Joint effects of elevated copper and temperature in juvenile Tambaqui exposed in black and white waters of the Amazon

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.