New research sheds light
Researchers have pinpointed the cause of fish kills that often follow floods on the Richmond River floodplain.
Studies during the past 20 years have shown that a large amount of deoxygenated water flows into the Richmond estuary system as floodwaters drain away, but its source has been unclear.
Acid run-off from oxidised acid-sulfate soil also has been suspected of causing the kills. Evidence now shows this is not the case.
A two-year study, led by Southern Cross University environmental scientist Dr Vanessa Wong, is complete and the results are due to be published soon.
Dr Wong said a team of more than 20 people including fishers, Department of Primary Industry (DPI) Fisheries staff, Richmond River County Council (RRCC) staff, NSW Sugar staff and SCU students took more than 200 water samples from across the estuary following the January 2008 flood and the one in May this year.
A major fish kill followed the 2008 flood, while there was no such event this year.
“We’ve tied down exactly where the deoxygenated water is coming from,” Dr Wong said. “There was some concern previously that these deoxygenated waters were coming from the upper estuary.
“We found that it wasn’t the upper estuary at all. The source of the deoxygenated waters was the mid-estuary ‘back swamp’ basins and other low-lying areas.”
On site, the team members measured dissolved oxygen, salinity, pH and temperature. They then collected samples to test for trace metals, nutrients, chemical oxygen demand and dissolved organic carbon in the laboratory.
“During the flood recession in January 2008 the pH was near neutral,” Dr Wong said. “There was very little or no acidity that was discharging at the time of the fish kills. We know now that the fish kills are not caused by acidic discharges.”
Dr Wong said the major difference between the two floods was the temperature, which was much higher during the 2008 event.
“During the January 2008 flood, the entire river was deoxygenated all the way down to the river mouth at Ballina. Even at Ballina, there was no dissolved oxygen,” she said. “However, recovery of the river took place in an upstream direction and this started two weeks after the flood peak. So dissolved oxygen returned to normal levels at Ballina first, and then it moved up the river. Recovery occurs when regular tidal movement re-introduces oxygenated water into the river.
“Having the two floods, one followed by a big fish kill and one without, was actually quite useful in terms of research, because we could really start to determine what is driving deoxygenation.
“Firstly, there’s the effect of temperature, which will play an important role when we start linking in climate change.
“From the 2008 flood we were able to tie down the sources of the deoxygenated water, because of this fantastic group of volunteers who went out and took samples for a month after the flood peak across the whole estuary – from Lismore, downstream all the way to Ballina including the back swamps and the main river channel.
“It was a really good spread of samples across the estuary and it was a really good spread of samples in terms of time as well.
“When we analysed these samples we found that the upper estuary waters, upstream of Coraki, remained oxygenated. The river channel remained oxygenated in the upper estuary.
“So, we were able to say that the deoxygenated waters were coming from elsewhere.
“Deoxygenation of the river channel usually happens downstream of the back-swamp basins, in the Tuckean, Rocky Mouth Creek, Bungawalbyn Creek systems. We could determine this by looking at dissolved trace metals and other indicators of deoxygenating processes. And this all starts to happen within about a week of the flood peak.”
Dr Wong said floodwaters were stored on the floodplain in sub-catchment storage areas. These waters were held behind natural riverbank levees and could not flow out until the main river dropped, which could take six days to nine days. When the main river dropped the stored floodwaters started to discharge with high velocity.
In the past, back swamps in these storage areas remained inundated for about 100 days after the main floodwaters had receded. However, now drainage works connected these wetlands to the river and instead of slowly evaporating floodwaters flowed from the back swamps in a matter of weeks, which had led to wetland degradation.
“The drainage systems have also changed the vegetation on the floodplain. Native vegetation was dominated by wetland species, which were a lot more tolerant of water-logging. Now, the back swamps are a lot drier and dominated by pasture species which are intolerant of flooding.
“When these pasture species are flooded they die. Once they die, micro-organisms decompose the dead vegetation which consumes the oxygen in the floodwaters. Decomposition processes drive the oxygen down, resulting in very little oxygen in the over-lying floodwaters.
“When these floodwaters, which have no oxygen, recede they are exported into the main channel. That’s basically what causes the big fish kills.
“So there are two main drivers for deoxygenation – the change in vegetation and the extensive drainage systems on the floodplain.
“The other factor in these events is temperature. In January 2008 higher temperatures presented an ideal environment for the micro-organisms which decompose vegetation. The warmer it is, the faster these ‘bugs’ can decompose vegetation and the faster they consume oxygen.
“In May this year the water temperature was a lot lower. Water temperature at Wardell, for example, was about 17 degrees, which was about 10 degrees less than what was found in January 2008.
“So, even though the floodplain was deeply inundated, causing a lot of vegetation to die, the lower temperatures slowed down decomposition. As a result, there was a drop in dissolved oxygen but no deoxygenation event and no fish kill.”
Dr Wong said that now the source of the deoxygenated water had been confirmed there were management implications.
She said there would be many impacts of climate change.
“Up here they are predicting more extreme events, which suggests more frequent flood events,” she said. “That could lead to more deoxygenation and fish-kill events.
“They are predicting higher temperatures, which also drives these events. Where once it was cooler, with an increase in temperature you end up with an event where you didn’t have one before.”
Dr Wong said many areas on the floodplain were already either at sea level or below it.
“The other big impact will be from sea-level rise,” she said. “If the sea level is higher, these areas are going to be inundated more often and the water will take longer to drain off the floodplain.
“So, with climate change, there are a whole lot of factors which will probably increase the frequency and severity of these events.”
The study, titled ‘Impacts of sea-level rise on surface water quality on coastal floodplain wetlands’, is supported by the RRCC, the Northern Rivers Catchment Management Authority and the Australian Research Council.