Deakin research shows complexity of Curdies River problems

Deakin researchers presented their initial results at a community meeting in Peterborough on October 8, and while there were some minor positives, the overall picture was sobering.

The study focused on phosphorus and nitrogen compounds, the two nutrients that in high concentrations can trigger devastating blooms of blue-green algae.

A three-month algal bloom killed tens of thousands of fish and wiped out a range of other aquatic life in the Curdies River earlier this year, prompting the Corangamite Catchment Management Authority to bring in Deakin's expertise.

The researchers looked at phosphorus and nitrogen readings taken at two gauges, one on the major tributary of the Curdies called Scotts Creek, and another further downstream after Scotts Creek meets the Curdies. Because the concentration levels bounced around throughout the year, they looked at "high" and "low" readings over time.

For phosphorus, which is the main contributor to blue-green algal blooms, the high concentrations barely changed at the Curdies gauge over the 30 years, while the low concentrations decreased slightly from the mid-2000s.

Phosphorus attaches itself to soil particles and gets washed into waterways when rain sweeps sediment into rivers. The process is often driven by erosion especially on unfenced and unvegetated riverbanks. CCMA community and catchment services manager Sarah Holland Clift said the declining low phosphorus concentrations suggested the authority's fencing and vegetation programs were making a difference.

But in a discouraging sign, the gauge on Scotts Creek showed rising phosphorus levels since 1990, especially in the high concentrations. Deakin researcher George Cunningham said the high phosphorus concentrations also coincided with high flow levels, meaning a large quantity of nutrients were being washed downstream.

Dr Cunningham said the vast majority of nutrients were "going into the water between July and October" with the winter and early spring rain. They were swept down the river and into the estuary, where the slower water flow allowed them to sink to the bottom, building up over time. Then, in the summer months, warmth and sunlight promoted the growth of the blue-green algae, which fed on the nutrients deposited in the estuary.

There were algal blooms in the Curdies in 2020 and 2021, but this year's outbreak was by far the worst, with bacteria counts showing the 2022 bloom was 40 times worse than 2021 and 400 times worse than 2020.

While the marginal improvement in low concentration levels was important, the major driver of nutrient levels in the estuary was the high flow, high concentration levels, which hadn't improved over time.

Ms Holland Clift said the Deakin research showed there were no "quick fixes" and "more (was) yet to be done to address sources at high flows". "It is important to remember that these issues are not easily solved and won't realistically be solved quickly," she said. "There remains a large amount of work to be done in the river system."

The Deakin researchers will submit a final report in December, which will be used to produce new programs to tackle the health of the river.