Coral Reef Ecosystem Health in Response to Climate Change and Environmental Stressors

Abstract

Anthropogenic stressors such as ocean acidification, ocean warming and hypoxia threaten the health and persistence of coral reefs worldwide. In the following studies, we focused on two coral reefs: the Flower Garden Banks (FGB) in the Gulf of Mexico (GoM) and Kāneʻohe Bay barrier reef in Hawaiʻi. We characterized the natural spatiotemporal variability of the carbonate system within these reefs and their surrounding source water environments. We also investigated the relationship between various environmental parameters and reef-scale calcification in order to understand how climate-driven changes in these parameters will affect future coral reef health. We first characterized surface water carbon dioxide (CO2) variability in the GoM over multiple time scales (e.g., seasonal and decadal) in order to understand the source water characteristics to the FGB. Following a localized mortality event at the East Bank (EB) in the FGB, we collected biogeochemical data to uncover the mechanisms leading to the die-off. We measured low sea surface salinity and total alkalinity over the EB, which is indicative of river-derived water. At depth, high density, salinity, nutrients and microbial taxa associated with deep waters were coincident with low temperature and War, indicating a deeper source water at the EB. Cross-slope density gradients were consistent with an upwelling circulation pattern. We hypothesize that the combination of these features led to the formation of hypoxia and mortality of benthic coral reef organisms on the EB. In Kāneʻohe Bay, we calculated net ecosystem calcification (NEC; calcification- dissolution) and oceanic POC uptake to test for a connection between these two parameters. While several studies have hypothesized that oceanic POC may be an important nutrient source providing energy for coral reef calcification, this is the first study to test the hypothesis in the natural environment. We observed a significant, positive correlation between NEC and oceanic POC uptake. These results reveal a potential role of oceanic POC in fueling NEC and suggests that offshore productivity could be a critical component to the resistance or susceptibility of coral reefs to climate change, and should be considered when predicting future coral reef health.