Prior stress by marine heatwaves and micro-habitat fragmentation drive the colonisation of epifaunal assemblages in marine forests

Macroalgae species of the genus Cystoseira s.l. (Fucales, Phaeophyceae) and genera Cystoseira, Ericaria, and Gongolaria are habitat-forming species from temperate areas characterised by their high productivity and associated biodiversity. These key species are declining worldwide due to multiple anthropogenic and biotic pressures, such as habitat loss or climate change-related effects. These impacts are leading to local/regional extinctions of marine forests and increasing their fragmentation and isolation. Discrete extreme temperature events in the ocean, known as marine heatwaves (MHWs), are emerging as threats to marine systems in many areas of the planet due to climate change. We analysed 74 yr of daily sea surface temperature data on the occurrence of MHWs in the Cantabrian Sea (northern Iberian Peninsula), finding a clear increase in their frequency, duration and intensity in the last 20 yr. We also performed a 10 d mesocosm experiment to assess how MHWs affect fragmented and non-fragmented patches of Ericaria selaginoides. After this experiment, the experimental patches were deployed in a tidal rock pool for 10 d to detect effects on composition and structure of associated epifaunal assemblages. Mesocosm experiment results showed how the biomass, productivity and oxygen consumption of E. selaginoides significantly decreased as MHW intensity increased. Field deployment revealed that abundance, composition and structure of the epifaunal assemblages were significantly affected by the interaction of prior stress by MHWs and patch fragmentation. Overall, our results showed how a canopy-forming macroalga is affected by extreme temperature events and, consequently, having effects on the colonisation of epifauna according to the level of microhabitat fragmentation with potential implications for the conservation of these endangered systems.