The influence of grain size, grain color, and suspended-sediment concentration on light attenuation: Why fine-grained terrestrial sediment is bad for coral reef ecosystems
Sediment has been shown to be a major stressor to coral reefs globally. Although many researchers have tested the impact of sedimentation on coral reef ecosystems in both the laboratory and the field and some have measured the impact of suspended sediment on the photosynthetic response of corals, there has yet to be a detailed investigation on how properties of the sediment itself can affect light availability for photosynthesis. We show that finer-grained and darker-colored sediment at higher suspended-sediment concentrations attenuates photosynthetically active radiation (PAR) significantly more than coarser, lighter-colored sediment at lower concentrations and provide PAR attenuation coefficients for various grain sizes, colors, and suspended-sediment concentrations that are needed for biophysical modeling. Because finer-grained sediment particles settle more slowly and are more susceptible to resuspension, they remain in the water column longer, thus causing greater net impact by reducing light essential for photosynthesis over a greater duration. This indicates that coral reef monitoring studies investigating sediment impacts should concentrate on measuring fine-grained lateritic and volcanic soils, as opposed to coarser-grained siliceous and carbonate sediment. Similarly, coastal restoration efforts and engineering solutions addressing long-term coral reef ecosystem health should focus on preferentially retaining those fine-grained soils rather than coarse silt and sand particles.
KeywordsTurbidity Light attenuation PAR Grain size Grain color Suspended-sediment concentration
This research was carried out as part of the US Geological Survey’s Pacific Coral Reefs Project in an effort in the USA and its trust territories to better understand the effects of geologic and oceanographic processes on coral reef systems and was supported by the USGS Coastal and Marine Geology Program. This work was inspired by numerous conversations with Paul Jokiel (UH-HIMB) and Greg Piniak (NOAA). We would also like to thank Amy East (USGS), Chris Perry (UE), and two anonymous reviewers who contributed numerous excellent suggestions and a timely review of our work. Use of trademark names does not suggest USGS endorsement of products.
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