Negative impacts of elevated nitrate on physiological performance are not exacerbated by low pH

Multiple environmental stressors, including nutrient effluents (i.e. nitrates [NO₃^-]) and altered pH regimes, influence the persistence of freshwater species in anthropogenically disturbed habitats. Independently, nitrate and low pH affect energy allocation by increasing maintenance costs and disrupting oxygen uptake, which ultimately results in impacts upon whole animal performance. However, the interaction between these two stressors has not been characterised. To address this, the effects of nitrate and pH and their interaction on aerobic scope and physiological performance were investigated in the blueclaw crayfish, Cherax destructor. Crayfish were exposed to a 2 × 3 factorial combination, with two pH levels (pH 5.0 and 7.0) and three nitrate concentrations (0, 50 and 100 mg L^-1NO₃^-). Crayfish were exposed to experimental conditions for 65 days and growth and survival were monitored. Aerobic scope (i.e. maximal - standard oxygen uptake) was measured at six time points (1, 3, 5, 7, 14, and 21 days) during exposure to experimental treatments. Crayfish performance was assessed after 28 days, by measuring chelae strength and whole animal activity capacity via the righting response. Survival was reduced in crayfish exposed to pH 5.0, but there was no exacerbation of this effect by exposure to high nitrate levels. Aerobic scope was compromised by the interaction between low pH and nitrate and resulted in prolonged elevations of standard oxygen uptake rates. Exposure to nitrate alone affected aerobic scope, causing a 59% reduction in maximum oxygen uptake. Reduced aerobic capacity translated to reduced chelae strength and righting capacity. Together, these data show that low pH and elevated nitrate levels reduce aerobic scope and translate to poorer performance in C. destructor, which may have the potential to affect organismal fitness in disturbed habitats.