Turf algae redefine the chemical landscape of temperate reefs, limiting kelp forest recoveryResistance, Resilience and Phase Shifts

Shane Farrell^1,2, Daniel Petras^3,4, Paolo Stincone⁴, Dara Yiu^2,1, John Burns¹, Abzer Kelminal Pakkir Shah⁴, Aaron Hartmann⁵

¹Bigelow Laboratory for Ocean Sciences, USA - ²University of Maine, USA - ³University California Riverside, USA - ⁴University of Tubingen, Germany - ⁵Harvard University, USA

In the southern Gulf of Maine (USA) kelp forests have collapsed due to ocean warming, marine heatwaves, and species invasions. Simultaneously, filamentous “turf” algae have proliferated, leading to an alternative phase state. This shift from kelp-dominated reefs to chemically-rich turf-dominated reefs is happening in temperate regions around the globe. Yet, the extent to which these turf algae alter the surrounding chemical environment or impact the rebound potential of kelp forests via allelopathy (chemically-mediated competition) remains unknown. Here, we used underwater visual surveys, comprehensive chemical profiling (nontargeted metabolomics), and laboratory experiments to reveal that turf algae release bioactive compounds into the water that fundamentally alter the “chemical landscape” and directly suppress kelp recruitment. Here, we show that 5 of the most dominant algal turf species all exude waterborne chemicals that directly lead to the mortality of S. latissima gametophytes. Our findings reveal a novel mechanism by which turf algae may create and perpetuate harmful ecological feedback loops, locking the system into a degraded state. Therefore, our work illustrates that allelopathy is critical in shaping modern kelp forests and their resilience. Further, it demonstrates that reversing climate-driven state shifts will require limiting global carbon emissions and regional interventions that break ecological feedback loops and foster recovery.