Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River. Part 2: Impact of Sediment Movement on the Aquatic Ecology of the Ovens River

"November 2005".

Project Number: Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River - M/BUS/79.

MDFRC item.

17 pages. Part 2 of 2 part report + executive summary: (see Executive Summary: Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River) and (Part 1: Regional patterns of riparian vegetation, erosion and sediment transport in the Ovens River basin).

The Ovens is one of the last remaining unregulated rivers in Victoria, with an active floodplain and healthy native fish community. It is also an important source of water for major regional towns and agriculture. River Health Assessments have revealed depauperate macroinvertebrate fauna and there is also anecdotal information that submerged macrophytes have been lost from the lower section and that gravel bars are not as common as they once were. One explanation for these changes is increased sediment inputs associated with changes in land management and riparian clearing. All streams are expected to carry some suspended solids under natural conditions but anthropogenic disturbance has influenced the loads and frequency of high suspended solid events (Ryan 1991). These solids may be organic; for example, riparian litter, detritus, plankton and algae or inorganic; for example, aggregates and sands. The nature and load of these solids may be highly variable, depending on rainfall in the catchment, percolation and run off and tributary flows as well as seasonal inputs of litter and algal primary production. Entrained sediment has two major effects on a stream, firstly, high turbidity affects water quality and secondly, siltation affects the structural composition of stream bed by filling interstitial spaces. Both of these processes can impact on the type and abundance of invertebrates either directly though habitat modification of indirectly through effects on primary production. Increased turbidity due to suspended sediment reduces light penetration and hence the productivity of algae and submerged aquatic plants.

Shading due to turbidity is known to reduce the biomass of attached algae (Cook 1999) the density of macroinvertebrates (Cook 1999, Bennisson et al 1989) and productivity of aquatic plants if light is limiting (Ryan 1991). In- stream sediment characteristics are generally correlated with stream gradient (Hawkins et al 1982). Upper (high gradient) stream reaches tend to be characterised by boulders and cobble (Hawkins et al 1982) and as the gradient decreases size of the bed sediment particles decreases with greater proportions of gravel, sand and silt. Increased sediment inputs can significantly alter the sediment composition at a site through infilling of interstitial spaces or blanketing existing substrates. The movement of sand and silt is of major concern in the Ovens River, these smaller particles transported primarily during high flow events are suspected to be responsible for the loss of aquatic macrophytes in the downstream reaches in the past 20 years where gravel beds have been almost buried by sand (J. Hawking pers. com.). The objectives of the study were to characterise water quality and measure sedimentation in streams with high and low suspended concentrations identified by SEDNET analysis and to determine the impacts of sedimentation on water quality and macroinvertebrate diversity.