Harmful Algal Bloom in South Australia
SPRING SITUATION
By Janine Baker
Marine Ecologist & Educator, for Great Southern Reef Foundation
The extensive harmful algal bloom (HAB) that wreaked havoc over thousands of kilometres of South Australia’s marine environment, has continued into mid-spring in some parts of the State. Great Southern Reef Foundation provided detailed reports on this unprecedented algal bloom in April and July.
Since that time, metropolitan Gulf St Vincent and parts of Yorke Peninsula and western Spencer Gulf continue to be affected. Some part of southern Yorke Peninusla, Kangaroo Island and the Encounter surf coast that were severely impacted in autumn, are now heading towards the long, uncertain and largely undocumented process of recovery.
Chlorophyll concentration map from CopernicusEU Sentinel-3 on 13 August 2025 Multiple Genera Detected
Within the metro Adelaide coastal waters, data from phytoplankton specialists have shown ‘pulses’ of harmful Karenia dinoflagellates blooming and dying over the weeks and months. Whilst the location and relative density of phytoplankton can be tracked by satellites and confirmed by water samples, the exact agents and the mechanism of their harmful actions are still being investigated.
Across the central South Australian coast, the bloom remains dominated by a mix of Karenia species – some confirmed and others still being identified, and these are blooming at hundreds of thousands to millions of cells per litre in some locations. The bloom mix comprises lesser quantities of many other harmful dinoflagellate species, in the genera Karlodinium, Katodinium, Gymnodinium and related groups.
Some of these algal species can harm at low densities, and the unpredictability and severity of this multi-species bloom has become frighteningly clear over the months. In some ways, the bloom is “self-perpetuating” - many of the species can both produce their own food from sunlight but also concentrate and fix carbon from the water, for growth and toxin production.
WHAT’S STILL LIVING IN GULF ST. VINCENT?
Port Jackson Sharks, which often begin aggregating for mating in Gulf St Vincent in Spring, by Stefan AndrewsAs the HAB raged on into winter, close to 90% of the 11,800 community records during July came from metropolitan beaches, where more than 500 people photographed the early winter mortalities. Fiddler rays, Port Jackson sharks, shovelnose rays; a multitude of fishes from reefs, seagrass beds and sand habitats; razorfish, abalone, trough shells all died and washed up, and the long list goes on.
The onslaught continued over August and September, and questions that would have seemed absurd pre-bloom, now warrant closer investigation:
Do sharks and rays still live in Gulf St Vincent?
Will the razorfish beds ever recover?
Are there any live seadragons left?
Where have all the calamari gone?
latest impacts
Throughout September, leatherjackets continued to wash up in the thousands, particularly schooling, scavenging species such as juvenile bluefin and rough leatherjackets, fishes that are part of “nature’s clean-up crew”. Sadly, all life stages are being observed dead on beaches, from young-of-the-year through to large adults of both sexes. Larger reef leatherjackets such as spinytail and six-spine have also washed up in abundance.
A common sight across Gulf St. Vincent beaches in recent months: high densities of small leatherjacket fish. Seagrass-dwelling fishes such as blue weed whiting in Gulf st Vincent continued to be killed by the bloom over spring. Some of the common, shallow water reef fish species in Gulf St Vincent have also been affected. Examples include long-snout boarfsh, talma, scalyfin and old wife.
Blue Weed Whiting at Tennyson Beacvh by Andy Burnell Crab.e.camSlow-swimming, site-associated fishes such as cowfishes and pufferfishes continued to wash up in abundance. These fishes have small gill slits that would easily be clogged by the dinoflagellate cells. No studies have been undertaken to determine the impact of whole Karenia cells when in direct contact with these scale-less fishes. The cowfishes can secrete their own lethal toxins through the skin when under stress.
Abalone wash-ups at Glenelg by Johanna WilliamsMany kinds of crabs have washed up in waves of abundance over the months, with spring sightings dominated by rough rock crabs, large spider crabs, decorator crabs and sand crabs. Abalone continue to die around bloom-affected regions, with 745 observations uploaded to iNaturalist since spring began. Those community-reported abalone observations collectively represent thousands of individuals, particularly blacklip, staircase and circular abalone.
