This study showed the effects of ocean acidification on ecosystems at coastal sites where volcanic CO2 vents lower the pH of the water. Along gradients of normal pH (8.1–8.2) to lowered pH (mean 7.8–7.9, minimum 7.4–7.5), typical rocky shore communities with abundant calcareous organisms shifted to communities lacking scleractinian corals with significant reductions in sea urchins and coralline algae. Seagrass production was highest in an area at mean pH 7.6 where coralline algal biomass was significantly reduced and gastropod shells were dissolving due to periods of carbonate sub-saturation.