Search

Purpose: In many streams worldwide including those on the south coast of Western Australia (WA), sediments of the > 2-mm fraction often contribute up to 50% of the streambed. However, most analysis and interpretation of sediment chemistry, including phosphorus (P), is conducted on the < 2-mm fraction as this fraction is considered the most chemically reactive. This paper aims to identify the contribution of the > 2-mm fraction to P retention and release in sandy-gravely streams. Material and methods: Sediment samples were collected from streams in agricultural catchments, and P retention and release by the < 2-mm and > 2-mm (typically lateritic; iron rich) sediment fractions were examined using fluvarium and batch experiments. Phosphorus sorbed by sediment was estimated on a mass (mg P kg −1 ) and area basis (mg P m −2 ). Results and discussion: Phosphorus sorption measurements suggested that mineralogy as well as particle size were important factors influencing P retention by stream sediments. Stream sediments retained approximately 30% of added P. In a desorption phase, approximately 8% of the retained P was released into stream water. Conclusions: Stream sediments in south western WA appear to be net immobilisers of P, retaining more P than they release, dependent on the stream P concentration. Exclusion of the > 2-mm fraction when determining stream sediment P dynamics may therefore underestimate whole stream sediment P retention and release.

Southern Australia is becoming warmer and drier as climate change progresses, creating serious threats to freshwater ecosystems that are dependent on the presence of water for their existence. The overall aim of this research project was to develop and evaluate four potential methods for enhancing the role, function and resilience of refuges for freshwater biodiversity in southern Australia. It focussed on means to maintain the physical conditions in refuges within ranges tolerable for species and to maintain connectivity that allows species to retreat to, and expand from, refuges. The four approaches studied were: • the feasibility of using cool-water releases (CWR) from reservoirs and shandying to control water temperature in rivers; • a method for deciding where streamside re-vegetation should occur in catchments to ensure maximum long-term negative effects on stream temperature; • the potential for artificial urban wetlands (i.e. anthropogenic habitat) to act as refuges for freshwater biodiversity against climate change; • a method for identifying redundant river regulation infrastructure and prioritizing artificial structures for removal during river restoration to improve connectivity along river channels for fauna movement. These four approaches were found to have the potential to address a range of objectives for refuge management, such as: reduce temperatures in refuges (1 & 2), increase number of refuges that act as colonization sources (all), assist dispersal into and out of refuges (all), increase biodiversity within refuges (all), increase permanence or resilience of refuges (all) and increase resistance or resilience of refuges during extreme events (1, 2 & 3). In particular, CWR could potentially be used to mimic natural thermal regimes, reduce the frequency and duration of extreme high temperature events and to assist movement of fish between thermal refuges, but further information and trials are required (1). Riparian planting can be used to reduce