Coral reef status in Mauritius: Historic trends and recent perturbations

Coral reefs are under pressure from numerous natural and anthropogenic stressors that are changing drastically the structure and functioning of these important ecosystems. This dissertation investigated impacts of human development, and an invasive sponge species on the benthic community structure of fringing coral reefs around the Mascarene island of Mauritius, in the southwestern Indian Ocean. The research used historical datasets, field surveys, and laboratory manipulations to understand the tempo and scale of changes in Mauritius. The dissertation is divided into three chapters: Chapter 1 examined a 13-year dataset on benthic community cover from 22 sites around Mauritius. The goal was to not only investigate the spatiotemporal trends in the benthic community structure and stability, but to also discriminate between the effects of four local and four global stressors on the coral reef communities. The results showed that there was a general decline in total live coral cover at many sites, both on back- and fore-reefs. However, most sites did not experience a phase shift to macroalgal dominance. Sea surface temperature (SST) and tourism were the two most important stressors affecting community structure. Physical location with respect to oceanographic conditions was another important factor structuring the benthic communities. Unlike the back-reef sites, the fore-reef sites did not appear to respond to any of the local stressors, but they were still sensitive to SST. Chapter 2 addressed aspects of the autoecology of Terpios hoshinota, a sponge likely to be a recent arrival in Mauritius. T. hoshinota is an encrusting sponge and a fierce space competitor. It is a poorly studied organism that kills stony corals by overgrowing them, and can impact reefs on the square kilometer scale. This is the first documentation of this sponge in Mauritius (published in Marine Biodiversity - Elliott et al. 2015). The success of T. hoshinota as in invader on coral reefs has been attributed to its relationship with its symbiotic cyanobacteria. An ex-situ experiment using light and dark treatments was performed to investigate the photosymbiotic relationship between the sponge host and its cyanobacterial symbionts. The results showed that variation in weight change (growth rate) of the sponge hosts was not driven by the abundance of the cyanobacteria at least in the short term. Chapter 3 investigated an outbreak of T. hoshinota at one lagoon in Mauritius to determine its impacts on coral reef community structure (published in Coral Reefs - Elliott et al. 2016), and complemented the historical investigation of decadal change in Chapter 1. The results showed for the first time a statistically significant preference by T. hoshinota to overgrow the stony coral Acropora austera. T. hoshinota also had a higher photosynthetic capacity than A. austera, a possible explanation for its high spreading rate. The long-term implications of T. hoshinota's proliferation on coral reef community structure are discussed. To conclude this dissertation, recommendations are provided to improve conservation management strategies for coral reefs in Mauritius.