Flux measurements as indices of H⁺ and metal effects on freshwater fish

The gills are the principal site for uptake and losses of major body electrolytes, acid-base equivalents, ammonia, respiratory gases, and many waterborne contaminants in freshwater fish. Measurements of these fluxes offer a powerful tool for assessing the toxic effects of extreme water pH's and dissolved metals. Advantages of the flux approach over conventional methods based on blood or tissue sampling include: (i) 10-400-fold greater sensitivity; (ii) non-invasiveness; (iii) simplicity and applicability to field work; and (iv) measurements made at the site of toxic action (the branchial epithelium). Limitations of the approach include: (i) changes in water quality during the flux determination; (ii) extra-branchial routes of flux; and (iii) adsorption and precipitation phenomena leading to artifactual flux values. Current ideas on the mechanisms of O₂, CO₂, Na⁺, Cl^-,Ca²⁺, K⁺, acid-base equivalent, and ammonia fluxes across the gills are reviewed, together with the actions of low pH, high pH, and metals (Al, Cu, Cd, La, Zn, Hg, Mn) on these fluxes. Recent kinetic analyses of flux relationships are described which provide new information on the mechanism(s) of toxicant action.