Sediment and nutrient sources and sinks in a wet-dry tropical catchment draining to the Great Barrier Reef

Abstract

Many tropical river systems have altered water quality due to human land use, impacting the biodiversity of freshwater and coastal ecosystems. Long-term, catchment-scale monitoring is needed to understand pollutant sources, controls, and trends. This 12-year study monitored baseflow and flood event nutrient and sediment concentrations, and estimated sediment loads across the Normanby Basin in northern Australia. Suspended sediment concentrations and yields were highest in upper catchment areas where cattle grazing occurred on erosion-prone sodic soils. Mid- and lower catchment rivers and floodplains were a sink for sediments and nutrients, trapping around 75% of suspended sediments during events. Clays (<4 μm) were preferentially transported to the estuary, with an estimated 46% sediment delivery ratio. In the estuary, suspended sediment concentrations were influenced by tidal resuspension processes and there were significant sources of DIN. These findings can help prioritise land management investments for the protection of Great Barrier Reef and freshwater ecosystems.

Keywords: Estuary; Nutrient concentration; Sediment load; Sediment transport; Tropical catchment; Water quality.