Many Caribbean coral reefs are changing from coral to macroalgal dominated. Dead reefs are less able to provide goods and services, such as shore protection, tourism, and seafood production. Consequently, a coral reef dominated by macroalgae will eventually have relatively poor ecological and economic value. Top-down and bottom-up processes influence macroalgal phase-shifts, but the importance of particular drivers and their interactions is not well understood. The lack of knowledge on the synergistic effects of top-down and bottom-up processes implies that valid management efforts may fail in producing desired outcomes, and hence be terminated. For example, when the conservation intervention to prevent a macroalgal phase-shift is to protect herbivores, but the macroalgae that is blooming is controlled by season. The research around macroalgal phase-shifts is intensive, but it is still far from complete.
My thesis takes a holistic approach to explain macroalgal phase-shifts and contributes key information for reversing and preventing them. Chapter 2 investigated patterns of turf gross primary production in reefs with contrasting exposures and along a depth gradient from 5 to 20 m in Glovers atoll. Light was the limiting factor, accounting for the decline in production with depth; water flow was also a modifying driver in shallower sites, generating differences in production across the atoll. Chapter 2 provided the foundations for the following chapters, because thresholds that determine trajectories of benthic dynamics may change depending on productivity. Greater understanding of primary production is important to explain coral reef benthic dynamics.
Chapter 3 considered the effects of seasonality and herbivory on the dynamics of the three macroalgal species that are most often implicated in Caribbean phase-shifts: Lobophora variegata, Dictyota pulchella and Halimeda opuntia. Herbivory was a pivotal process controlling the abundance of L. variegata and H. opuntia, but it did not affect D. pulchella. The abundance of both L. variegata and H. opuntia was correlated with seasonal changes in the environment, but was depleted in treatments that allowed herbivory. Seasonal changes in water temperature and light controlled the abundance of D. pulchella; which increased in summer and decreased during winter. Chapter 3 highlights the importance of incorporating seasonality to fully understand ecological processes and drivers around macroalgal phase-shifts.
Chapter 4 used a manipulative experiment to evaluate the effect of macroalgal competition on the growth rate of corals with an emphasis on colony size, species identity, and intensity of competition. Coral-algal competition was studied for one year between three coral species (Porites astreoides, Agaricia agaricites and Colpophyllia natans) and two macroalgal species (Lobophora variegata and Halimeda opuntia). Two coral colony sizes (10 cm and 30 cm in diameter) were used and, for the small corals, two levels of competitive intensity (25% and 100% contact with the coral perimeter). Coral size had the greatest impact on competitive outcome; two species of large corals grew when in competition and the third species did not loose tissue. All small colonies shrunk after a year of macroalgal competition. The competitive outcome did not differ between algal species, but it did vary among coral species, C. natans being the most susceptible to macroalgal competition. The presence of algae was detrimental to coral, but there was no difference between competitive intensity levels of 25% and 100% among small corals. Chapter 4 provides further empirical support to the theory that algal blooms can inhibit coral population dynamics by causing a bottleneck in the survivorship of smaller size classes. Chapter 4 identified a trait (size) that confers corals resilience against macroalgal competition, while Chapter 5 identified habitat features that provide greater resilience to reefs against macroalgal phase-shifts.
Chapter 5 assessed the efficacy of 19 habitat features to predict the spatial distribution of grazing. Empirical data was used to model the spatial distribution of grazing within territories of two common parrotfish with contrasting grazing strategies: Sparisoma viride and Scarus iseri. The feeding rate of both species intensified near cleaning stations. Structural complexity significantly influenced the feeding rate of both species but in opposite directions, negative for S. iseri and positive for S. viride. Counter intuitively; the number of damselfish, which aggressively defend their territories against herbivores, increased the probability of bites by S. viride. The percent of grazable surface area increased the bite probability by S. iseri. Chapter 5 highlights the importance of key habitat features on coral reefs’ benthic dynamics.
These results can significantly improve predictions on the response of modern coral reefs to phase-shifts by incorporating spatial and temporal changes in primary productivity and seasonality, identifying coral size thresholds, considering cleaning stations and ascertaining how small-scale changes in substrate complexity influence grazing. These results are being incorporated into simulation models of tropical ecosystems to predict the recovery of coral reefs following large-scale disturbances. The data from the models enables the adequate level of management and protection of coral reefs, building the ecosystems’ resilience and allowing their maximum recovery in the shortest period possible.