Ocean acidification and hypoxic events are an increasing worldwide problem, but the synergetic effects of these factors are seldom explored. However, this synergetic occurrence of stressors is prevalent. The coastline of Chile not only suffers from coastal hypoxia but the cold, oxygen-poor waters in upwelling events are also supersaturated in CO2, a study site to explore the combined effect of ocean acidification and hypoxia. We experimentally evaluated the metabolic response of different invertebrate species (2 anthozoans, 9 molluscs, 4 crustaceans, 2 echinoderms) of the coastline of central Chile (33°30’S, 71°37’W) to hypoxia and ocean acidification within predicted levels and in a full factorial design. Organisms were exposed to 4 different treatments (ambient, low oxygen, high CO2, and the combination of low oxygen and high CO2) and metabolism was measured after 3 and 6 days. We show that the combination of hypoxia and increased pCO2 reduces the respiration significantly, compared to a single stressor. The evaluation of synergistic pressures, a more realistic scenario than single stressors, is crucial to evaluate the effect of future changes for coastal species and our results provide the first insight on what might happen in the next 100 years.

The exposure to environmental variations in pH and temperature has proven impacts on benthic ectotherms calcifiers, as evidenced by tradeoffs between physiological processes. However, how these stressors affect structure and functionality of mollusk shells has received less attention. Episodic events of upwelling of deep cold and low pH waters are well documented in eastern boundary systems and may be stressful to mollusks, impairing both physiological and biomechanical performance. These events are projected to become more intense, and extensive in time with ongoing global warming. In this study, we evaluate the independent and interactive effects of temperature and pH on the biomineral and biomechanical properties of Argopecten purpuratus scallop shells. Total organic matter in the shell mineral increased under reduced pH (~ 7.7) and control conditions (pH ~ 8.0). The periostracum layer coating the outer shell surface showed increased protein content under low pH conditions but decreasing sulfate and polysaccharides content. Reduced pH negatively impacts shell density and increases the disorder in the orientation of calcite crystals. At elevated temperatures (18 °C), shell microhardness increased. Other biomechanical properties were not affected by pH/temperature treatments. Thus, under a reduction of 0.3 pH units and low temperature, the response of A. purpuratus was a tradeoff among organic compounds (biopolymer plasticity), density, and crystal organization (mineral plasticity) to maintain shell biomechanical performance, while increased temperature ameliorated the impacts on shell hardness. Biopolymer plasticity was associated with ecophysiological performance, indicating that, under the influence of natural fluctuations in pH and temperature, energetic constraints might be critical in modulating the long-term sustainability of this compensatory mechanism.

Understanding how physical attributes of artificial substrates influence ecological processes is essential for the design of habitat-enhancing structures in coastal ecosystems. Here, we examine how structural heterogeneity (flat vs. heterogeneous tiles) and colour (black vs. white) of 3D-printed substrates are associated with the thermal environment and recruitment of two intertidal barnacle species, Notochthamalus scabrosus and Jehlius cirratus. Surface temperature during low tide was quantified using infrared thermography. Black tiles heated significantly more than white tiles, with temperature increases of ∼6–12 °C and 1–4 °C, respectively. However, surface heterogeneity produced measurable thermal buffering, with crevices on black tiles remaining 3–6 °C cooler than adjacent ridges, generating fine-scale thermal refugia whose magnitude varied with substrate colour. Barnacle recruitment was primarily associated with habitat structure, with high recruitment on heterogeneous tiles, especially within crevices where recruitment was two orders of magnitude greater than ridges for both species. Species-specific responses to colour were limited: J. cirratus responded only to microhabitat identity, whereas N. scabrosus exhibited additional sensitivity to substrate colour. These results suggest that structural heterogeneity can mitigate thermal stress by providing stable microhabitats that may enhance post settlement survival and recruitment, even on thermally extreme artificial substrates. Our findings highlight the roles of substrate colour and microtopographic complexity in shaping thermal environments and recruitment patterns and provide a mechanistic basis for disentangling structural and functional components of thermal heterogeneity in artificial substrates under warming conditions.