Investigating variation in coral trophic dynamics using stable isotope analysis and implications for nutrient pollution source monitoring

Abstract

Coral reefs host the highest species richness of any marine ecosystem, and ecologists have long struggled to explain the coexistence of such diverse community assemblages. Competitive exclusion explains how competition for resources applies evolutionary pressure on organisms to expand or shift ecological niches, avoiding competition and increasing fitness. Ecologists have argued that niche partitioning cannot be fine enough to support the high biodiversity on coral reefs, however this may be due our limited ability to measure the trophic niches of corals. Corals maintain complex trophic dynamics: they feed on plankton, they maintain a nutritional endosymbiosis with Symbiodiniaceae algae allowing them access to phototrophic nutrients, and they associate with diverse bacterial and fungal assemblages that can fix carbon and nitrogen. This multitude of pathways confounds coral trophic niche definition. Further, a clear understanding of coral nutrition is necessary to accurately interpret coral stable isotope data used to identify nutrient pollution sources. In this thesis, I used stable isotope analysis (SIA) of corals to identify the source of nitrogen inputs into coastal seas as well as compare the nutrition of coral species. First, I used gorgonian soft corals from Barbados to map δ15N values and evaluate the impact of sewage discharge and mitigation efforts across space. Further, I extracted material from annually secreted growth rings in the skeletons of these corals to reconstruct δ15N values, and therefore the magnitude of sewage pollution, in the past. Additionally, I used the δ15N values of the algal symbionts of a Myanmarese hard coral to evaluate the impacts of sewage and agricultural fertilizer. Next, I used SIA to characterize the relative size and placement of the isotopic niches of soft corals that lack symbiotic algae. These data revealed trophic niche partitioning, where some genera were generalists feeding on diverse particles while others were specialists, targeting a subset of particles. Third, I investigated the use of nutritional pathways aside from suspension feeding in Hong Kong soft corals without algal symbionts. I demonstrated that some of these corals assimilate nitrate, and I identified members of their associated bacterial communities that have the capacity to use this inorganic nitrogen source, suggesting that these corals use their microbial communities to access limiting nitrogen and that nutrient limitation is a precursor to nutritional symbioses. Finally, I assess trophic niche partitioning across corals with algal symbionts by quantifying the amount of coupling between the nutrition of these two partners. SIA of host and symbiont revealed high coupling in some coral species, and limited coupling in others. I used a thermaltolerance experiment to demonstrate that increased dependence of the coral host on their symbionts is correlated with higher bleaching susceptibility, illustrating a tradeoff of autotrophy vs. heterotrophy. This thesis shows that trophic niche partitioning across corals both with and without algal symbionts is more fine-scale than previously thought, suggesting that niche partitioning is a major mechanism supporting high biodiversity on reefs. An understanding of the trophic niches of corals can be used to interpret SIA data collected for nutrient pollution monitoring.