Coral reefs, some of the most diverse ecosystems in the world, face increasing pressures from global and local-scale anthropogenic stressors. Nutrient enrichment and sediment loading could make corals more vulnerable to global climate change by suppressing carbonate productivity and reducing their photosynthetic capacity. Yet, some nutrient-enriched reef environments have exhibited higher coral growth rates, coral productivity, and coral thermal performance. To properly inform management in coastal coral reef ecosystems, a better understanding of warming and land-based inputs (i.e. sedimentation rates and nutrient loading) on coral reef ecosystems is necessary. This thesis is comprised of two studies that address the ecological and physiological influence of land-based inputs on branching coral species to thermal stress. The first study tested how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including algal endosymbiont and coral host response variables, holobiont metabolic responses, and percent cover of Pocillopora acuta colonies on the north shore fringing reefs in Mo'orea, French Polynesia. Algal endosymbiont % nitrogen content, algal endosymbiont densities, and total chlorophyll a content increased with nutrient input, while algal endosymbiont nitrogen content cell-1 decreased, likely representing competition among algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and rate processes, and the percent cover of P. acuta colonies decreased by nearly two orders of magnitude along the nutrient gradient. The second study focused on the direct effects of nutrient enrichment on coral thermal performance on a fore reef in Mo’orea, French Polynesia. In this study, we measured how long-term nutrient enrichment affected coral physiology, including algal endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies next to and ~20m away from a nutrient-enriched reef environment. Algal endosymbiont densities, total chlorophyll a content, and tissue biomass were higher at the nutrient-enriched site, while algal endosymbiont nitrogen content cell-1 was higher at the control site, likely representing competition among algal endosymbionts. Nutrient enrichment increased the metabolic responses of corals to thermal stress in terms of their performance and rate processes on the fore reef. These findings illustrate that the relationship between land-based inputs, such as nutrient and sediment loading, and coral physiology are dependent on local environmental context. Together these results help us better understand how local-scale anthropogenic stressors influence coral performance and can inform coral reef management, particularly on where to mediate the influx of land-based inputs into coastal coral reef ecosystems.
Thesis or Dissertation
Department of Biology, CSU Northridge