Nutrient impacts on coral reefs captured through macroalgal isotopes

Anthropogenic nutrient runoff is a major local stressor on coral reefs but compared to research on global climate change and overfishing, progress has been slower at quantifying its effects, particularly at the ecosystem scale. This is due to the difficulties in cost-effectively capturing the high spatio-temporal variability of bioavailable nutrients in reef systems. In this thesis, I examine common bioindicators and associated methodologies for assessing nutrient regimes as well as the relationships between nutrient and biological responses of the bioindicators. I first compare the precision and cost-effectiveness of five nutrient signatures (δ15N, δ13C, %N, %C and C:N Ratio) in a suite of eight indicators across 21 reefs around the inner Seychelles islands. I show that the congruency between the three most precise types (brown macroalgae, green macroalgae and zoanthids) was low, which was likely due to differences in species-specific ecological strategies (e.g. nutrient uptake and/ or storage capacity). I then test the theory that species within the same functional groups should respond similarly to nutrient enrichment using a) passive biomonitoring (sampling along a nutrient gradient) b) active biomonitoring (in situ reciprocal transplant experiment), and c) manipulative laboratory experiments (nutrient supply rates). Overall, these studies suggest that even the responses of morphologically-similar macroalgae with different strategies for nutrient uptake can vary over fine spatio-temporal scales, particularly if they are not nutrient-limited. Finally, I use one of these methodologies in a real-world scenario to investigate the influence of mass coral mortality events on δ15N signatures of transplanted macroalgae 1) before and after the 2016 bleaching event in the Seychelles, and 2) during the 2019 bleaching event in Mo’orea. Both case studies strongly imply that macroalgae can potentially take up this mass release of dead coral tissue, and possibly locking them into local biogeochemical cycles for up to a year after a bleaching event. I conclude that a traits-based approach, using a suite of congruent bioindicators with the same functional traits (i.e. rapid nutrient uptake), would be most cost-effective for future research and monitoring programs