Abstract
Abstract. Forest clearing for reasons of timber production, open pit mining and the establishment of oil palm plantations generally results in excessively high sediment loads in tropical rivers. The increasing sediment loads pose a threat to coastal marine ecosystems, such as coral reefs. This study presents observations of suspended sediment loads in the Berau River (Kalimantan, Indonesia), which debouches into a coastal ocean that is a preeminent center of coral diversity. The Berau River is relatively small and drains a mountainous, still relatively pristine basin that receives abundant rainfall. In the tidal zone of the Berau River, flow velocity was measured over a large part of the river width using a horizontal acoustic Doppler current profiler (HADCP). Surrogate measurements of suspended sediment concentration were taken with an optical backscatter sensor (OBS). Averaged over the 6.5 weeks covered by the benchmark survey period, the suspended sediment load was estimated at 2 Mt yr−1. Based on rainfall-runoff modeling though, the river discharge peak during the survey was supposed to be moderate and the yearly averaged suspended sediment load is most likely somewhat higher than 2 Mt yr−1. The consequences of ongoing clearing of rainforest were explored using a plot-scale erosion model. When rainforest, which still covered 50–60% of the basin in 2007, is converted to production land, soil loss is expected to increase with a factor between 10 and 100. If this soil loss is transported seaward as suspended sediment, the increase in suspended sediment load in the Berau River would impose a severe stress on this global hotspot of coral reef diversity.
Highlights
Indonesia is the country with the largest area of coral reefs, which hosts the global biodiversity center of several groups of marine organisms (Tomascik et al, 1997; Spalding et al, 2001)
The Berau River discharges to the Berau continental shelf, which is an ecological hotspot of global importance
The suspended sediment load was observed in the tidal zone of the river
Summary
Indonesia is the country with the largest area of coral reefs, which hosts the global biodiversity center of several groups of marine organisms (Tomascik et al, 1997; Spalding et al, 2001). Sediment loads from Indonesian islands are disproportionally large. The disproportionally high sediment load from the Indonesian islands is due to the high topographical relief, the small size of the drainage basins with eroding rocks and heavy rainfall that characterizes the tropics (Milliman et al, 1999). Sediment loads in Indonesia are increasing at a higher rate than in other tropical regions because of large-scale deforestation (Syvitski et al, 2005; Milliman and Farnsworth, 2011). The increasing sediment loads pose a serious threat to coastal coral reef ecology (Edinger et al, 1998; Spalding et al, 2001; Fabricius, 2005)
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