Page 1125 - Week 03 - Wednesday, 30 March 2011
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seen around the edge of the lake. Instead of just being stocked with native fish we have such high levels of carp in this part of the river system that we could hold an annual “carp out” event to try and remove them. Is this really the best environmental management we can offer?
We need to understand the lake in the context of its catchment, the land that captures the rainwater that drains into the waterways that feed the lake. The lake as designed provides an essential ecosystem service. It improves water quality downstream by retaining water which is slowly released into the river. Sediments and nutrients settle to the bottom of the lake. But how much pollution can this lake deal with? Something is telling me that it has simply had enough.
Our city has become more urbanised and the areas of non-porous surfaces such as roads and car parks have therefore increased. So instead of soaking into the ground and being filtered through the earth, more water is flushed directly into the stormwater system. That stormwater brings pollutants such as dirt, fertilisers, oil and animal faeces into the waterway and, ultimately, the lake. Pollutants increase the level of nutrients, primarily nitrogen and phosphorus from detergents and fertilisers, which in turn encourages the growth of algae and bacteria. To make things worse, the carp, those wonderful feral beasts, have been accused of stirring up the nutrients at the bottom of the lake and making the water quality even worse. We must not forget about the sewerage plants which, even when operating to the highest current environmental standards, are also a significant source of nutrients.
Nutrients are a key contributing factor for the growth of blue-green algae, as well as light, warmth, still water and low flows. While they are a part of the natural system, blue-green algae can out-compete other species of algae and in bloom conditions can produce toxins. Government alerts warn that some susceptible people may experience symptoms such as skin irritation, flu-like symptoms or gastrointestinal illness. Then there is the impact on dogs, which seem to suffer even worse than humans when it comes to blue-green algae.
What about other bacteria? It seems that even moderate rain events lead to short-term spikes in faecal contamination in the lake, as rain washes animal faeces from the catchment. The National Capital Authority are not clear how much of this is from dogs or from waterbirds. At higher concentrations, this faecal contamination can be a risk to human health, and high levels have led to lake closures five times over the past five years.
This faecal contamination is picked up by the water quality testing conducted by the NCA under ACT health guidelines for recreational water quality, using intestinal enterococci which is the World Health Organisation’s preferred indicator of faecal pollution. The NCA currently conducts tests and reports on water quality in the lake in accordance with the approved Australian water quality guidelines. Physical, chemical and biological parameters are reported annually. Microbiological monitoring samples are also taken weekly in summer months at a number of beach locations. Algae is monitored visually every week and samples are taken on a monthly basis.
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