In the coastal waters of Maine, a profound ecological shift is quietly reshaping the structure and energy base of marine ecosystems. Over recent decades, kelp forests lining the southern coast of Maine have suffered drastic declines in abundance—by as much as 80%. These towering underwater forests, once vast and vibrant, have been largely supplanted by sprawling mats of turf algae. This transformation is more than a superficial makeover of seabed flora; it signals a fundamental reorganization of food webs and energy pathways that sustain diverse marine life. New research led by scientists at Bigelow Laboratory for Ocean Sciences delves deeply into these changes, revealing how the collapse of kelp forests redefines the flow of energy to predators and alters predator-prey dynamics in a warming Gulf of Maine.
Kelp forests are widely recognized as foundational marine habitats, offering shelter and nourishment for myriad species. However, the consequences of their loss for food web energetics have been less clear—until now. By combining extensive visual dive surveys with advanced stable isotope techniques, the research team has quantified the relative contributions of kelp-derived carbon to the diets of dominant predatory fishes, such as cunner and pollock. These species illustrate the broader ecosystem response to habitat shifts. In kelp-dominated zones, predatory fish derive a significant majority of their energy from kelp-associated carbon, underscoring the critical role of kelp as a basal energy source.
Conversely, in regions where turf algae dominate, these same predators shift their reliance away from kelp and instead tap into phytoplankton, microscopic drifting photosynthetic organisms, for their energetic needs. Notably, the turf algae, despite their abundance, do not directly fuel higher trophic levels. This decoupling of energy flow from benthic primary producers to consumers upends traditional assumptions about energy pathways in temperate coastal ecosystems and indicates a restructuring of the reef’s trophic foundation.
A novel approach based on analyzing the carbon isotope ratios in essential amino acids extracted from fish muscle tissue underpinned these groundbreaking insights. Unlike traditional “bulk” stable isotope analyses, this amino acid-specific method enables tracing discrete energy sources with unprecedented precision because animals cannot alter the carbon isotope signatures in essential amino acids inherited from their primary producers. Dr. Dara Yiu, the study’s lead author and a University of Maine PhD candidate, emphasized how this refined molecular technique allowed the researchers to pinpoint the carbon origin in fish diets at a level of detail previously unattainable in marine trophic ecology studies.
This technique not only confirmed the dominance of kelp-derived carbon in healthy kelp forest food webs but also made clear the compensatory shift to phytoplankton energy sources in turf-dominated habitats. The research thereby illuminates the broader ecological impacts of kelp forest loss beyond structural habitat changes, extending to ecosystem energetics and food web stability.
Temperature-driven changes appear central to these transformations. As ocean temperatures in the Gulf of Maine have risen, kelp forests have declined precipitously, unable to persist under the warmer conditions. Turf algae, more tolerant to temperature increases, have expanded rapidly in the void left by kelp. Unlike kelp, turf algae are less productive at transferring energy efficiently to higher trophic levels, interrupting the traditional flow of carbon and potentially weakening the energetic foundation for commercially and ecologically important fish species.
In this context, the Gulf of Maine serves as an important natural laboratory for understanding “state shifts” — ecosystem-wide transitions from one dominant habitat type to another, often with profound ecological ramifications. While the consequences of such shifts have been studied extensively in tropical coral reefs and terrestrial forests, similar dynamics in temperate kelp forest systems remain poorly understood. This research thus represents a vital step toward filling that knowledge gap, revealing how energy pathways are altered during such ecological regime changes in temperate marine environments.
The complexity of food web interactions observed in remaining kelp forests further illuminates the ecosystem consequences of kelp loss. Using stable isotope analysis of carbon and nitrogen in whole fish tissues, the researchers found that predatory fish in kelp forests exhibit broader ecological niches and less dietary overlap compared to their counterparts on turf-dominated reefs. This suggests that kelp forests, by supporting diverse energy sources and habitat complexity, facilitate more complex and potentially more resilient predator-prey dynamics.
Moreover, the study’s findings challenge prevailing assumptions that phytoplankton universally dominate as the primary energy source in highly productive temperate marine ecosystems like the Gulf of Maine. Instead, kelp forests locally serve as potent sources of carbon that sustain fish populations and nearshore food webs. These results imply that the degradation of kelp forests could trigger cascading effects on marine biodiversity and fisheries productivity by altering the basal energy inputs that underpin the food web.
The research team plans to extend these investigations to Cashes Ledge, a remote offshore seamount characterized by thriving kelp forests and abundant fish populations. Exploring this relatively intact kelp ecosystem could offer a glimpse into the historical baseline conditions of Maine’s coastal reefs and provide insights into how resilient kelp forests might shape food web dynamics under future environmental changes.
Efforts to understand the wider ecological consequences of kelp loss are especially urgent given the accelerating pace of ocean warming and human impacts on coastal habitats globally. As Dr. Doug Rasher, senior author and senior research scientist at Bigelow Laboratory, notes, this study only scratches the surface of the complex processes shaping food web resilience in the Gulf of Maine. Continued research integrating sophisticated molecular tools and long-term ecological monitoring will be critical to predict and mitigate the impacts of climate-driven habitat shifts on marine ecosystems.
In sum, this study constitutes a milestone in marine ecology by not only documenting a dramatic state shift from kelp to turf algae but by elucidating the subtle, yet profound, changes in energy flow that accompany such habitat transformations. It highlights the indispensable role of kelp forests as energetic foundations of nearshore food webs and underscores the urgency of conserving these habitats amid a rapidly warming ocean.
Subject of Research: Animals
Article Title: Kelp forest loss and emergence of turf algae reshapes energy flow to predators in a rapidly warming ecosystem
News Publication Date: 6-Jun-2025
Web References: http://dx.doi.org/10.1126/sciadv.adw7396
References: Science Advances, Volume and page numbers unavailable
Image Credits: Brian Skerry
Keywords: Marine food webs, Habitat loss, Marine conservation, Ocean warming, Algae, Isotopes
