Yarra River Ecological study

CRC for Freshwater Ecology project B110

Background

River The Yarra River, pictured here at suburban Eltham, is perhaps best known as the River on which the city of Melbourne was settled. It has a catchment area of about 4000 km², about a third of which has been set aside in a pristine state as water supply catchments. About half of the catchment is used for agriculture: the Yarra valley is increasingly recognised as an important wine-growing region. The suburbs of Melbourne account for less than a fifth of the catchment.

Despite its proximity to Melbourne and its importance to the regional economy, the Yarra River has received little scientific study. To redress this shortcoming, the CRC for Freshwater Ecology is conducting a study on the Yarra’s ecological function and structure. A major impetus for the commissioning of this project was the CSIRO's Port Phillip Bay study (Harris et al. 1996). That study identified the Yarra as a major source of nutrients to the Bay, and concluded that the protection of the bay ecosystem is dependent on the reduction of nutrient (in particular nitrogen) inputs into the Bay.The Yarra River at Griffith Park, ELtham

Objectives and methods

The study asked:

What are the spatial patterns in stream community structure and function in the lowland segments of the river (see map)?
What is the impact of catchment landuse, particularly urbanisation, on the patterns in ecosystem structure and function?

Water quality was measured in a series of spatial snapshots. Sixty sites were tested in a series of one-day field trips in a spatially and temporally stratified sampling program. Community composition was evaluated by sampling benthic and snag macroinvertebrate fauna over two seasons and 45 sites on the river. Community function was evaluated by measuring production to respiration ratios (P/R ratios) in whole river reaches (12 sites), in the water column (6 sites) and in the benthos (3 sites). Nutrient cycling was also evaluated by measuring sediment denitrification potential (3 sites).

Major findings

Water Quality - was spatially variable, with much of the variability being explained by tributary inputs. Both rural and urban landuse impacts on water quality were evident. Filterable reactive phosphorus was generally very low (and may be limiting in the rural segment), while nitrate concentrations were generally very high

Community Structure - three macroinvertebrate community groups were evident. The rural community group degraded with distance down stream. With further distance downstream some recovery was evident in macroinvertebrates (the gorge community). Further downstream significant degradation was evident in the urban community. Impact on community structure in the rural segment was not well explained by tributary inputs and appears more related to variables such as riparian condition and perhaps by catchment imperviousness.

Community Function - the Yarra River was found to be generally heterotrophic (P/R ~ 0.25). Gross primary production was low in the rural segment (may be phosphorus limited), highest in the gorge segment (due to the occurrence of aquatic macrophytes), and decreased in the urban segment (due to turbidity-related light limitation). There were no spatial differences evident in gross community respiration within the river.

Project team

Dr Peter Breen, Project leader
Dr Chris Walsh, Dr Mike Grace, Dr Sophie Bourgues, Prof. Barry Hart, Janelle Boyall, Claire Sellens, Brett Cole

Associated student project:

Kylie Waller, BSc(Hons) Biological Sciences, Monash University, A comparison of longitudinal patterns in snag and benthic macroinvertebrate communities in the Yarra River.

The Yarra bibliography (for the 'Yarra Forum')