Fine sediment in rivers : scale of ecological outcomes

Abstract

It is well recognised that the accumulation and transport of fine sediment (particle size 1000 km2) such as fine sediment accumulation and transport can improve broad-scale assessments of stream ecological condition. Therefore, the two aims of this thesis are to determine: 1. The relationship between fine sediment accumulation/transport and changes in macroinvertebrate community structure, as an indicator of stream ecosystem condition, from evidence at the habitat, reach and catchment scales. 2. Whether representing a physical process (i.e., bed material accumulation/transport) affecting macroinvertebrate communities can improve broad-scale assessments of stream ecological condition. This study has associated bed material accumulation and transport predictions from the Sediment River Network (SedNet) sediment delivery process model with direct measurements of fine sediment in the stream bed and associated these measurements with ecological outcomes and the four main conclusions of the thesis in relation to aims of the thesis are: 1. Based on the evidence from published studies and from this study, fine sediment accumulation causes damage to macroinvertebrate communities at habitat, reach and catchment scales. However, the present study was not able to disentangle the effects of fine sediment transport and accumulation on macroinvertebrate communities and this remains an area in need of investigation. 2. By the time an accumulation effect was predicted by SedNet (i.e. medium accumulation) the fine sediment threshold of 10% of substrate composition where there was a distinct change in macroinvertebrate community composition had been passed. Thus, predictions of medium accumulation can be interpreted as indicating damage to macroinvertebrate communities. Therefore, sediment process models such as SedNet are useful for identifying sediment sources in large catchments and sections of river where bed material accumulation and transport may be affecting stream ecological condition. 3. A fine sediment accumulation threshold where there were distinct changes in macroinvertebrate community structure with lower numbers of habitat disturbance sensitive taxa (e.g. Ephemeroptera, Plecoptera, Trichoptera) existed at low accumulation levels (10% of substrate composition see Chapter 3). This threshold provides a guideline for the management of fine sediment accumulation in riffles and is lower than the 20% and 30% thresholds suggested by other studies (see Chapter 2 Table 2.4). While this threshold could be refined with further work, both the present study and work previously reported clearly indicates that closer consideration should be given to low levels and early stages of sediment input to rivers. This threshold provides a guideline for the management of instream fine sediment in the Upper Murrumbidgee catchment and similar upland catchments.

Date of Award	2010
Original language	English
Supervisor	Richard Norris (Supervisor) & Scott Wilkinson (Supervisor)