THE drying out of Bushells Lagoon is a natural event and wildlife will return to the area once it refills with water according to locals and wetlands experts.
Concern was raised about the health of the Lagoon at Wilberforce, after hundreds of dead carp were found, along with some dead eels, turtles and pelicans.
Dead wildlife
When the Gazette visited the lagoon on the morning of February 14, the dead carp were there. The Hawkesbury Environment Network has posted photos of dead eels, turtles and pelicans.
There were about 50 live pelicans when the Gazette visited, as well as other wildlife around the wetlands.
The dried up lagoon
Bushells Lagoon is located at Wilberforce, and is surrounded by farm land.
Hawkesbury Council general manager Peter Conroy confirmed the Lagoon is ephemeral, meaning it experiences regular periods of drying out.
Hawkesbury Environment Network committee member Richie Benson said he first noticed the water levels dropping about two weeks ago.
“On Sunday I went down there and found thousands of dead carp,” he said.
“Most eels were dead and some smaller ones were getting out of the water to get air and dying on the mud.
“What we're seeing is that because the water is so shallow, it has heated up a fair bit, and it is too hot for dissolved oxygen levels to be safe and things have just suffocated.”
Benson said more trees were needed to shade the lagoon and prevent erosion into it, to stop it silting up.
“When we do get floods it carries a lot more silt and sediment...[the water] is not as deep anymore and then the water heats up a lot quicker.”
Local knowledge
Keith Brandwood can see Bushells Lagoon from his house at Wilberforce and said this was a natural occurrence that happened from time to time.
“It is a normal thing, once it fills up again you have a natural bloom of vegetation. It is beautiful when they fill back up again,” he said.
“Obviously as lagoon levels drop the water temperature and pollution rises and some species won’t cope but that is normal.”
Mr Brandwood, a bird watcher, said the carp dying off would allow the vegetation to thrive once the water levels rose again.
“All the seeds collecting on the dry surface will bloom once again and it will be like that until the carp grow to such a proportion and eat away at the vegetation,” he said.
He added that this was a phenomenon that happened to many wetlands across the Hawkesbury.
“Pitt Town lagoon, if you had gone there a few weeks ago, there were 3000 birds but there is no water now,” he said.
Natural cycles
University of Technology Sydney associate professor Simon Mitrovic studies fresh water ecology.
Dr Mitrovic said while he could not comment specifically on Bushells Lagoon, he said generally speaking, ephemeral wetlands did dry out naturally, this could include large fish kills.
“In times of low water, you can get increased algal growth and this can lead to low dissolved oxygen at night when the algae respire and use up the oxygen and this can often lead to fish kills,” he said.
“Warmer temperatures in the water can contribute to that as well.”
Dr Mitrovic said aquatic life noramlly returned after such events.
“Ephemeral wetlands do dry out under natural cycles,” he said.
“Once the water returns fish will come. There is usually natural connectivity between wetlands and creeks and pools, so when the water comes back the aquatic life will come back.”
Dr Mitrovic added that the break down of organic matter could also contribute to fish kills by removing oxygen from the water.
He said to determine the actual reason why a fish kill occurred, a detailed study would need to be conducted.
Hawkesbury Council response
Hawkesbury Council general manager Peter Conroy has told the Gazette that Bushells Lagoon is owned by the Department of Crown Lands, but that Council is responsible for its management.
“The Council currently have a role in the control of noxious and environmental weeds at the Lagoon,” he said.
Mr Conroy said Council was dealing with the EPA, who had been alerted to the problem by a resident.
“Council assumed that the EPA is taking a lead role in this investigation in response to the number of fish kills,” he said.
“At this stage Council are unsure of the specific actions EPA are taking in regard to the water testing and the investigation of dead birds but we understand that Taronga Zoo may be involved.”
