Simulation of acid-base condition and copper speciation in the fish gill microenvironment

pH, alkalinity, and mucus content in the fish gill microenvironment of carp (Cyprinus carpio) were measured by exposing fish to copper at various wate r pH levels using an apparatus which separates inspired acid expired water. The relationship between pH levels inside and outside of the gill microenv ironment, between pH and alkalinity, and between mucus secretion, pH, and c opper exposure concentration were modeled. Copper speciation in the surroun ding water and in the fish gill microenvironment was simulated using MINTEQ A2 chemical equilibrium calculation software. The results of the modeling f or pH, alkalinity, and mucus calculation were then adopted as inputs for pu rposes of parameter identification in the speciation modeling. The differen ces observed in the copper species distribution between that of the fish gi ll microenvironment and the surrounding water were based on the speciation modeling. The change in copper bioavailability for fish uptake was also exa mined. The results indicate the presence of an experimental pH balance poin t at 6.9, where the pH in the fish gill microenvironment is identical to th at of the surrounding water. The observed deviation range in pH levels betw een that found at the gills and that of the surrounding water varied from - 0.4 to 0.8 units. A sinusoidal model was developed for calculation of gill pH based on the pH of the surrounding water. Models calculating alkalinity either in the gill microenvironment or in the surrounding water and for est imating mucus secretion were also developed. The results of the chemical eq uilibrium calculations demonstrate that, within a pH range of 6-9, the domi nant species of copper in bulk solution shifted from free ions to that of t he hydroxo complex. With respect to the fish gill microenvironment, the dom inant species found under acidic conditions were the mucus-copper complex a nd free ions. Because of the influence of mucus complexation and pH change, bioavailable copper species in the fish gill microenvironment were signifi cantly lower than that in the bulk solution, especially under acidic condit ions. (C) 2001 Elsevier Science Ltd. All rights reserved.