Periphyton and phytoplankton response to reduced dry season flows in the Daly River

Final Milestone Report: National River Health Program – Environmental Flows InitiativeThis project evaluates whether phytoplankton, benthic diatoms and macroalgae are directly, or indirectly, responsive to dry season river flow, and provide information for the allocation of water for the environment. Owing to their rapid replication rate of a couple of days, these algae are responsive over a time scale of weeks to changes in the aquatic environment, notably flow and water quality. Flow in the Daly River and its major tributaries, during the dry season, is maintained by groundwater. The extraction of water directly from these rivers or from the groundwater during the "dry" will reduce flows in the Daly River and its tributaries. There is also potential for the river's water quality to be directly affected. In the upper reaches of the catchment, dry season flows originate predominantly from aquifers within Cretaceous sediments. With groundwater inflow from the Daly River Basin, the conductivity of the Daly River increases 20-30 fold, pH and the carbonate buffering capacity increases at least an order of magnitude, whilst soluble phosphorus and nitrate concentrations more then double. In the Douglas River, inflow from the Tindal Limestone results in an almost 100 fold increase in nitrate concentrations but has not resulted in high phytoplankton concentrations due probably to phosphorus limitation. Such a marked increase in nitrate concentrations was not measured elsewhere in the catchment, and may be due to modified land-use and management practices. Extraction from the Daly River Basin for consumptive use would be expected to alter, in addition to flow, river water quality, depending of the change in the mix of river sources.Simon Townsend, Peter Gell, Sophie Bickford, John Tibby, Roger Croome, Malgorzata Przybylska, Armando Padovan and Rodney Metcalf