Constructing a Toolbox of Geochemical Indicators of Community Composition Shifts in Coral Reef Ecosystems Under Stress

Abstract

Modern coral reef ecosystems house 25% of the planet’s entire marine biodiversity in ~ 0.1% of the surface area cover of the ocean and provide numerous ecosystem services for human populations. Unfortunately, these marine rainforests are known to be in a global state of decline, largely due to the impacts of sea-surface warming, ocean acidification, pollution, disease, and other direct human impacts. Numerous studies have proposed that as these stressors increase in reef environments, a pronounced shift from precipitation to dissolution of calcium carbonate (CaCO3) substrates will induce the collapse of habitat-forming coral structures and lead to dominance of macroalgae. Together, these changes lend themselves to a decline in the overall biodiversity of marine organisms hosted in today’s reef ecosystems in addition to major losses to global fisheries,erosion of coastal protection, and dissolution of the foundations of many tropical islands. As a means to improve future reef management and adaptation strategies, there has been a recent push for the development of new tools for monitoring the health of coral reefs under environmental stressors. My research follows a forensic geochemistry approach to understand ecological shifts within and between reef ecosystems in the age of anthropogenic climate change. Specifically, I combine discrete and continuous measurements of seawater composition in coral reef environments, mathematical modeling, and high-resolution mass spectrometry to determine the most effective geochemical proxies for changes in reef metabolism and community composition under stress. Here, I will report on (1) the limitations of exploring variability in modern reef metabolism through assumptions of canonical relationships between carbon and oxygen budgets, (2) the development of a high-precision method for detecting variability in the seawater dissolved strontium-to-calcium (Sr/Casw) ratio and its potential as an indicator of calcifier community composition, and (3) variability in the stable carbon isotope (δ13C) composition of seawater dissolved inorganic carbon (DIC) on reefs as a potential proxy for benthic primary producer community composition. With the current unprecedented decline of coral reef ecosystems, the ultimate goal of my research is to construct a sophisticated and quantitative toolkit for detecting ecological transitions within these dynamic marine habitats.