This report reviews the significant physico-chemical processes and biological communities of five estuaries managed by Parks Victoria in southwest Victoria: Glenelg, Fitzroy, Curdies, Gellibrand, and Aire.
Estuary management can involve the periodic opening of sand barriers at the entrance to release water out to sea. This report aims to synthesise current information regarding how the timing and extent of these artificial opening events affect the various physical processes within the estuary systems. It is expected that this knowledge will facilitate management decision-making.
The estuaries examined in this report have the following characteristics:
ï Their marked seasonal inequality in river discharge means they may be totally fresh in winter or spring and marine dominated in summer and autumn.
ï Low summer and autumn river discharge results in sand bars obstructing mouths, often for extended periods.
In addition, these estuaries tend to be highly stratified: layers of fresh and salt water that do not mix. This is influenced by the low tides on the west coast of Victoria (low tidal mixing energy) and the state of the mouths.
When the estuary mouths are open, estuaries can fill and empty in response to the forcing of oceanic tides. Marine water flows into the estuaries via their mouths during flood tides and out again on ebb tides. When the mouth is narrow it can have a restrictive or "throttling" effect on water currents. This in turn influences the volume of water exchanged in a tidal cycle.
Other factors that may influence the level of salt water in an estuary include atmospheric pressure, wind, wave action, sedimentation, and seasonal variation in river flows.
The large difference in seasonal flows has a marked impact on the turbulent energy regime of the river's estuaries. Winter floods often flush salt water from the estuaries completely for several weeks at a time. As discharge drops in spring, fresh, well-oxygenated seawater re- enters the estuary. Over summer and autumn the mouth becomes more restricted as it sands up and tidal exchange is reduced. The bottom salt wedge layer stagnates and may become anoxic (i.e., no dissolved oxygen is present).
Dissolved oxygen (DO) concentrations are generally taken as an important measure of the health of aquatic ecosystems. Very low DO or anoxic conditions raise concerns about threats to estuarine productivity and communities.
The highly stratified nature of southwest Victorian estuaries, coupled with the common occurrence of anoxia in deeper saline waters places their ecosystems at a particular risk during mouth opening. Typically the top 1Ò2 m of surface water flows out to sea once the mouth is opened. Most estuarine organisms require water having DO > 5 mg/L and this concentration is typically found in the surface water layer due to physical exchange with the atmosphere and photosynthetic activity. If all water having adequate DO is lost from the estuary, leaving only anoxic or poorly oxygenated deeper water, extensive mortality of estuarine organisms may result.
The point at which artificial opening of estuary entrances is triggered under management license conditions has been determined according to anthropogenic concerns, such as inundation of farmland, rather than by reference to the consequences of the trigger level on estuarine ecology, particularly wildlife or wetland habitat.
Inadequate records of natural and artificial openings and a lack of relevant research mean that it is nearly impossible to say what the impacts of artificial opening have been, what the natural regime was or what the best managed entrance opening regime has been. Artificial opening seems goes back at least 50 years in most estuaries, and it is not known if the frequency of mouth opening has changed over this time.
To attempt to redress the lack of comprehensive information, each of the estuaries considered in this report is given a general physical description, a description of its catchment and coast adjacent to its mouth as well as any alterations to the estuary over time. The estuaryÌs environmental values and the status of the ecological understanding of the estuary are provided. Finally, the details of existing management plans and past and present opening regimes and protocols are provided for each of the estuaries under consideration.