Lagoon Problems
Poor water quality and the organic ooze
A significant problem with the lagoon is the poor water quality. This is partly caused by the existence of an extraordinary large layer of organic material lying on the bottom of the lagoon, which is up to depths of greater than 2.4 metres in the centre in the lagoon.
What has caused the problem of the organic ooze?
The organic black sludge on the bottom of the lagoon is a result of a large amount of organic material that has sunk to the bottom of the lagoon since the lagoon was closed off to Gasworks Creek many decades ago.
A lack of flushing
The only water that enters the system comes from rain falling directly on the surface of the lagoon, some stormwater runoff from a small catchment (only 50% greater then the water catchment itself) and negligible ground water recharge. So essentially the lagoon only gets its water from rain which is intermittent and, because of the recent drought, has been severely limited. The average depth of the lagoon is less than one metre.
The Chironomid midges
In its current condition, the lagoon, is an excellent breeding environment for Chironmond midges (these are the thousands of little annoying insects which can be seen flying around the lamp posts of the lagoon at night). This situation has come about because of an excessive amount of nutrients (such as nitrogen and phosphorous) into the water and, because the water is so shallow and wide, the water volume is easy to heat up.
Although chemical methods, such as Abate and s-Methoprene pellets, have been used to control these insects the preferred method is the long term improvement in the water quality of the lagoon.
Eutrophication and algal blooms
The low water level, high water temperatures and high concentrations of nutrients (such as nitrogen and phosphorus) have caused eutrophication (a process that depletes oxygen from the water limiting plant and animal life) of the water and the growth of toxin producing algae. These algae are the green or purple ‘mist’ which can be seen suspended in the water and they make it difficult for many native species to survive.
Reduction in biodiversity and habitat
Biodiversity has also been reduced by the low water quality. The main reasons for these losses are:
- the number of aquatic habitats supported by different water depths and temperatures has been reduced. For example, different aquatic plants need different water depths to live in.
- increased turbidity caused by suspended particles from the peat causing less penetration of light into the water column.
- a low level of dissolved oxygen allowing anaerobic organisms to flourish and aerobic organisms to flounder.
- the high pH of the water that is quite alkaline.
- toxic algal blooms.
These conditions are not conducive to healthy habitats with a diverse range of species. In conditions such as these there tends to be a couple of species that will dominate simply because they are able to adapt more readily and are less susceptible to fluctuations in temperature, dissolved oxygen and algal toxins. A healthier water body could accommodate a much larger diversity of species.
Visual and social amenity
As part of the rehabilitation of Dowse Lagoon it is proposed to improve the visual and social amenity of the lagoon and the surrounding parkland.
