Effect of ocean acidification on reef-building corals: understanding variability and projecting population-level impacts

Understanding the long-term ecosystem level impacts of ocean acidification on marine environments is critical to informing national and international policies on carbon emission targets. However, key to understanding such long-term impacts is the ability of projection models to scale up short-term physiological responses to long-term ecosystem-level impacts. This thesis aims to improve upon previous models projecting the impacts of ocean acidification on coral reefs by incorporating the effects of ocean acidification on every stage of the coral life cycle, and also by deepening our understanding of how flow potentially influences the effects of ocean acidification on calcification. Experimental studies investigating the effects of ocean acidification on calcification have produced a wide range of responses. However, as yet there have been no attempts to produce a summary response that can be used in projection models. Thus in Chapter 2, I use regression-based meta-analysis to produce a quantitative summary of the effect of ocean acidification on calcification from all existing experimental studies. I also test several factors that may explain significant amounts of variability in experimental results so far. The effect of ocean acidification on calcification was found to be less sensitive than originally thought, ~15% per unit change in aragonite saturation state (Ω(Arag)). I also found that studies employing buoyant weighting found significantly smaller decreases in calcification per unit Ω(Arag) (~10%), compared to studies using the alkalinity anomaly technique (~25%). Despite recent studies suggesting that coral recruitment (when including the effects on crustose coralline algae) is very sensitive to ocean acidification, projection models to date have yet to take into account the effects of ocean acidification on pre- and post-settlement stages of corals. In Chapter 3, I used the quantitative summary from Chapter 2, combined with a similar meta-analysis of recruitment, in an integral projection model to evaluate the effects of ocean acidification on growth and recruitment on long-term population growth. I found that the effects of ocean acidification on recruitment potentially exert more influence on long-term population growth rate than the effects of ocean acidification on growth, although there is substantial uncertainty associated with recruitment-mediated effects, due to the comparative paucity of studies of effects of ocean acidification on this aspect of the life cycle. The meta-analysis of the effects of ocean acidification on calcification also revealed that among-study variation is large. It had been suggested that interactions between ocean acidification and other factors might account for a proportion of the variability between experimental results. One potential interacting factor that has not received any attention is flow, despite its long recognized role in shaping reefs through its influence on mass-transfer rates. In Chapter 4, I show, using flume experiments, that flow, through its effects on photosynthesis, mediates the effect of ocean acidification on calcification of Acropora secale. The interactive effect is large, with the sensitivity of calcification to decreasing Ω(Arag) increasing by ~0.5% per cms⁻¹ increase in flow. To elucidate the mechanisms behind the flow-ocean acidification interaction, I then use an experimental micro-sensor study to parameterize a basic diffusion-reaction-uptake model (Chapter 5). The model predicted tissue surface pH well and showed that low flow, through thickening of the diffusive boundary layer, increases tissue surface pH. However, these elevations in tissue surface pH have been found at relatively low flows that are rarely encountered by corals in nature, suggesting that the DBL effect is unlikely to ameliorate the decreases in coral calcification under ocean acidification. The overarching aim of my thesis was to improve upon previous models projecting the impacts of ocean acidification on coral reefs. This is achieved both by incorporating the effects of ocean acidification (from all existing experimental studies) on every stage of the coral life cycle into projections, and also by deepening our understanding of an important interactive factor (flow) that is driving variability in sensitivity of calcification to ocean acidification. In a broader context, this thesis provides a template for quantitatively summarizing existing knowledge of how demographic rates change in response to a stressor, and a modelling framework that can be used to assess the impacts those changes on population growth and stability