Environmental groups say the state of the Giralang Pond should sound a warning to the ACT government about the fragile health of one of the city's important waterways.
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Large dead carp floated in the black water of the pond, located on the Ginninderra Creek, this week and a connecting storm water drain became so choked with rubbish its smell forced people living nearby to remain indoors.
Ginninderra Catchment Group Waterwatch co-ordinator Damon Cusack said that for the sake of water quality, the ACT government should consider prioritising cleaning out the pond, ideally dredging it, over building wetlands in the new suburbs of Gungahlin.
''We're very happy that new suburbs have these nice areas to go to and appreciate everything associated with the wetlands, it's fantastic and communities should have that, but we've also got to look at what impact is happening on existing wetlands,'' Mr Cusack said.
He said Giralang Pond was experiencing a ''black water event'', in which drastically low levels of oxygen led to the death of large fish.
Mr Cusack said high volumes of silt and mineral run-off from building sites associated with Gungahlin's rapid expansion were entering the pond, along with a large amount of tree leaves, and called for exclusion zones for leaf-dropping non-native trees along drainage lines.
This January was one of the hottest and driest on record in Canberra, which resulted in water temperatures being high in the pond. Vegetation broke down in the water and had drawn the oxygen out of it, Mr Cusack said.
''[Carp death] is a natural event in normal systems, in bigger systems, but we're increasing the chances of it happening with all the issues we're adding on top of that,'' he said.
Territory and Municipal Services Minister Shane Rattenbury said a City Services officer had visited the site on Tuesday and discovered about 10 dead medium-size to large carp.
He said officers would conduct water quality tests at Giralang on Wednesday and the fish would be removed within the next 24 hours.
''A preliminary assessment indicates that due to a lack of water inflows, combined with the recent extremely hot weather conditions, the available oxygen in the water was being depleted, resulting in larger fish species struggling to get enough oxygen,'' Mr Rattenbury said.
Mr Cusack said oxygen saturation levels for an on-stream pond should sit at about 80 per cent. He said Giralang Pond Landcare Group convenor Denise Kay had recently tested the pond water and found the oxygen level to be at 10 per cent.
Ms Kay said during the recent hot weather a stormwater drain running into the pond had been clogged with rubbish and became so foul smelling she had been forced to stay inside during evenings.
Mr Rattenbury said the services officer reported the gross pollutant trap, which the stormwater passed through, was blocked and Roads ACT had said it would inspect or clean it out on Wednesday morning.
