Kelp forest loss and emergence of turf algae reshapes energy flow to predators in a rapidly warming ecosystem

Climate change is decimating habitat-forming species in ecosystems around the world. Yet, the impacts of habitat loss on the energetics of the wider food web remain uncertain for many iconic ecosystems, including cold-water kelp forests. Here, we assessed how the loss of kelp forests and the subsequent proliferation of low-lying turf algae in the Gulf of Maine have altered the trophic niches of, and energy acquired by, predatory reef fishes. Bulk tissue δ13C and δ15N analysis showed that fishes in kelp forests had larger trophic niches and greater interspecific niche separation than fishes on turf reefs. Moreover, δ13C analysis of essential amino acids revealed that kelp-derived energy accounted for most of the energy used by kelp forest fishes (> 50% on average), whereas fishes on turf reefs compensated for kelp decline via greater reliance on a phytoplankton-based energy channel. Therefore, ecosystem state shifts to turf algae—now a global phenomenon—may have far-reaching impacts on food web energetics and resilience.