Sclerochronological and geochemical records from Porites spp. in Fiji: Assessing massive coral response to environmental changes

Abstract

Coral reefs are one of the most biologically diverse systems on Earth and the ecosystem services they provide are of high importance for coastal communities and beyond. Yet, they are becoming increasingly threatened by global (climate change) and local stressors (e.g., land-derived pollution, overfishing). Many studies have focused on understanding how scleractinian (reef-building) corals in inshore reefs are responding to these environmental stressors and their potential compounding effects. However, this is complicated due to the lack of high-resolution records of both environmental parameters and coral growth. Records of coral growth and geochemistry from the aragonitic skeletons of massive scleractinian corals can be used as effective archives of modern and past changes in reef water quality, coral calcification rate and physiology, improving our understanding of reef-specific and species-specific responses of corals to multiple stressors. In this thesis, I aim to disentangle the relationship between coral growth, skeletal deposition, geochemical composition, and environmental conditions in Fiji. I acquire sclerochronological and geochemical records from massive Porites spp. corals from four inshore reefs adjacent to different catchment environments in Viti Levu, Fiji, alongside remotely sensed environmental data. I reconstruct the sensitivity of Porites spp. to environmental changes, explore the fundamental physiological controls on the incorporation of SST proxies into skeletal aragonite, and explore site-specific environmental and hydrological controls on reef water quality and the use of proposed proxies for terrigenous input. Sclerochronological records of coral growth (linear extension, density, and calcification) are obtained from Porites spp. cores using Computed Tomography. Geochemical records include measurements of a suite of trace and minor elemental concentrations in the coral skeletons using Laser Ablation – Inductive Coupled Plasma Mass Spectroscopy to assess a range of SST proxies (Sr/Ca, Mg/Ca, Li/Ca, and U/Ca) and terrigenous input proxies (Ba/Ca, Mn/Ca, Y/Ca, La/Ca, and Ce/Ca). Coral growth records from inshore reefs show that although water quality plays a significant role in driving mean linear extension and calcification rate between 1998 and 2016, persistent thermal stress has the capacity to reduce coral growth across all locations, regardless of local conditions. This is important because under a scenario of amplified global warming and persistent thermal stress local management might not be enough to palliate the effects of climate change. Traditional coral-derived SST proxies in inshore reefs in Fiji reflect SST variability to some degree and are applicable for paleotemperature reconstructions. However, multiple core replicates across locations are needed to build a composite record that accurately reflects SST. Results show the existence of biological controls ('vital effects') on the elemental composition of the corals as a response to reef-specific environmental factors, complicating the extraction of an SST signal. In addition, application of terrigenous input proxies, although successfully recorded by the corals of this study, need to be considered on a site-by-site basis. This is because variable environmental and hydrological mechanisms lead to reef-specific changes in water quality as a response to climatic events (e.g., rainfall seasonality, tropical cyclones). As such, future interpretations of terrigenous input proxies will require of a deep understanding of hydrological and climatic catchment-coastal linkages. Overall, this work showcases the use of coupled sclerochronological and geochemical methods for providing robust growth and environmental records and a better understanding of how environmental conditions have affected coral growth in vulnerable inshore reefs in the past