The acute osmoregulatory effects of low <scp>pH</scp> and Cu, alone and in combination, on the dwarf cichlid (<scp><i>Apistogramma agassizii</i></scp>) in Rio Negro blackwater and Rio Solimões whitewater: Flux rates of ions, nitrogenous wastes and wa

Increases in anthropogenic activities in the Amazon have led to pollution from trace metals, including copper. Dissolved organic carbon (DOC) is known to protect against metal toxicity and ionoregulatory disturbances in Amazonian fish, particularly at low pH. However, little is known about the effects of DOC and trace metals, such as copper, on the branchial water transport pathways. Water moves across the gills of fish through two distinct pathways: paracellularly through tight junctions and transcellularly by diffusion through aquaporins. In the present study, we evaluated the effects of copper (nominally 200 μg L^-1) on diffusive water flux rate (transcellular water movement), paracellular permeability ([³H]-polyethylene glycol-4000 clearance), ion balance (net sodium, potassium and chloride fluxes) and nitrogenous waste (ammonia and urea) excretion in the dwarf cichlid, Apistogramma agassizii. Exposures were conducted in control water (low ions, very low DOC), in filtered Rio Negro (RN) blackwater (low ions, high DOC) and in filtered Rio Solimões (RS) whitewater (higher ions, intermediate DOC) at pH 7 and pH 4. Copper increased ion losses in control water, especially at low pH; RN water protected against these effects, whereas RS water did not, reflecting greater complexation of free Cu²⁺ ions by RN DOC. Our results are the first to show that copper tends to inhibit urea-N excretion as well as ammonia excretion, and also decreases branchial water transport both transcellularly and through tight junctions. The protective effects of DOC against the disturbances caused by copper were dependent on the source of the DOC and the water pH.