Land use change has caused an increase in sediment and associated particulate nutrient loads in rivers draining to the Great Barrier Reef (GBR). Recent research in GBR catchments has indicated that particulate nutrients in sediment are bioavailable to both freshwater and marine phytoplankton but, the relative importance of this source to GBR eutrophication is unknown. In this research, we quantified the contribution of this source of bioavailable nitrogen to GBR freshwater and marine environments.
The different processes through which dissolved inorganic nitrogen (DIN) is generated from eroded sediment were identified and related to bioavailable nutrient pools. These pools and processes were quantified in a range of sources (e.g. surface and subsurface soil) in a dry tropics grazing catchment. Using this data we modelled the contribution from different sources and processes/pools to the end-of-catchment DIN load. Additionally, suspended sediment samples were taken at different positions in riverine sediment plumes from the Burdekin River. Samples were incubated in the lab to quantify DIN generation rates and estimate DIN generation loads from plume events.
The modelled load of DIN generation from sediment accounted for all the DIN load in the catchment. Surface sediment erosion contributed disproportionally to the modelled DIN generation. The measured DIN generation load from sediment in the plume in seven days was almost double the end-of-catchment load. Fast solubilisation of DIN was the main process in the catchment. Desorption of ammonium at the river estuary was an important source in short timeframes. The importance of mineralisation of the organic fraction increased with the time the sediment is in suspension.
Particulate nutrients in sediment are a significant source of bioavailable nitrogen in eroding grazing catchments and their marine sediment plumes. The processes that drive this bioavailability are complex, vary with sediment source and operate at different timeframes and spatial scales.