (Credit: Shane Farrell).