title: The Transformation of Catchment Phosphorus in Dry Subtropical River Systems
Creators: name: Kerr, Jason Grainger
Publisher: name: Griffith University
Year: 2009
Subjects: subject: Phosphorus
subject: subtropical river systems
subject: river systems
subject: subtropical
subject: catchments
subject: Equilibrium Phosphate Concentrations
subject: sediment
subject: acquatic ecosystems

description: Phosphorus (P) derived from catchment sources is a key factor in the eutrophication of many aquatic ecosystems. Increasingly the protection of receiving waters has been focused on improving water quality through changing land management to decrease the supply of nutrients from the catchment. Rivers are an important link between management actions in the catchment and water quality downstream and therefore an understanding of P dynamics in river systems is central to describing and predicting P fluxes through the catchment. Much of the knowledge about P transformations and transport in rivers comes from studies of temperate systems and to date there have been no studies which define the underlying drivers of P dynamics in dry subtropical rivers. This thesis addresses this gap in knowledge by describing P dynamics in the upper Brisbane River (UBR), a grazing impacted, dry subtropical, river system. A conceptual model of P dynamics was proposed which described the key processes occurring in dry subtropical rivers in the context of the highly variable flows which characterize these systems. The model focuses on three key phases; a drawdown phase whereby flow ceases and P dynamics are dominated by biological and chemical processes; a transport phase during event flows where P dynamics are dominated by physical transport processes; and an intermediate phase where P dynamics are governed by the interaction between biological/chemical and physical processes. The model was used to contrast P dynamics in dry subtropical rivers with the more widely studied temperate systems and to provide a framework for the design and interpretation of experiments. Experiments comprised quantification of instream P storage in key ecosystem compartments coupled with P fractionation and P sorption measurements in soils and sediments under varying extremes of river flow. Bed sediments were shown to be the dominant store of P in most sections of the river examined. P storage was measured in the standing biomass (macrophytes, leaf litter and periphyton), bed sediment and water column of five reaches of varying geomorphology and compared to total P fluxes from the river during the wet season of December 2005 to March 2006. Bed sediments contained the majority of P comprising more than 87% of total reach P at four of the five study reaches. Estimated P storage in the top 2 cm of bed sediments (15.8 T) was approximately 6 times greater than total wet season export from the river system (2.7 T), and almost 7 times higher than the P stored in macrophyte biomass (2.3 T). This suggests that the bed sediments may be an important source of particulate P to downstream ecosystems during event flows and a source of P to primary producers (e.g. macrophytes) during periods where inputs to the river system are reduced...