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 C and N analysis showed that fishes in kelp forests had larger trophic niches and greater interspecific niche separation than fishes on turf reefs. Moreover, C 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.