Nagelkerken, IvanRavasi, Timothy (Okinawa Institute of Science and Technology)Cipriani, Vittoria2025-05-072025-05-072024https://hdl.handle.net/2440/144391In the face of rapid climate change, marine organisms are undergoing profound transformations, from individual to population levels. Ocean warming and acidification will influence their fitness, ecological interactions, and community-level responses. However, there is limited empirical evidence of how wild fish communities may respond under future climate change. To address these research gaps, I explored the direct and indirect effects of ocean warming and ocean acidification, both in isolation and in combination. This thesis introduces a novel approach by integrating multidimensional niche and mechanistic models with natural analogues for future ocean conditions, incorporating species interactions and indirect effects such as habitat and food changes alongside direct environmental impacts on fish. By using natural CO2 vents, I examined ocean acidification’s effects on niche modifications within a temperate rocky reef fish assemblage. I revealed that plasticity and generalist behaviour are crucial for fish species' success amid changing environments (Chapter 2). Fish adapted to ocean acidification likely modifying their niche following climate-driven habitat simplification, marked by a transition to a turf algae-dominated system. Ocean acidification can directly alter fish ecological niches through behavioural modifications, and indirectly through habitat simplification and food webs alterations, modifying interactions among fish assemblage members. The observed niche plasticity suggests a potential adaptation strategy for fishes to thrive in a high CO2 environment. These traits may enable generalist fish species to endure climate change, as discussed in Chapter 4. Using natural analogues for ocean acidification and warming in a temperate reef at the cold range edge of a group of range-expanding tropical reef species, I revealed that habitat plasticity or preference for projected future habitats (i.e. turf), allowed some fishes to increase, maintain, or partly reduce their population size. In contrast, species with low plasticity experienced complete populations collapse (Chapter 4). The indirect climate effects through habitat phase shift appear to play an important role in determining fish community structure. Species with flexible habitat dependency are more likely to persist in simplified ecosystems under climate change. Further, by analysing climate-change stressors in isolation and in combination, in a natural setting, I reveal that elevated temperature poses a more significant threat to fish persistence than ocean acidification alone (Chapter 3). Using a Dynamic Energy Budget (DEB) model, I showed that ocean warming negatively affects the reproductive output of a small temperate reef fish species, but ocean acidification can mitigate these negative effects by indirectly enhancing its fitness. This thesis provides empirical evidence that ocean acidification could buffer the detrimental effects of ocean warming on the reproductive output of temperate reef fish species and their adaptability. As climate-driven poleward shifts occur, ocean acidification may slow the pace of range contraction in generalist temperate species, able to capitalise on resources. Overall, species can enhance their fitness under climate change by employing coping strategies involving physiological and behavioural adjustments. In the face of warming and acidifying oceans, a multidimensional framework, using natural analogues can provide mechanistic insight, enhancing our understanding of species sensitivity and their capacity to acclimate to environmental changes.enclimate changenatural analogueniche plasticityocean acidificationocean warmingDEBThesis