Supplementary material from "Diel Variability and Decoupled pH-Oxygen Dynamics Drive Metabolic Plasticity in Kelp Crabs from an Upwelling Seascape"

Coastal upwelling zones, shaped by global change and human impacts, naturally impose hypoxia and acidification on marine species, creating selective pressures that influence physiological responses and drive phenotypic variability. Understanding these responses is crucial for predicting marine biodiversity dynamics in heterogeneous seascapes. We explored diel cycles of pH and dissolved oxygen (DO) and their influence on the metabolic performance of the kelp crab Taliepus dentatus, a species with limited larval dispersal. Kelp crabs from two environmentally contrasting sites along an upwelling seascape in Central Chile—an upwelling shadow and an active upwelling zone—were studied using field sensor data and laboratory experiments. Active upwelling disrupts the regular diel pH cycle, resulting in persistently low pH (pHT= ~7.5) decoupled from oxygen dynamics. Experimental simulations of diel pH-DO fluctuations revealed that nocturnal low DO/low pH conditions (DO= 1 and 5 mg L-1 pH=7.5 and 7.8 for ‘upwelling’, and ‘downwelling’ conditions, respectively) reduced metabolic rates and respiratory quotient in crabs. Crabs from the ‘active upwelling zone’ exhibited elevated metabolic rates, haemolymph pH, and lactate accumulation under extreme low pH/low DO conditions in comparison to those from the ‘upwelling shadow’, suggesting site-specific physiological adjustments. These findings underscore the importance of incorporating natural variability into experimental designs and management frameworks aimed at predicting species resilience under climate change.