The impacts of multiple stresses on the replenishment of coral communities (PhD Thesis)

Abstract

Coral reefs are threatened by multiple stresses and have been suffering severe degradation world-wide. Replenishment processes are thus important to the resilience of coral reefs and their ability to recover from human and natural disturbances. However, there is a lack of knowledge on the effects of multiple stresses on the replenishment of coral communities. This thesis focuses on the impacts of fishing, sedimentation and hurricanes on coral communities, juvenile coral assemblages, their growth and mortality, and coral larval settlement. The study was conducted on reefs on the west coast of St. Lucia, West Indies. Several no-take marine reserves in the study area provided sites with exposure to different levels of fishing. Rivers caused sediment input along two coastal stretches. A tropical storm and hurricane caused sediment runoff and reef destruction prior to and during my research. Although chronic sediment input was continuous throughout the study period, the greatest coral mortality occurred after the storm and hurricane events. However, especially in deeper water on reefs close to the river mouths, sedimentation was responsible for a steady decline in coral cover and inhibited reef recovery. Generally, coral losses were immediately replaced by algal cover. Juvenile coral numbers, diversity and growth rates were negatively affected by sedimentation. Also juvenile species composition was modified by sediment input. Coral larval settlement increased with decreasing sedimentation on both natural and artificial substrata. Juvenile coral mortality was lowest on reefs with intermediate levels of sediment input. Herbivorous fish may increase coral settlement and post-settlement survival by grazing on algae, but their role on these reefs remains unclear. Even in marine reserves, where herbivorous fish stocks tripled in biomass over the study period, macroalgal cover was not affected. Longer-term studies are needed to investigate whether marine reserves are successful management tools in reversing phase shifts between coral and algal dominated states. In addition, land-based management is required to reduce sources of sediment which as this study shows are detrimental to the replenishment of coral populations