Symbiotic scleractinian corals (corals) are mixotrophs that can obtain nutrients through photosynthesis (autotrophy) by the endosymbiotic algae Symbiodinium (symbiont) and assimilation of particulate organic matter (heterotrophy) by the coral animals (host). Nutrients are also efficiently exchanged and recycled between the symbiotic partners. The dynamics of coral trophic strategies are species-specific and are influenced by a suite of environmental factors, particularly light and nutrient availabilities, which vary spatially and temporally. In this thesis, I explored how the coral trophic strategies and nutrient sources vary spatially across Hong Kong’s unique west-east environmental gradients of water quality created by Pearl River (PR) freshwater discharges. I measured the stable iso...[ Read more ]

Symbiotic scleractinian corals (corals) are mixotrophs that can obtain nutrients through photosynthesis (autotrophy) by the endosymbiotic algae Symbiodinium (symbiont) and assimilation of particulate organic matter (heterotrophy) by the coral animals (host). Nutrients are also efficiently exchanged and recycled between the symbiotic partners. The dynamics of coral trophic strategies are species-specific and are influenced by a suite of environmental factors, particularly light and nutrient availabilities, which vary spatially and temporally. In this thesis, I explored how the coral trophic strategies and nutrient sources vary spatially across Hong Kong’s unique west-east environmental gradients of water quality created by Pearl River (PR) freshwater discharges. I measured the stable isotope values (δ¹³C, δ¹⁵N, and δ³⁴S) of coral and symbiont tissues belonging to Acropora, Pavona, Platygyra, and Turbinaria collected along the environmental gradients during the wet season when PR influences peak. Statistically, I analyzed the isotope data to delineate their spatial patterns and linked those patterns to water quality parameters extracted from Hong Kong’s Environment Protection Department. The results suggested that the contrasting west-east environmental conditions (light and nutrient availabilities) affected the coral’s trophic strategies, with Platygyra and Pavona employing more flexible while Acropora and Turbinaria employing stricter trophic strategies. The data also suggested high spatial variabilities in nutrient sources subjected to PR influences and biochemical processes. In this thesis, I also proposed a novel Coral Heterotrophy Index (ΔCN) and proved it useful in capturing the shifts in coral trophic strategies and degrees of trophic plasticity. This thesis revealed complex responses of the coral host and their associated symbionts to the multiple nutrient sources and different environmental settings across sharp environmental gradients. Finally, I identified directions for future research endeavors, including considering temporal dynamics, applying more advanced compound-specific stable isotope analyses (CSIA), and repeating these approaches to study coral trophodynamics across “vertical” environmental gradients from shallow-water coral reefs to mesophotic coral ecosystems (MCEs).