Spatial and Temporal Scales of Coral Recruitment and Key Ecological Processes

Over 75 % of Caribbean reefs are considered threatened, and rates of recovery are slow or imperceptible. Post-settlement processes from the time corals settle (i.e., attach to the benthos) to recruitment (i.e., survive to some later phase) determines much of coral demography. As environmental conditions become more hostile to settling corals, understanding the process of successful coral recruitment will be essential to properly steer management efforts. This study had three major components. First, I investigated scales over which coral recruitment operates along a stress gradient on the Mesoamerican Barrier Reef. Second, I studied effects of benthic organisms (at mm-cm scales) on newly settled corals and their survivorship. Third, through experimental manipulation, I tested the hypothesis that parrotfish, as regulators of fore reef algae, improve the recruitment potential of the reef for coral recruitment. Key findings included the discovery of hot (Roatan Banks, Honduras) and cold (Punta de Manabique, Guatemala) spots of coral recruitment on the Mesoamerican Barrier Reef. Whereas rates of recruitment were stochastic along the stress gradient, iii impacts of stress may be manifested in the survivorship of coral recruits. Juvenile coral abundances on natural substrata were largely regulated by algal abundance. Furthermore, recruitment failure may be exacerbated for corals settling into coraldepauperate reefs where succession in nursery microhabitats moves rapidly towards heterotrophic organisms inhospitable to settling corals, such as invertebrate crusts. During year 2 and year 3 of the study, annual recruitment rates declined by over 50 % from the previous year. This suggests that immediately after disturbances that create primary substrate, a “recruitment window” opens for settling corals. With time, the window closes as nursery microhabitats become overgrown by organisms hostile to coral settlement and survivorship. Finally, while Caribbean reefs are infamous for “phase shifts” from coral to macroalgal dominance, not all reefs have shifted to this alternative state. Results from Bonaire illustrate how maintaining “surplus herbivory” is instrumental in avoiding phase shifts. However, vastly depleted grazing resulting from herbivore exclusion cages caused microphase shifts to algae and reduced coral recruitment, suggesting the sensitivity of Bonaire’s reefs to parrotfish exploitation. These data shed light on the importance of locally improving land use and herbivorous fish populations in an effort to enhance recruitment and survivorship of corals