The dynamics of coral-algal interactions on coral reef ecosystems

Abstract

The interactions between coral and macroalgae strongly influence the structure and function of coral reef ecosystems. Anthropogenic stressors have been shown to disrupt coral-algal dynamics, often shifting competitive advantage in favor of macroalgae. As anthropogenic stressors intensify, altered competitive dynamics between coral and macroalgae are expected to exacerbate ecosystem shifts. However, the mechanisms by which coral and macroalgae compete and their implications for ecosystem dynamics remain poorly understood. By bridging small scale physiological techniques with large scale ecological monitoring, this dissertation explores the ecology and physiology of coral-algal interactions to gain a better understanding of the processes that govern coral-algal competitive dynamics.The first series of experiments of this thesis explored how seasonal fluctuations in environmental parameters influence the relative abundance of macroalgae and how changes in species abundance affect coral-algal composition and interaction frequency. Environmental conditions, benthic community composition, and coral-algal competition were monitored over two years across eight distinct reef zones on the high-latitude Heron reef, southern Great Barrier Reef. By examining reef habitats that had little anthropogenic influences, we were able to detect seasonal and spatial shifts in macroalgal abundance that closely tracked environmental conditions, revealing that temperature and light interact to exert considerable control over the abundance of macroalgae. Macroalgal taxa had disproportionate effects on corals across reef habitats and seasons, with species composition varying distinctly between the lagoon and reef slope, and the greatest frequency of coral-algal interactions at the reef flat and shallow lagoon and in spring and winter. The results of this long-term experiment improve our understanding of how environmental conditions control spatio-temporal changes in macroalgal communities, and how changes in benthic cover influence the frequency and composition of coral-algal interactions.In the next series of studies, we investigated if and to what extent a natural gradient of human population influences the benthic cover as well as the diversity, frequency, and outcomes of coral-algal interactions in the central Maldives. The natural clumped nature of human populations in the Maldives presented a unique opportunity to study the direct effects of human influences where the main local anthropogenic stressors are predominantly sedimentation and eutrophication, not the overfishing of herbivorous reef fish. Reefs exposed to the highest human population pressures showed a reduction in the abundance of particular coral categories, as well as an increase in the abundance of dead hard coral and filamentous algae. Furthermore, the diversity of coral-algal interactions was significantly reduced. Interactions between crustose coralline algae and Halimeda were not only the most common and least harmful to coral, but were also positively correlated with coral cover, emphasizing the role that positive species interactions can play in regulating community structure and function. These results highlight that proximity to local human population can lead to changes in benthic cover as well as changes in the composition and diversity of coral-algal interaction abundance, and add valuable information to the debate on how local human populations influence coral reef ecosystems.The final set of manipulative experiments examined the effects of temporal variability and ocean warming and acidification on organismal physiology and the mechanisms of coral-algal interactions. The metabolic rates of Acropora intermedia and Halimeda heteromorpha were highly dependent on season, both displaying peak calcification and photosynthetic rates under present day conditions in summer. Current projected levels of ocean warming and acidification expected to occur by mid-to-late century resulted in highly negative outcomes for both A. intermedia and H. heteromorpha only in summer, significantly reducing the survivorship, calcification and productivity of A. intermedia and the calcification rates of H. heteromorpha. The effect of H. heteromorpha contact on A. intermedia did not intensify under ocean warming or acidification and included positive, neutral and negative effects on A. intermedia, changing with season and under projected ocean warming and acidification. This study highlights the importance of seasonality in determining the direction and effect of interactions between coral and macroalgae as well as the susceptibility of coral and macroalgae to multiple global change stressors.The results presented here provide novel insights into how interactions between coral and macroalgae vary naturally with location and environmental conditions. Both local and global anthropogenic impacts have the potential to disrupt the processes that govern coral-algal competition, with the direct and indirect impacts of human activities altering coral-algal interactive dynamics by changing species abundance and affecting competitive ability. Macroalgal competitive strength did not increase under ocean warming and/or acidification; instead it was the combined effect of warming and acidification which generated the most harmful impacts on both coral and macroalgal physiology. The current business-as-usual pathway is very likely to have serious impacts on coral reef ecosystems. A reduction in CO2 emissions to levels set in the Paris agreement, in combination with management that reduces the local impacts, are very likely to maintain ecosystem functions and ecological resilience in the face of a changing climate.