Abstract
The long-term success of coral reefs depends on a positive balance of calcium carbonate production exceeding dissolution, erosion, and material export. As a result of ocean acidification, coral reefs could transition from net accretion to net erosion owing to decreasing rates of calcification and increasing rates of chemical dissolution and bioerosion. Here, I present a fundamental paradigm that aims to explain the main driver of carbonate sediment dissolution on coral reefs based on theory and a new empirical dataset of pore water carbonate chemistry from the Bermuda coral reef platform. The paradigm shows that carbonate sediment dissolution is most strongly controlled by the extent of organic matter decomposition in the sediments, but that the magnitude of dissolution is influenced by how much decomposition is required to reach pore water undersaturation with respect to the most soluble bulk carbonate mineral phase present in the sediments, a condition defined as the Carbonate Critical Threshold (CCT). Decomposition of organic matter beyond the CCT under aerobic conditions results in stoichiometric proportional dissolution of carbonate sediments. As ocean acidification proceeds over the next several decades, the extent of organic matter decomposition required to reach the CCT will decrease, carbonate dissolution will increase, and subsequently the accumulation of carbonate sediments will decrease. Since drastic reductions in anthropogenic CO2 emission are unlikely in the foreseeable future, the paradigm shows that active controls and reduction of organic matter input to coral reefs at the local scale might be an effective mitigation strategy to prevent or delay coral reefs transitioning to a state of net dissolution.
Highlights
Reviewed by: Shoji Yamamoto, The University of Tokyo, Japan Atsushi Watanabe, Tokyo Institute of Technology, Japan
The paradigm shows that carbonate sediment dissolution is most strongly controlled by the extent of organic matter decomposition in the sediments, but that the magnitude of dissolution is influenced by how much decomposition is required to reach pore water undersaturation with respect to the most soluble bulk carbonate mineral phase present in the sediments, a condition defined as the Carbonate Critical Threshold (CCT)
Since drastic reductions in anthropogenic CO2 emission are unlikely in the foreseeable future, the paradigm shows that active controls and reduction of organic matter input to coral reefs at the local scale might be an effective mitigation strategy to prevent or delay coral reefs transitioning to a state of net dissolution
Summary
Oceanic uptake of anthropogenic CO2 has resulted in significant changes in the surface seawater dissolved inorganic carbon system and acid-base balance. Models and mesocosm experiments suggest that coral reefs in general will transition from a condition of net accretion to net erosion as a result of the projected decreasing rates of calcification and increasing rates of chemical dissolution and bioerosion, but the timing of when, how fast, and at what seawater CO2 conditions this will occur varies (Andersson et al, 2009; Silverman et al, 2009; Dove et al, 2013) Despite these dire projections with potential devastating consequences to coral reef ecosystems and the millions of people dependent on these systems for nutrition, financial revenue, and protection from storms, the effect of ocean acidification on CaCO3 dissolution has received relatively little attention from both the scientific research community and the general public (Eyre et al, 2014). Note that throughout this manuscript the term CaCO3 refers to the general group of carbonate mineral phases (i.e., aragonite, Mg-calcite, and calcite) that make up contemporary coral reef carbonate sediments
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