Tracking trends in hypoxia: a freshwater perspective

Abstract

Hypoxia or low dissolved oxygen occurs in both marine and freshwater systems and is a threat to aquatic life. Historically hypoxia arose naturally and intermittently however, anthropogenic influences and climate change have exacerbated its severity and frequency. I define a hypoxic event as one where levels of dissolved oxygen in the water are low enough to cause a reduction in overall fish health based on the most sensitive species in the system. The primary aims of my thesis were to determine the long-term physiological impacts of hypoxia to freshwater fish and develop the science to trace its occurrence through time. I examined the physiological effects and tolerance of fish to long-term hypoxia exposure at different temperatures. Higher water temperatures limit the amount of available dissolved oxygen occurring in water therefore, it was likely that high temperatures in combination with hypoxic conditions would also limit performance. I investigated the effects of hypoxia on three key species from the Murray Darling Basin, Australia, golden perch (Macquaria ambigua), silver perch (Bidyanus bidyanus) and Murray cod (Maccullochella peelii). In my first data chapter I found golden perch had a reduced metabolic scope for activity after long-term exposure to hypoxia, which was also influenced by temperature. Additionally, golden perch exhibited an acclimation response, whereby prolonged hypoxic exposure improved tolerance to low oxygen conditions. However, silver perch, a sympatric species, had a poor tolerance to hypoxia and all individuals died after one month’s exposure. In my next chapter, I investigated if acclimation ability was affected by exposure time (7, 14 and 30 days) for Murray cod. Similarly long-term exposure to hypoxia improved the tolerance of Murray cod suggesting fish had acclimated. However, acclimation was inversely related to exposure time. After documenting the physiological impact hypoxia exposure had on fish, I investigated a means to track its occurrence through time. Otoliths or the ear bones of fish accrete daily layers of material on a calcium carbonate matrix that reflect the environmental conditions experienced by fish. I investigated elemental signatures that represented hypoxic occurrence under controlled conditions. I reared juvenile golden perch under combined differing temperature and oxygen conditions for a month and analysed trace element concentrations in the otoliths. Trace elements measured in the otoliths, however, did not differ among hypoxic and normoxic treatments. By running transects along the otoliths of golden perch and Murray cod from modern and historic collections of fish that either died or experienced a hypoxic event I could reconstruct the long-term occurrence of hypoxic events. Records of hypoxia frequency in most waterways only go back a few decades so this technique could determine hypoxia histories of water bodies that would be unattainable using traditional methods. I highlight that any prolonged exposure to hypoxic conditions benefits individuals’ ability to remain in low oxygen environments longer, and that coexisting fish have species-specific responses. Furthermore, I highlight otoliths acting as natural tracers of hypoxia, such that given the right conditions elements routinely physiologically regulated act as natural tracers for low-oxygen events. Thus, our ability to reconstruct hypoxia through time using otoliths is reliant on the physiological disruption it creates.