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Abstract

The ecological roles of dissolved organic carbon (DOC) in aquatic ecosystems are widespread although not yet fully understood, but DOC is likely to impact physiological processes in fish. Generally, DOC is supportive to ionoregulation in freshwater fish, especially in acidic or metal contaminated waters, although the exact protective mechanisms and the influence of water chemistry remain unknown. DOCs can be characterized by optical and chemical indices but the effect of pH and their inter-relationships are not well understood. I showed that the indices used to characterize DOC should consider pH and be interpreted carefully as some changes reflect alterations in conformation (i.e. spatial arrangement of the molecule) while others reflect changes in composition (i.e. chemical makeup of the molecule) resulting from proton binding. I also investigated the independent and interactive effects of DOC and pH, major ion concentrations (Na⁺, Ca²⁺) and/or a potential metal pollutant (copper, Cu) on fish. To explore the physiological effects of these environmentally relevant DOC-water quality interactions, fish native to specific habitats with different ionoregulatory strategies were utilized: rainbow trout, Pacific sanddab and rock sole, and the Amazonian dwarf cichlid. There were independent effects of major ions (Na⁺ and Ca²⁺), and Cu as well as interactive effects with DOCs which were altered by a change in pH. Generally, DOC was supportive to ionoregulation against physiological disturbances caused by ambient pH, major ions or copper. The pH-dependent physiological responses to DOC may be due to conformation changes in DOC such as condensation or the titration of the negative net charge on both DOC and the gill leading to more interaction. Natural DOC sources spanning a wide variety of characteristics, together with model compounds of known chemistry representing key chemical moieties of natural DOCs, were used. I demonstrated that specific chemical groups of the DOC molecules (e.g. carboxylic rings, aliphatic chains, and amino acids (i.e. peptides)) likely play a role in the physiological responses to natural DOCs. This thesis highlights that the variability in DOC structure and function is reflected in the physiochemistry of the DOC and in the physiological response of fishes, both of which are pH-dependent.