Mark Tozer, a diver of 38 years’ experience, said he was left “stunned” and “heartbroken” yesterday after going diving at the site of a shipwreck, which lies some 12km off the coast of Adelaide’s Glenelg Beach.

Tozer, who founded the Rodney Fox and Mark Tozer’s Museum and Research Centre, told 9news.com.au he had visited the shipwreck many times before, but this was the first time since a toxic algal bloom began affecting waters off parts of the state’s coast last March.

According to the SA Department of Environment, there is nothing that can be done to dilute or dissipate the bloom naturally, and similar blooms around the world have lasted from weeks to several months.

The bloom, believed to have been caused by an ongoing marine heatwave and calm conditions, has started to clear somewhat thanks to cooler winter temperatures.

“What this species does is that when conditions are less favourable, it changes its mode of reproduction, and it ends up producing cysts, which sink to the sea floor, and they float around, and they wait, basically, in simple terms, until the conditions are better again,” he said.

“When there’s more light, more water, warm water, the appropriate salinity level, and enough nutrients, then they’ll start reproducing again.”

Rod Ness, a commercial fisherman in Victor Harbor for three decades, said last week sales at his fresh fish shop had dropped 70 per cent in the last three weeks.

The slump has prompted calls for the algal bloom to be declared a national disaster by the federal government.