Dissolved organic carbon in the lower Namoi River, New South Wales : determing responses to flow, loads and food web linkages

University of Technology, Sydney. Faculty of Science.Like many lowland rivers around the world, the Namoi River in north-west New South Wales (NSW) has greatly reduced rates of flow due to effects of regulation and abstraction. This has likely led to reduced amounts of allochthonous dissolved organic carbon (DOC) entering main channels through less frequent wetting of benches, flood runners and floodplains. However, these changes in DOC have not been quantified. In addition, the benefits that increased delivery of DOC may have to aquatic food webs are not well understood for lowland rivers. The NSW Government has focussed on the sustainable use of natural resources with a strong environmental protection component. Part of the legislative requirements was to supply environmental flows to rivers and to quantify ecological benefits. A scientific approach was developed, and monitoring of these releases began in 1999 under the Integrated Monitoring of Environmental Flows (IMEF) program. In this study DOC concentrations were quantified over different flow regimes across three sites at a variety of temporal scales with export loads and diel variation determined. DOC concentrations over low flow periods were fairly similar between sites and ranged between 5 and 10 mg L-1. DOC concentrations during a major flood increased substantially with a mean of 20.4 mg L-1 and a maximum of 44 mg L-1. A significant (P<0.05) positive linear relationship was found between median DOC concentration and flow. The relationship between DOC and flow was used to estimate DOC loads to the river under different modelled flow scenarios including without environmental flow, with environmental flow, and simulated natural flow. Environmental flows were found to increase DOC delivery relative to current flows and approximated that delivered under natural conditions. To determine diel variability, DOC was sampled at 4 hr intervals from two sites across two distinct flow regimes. This included a large flood (mean flow 224 m3 sec-1 and a peak flow of 376 m3 sec-1) sampled four hourly for ten consecutive days. DOC concentrations were significantly greater at night than during the day (P