Spatiotemporal stressors, not secondary structures or small temperature increases, control rapid facilitation of intertidal epifauna.

Small animals (epifauna) are ubiquitous in marine systems. Epifauna have high secondary production and provide trophic linkages between primary producers and higher-order consumers, like fish. Despite their importance, little is known about how these animals respond to warming or how their communities vary across spatiotemporal gradients. Here we use mimics of turf seaweed and invasive kelp holdfast to test, in a 5-factorial field experiment, whether intertidal epifauna are facilitated by different habitat structures, temperature conditions, and along cooccurring spatiotemporal gradients. We found that facilitation of epifauna by intertidal turf seaweed peaked in summer, at low elevation, in older habitats and at a less wave-exposed site. However, epifauna were not affected by the presence of a secondary structure like kelp holdfast mimics or small temperature increases from passive solar heating of black and white mimics. There were many significant two-way, but few higher order interactions, showing stronger facilitation under specific environmental conditions, like at low elevation in summer, or low elevation in old habitats. These results highlight that turf-associated epifauna are controlled by vertical elevation, season, hydrodynamics, and habitat age, and appear to be resilient to small temperature increases. Findings are important to better understand linkages between primary producers and higher order consumers and system-wide productivity, and because fast growing turf, facilitated by global warming and eutrophication, are increasingly outcompeting slower growing large perennial canopy forming seaweeds, like kelp and rockweeds.