Loss of an Ecosystem Engineer
The widespread loss of long-lived, sporadically-recruiting, structurally-supportive shellfish species such as razorfish Pinna dolabrata is of great concern. Due to limited investment in reproduction over the 2-decade life span, Butler et al. (1993) reported that razorfish populations cannot survive high mortality events. Around 1,000 records of dead razorfish – ranging from single shells through to aggregations of hundreds - have been uploaded to iNaturalist, collectively comprising at least tens of thousands of razorfish.
Major mortality events have occurred on north-eastern Kangaroo Island and both sides of Gulf St Vincent. Almost half of these washed up later in the bloom cycle, in September. Razorfish have a major role as “ecosystem engineers”, providing a hard surface for other species to grow on, in soft sediment habitats and seagrass beds. Attached organisms such as barnacles, hydroids, sponges, sea squirts, small seaweeds and native oysters grow on the razorfish shells, and also provide feeding and living habitats for various mobile invertebrates and small fishes.
The 3D structure of the razorfish themselves also provide shelter and breeding opportunities for marine life. The ecological implications of “whole bed” loss of razorfish are still being determined, but this structurally significant species should be a strong candidate for habitat restoration efforts, well into the future. Smaller numbers of other structural species such as hammer oyster Malleus meridianus - especially in metro waters, native Ostrea angasi oysters and have also been recorded in the beach mortalities.
Dead and living razorfish by Stefan Andrewsloss of living habitats
The continued death and wash-up of tonnes of seagrass - roots and all - and brown canopy seaweeds and mixed seaweed understory in HAB-impacted areas over spring is equally concerning. Living, structural habitat with many critically important ecological functions is being lost. The June update provides more information about the ecosystem services of marine vegetation.
Seagrass beds under stress in Upper Gulf St Vincent.signs of resilience
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Giant Spider Crab
Spotted at Edithburgh by Elizabeth Solich on September 21
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magpie perch
Spotted at Edithburgh by Troy Johnson on October 7.
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Moonlighter
Spotted at Edithburgh by Troy Johnson on October 7
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Western Talma
Spotted at Edithburgh by Troy Johnson on October 8
In areas where the harmful bloom damaged habitats and biodiversity for months over autumn and winter, there are some signs of life - species that remained alive and have been photographed throughout winter and spring, and new recruits that are appearing at previously bloom-ravaged sites.
At Edithburgh and Wool Bay, divers participating in a multi-faceted community monitoring project have been recording the damage to (and loss of) attached biota on the jetty piles and sea floor, but also the indicators of resilience – species that survived months of Karenia-laden waters - and recovery. Even the appearance of a live stargazer - a large ambush predator that lives on the sea floor - is a heartening sign. These site-associated, long-lived fishes have been recorded dead in the hundreds in Gulf St Vincent over more than a half year of the HAB. Live adult cowfish, pufferfish and beaked salmon also show that not all individuals of susceptible species succumbed.
The mollusc, crustacean and fish fauna in the Edithburgh area have been severely impacted since April. Five months on, September photos of live sand crabs, blue crabs, gastropod shell eggs and young-of-the-year reef fishes such as brown-spotted wrasse, young talma, magpie perch and moonlighter offer some hope for future recovery of the reef fish fauna in that south-western side of Gulf St Vincent. Prior to the bloom, more than 115 bony fish species had been recorded by divers at Edithburgh.
The Great Southern Reef December update will showcase more of these signs of resilience, recovery and hope for the future.
The bloom has impacted thousands of square kilometres of SA coastal waters, but some area remain resilient. At the time of writing, the higher salinity waters of Northern Spencer Gulf remained free of the harmful algal bloom. It is hatching time for the next generation of the iconic Giant Cuttlefish, and time will tell what fate awaits those recruits, as they travel south towards the Cowell - Wallaroo line.
Stargazer and Southern Cardinalfish at Edithburgh by Troy Johnson 
Janine Baker is a marine ecologist and educator with 35 years of experience in South Australian marine research. She is an expert in species identification and biodiversity of the Great Southern Reef, contributing significantly to marine conservation efforts.
Read the comprehensive previous update by Janine for a deep dive into the marine crisis. She tracks the bloom’s persistence into autumn, the staggering loss of marine life and habitat, the patterns in species mortality, and the evidence pointing to widespread ecosystem collapse
Read Janine’s initial analysis of the harmful algal bloom. In this article Janine lays out what we know so far: the species involved, where the bloom is spreading, how marine life is being hit, and what data and citizen science are telling us.
