| dc.description.abstract | Global climate change is causing warming of the world’s oceans as well as reductions in O2 levels and increases in CO2. Concern has been raised that interactions between these three factors may be non-linear, meaning that their combined impacts cannot be predicted from responses to an individual factor. Therefore, multi-factor studies are needed to quantitatively assess the interactive effects of these three variables to improve predictions of how species will respond to climate change. This thesis investigates the physiological impacts of combined changes in temperature, O2, and CO2 on acid-base regulation, aerobic respiration, and tolerance limits of fish. In chapter II a review of the primary literature is conducted to identify studies in which interactions between combinations of at least two of temperature, O2, and CO2 on the biology (physiology and behaviour) of marine fish were investigated. This review highlights where interactive effects are most commonly observed, considers possible explanations for the high variability between studies in the types of interactions observed, and proposes future work to fill key knowledge gaps and enhance prediction of population level responses. Chapter III examines the time scale of acid-base regulatory responses in European sea bass (Dicentrarchus labrax) acutely exposed to high CO2. Sea bass showed one of the fastest acid-base regulatory responses observed in any fish species enabling it to rapidly restore blood O2 carrying capacity (within 40 minutes at 10,000 µatm) after initial blood acidosis caused a ~3-fold decrease in haemoglobin’s affinity for oxygen. Rapid acid-base regulation capacity in sea bass may be a result of adaptations to natural environmental variability in ecosystems they inhabit or to physiological challenges related to their ecology. In nature CO2 increases when O2 declines – as such chapter IV investigates the impacts of environmentally realistic rises in CO2 on responses to hypoxia in sea bass. Sea bass exhibited greater hypoxia tolerance (~20% reduced O2crit), associated with increased haemoglobin-O2 affinity (~32% fall in P50) of red blood cells, when exposed to reciprocal changes in O2 and CO2. This result suggests that increasing environmental CO2 during hypoxia facilitates increased O2 uptake by sea bass in hypoxic conditions. Consequently, considering environmental CO2 may be vital to accurately assess impacts of hypoxia on fish species more generally. Chapter V examines the potential for acutely increased CO2 to affect critical thermal maximum (CTmax) of rainbow trout (Oncorhynchus mykiss) via either respiratory acidosis or by limiting O2 supply capacity. Trout were exposed to combinations of high CO2 (~10,000 µatm) and hyperoxia (~42 kPa = 2x atmospheric) before blood acid-base chemistry, Hb-O2 affinity, aerobic scope, and finally CTmax were measured. Despite large changes in acid-base chemistry, pH, and aerobic scope between treatments we saw no impacts of any treatments on CTmax of trout. These results suggest that mechanisms determining CTmax of trout are independent of blood pH or O2 and that combined changes in environmental O2 and CO2 are unlikely to affect critical thermal tolerance limits of fish. Furthermore, it provides evidence that processes such as disruption of lipid membranes may be the true mechanism by which upper critical thermal limits are set. Lastly, chapter VI investigates interactive effects between combined changes in temperature, O2, and CO2 on aerobic performance of European sea bass. Fish were exposed to combinations of temperature (14, 18, or 22°C) and CO2 (~400 or ~1000 µatm) for at least two weeks before we measured aerobic scope at four O2 levels. Combined impacts of temperature and O2 resulted in a negative synergistic interaction on aerobic scope so that hypoxia had greater effects on aerobic scope at warmer temperatures. In contrast CO2 showed no interactive effects with either temperature or O2, either individually or in combination. This result suggests that responses of fish to climate change will primarily be a result of interactive effects of changes in environmental temperature and O2, not CO2. This thesis demonstrates the complexity of multi-factor interactions between temperature, O2, and CO2 on the physiology of fish species. Results from each study improve knowledge of when and how interactive effects of combined environmental changes on respiratory physiology might occur. Ultimately, these findings will contribute to the growing evidence base which will enable more accurate predictions of the impacts of climate change on fish species. | en_GB |
