The role of terrestrial sediment on turbidity near Singapore's coral reefs

• Ebb-dominant tidal asymmetry promotes export from the Johor Estuary.
• Import by density-driven flow is weak because of vertically mixed sediment.
• Much terrestrial sediment is therefore discharged to Singapore's coral reefs.
• Offshore mixing rates and residual transport rates are high.
• Terrestrial sediment therefore probably has a low impact on rising turbidity levels.

In past decades, the turbidity in Singapore's coastal waters has been increased. This has led to reduced visibility and increased siltation rates, detrimental for the coral reefs and other sensitive ecosystems around Singapore. The reasons for this increased turbidity are poorly known because little quantitative information exists on sediment dynamics around Singapore, on changes in sediment sources, and on the physical environment. Therefore we set out to quantify the effect of fluvial contributions on changes in sediment dynamics, using a combination of numerical models, satellite images, and hydrodynamic and sedimentary data. Results indicate that the main fluvial source enters an ebb-dominant estuary, with sediment export primarily balanced by settling/scour lags rather than estuarine circulation. A large part of the sediment load enters the Singapore Strait, where the large-scale marine currents effectively transport most sediment towards the coral reefs. However, mixing with marine water masses in both the estuary and the adjacent Singapore Strait sufficiently dilutes this fluvial sediment source to have a negligible impact on Singapore's coral reefs.