Alison Bennett Taylor, Denmark Bulletin
Wilson Inlet recorded its sharpest increase in water flow ever in one of the wettest starts to winter on record, according to a local research scientist.
Department of Water and Environmental Regulation environmental officer Dr Elke Reichwaldt told the Wilson Inlet Catchment Forum that climate change was already impacting water flows into the inlet. June was the fifth wettest month ever recorded in Denmark, with April also much wetter than average followed by an exceptionally dry May – unpredictable weather patterns that could be attributed to climate change.
“We had no flow into the inlet at all and then suddenly in a matter of days we had maximum flow,” Dr Reichwaldt said.
“In 2021 we had some flow but 2023 had the fastest increase in water flow ever recorded.”
The sharp increase in water flow resulted in an increase of 5cm a day in the inlet’s water level until the sandbar at Ocean Beach was breached on June 29.
About 50 people attended the annual forum hosted by the Wilson Inlet Catchment Committee and DWER’s Healthy Estuaries WA at the Riverside Club.
Dr Reichwaldt carried out continuous monitoring of the inlet throughout the year, sampling water from the surface and close to the sediment that lines the estuary’s bottom at seven different sites.
She told the forum that Wilson Inlet remained a healthy eco-system despite higher nutrient levels being recorded at some sites when the inlet was open to the sea.
Low oxygen levels caused by stratification – fresh water sitting atop the denser saltwater pushed in by ocean tides and swells – had contributed to high levels of phosphate being emitted from the sediment.
Nitrate levels were also higher than desirable during the wetter months.
DWER catchment modeller Karl Hennig told the forum that three sub-catchments making up just nine per cent of the 2300sqkm catchment – Cuppup, Sunny Glen and Hay – produced 88 per cent of the excess phosphorus loads.
Cuppup was responsible for 46 per cent, Sleeman for 30 per cent and Sunny Glen for 12 per cent.
The same three catchments produced 46 per cent of the nitrogen load, with the Upper Hay catchment responsible for 31 per cent of the excess.
Researchers were still trying to ascertain how much of the nitrate entering the inlet was biologically produced by clover pastures and therefore not readily available to microalgae. Management options for reducing excess nitrogen were limited.
WICC chief executive Shaun Ossinger said 60 per cent of local farmers were now actively involved in soil testing, reducing fertiliser use, fencing waterways to exclude stock and revegetating creeklines, which helped filter the water entering the catchment.
DWER aquatic science branch manager Malcom Robb said Healthy Estuaries WA was working with the fertiliser and dairy industries to reduce farm inputs, resulting in a 46 per cent reduction in phosphorus applied on participating farms and saving an estimated $3.3 million on fertiliser since 2016.
“It is working together to solve local problems on a local level with local people.”
This article appeared in the Denmark Bulletin, 7 December 2023.
