Clay trial reduces phosphorus to prevent algal growth

Recent trials of phosphorus-binding hydrotalcite clay (HT-clay) in a drain and a small lake in the Peel–Harvey estuary catchment have successfully reduced the amount of phosphorus available for algal growth.

Dr Svenja Tulipani, environmental chemist from the Department of Water and Environmental Regulation (the department) said that the trials are part of the Healthy Estuaries WA program’s work to reduce the nutrient loads in waterways from past and current land use.

“Many of our waterways can have excessive nutrients, especially phosphorus, which flow in from their catchments. These nutrients can cause algae to grow excessively and can lead to nuisance algae blooms and fish kills,” said Svenja.

The latest two trials have targeted Gull Road Drain, a tributary of the Serpentine River, and Cox Bay North Lake, an urban lake next to the Peel–Harvey estuary in Mandurah.

“In late 2023, we applied the clay to a flowing drain upstream of the Serpentine River, where it successfully bound up to 95 per cent of phosphorus at the treatment site,” said Svenja.

“Our latest trial in early 2024 has been at Cox Bay North Lake next to the estuary in Mandurah, which has been the first time we have applied the clay to a lake rather than a flowing waterway.”

Clay was applied in Cox Bay North Lake using a small boat over two one-day periods, with the first application in February and the second in March 2024.

Environmental scientist Ryan Kam said “The first application initially reduced phosphate concentrations by 95.7 per cent. Phosphate is a form of phosphorus that is readily available for algae to use for its growth.

“After this, the phosphate levels then started to increase gradually when the water levels in the lake increased, indicating phosphorus input from groundwater that feeds the lake,”

“We applied the clay again one month after the first application, and this second application reduced phosphate concentrations to almost non-detectable levels.

“Chlorophyll a concentrations, which are a good indicator for algal growth, reduced by 60 per cent from before the trial to five days after the second application. We expect these will continue to decline into winter in part because of reduced phosphorus availability, and because of slower algal growth in the cooler months,” said Ryan.

“The water clarity in the lake has also improved measurably after the two applications, although it does still appear green because of continuing presence of algae in the small lake.

“These results are really encouraging, although we don’t expect them to be a permanent fix given that there is likely to be continuing phosphorus input from groundwater,” said Svenja.  

“We will continue monitoring and investigating to see whether there are any sources of phosphorus that can be addressed to prevent future algal blooms in the lake, and whether the clay would be a feasible tool for the lake long term.”

These trials are funded by the Bindjareb Djilba Protection Plan and Healthy Estuaries WA – State Government initiatives that aim to improve the health of our waterways.

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