Abstract
This study aimed to investigate how exposure to elevated water temperature and metal concentration jointly affect the physiology of Amazonian fish. Aboard a research vessel in the Amazon, we evaluated the effects of water temperature (river T°C at 31.5°C and a + 4°C increase to 35.5°C) and of 3-h copper (Cu) exposure (up to 600 μg/L) in juvenile Tambaqui (Colossoma macropomum) exposed in freshly collected Rio Negro ('black water') and Rio Solimões ('white water') waters. In Cu-free water, the +4°C raise accelerated physiological Na(+) influx and efflux rates, but only in Rio Negro water. Temperature had no effects on the other physiological fluxes (Cl(-), K(+) and ammonia fluxes). Cu exposure led to net losses of Na(+) (via increased efflux), Cl(-) and K(+) and decrease in acute upper thermal tolerance (CT(max)). These Cu effects were more prominent in Rio Negro water, where Cu bioavailability was the greatest. The +4°C change had no effect on gill Cu accumulation and, overall, there was limited evidence that warming worsened Cu-induced ionoregulatory disturbances. However, in Rio Negro, as Cu and heat both separately promoted Na(+) net losses, fish Na(+) balance was the most compromised in the presence of the two stressors. Altogether, the impaired thermotolerance and ionoregulation under combined Cu and heat exposures suggest a cumulative physiological interaction between two stressors that are increasing threats to the Amazon basin.