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    Bioerosion in the Hawaiian Archipelago

    The Research Problem

    Coral reef ecosystems persist in an accretion-erosion balance.  This considers the process of coral growth by gradual accumulation. Corals and other calcifying organisms build reefs through this build-up of calcium carbonate skeletons.  Bioeroding organisms erode reefs directly, through grazing on (eating) and boring into (living) calcium carbonate reef substrate, and, indirectly, by increasing reef susceptibility to physical erosion from mechanical stress such as ocean currents and waves.  Erosion due to biological processes can exceed physical effects and facilitate physical erosion by increasing porosity (the amount of empty spaces in the coral) and weakening the coral skeleton. While management efforts to sustain coral reef ecosystems often focus on coral health and coral growth rates, reef resilience also depends on rates of bioerosion.

    Methods

    Using a novel approach, we are characterizing the bioeroder community and bioerosion rates across the Hawaiian Archipelago. Micro computer-aided tomography (μCT ) is a powerful technology for visualizing the internal structure of solid objects.  In medical imaging, this is often called a CAT-scan; in this application, the resolution is much higher, allowing precise examination of coral skeletal density and the size, shape, and location of bore holes created by bioeroders.  Using pre- and post- scans of coral blocks deployed throughout the Hawaiian Archipelago, we can precisely measure the loss of calcium carbonate (coral skeleton) and characterize what type of bioeroder caused the loss. Additionally, we are investigating the environment of bioeroding organisms using ReefSense, a sophisticated array of instruments that measures physical and chemical processes at the small scale experienced by bioeroding communities.  This high-intensity sampling gives us greater power to understand the environmental drivers behind bioeroder community composition and bioerosion rates.  Understanding how these communities respond to that environmental variation can help us predict responses to anthropogenic climate change.

    Other research

    Using a modern genetics method, the ReefChip microarray takes advantage of the abundance of DNA sequence data being generated by the BarCode of Life to develop a next-generation Biodiversity Gene Chip. ReefChip will be capable of quickly and efficiently detecting, identifying, and quantifying marine species in mixed environmental samples.  We are working to add a database of Hawaiian bioeroder sequences to the ReefChip to allow rapid processing of bioeroder community samples.

    Importance

    Enhanced rates of bioerosion can compromise the function of the reef framework, undermining the mechanical stability, structural complexity, and net accretion of coral reefs. Coral reefs absorb energy from wind-driven waves, providing natural shoreline protection to tropical ecosystems. When mechanical stability is compromised, coastal property is more susceptible to storm damage and wave action. Structural complexity in coral reefs provides habitat and protection for reef organisms, and reduced structural complexity can result in decreased abundance and diversity of fishes. Finally, if increased rates of bioerosion exceed accretion, coral reefs will be unable to keep up with sea-level rise. The services humans receive from coral reef ecosystems depend on the response of bioerosion rates to local and global human impacts, including coastal development and increased atmospheric carbon dioxide.