Part 1 - A Review of the Existing Studies on Lake Mulwala and its Catchment

"June 2004".

Project Number: Lake Mulwala Water Quality Study.

MDFRC item.

Part 1 of 4 part report (see Executive summary: Lake Mulwala Water quality study), (Part 2: Sampling program to address critical knowledge gaps in Lake Mulwala), (Part 3: Results of sampling program to address critical knowledge gaps identified in assessing the environmental condition of Lake Mulwala) and (Part 4: Developing a monitoring strategy for Lake Mulwala).

The purpose of the current review is to examine what data already exists on water quality in Lake Mulwala in order to make recommendations for the ongoing monitoring and assessment of the lake. This review considers a wide range of water quality parameters, (including water chemistry) algae, bacteria, macroinvertebrates, fish, plants and other species. In addition to published articles, reports and syntheses, we have considered primary data obtained from the Murray-Darling Basin Commission (MDBC) long-term monitoring program, the Department of Infrastructure, Planning and Natural Resources (DIPNR), Goulburn Murray Water (GMW), North East Water, and the Victorian Water Resources Data Warehouse at sites on the River Murray and Ovens River upstream of Lake Mulwala, within the lake itself, in the Mulwala Main Canal and downstream of Yarrawonga Weir. The main conclusions regarding existing data are: - The concentration of nutrients in Lake Mulwala are consistently higher than the Australian and New Zealand Environment and Conservation Council's trigger levels for lakes and reservoirs but generally lower than the trigger levels for lowland rivers (ANZECC, 2000). Phosphorus levels in particular are consistently higher than levels need to sustain algal blooms. - The electrical conductivity of the water in Lake Mulwala is significantly below the thresholds that impact on aquatic organisms and the level would generally be considered low for River Murray waters. The movement of salt through the lake is highly episodic, the largest flux occurring during floods. The Ovens River has a strong influence on the transport of salt into the lake. - Turbidity levels in the lake generally fall within acceptable levels and while the lake is slightly clearer than the upstream sources, it does not appear to significantly influence downstream water clarity. - Lake Mulwala may act as a sink for metals through sedimentation of suspended particles. - Limited monitoring of pesticides in the system has detected pesticides in the Ovens River, the Mulwala Main Canal and within Lake Mulwala, but only on a very few occasions. Pesticide concentrations in lake sediment (potentially the largest sink for pesticides) have not been studied. - Algal blooms are a significant water quality issue in Lake Mulwala and a number of severe outbreaks of blue-green algae have occurred in recent years. However there are serious inconstancies in the available algal data sets that need to be resolved. - Long-term trends in the vegetation of the lake cannot be assessed as only a single extensive survey has been carried out (providing a “snapshot” of the vegetation distribution at that time). There is some anecdotal evidence that vegetation on the northern side of the lake may be impacted by saline groundwater intrusions. - Lake Mulwala is an important recreational fishery for Murray Cod. However, the average size of cod caught in fishing competitions has declined over the last decade, suggesting that the population may be under stress. Several key processes which may be threatening to the water quality in Lake Mulwala have been identified. They include: - Remobilisation of pollutants from sediments: Sediment quality is poorly understood in Lake Mulwala, but remobilisation of pollutants (nutrients, metals and pesticides) is a potential threat to water quality in the lake. - Stormwater: Loads of pollutants from stormwater draining directly into the lake and the resultant impact on the lakes biota are poorly quantified. - Hydrocarbons: Hydrocarbons entering the lake with stormwater, from fuel storage facilities and recreational activities are potential threats to water and sediment quality in the lake. - Aerial deposition: Aerial deposition of pollutants (particularly nitrogen) into the lake from industrial activities is a potential (but unquantified) risk to the water quality. - Bushfires: Bushfires upstream in the catchment have the potential to cause severe degradation of water quality in Lake Mulwala by increasing the amount of sediment and toxic substances that enter the system following storm events. Sediment mobilised following the last bushfire in the Ovens River catchment will eventually reach Lake Mulwala. - Weeds: Patches of weeds within the lake have to potential to spread and become problematic for lake users if appropriate controls are not put into place. - Blue-Green Algae: The apparent increase in the number of serious blue-green algal blooms requires further investigation. - pH. An apparent decreasing trend in the pH of the water in both the lake and surrounding catchments is a potential long-term threatening process. Recommendations for areas of further study include: - A survey of the sediments to determine the extent to which nutrients, metals and pesticides have been accumulating within the lake and the potential for release of these substances back into the water column. - Determining the impact of stormwater on the water quality within the lake. - A study to determine whether stratification occurs within the lake. Stratification has important influences on the cycling of metals and nutrients within the system. - Clarification of the causes of differences in the various algal monitoring programs data in order to establish whether or not the incidence of blooms in the lake is actually increasing. - Improved monitoring programs to allow accurate calculation of loads for various important chemical parameters entering and leaving the lake. The current monitoring regime is not sufficient to calculate loads accurately. - Ongoing monitoring of vegetation distribution and health in the lake and its foreshore.