Gene-to-Population Level Responses to Multiple Stressors on the Rocky Shore.

Coastal ecosystems are exposed to both global and local stressors operating across multiple scales. However, research rarely considers how their combined effects propagate over time and across levels of biological organisation. Here, we employ an in situ warming experiment across two rocky shore sites with contrasting sewage pollution to quantify independent and interactive effects of warming and nutrient pollution from genes to populations. Passively warmed and control settlement plates were deployed at polluted and non-polluted sites and surveyed across a summer to quantify temporal dynamics in the responses of key intertidal taxa. Barnacles were further employed as a model for comparing responses across biological levels, including body size, stable isotope analysis and RNA sequencing. Pollution consistently increased invertebrate abundance and macroalgal cover, alongside transient positive effects on barnacle size, whereas warming reduced barnacle abundance and suppressed macroalgal cover late in the season. Warming and pollution interacted synergistically on barnacle abundance, with pollution remaining the dominant stressor. Microphytobenthos groups similarly showed distinct pollution-driven increases with warming primarily modifying temporal trajectories; cyanobacteria showed both date-specific and season-wide synergistic interactions, against a backdrop of temporal variability across stressor treatments. Consistent with these patterns, pollution shifted barnacle δ