title: Paying for Oxygen: The Mechanics and Ecological Consequences of Breathing Water
Creators: name: Leighton, Garrath
Publisher: name: University of Liverpool Repository
Year: 2024

description: The environmental impacts on oxygen availability in aquatic ectotherms remains poorly understood and ecologically important. Knowledge is required of not only the physical factors determining rates of oxygen movement from the environment to tissues, but also its energetic costs; the difference between these measurements is the aerobic energy available for all other biological functions. The chapters of this PhD explore oxygen movement and its energetic costs simultaneously, applying principles of mass transfer theory and energetics of oxygen movement under different environmental and biological conditions, including temperature change, environmental hypoxia, and increasing body size. 

After introducing the thesis background in Chapter 1, Chapter 2 introduces principles of mass transfer theory, applied to oxygen movement at gills of amphipod crustaceans, before demonstrating theoretically how thermal sensitivity of oxygen availability can be estimated, relative to the aerobic costs of oxygen movement. Predictions are made of temperature-induced body size change in amphipods, which are then compared with observed decreases in maximal body length with increasing temperature in marine amphipods, demonstrating a strong positive correlation. These results provide support for the theory that the maintenance of aerobic scope (the oxygen bioavailability above that used when at rest) drives body size responses to temperature. 

Chapter 3 develops a dynamic model of oxygen movement at fish gills, using mass transfer correlations relevant to the different phases of oxygen movement at the respiratory surface (water, membrane and blood phases). Concepts that are theoretically developed include the effect that the haemoglobin-oxygen dissociation curve has on resistance to oxygen movement, and increases in functioning gill surface area for oxygen uptake. By incorporating the energetics of both water and blood flow through the gills, predictions can be made not only of the rates of oxygen movement in different environmental and exertional scenarios, but also of its associated aerobic costs. 

Chapter 4 applies the model developed in Chapter 3 to a previous study of the predatory teleost dourado, exposed to graded hypoxia. Two of the principal model outputs (rate of oxygen movement and arterial partial pressure of oxygen) are used to compare model output with observed data. A key outcome is that, despite dourado apparently maintaining aerobic metabolism with falling environmental oxygen (oxyregulating), aerobic energy available for tissues not involved in oxygen movement falls, due to rising costs of gill ventilation, until a critical restriction causes anaerobic metabolism, which is then linked to a failure to increase the diffusing capacity for oxygen proportional to the declining partial pressure gradient of oxygen, leading to a fall in whole-organism metabolic rate. Thus this modelling provides a mechanistic explanation for the failure of oxyregulation that occurs at a critically low environmental partial pressure of oxygen (pcrit).

Chapter 5 uses the theoretical upper limit to metabolic rates brought about by the escalating cost of oxygen movement, developed in Chapter 2. The constraints of this metabolic upper limit are applied to the body mass-dependence of maximal metabolic rate in active-ventilated teleosts. Equations are developed for the body mass scaling of maximal metabolic rate (MMR) and resistance to gill water flow, based on key gill dimensions, before predicting a fixed relationship between the mass scaling of secondary lamellar length, spacing and the efficiency of muscles of gill ventilation. Interspecific analysis of gill dimension-body mass scaling data across 14 studies demonstrates no significant difference between predicted and observed secondary lamellar length and spacing relationships. No limit to MMR-body mass scaling is identified in these species because expected 2-D space filling limits to the increase in gill cross-section with body mass can apparently be overcome. 

Chapter 6 discusses implications of the research, given the multiple stressors facing aquatic environments.