The Ecology of Turf Algae on Coral Reefs

Coral reefs are one of the most diverse and productive ecosystems on the planet. Globally, corals are declining and algae are increasing due to anthropogenic activities. While most research and conservation efforts focus on the proliferation of macroalgae, in fact most algae on coral reefs are turf algae: heterogeneous assemblages of many species of small algae. These ubiquitous and abundant algal “shag carpets” are typically overlooked or studied as a homogenous functional group. I examine characteristics that make turf algae functionally unique from other types of algae, focusing on their ecological importance as food for reef organisms, major sources of productivity, and competitors with reef-building corals. Further, I argue that turfs will become more abundant in the future because they respond positively to the many anthropogenic threats facing coral reefs, including overfishing, pollution, acidification and warming. First, I explore the spatial patterns in turf assemblages, showing that, although turfs appear visually homogenous to the naked eye, they are highly variable at the centimeter scale. This descriptive work provides a foundation for testing ecological processes that may be causing those small-scale patterns. Second, I use turf algae as a tool to demonstrate that the relative influences of top-down and bottom-up processes are context-dependent. I measured the greatest response to grazer exclusion where background herbivory was lowest, suggesting a positive feedback between the loss of herbivory and loss of resilience. Further, in contrast to high profile findings in other ecosystems, my data revealed that nutrient enrichment increased, and consumers reduced, turf diversity. Finally, I expand the traditional perspective of coral reef herbivores to include small infaunal invertebrates living in turf algae. By partitioning herbivory among fish and small invertebrates, I provide some of the first evidence that infaunal invertebrates significantly impact turf algae, but their role depends on either competition with or predation by fish. Overall, my dissertation provides a new perspective on turf algae and opportunities to test classic ecological concepts. Additionally, a better understanding of the ecology of turf algae has urgent conservation applications, including insight into how reefs of the future will function and how to best protect them