A deadly algal bloom continues to spread across South Australia coastlines. Source: 9News & Shark Watch SA

A deadly algal bloom that has killed hundreds of marine animals and left swimmers sick in Australia's south has now reached city beaches. Authorities have warned visitors to keep out of the water as 'nothing can be done' to prevent its spread.

After the karenia mikimotoi algae was first detected in March at popular surf spot Waitpinga Beach in South Australia, bringing foamy waves, washing ashore dead marine life and leaving swimmers sick. The state government has been tracking its progress and this week, testing confirmed it has now reached the Adelaide beach of West Lakes.

The state's Department of Environment and Water (DEW) told Yahoo News ongoing testing will be carried out by authorities to monitor the algal bloom and the public will remain informed. It warns people and dogs should avoid swimming or surfing in discoloured water, foam or where marine life is dead or in poor health.

"State government agencies are working together to monitor the algal bloom and provide updates and information to the public," the spokesperson said. "This includes regular water and wildlife testing at various locations to ensure South Australians have the latest environmental and health information they need to safely enjoy the coast."

Daily measures of chlorophyll-a — a marker for the algae — continue and increased levels of this marker have been found across Gulf St Vincent, Spencer Gulf, the eastern coastline between Port Adelaide and Aldinga, Investigator Strait on Kangaroo Island, Troubridge Point on Yorke Peninsula and along the coastline between Victor Harbor to Robe.

Dozens of dead marine animals wash ashore from algal bloom

Yahoo News has reported on multiple sightings of dead marine animals on beaches since the algal bloom began to spread, such as the millions of shellfish that washed up on Goolwa Beach in April and the string of sharks that have been found along the state's coastline.

State Senator Sarah Hanson-Young urged beachgoers to report any wildlife sightings to authorities, with the deaths continuing along the coastline.

"Over the weekend it was clear the algal bloom had spread as locals found a large amount of dead marine life at several metropolitan beaches, including at Brighton and Hallet Cove," she wrote online. "The science is clear — if we want to protect our oceans and the marine life that call them home, we need urgent climate action."

Hundreds of marine animals have died and washed up on shore. Source: ABC/Jock Lawrie & Sophie Landau

Among the killed wildlife are penguins, dolphins, sharks, stingrays, fish, squid and seals. A shark, estimated by an eyewitness to have been almost four metres in length, was taken away by authorities at Port Broughton after it was found dead in the water.

The algae is not fatal to humans, however, it can cause temporary skin, eye and respiratory irritation.

Authorities warn 'nothing can be done' as algae sweeps coast

Despite keeping a close eye on the situation through testing and reporting, DEW explained to Yahoo that not much can be done to stop or mitigate the impact of the algae.

"The algal bloom is a dynamic situation. Its movement depends on weather and water conditions, and therefore makes the effect on people and wildlife unpredictable," the spokesperson said. "Similar blooms around the world have lasted from a week to several months, depending on wind, rain and ocean temperatures. Nothing can be done to dilute or dissipate the bloom."

You can report any marine mortalities to Fishwatch on 1800 065 522. Alternatively, you can upload sightings to the SA Marine Mortality Project on iNaturalist.org.

This article originally appeared on Yahoo News Australia at https://au.news.yahoo.com/aussies-urged-to-stay-out-of-water-as-deadly-substance-spreads-across-beaches-023350938.html