Toxic algal bloom off Adelaide kills more than 34,000 marine animals, turns beaches into 'marine graveyard'

A toxic algal bloom off South Australia's coast has killed more than 34,000 marine animals and left swathes of Adelaide's shoreline strewn with dead fish, sharks, rays and seadragons, local authorities and researchers said.

The bloom, first noticed by an experienced diver during a dive last September, now spans an area reported to be twice the size of the Australian Capital Territory and has already required extensive clean-up efforts. The City of Holdfast Bay, a coastal suburb in metropolitan Adelaide, has collected more than 37 tonnes of dead marine life in recent weeks.

The die-off has affected a range of species, including leafy seadragons, Port Jackson sharks, stingrays, fiddler rays and other bottom-dwelling sharks, according to accounts from divers and local officials. "This is a new thing for us," said diver and Department for Environment and Water employee Andy Burnell, 60, who first observed the bloom. "There've been some minor blooms in the past here and there, small-scale things, but this is totally out of left field for South Australia. It's the first bloom like this."

Algal blooms can be triggered by a combination of excess nutrients such as nitrogen and phosphorus entering coastal waters, warm temperatures and calm sea conditions. When a bloom intensifies it can reduce oxygen levels in the water, causing fish and other marine animals to suffocate. Researchers also noted that when waves disturb dense algal mats, toxin-containing aerosols can form; Burnell said surfers had emerged from the water "coughing and with headaches."

State authorities and scientists said options for controlling a bloom once it has developed are limited, because blooms commonly cover large areas and can persist or reappear in warm weather. Local councils have mobilised tractors and crews to scrape and remove algae and carcasses from beaches in an effort to limit public health risks and the local visual and smell impacts, but officials warned such measures address symptoms rather than the underlying marine event.

The scale and timing of the event have prompted scrutiny of monitoring and preparedness. Scientists from the Great Southern Reef Research Partnership told federal environment ministers in written requests that monitoring and mitigation funding was needed. In October 2023 the scientists wrote to then-environment minister Tanya Plibersek seeking what they described as $40 million over 10 years for algal bloom monitoring; they made a second appeal in May to the succeeding minister, Murray Watt. According to local accounts, those funding requests were not approved.

Local residents and business owners have reported economic and recreational impacts. Burnell said tourism to small regional towns along the coast has declined and recreational fishing has been avoided by some people. "There's also a huge economic impact," he said. "It's a huge, huge deal for South Australia. People are avoiding going to the small regional towns on the coast for their holidays. People have stopped fishing."

Authorities and researchers said the bloom was not unexpected in terms of the environmental conditions that can produce such events, though its scale and severity have been unusual for the region. Locals had hoped recent cold weather and storms might reduce the bloom, but scientists warned the algae could strengthen again as temperatures rise into the Southern Hemisphere summer.

Department and council staff continue to monitor the coastline and remove dead animals, while researchers seek to analyse samples to identify the dominant algal species, toxin levels and the bloom's drivers. Investigations will inform whether nutrient runoff from rivers, agricultural inputs, sewage discharges, or other factors contributed to the event.

The event adds to a series of high-profile marine disturbances in Australian waters in recent years, with researchers saying that climate-driven ocean warming and changing rainfall patterns can increase the frequency and intensity of harmful algal blooms in some regions. Federal and state officials have said they will assess scientific advice on monitoring and longer-term mitigation measures, while local governments focus on immediate clean-up and public-safety measures as coastal communities contend with the ongoing impacts.