Coral cavities are sinks of dissolved organic carbon (DOC)

We studied the removal of dissolved organic carbon (DOC) by coral cavities of 50-250 dm3 at a depth range of 5-17 m along the coral reefs of Curacao, Netherlands Antilles, and the Berau area, East Kalimantan, Indonesia. We found significantly lower DOC concentrations in cavity water compared with ambient reef water. On average, DOC concentrations in cavity water were 15.1 ± 6.0 µmol L−1 (Curacao) and 4.0 ± 2.4 µmol L−1 (Berau) lower than in reef water. When the cavities were closed, DOC concentrations in the cavities declined by 22% ± 8% and 11% ± 4% in Curacao and Berau, respectively, within 30 min. This corresponded to average DOC removal rates per cavity surface area of 342 ± 82 mmol C m−2 d−1 in Curacao and 90 ± 45 mmol C m−2 d−1 in Berau. Bioassays showed that bacterioplankton are not responsible for this DOC removal by coral cavities. DOC fluxes exceeded bacterioplankton carbon (BC) fluxes into cavities by two orders of magnitude. On average BC fluxes per cavity surface area were 3.6 ± 1.3 mmol C m−2 d−1 (Curacao) and 1.9 ± 1.3 mmol C m−2 d−1 (Berau area). The net DOC removal per square meter of cryptic surface likely exceeded the gross primary production per square meter of planar reef area. We conclude that coral cavities and their biota are net sinks of DOC and play an important role in the energy budget of coral reefs.

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