No oxygen, no fish: Investigating dead fish in Richmond River post floods

ABOVE: Dead fish in the Richmond River, downstream of Tuckean barrage at Bagotville. Photo: Damien Maher from Southern Cross University.

After the worst flooding event on record, followed a month later by renewed flooding, scientists are studying the severe deoxygenation of the Richmond River system.

Project leader Professor Damien Maher from the University’s Faculty of Science and Engineering said the deoxygenation of the water was extensive.

“After the first flood in late February there was no oxygen in the river between Ballina and Coraki. That’s around 60 kilometres of river and estuary with no oxygen and therefore no fish. A lot of the dead fish were washed out to sea during this event due to the magnitude and duration of the flood,” Mr Maher said.

Professor Scott Johnston said deoxygenation happens when floodwaters overtop the riverbanks and spill out onto the lower floodplain covering vegetation and organic matter such as grasses.

“When this vegetation starts to decay and rot in the floodwaters it can deplete all the oxygen in the overlying water,” he said.

While zero oxygen in the water can cause large fish kills and it’s just the visible tip of the ecological iceberg, Mr Johnston said.

As the flood waters slowly recede, researchers say a ‘second wave’ of impact will follow as the floodplains begin to drain.

Mr Maher’s research team is conducting experiments on the water draining off the floodplain to assess how it will exacerbate the current low oxygen event and contribute to other contaminants such as metals and acids.

“Large areas of the floodplain have acid sulphate soils in them at fairly shallow depth. So, once the flood recedes and the surface water drains away, it starts pulling the shallow ground water from the wetlands into the river which has high levels of sulphuric acid and metals,” Mr Johnston said.

“This is an ongoing issue that the Richmond has been facing. Often after these flooding events the river is not only subject to deoxygenation and fish kills, but also acidification and low pH levels, particularly in the tributaries like Broadwater and Rocky Mouth Creek.”

The metal contaminants draining off the floodplains can also be highly toxic to aquatic life, causing secondary fish kills.

Mr Maher said the scale of the flooding event provided a new opportunity.

“The sheer scale of this flood has meant that all those fine sediments that wash off from other smaller flood events and deposit on the riverbed have been scoured out. So, we’ve nearly had a reset of the system where it’s been taken back to something closer to the original morphology prior to developing the catchment,” he said.

“If there are ways to stop the excess sediment in the upper catchment that’s been moved around through these landslides and prevent that from getting back into the river, we may actually be looking at a river system that’s partly reverted back towards its natural state. So, there’s opportunity to enhance that trajectory in a positive way.”

The research team will continue water quality monitoring and research to track the recovery of the Richmond River system, integrating the data into a model which will help to provide a predictive tool for assessing fish kills into the future.

The Estuaries and Catchments Science unit of the NSW Department of Planning and Environment has been working closely with the university to provide data on fish kills in the Richmond River.

This has included boat-based surveys and the strategic installation of water quality loggers.

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