State of the Environment 2006 – A dying shame – Australian coastal freshwater lakes

Author: Professor Graham Harris

Citation: Harris G 2006, A dying shame – Australian coastal freshwater lakes’, paper prepared for the 2006 Australian State of the Environment Committee, Department of the Environment and Heritage, Canberra,

The Gippsland Lakes are the largest of a number of coastal freshwater lakes (or lagoons) that lie along the east coast of Australia from Tasmania to the central NSW coast. These beautiful lakes are the home of many rare and endangered species and contain many valuable ecological assets from seagrass beds to breeding sites for fish and birds. All are separated from the sea by a barrier of sand dunes and would originally have opened to the sea intermittently when rainfall caused lake levels to rise and a flush of water to break through the dunes. Understandably people wish to live or spend their weekends beside or close to these places. Unfortunately the rapid spread of tourism and coastal development along the coast means that they are now subject to many pressures. These coastal lakes are suffering from land use change and forest clearing in their catchments, suburban development, alteration of freshwater inflows, storm and waste water discharges, over fishing and resource development of various kinds. In addition, because they make excellent harbours for small craft along an otherwise exposed coastline (and local residents found the occasional changes in lake level inconvenient), many of the lakes have been opened to the sea, and some are now permanently dredged at their mouths.

The consequences of these actions are lakes that now suffer habitat loss, salt ingress and the destruction of freshwater wetlands, increased nutrient loadings, overfishing and various kinds of pollution. The Gippsland Lakes have all these problems and more. Salt ingress since the permanent opening of Lakes Entrance in the 1880s has caused long term changes in the Lakes which are still going on. Reduced freshwater inflows (through the construction of dams in the catchments and the use of freshwater for irrigation and cooling in power plants) has made the salt ingress worse. The main Lakes are now frequently stratified – with warmer freshwater at the surface lying over a zone of colder, denser, salty water lower down. Salt water is eating away at freshwater wetlands and, by killing off the freshwater plants in Lake Wellington during the 1967 drought, has led to the replacement of clear, fresh water and aquatic flowering plants with turbid, brackish water and toxic cyanobacterial (blue-green algal) blooms. Excessive nutrient loads from agricultural and urban development in the catchments increases the frequency of these blooms.

Increased plant production in the water column (and algal blooms at the surface) leads to decomposition in the now stratified waters and the reduction in oxygen concentrations in bottom waters. Deoxygenation and anaerobic conditions in bottom waters hampers the ability of the Lakes to deal with nutrient inflows and, indeed, the Gippsland Lakes export anaerobic decomposition products (ammonia) to the ocean on the falling tide. If this were not enough the Gippsland Lakes suffer from other problems associated with air pollution from the nearby power plants of the LaTrobe Valley and from oil and gas extraction offshore. A combination of tectonic adjustment and extraction of oil, gas and water from below the sea bed offshore means that the Lakes are subsiding. Storm surges have a greater impact as the land subsides.

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