Nitrate pollution is a pervasive threat to freshwater ecosystems worldwide. In Canterbury, New Zealand (NZ), intensive agricultural activities have resulted in large quantities of nitrate (NO3-) leaching into and contaminating freshwater streams and rivers. Exposure to elevated nitrate concentrations can cause adverse effects on aquatic life and is thought to be linked to declines in freshwater fish. To address this issue, the NZ National Policy Statement for Freshwater Management has mandated a new national bottom line of 11 mg L-1 NO3- in rivers (MfE, 2020). However, this limit is primarily based on laboratory-based data collected on non-native fishes, with little consideration of how native freshwater fish species cope with nitrate.

To assess this pressing knowledge gap, I investigated the physiological effects of nitrate exposure on upland bully (Gobiomorphus breviceps), an endemic species inhabiting streams in Canterbury, NZ. The energetic costs associated with living in nitrate-contaminated streams were assessed by measuring fish body condition and organosomatic indices along a nitrate contamination gradient (spanning 3 – 51 mgL-1 NO3-). In the field (Chapter 2). I found non- significant associations between habitat nitrate concentration and fish body condition and organosomatic indices, suggesting living in nitrate-contaminated habitats was not energetically expensive. I further investigated the energetic costs of exposure to nitrate pollution in G. breviceps, under tightly controlled conditions, by exposing fish to one of three ecologically relevant nitrate concentrations in the laboratory (0, 11, or 50 mgL-1 NO3-) for a maximum of 66 days (Chapter 3). Following exposure, aerobic scope (maximum – standard metabolic rate) and performance measures (growth, body condition and organosomatic indices) were measured. Aerobic scope and performance indicators were not compromised in G. breviceps exposed to elevated nitrate concentrations (11 and 50 mgL-1 NO3-), suggesting these fish maintain aerobic performance in the face of nitrate contamination.

Lastly, I investigated the effect of nitrate exposure on the susceptibility of G. breviceps to elevated temperatures, because species are often exposed to multiple stressors in current-day freshwater environments. The upper thermal tolerance limits (measured as critical thermal maxima; CTmax) were measured in G. breviceps exposed to nitrate (0, 11, or 50 mgL-1 NO3-) for 56 days. CTmax of G. breviceps was unaffected by exposure to nitrate. Taken together, the results of this thesis suggest that G. breviceps can maintain growth, performance, and heat tolerance up to concentrations as high as 50 mgL-1 NO3-. The new national bottom line of 11 mg L-1 NO3- appears to be sufficient to protect G. breviceps, however, future studies should assess these limits for other, potentially more susceptible freshwater species using the framework developed in this thesis.