Rare toxic algae that can prove fatal to humans have been detected in a South Coast river just weeks after a catastrophic silo collapse caused hundreds of tonnes of wheat to spill into the water.
Water NSW issued an alert after the algae Alexandrium minutum were picked up in the Shoalhaven River at Nowra, about a kilometre upstream from the Manildra starch factory, where two wheat silos collapsed last month.
The algae can cause paralytic poisoning in humans if they eat contaminated shellfish, which can prove fatal in extreme cases. Mild cases can trigger numbness and tingling, as well as headaches, vomiting and diarrhoea.
Oyster farms about 15 kilometres downstream from the factory were closed last Friday after the algae were detected, but the Department of Primary Industry reopened them on Thursday after testing found the harvest area, and the oysters, were safe.
The Shoalhaven is a key oyster region in NSW, with farms at Shoalhaven Heads and Greenwell Point. The department said testing would continue weekly as the industry heads into its busiest time of the year.
But the public have been told to avoid eating wild shellfish they might collect themselves from the river near the Nowra Bridge, and to avoid the water at the recreational spot, which is popular with water-skiers. Fin fish should be cleaned thoroughly.
Alexandrium minutum is normally found in clean water off the coast, but was detected this month in the Shoalhaven River. Credit: Philippe Garcelon/Creative Commons Attribution 2.0 Generic licence
The algae were detected during testing linked to the collapse of two 30-metre-high silos at Manildra on October 17. The silos each contained 1000 tonnes of wheat when they collapsed about 10pm, with the contents of one ending up in the river. A third silo was also destroyed.
SafeWork and the Environment Protection Authority are investigating the collapse at the factory, which makes wheat starch, gluten and ethanol.
The Department of Climate Change, Energy, the Environment and Water has not detected the algae at this concentration in the Shoalhaven River before but said further testing showed the amount was dropping.
UNSW algae expert Bojan Tamburic said it was uncommon to see Alexandrium minutum in estuaries, as it was normally detected in clean water off the coast.
Oyster farms on the Crookhaven River at Greenwell Point, which comes off the Shoalhaven River.Credit: Wolter Peeters
“You don’t need to have a huge number of cells in order for that bloom to be toxic,” he said. “Even a small nutrient input could shift the algae population towards Alexandrium.”
He said the timing of the algal bloom, and the fact it was decreasing, suggested the wheat spill could have helped it develop by creating a spike in nutrients to feed the algae.
“There are algae in all water ecosystems at all times, and many different species. What happens in the case of a bloom is the environmental conditions become really favourable towards one of those species, and the population expands rapidly ... if we’re unlucky, it’s a bloom of a toxin species, as is the case here.”
The bloom would normally die off within a month, he said.
Manildra did not respond to a request for comment.
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