Figure 2. Effects of CO₂ addition on rates of change in pH (total scale hr⁻¹) and pCO₂ (µatm hr⁻¹) in tide pools were reduced as macrophyte cover increased.

The effect of CO₂ addition was defined for each pool as the rate of change when CO₂ was added minus the rate when CO₂ was at ambient levels. As macrophyte abundance in the pools increased, (A) the effect of CO₂ addition on pH became more positive (R² = 0.45, P = 0.033) and (B) the effect of CO₂ addition on pCO₂ became more negative (R² = 0.43, P = 0.040). Rates of change in pH and pCO₂ in the absence of macrophytes are indicated by the y-intercept of the regression lines (pH:−0.07 ± 0.03 s.e. total scale hr⁻¹; pCO₂: +81.22 ± 33.80 µatm hr⁻¹). Regression lines are means ±95% c.i.