Characterising organic matter sources to the Daly River

The Daly River is located within the wet/dry tropical region of Northern Australia and as such experiences highly episodic flows with 90% of runoff occurring in the wet season (January-March). It has a catchment of 53 000km2 , the upper part of which is drained by the Katherine River and for most of it's path, the river and tributaries flow through savannah vegetation. A variety of pressures have been identified for the Daly river and these include cattle grazing, horticulture and cropping. These activities have the potential to alter surface and groundwater flows thereby affecting sediment, nutrient and organic matter delivery to the river system. As part of a Tropical Rivers and Coastal Knowledge (TRaCK) project we have been using a variety of techniques to understand the relative contributions of allochthonous and autochthonous material within one reach of the river. Samples of particulate material were collected at various sites during the wet season to understand organic carbon loads entering the river during this time. During two subsequent field surveys in July and September, sediment and water column samples were collected for analysis of their lipid biomarker content. This has allowed the major sources of arbon to be identified along with changes through the dry season. Lipids extracted from the water column samples indicate a mixed microalgal population but interestingly also suggest a relatively high bacterial population. Sediment samples are highly variable, reflecting the heterogenous nature of sediment types within the river. Diatoms appear to be a major contributor of organic carbon, along with other algae but terrestrial derived organic matter is apparently minor.