The erosion of reef substrates is driven by interrelated physical, biological and chemical processes, which constantly remove calcium carbonate (CaCO3) from coral reef ecosystems. Changes to the rate of such processes may potentially disrupt the ratio of calcification to erosion and the subsequent positive net accretion of this ecosystem. It has been predicted that increases in ocean acidity and temperature will significantly impact the calcification potential of reef calcifiers, yet alteration to persistent destructive processes under future climate scenarios have barely been investigated. This research aimed to understand how the combined impact of warming and acidification (CO2-T) affect biological and chemical erosion of dead reef carbonate substrates, as well as, the growth rates of living crustose coralline algae (CCA) subjected to ecological disturbance. The experimental component of this thesis incorporated past and present seawater conditions along with two elevated CO2-T scenarios predicted by the Intergovernmental Panel of Climate Change (IPCC).
The first data chapter (Chap. 2) investigated the effect of elevated CO2-T scenarios on the microbioerosion of coral skeletons with a further examination of physiological and ecological responses of their endolithic microborers to these environmental conditions. The experiment was performed on two distinct coral taxa. Rates of carbonate dissolution by microborers were increased under elevated CO2-T scenarios but the magnitude of the response varied between the two coral taxa. Predicted scenarios of ocean acidification (OA) and warming promoted changes in biomass, respiration and community structure of photosynthetic microborers, which were subsequently associated to increased dissolution rates. This suggests that alterations in microbioerosion processes may potentially disrupt the carbonate balance of coral reefs ecosystems in the coming decades. The second data chapter (Chap. 3) decoupled the relative contribution of photosynthetic microborers from the environmental effect of seawater carbonate chemistry in the dissolution of CCA skeletons. Remarkably, the presence of photosynthetic microborers reduced the effect of seawater saturation state (Ω) with respect to high-Mg calcite (ΩHMC) in the dissolution of CCA skeletons across all CO2-T scenarios. Nevertheless, the dissolution of CCA skeletons was significantly increased at the business-as-usual CO2-T emission scenario irrespective of the presence/absence of phototrophic microborers. These results suggest that future OA and warming conditions may affect the stability of shallow fringing reefs where CCA skeletons play a major role in cementing the benthic community.
Considering the high susceptibility of CCA to increased ocean acidity and temperature, the final data chapter (Chap. 4) combined field surveys and experimental manipulations to first explore patterns of parrotfish grazing on different CCA species and then to assess whether short vs. longer-term exposure to different CO2-T scenarios, applied in spring, altered regeneration and growth abilities of ecologically disturbed CCA. Shallow and deep grazing scars were inflicted on Porolithon onkodes specimens at the beginning and re-inflicted after 4 weeks of exposure. Field data revealed that the CCA P. onkodes dominate the shallow reef crests and is one of the most frequently grazed species. Experimental data indicated that differences in the depth between shallow and deep scars had an effect on tissue regeneration with shallow scars healing almost entirely in 2 weeks while deep scars only recovered 60% during the same period. Regeneration of grazing scars and calcification of grazed CCA also varied with the exposure time to acidified and warmer seawater. Short-term exposure (2 weeks) to elevated CO2-T treatments did not affect recovery or calcification, but when scars were re-inflicted both responses were significantly reduced at the medium and high CO2-T scenarios in the long-term exposure (6 weeks). Unexpectedly, lateral growth for both grazed and ungrazed specimens was unaffected by elevated CO2-T conditions based on a spring, as opposed to summer baseline. Nonetheless, observed reductions in the regeneration and calcification rates of ecologically disturbed CCA suggests negative impacts on survival and competitive abilities.
This thesis documented the effects of natural and anthropogenic disturbances on processes key to the carbonate dynamics of coral reefs. In particular, it distinguished the erosional role of endolithic algae from that of seawater chemistry and how projected CO2-T scenarios may alter biological and ecological responses of microborers and subsequent carbonate losses in coral ref ecosystems. This research also assessed the effect of future CO2-T scenarios on the growth strategies of living CCA following persistent ecological disturbances. The results suggest that greater rates of erosion, and reduced rates of calcification will threaten the stability of reef framework. Because the deposition and dissolution of main carbonate substrates, corals and CCA, are integral aspects determining the ability of reefs to preserve their three-dimensional structures, this research increases our understanding of how future climate conditions may disrupt the delicate carbonate balance in coral reef ecosystems.