The lowest measured pH was 6.0 and was found in the very high carbon dioxide group (Table 1). Ca, Mg and labile Al concentrations were low in all groups (Table 2). Mean gill Al concentration was below 8.0 μg Al g−1 dw for all the experimental groups (range 6.5–7.9 μg Al g−1 dw in Table 2), and the five means were not significantly different.
Plasma chloride decreased with increasing carbon dioxide concentration both on days 9, 20 and 37 for (Fig. 1a, b; pb 0.01; day 20 not presented). This linear relationship was established after only 2 days of exposure (Fig. 1a) and was maintained throughout the trial period (Fig. 1b). The mean reduction in plasma chloride for all three sampling days was 0.65 mM/(mg L−1 CO2). SGR decreased with decreasing plasma chloride concentration (pb 0.05). The relationship was described by a second-order polynomial model (Fig. 2).