| Summary: | Ocean acidification is the lowering of oceanic pH that has resulted from the increase of CO2 in the atmosphere. pH is a dominant factor driving the dissociation of inorganic carbon in the ocean and because the pH is lowering, the concentration of the inorganic carbon forms is changing. It is predicted the concentration of oceanic CO2(aq) will increase by around 200%, and the concentration of CO32- will decrease by 56% by the end of this century. CO2 is an energetically cheap carbon source for photosynthesis compared with the other highly abundant inorganic carbon form used for photosynthesis: HCO3-. Increased CO2 availability and lower concentrations of CO32- could influence macroalgal growth and lead to changes in macroalgal community structure. Before we can predict how macroalgae will respond to these changes in their carbon supply, we require a better understanding of the utilisation of inorganic carbon and the extent of carbon limitation in macroalgal communities. Field community surveys and stable isotope measurements showed macroalgal species at wave-exposed sites utilise a higher proportion of CO2 than wave-sheltered species, and there are more CO2-only using species present at wave-exposed sites. The higher CO2 availability caused by thinner diffusion boundary layers at wave- exposed sites compared with wave-sheltered sites is likely to drive these changes in carbon usage. In laboratory experiments, carbon uptake kinetics were examined for two common species of seaweed, Xiphophora gladiata (Labillardière) Montagne ssp. novae-zelandiae Rice and Hymenena palmata (Harvey) Kylin, under water motion (high and low) and pH (8.1 and 7.6) treatments. Results showed that water motion had a greater impact on carbon-uptake than pH. A 6-week laboratory experiment, in which early successional macroalgal communities were grown at two pH treatments (8.1 and 7.6), revealed that these coralline and diatom dominated communities are tolerant to ocean acidification and are able to grow, however, there was reduced growth for calcifying algae at pH 7.6. This project has shown that greater CO2 concentrations could positively influence photosynthesis in some species of fleshy macroalgae by reducing carbon limitation, however, calcifying algae are vulnerable to the oceanic chemistry changes caused by ocean acidification. These varying responses among species and the variability of communities under different levels of water motion is likely to lead to communities responding to ocean acidification at a local scale.
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