A Novel μCT Analysis Reveals Different Responses of Bioerosion and Secondary Accretion to Environmental Variability

Abstract

Corals build reefs through accretion of calcium carbonate (CaCO3) skeletons, but net reef growth also depends on bioerosion by grazers and borers and on secondary calcification by crustose coralline algae and other calcifying invertebrates. However, traditional field methods for quantifying secondary accretion and bioerosion confound both processes, do not measure them on the same time-scale, or are restricted to 2D methods. In a prior study, we compared multiple environmental drivers of net erosion using pre- and post-deployment micro-computed tomography scans (μCT; calculated as the % change in volume of experimental CaCO3 blocks) and found a shift from net accretion to net erosion with increasing ocean acidity. Here, we present a novel μCT method and detail a procedure that aligns and digitally subtracts pre- and post-deployment μCT scans and measures the simultaneous response of secondary accretion and bioerosion on blocks exposed to the same environmental variation over the same time-scale. We tested our method on a dataset from a prior study and show that it can be used to uncover information previously unattainable using traditional methods. We demonstrated that secondary accretion and bioerosion are driven by different environmental parameters, bioerosion is more sensitive to ocean acidity than secondary accretion, and net erosion is driven more by changes in bioerosion than secondary accretion.

Environmental drivers of coral reef carbonate production and bioerosion: a multi‐scale analysis

“…Carbonate production, bioerosion, and net accretion rates were calculated for each block by comparing before and after μCT scans of the entire blocks (Silbiger et al. , ). μCT is an x ‐ray technology that non‐destructively images the external and internal structures of solid objects, resulting in a three‐dimensional array of object densities.…”

“…Prior studies highlighted the need to analyze both carbonate production and bioerosion independently (Silbiger and Donahue , Silbiger et al. ), but net accretion rates are also necessary for understanding long‐term reef sustainability (Silbiger et al. ).…”

“…, Silbiger et al. , ). Here, we examine the patterns of carbonate production, bioerosion, and net accretion rates over multiple spatial scales at 29 sites across the Hawaiian Archipelago (~2,500 km).…”

“…In a within‐reef study (~30 m; Silbiger et al. , ), ocean acidity was the best predictor of reef bioerosion when compared to nutrients, chlorophyll, temperature, depth, and distance from shore. At a large‐scale (~16,000 km) comparison across the Pacific Basin (DeCarlo et al.…”

“…We use a newly developed μCT methodology (Silbiger et al. , ) to measure in situ rates of carbonate production, bioerosion, and net accretion (calculated as percent change in volume) from experimental blocks of CaCO 3 deployed across the Hawaiian Archipelago. In the first large scale application of this μCT method, we calculate carbonate production rates primarily from crustose coralline algae, and bioerosion rates from borers and grazers from an early successional community after a one‐year deployment period.…”