The waters along the coast of South Australia, near the city of Adelaide, have been showing an unusual and alarming picture for months. The sea has taken on a yellow-green color, signaling an ecological disturbance of enormous proportions. This phenomenon, first noticed in mid-March 2025 in the area around the Fleurieu Peninsula, is not just an aesthetic problem but an indicator of a massive algal bloom that has serious consequences for the entire marine ecosystem. High concentrations of chlorophyll, a key pigment indicating the presence of phytoplankton, have been detected along the entire coast, and satellite images reveal the scale of this phenomenon, which has been ongoing for more than five months with varying intensity and location.

The cause of the ecological crisis: the alga Karenia mikimotoi

Scientific analyses of water samples have confirmed that the cause of this phenomenon is a marine dinoflagellate known as Karenia mikimotoi. It is a single-celled organism belonging to the phytoplankton group, but unlike many other algae, this species lacks a solid cellulose wall, which makes it extremely sensitive and fragile. It is this fragility that contributes to its harmful effects; when the cells break down under the influence of waves or other mechanical stressors, they release a series of toxic compounds directly into the water column. These compounds, including hemolytic toxins, have a devastating effect on the surrounding wildlife. Blooms of this specific alga have been recorded worldwide, from Japan to Ireland, and are almost always associated with mass kills of marine organisms, leaving behind devastation in coastal ecosystems.

Devastating consequences for marine life

While Karenia mikimotoi poses a relatively minor threat to humans, it is lethal to marine life. The greatest danger lies in its direct impact on fish. The alga produces compounds that damage the delicate tissue of fish gills, causing suffocation and eventually death. The consequences are visible along the entire affected coastline, where mass kills of fish and other marine animals have been recorded. Citizens and scientists have recorded thousands of cases of dead animals, including various species of fish, mollusks, and crustaceans. The foams that form on the sea surface during the bloom further worsen the situation as they can damage the feathers of seabirds, destroying their waterproofness and ability to maintain body temperature, which also leads to death. The food chain is severely disrupted, and the long-term consequences for the populations of local marine species are yet to be assessed. This event is not just a temporary crisis, but a blow to the region's biodiversity with potentially long-lasting consequences.

A warning for humans too

Although the alga Karenia mikimotoi is not classified as highly toxic to humans, its impact is not negligible. Health authorities have issued warnings for residents and visitors to coastal areas. Exposure to the alga can cause a series of unpleasant symptoms. Contact with water where the alga is blooming can lead to skin and eye irritation, causing redness, itching, and rashes. An even bigger problem is the aerosol, i.e., tiny particles of water and algae that are dispersed into the air by wind and waves. Inhaling such air can cause respiratory problems, such as coughing, shortness of breath, and chest discomfort, even in people who are just walking along the coast. Swallowing seawater can, in turn, lead to digestive problems. Authorities have advised avoiding swimming and recreational activities on beaches with visible discoloration of the sea and foam. During some periods, the most affected beaches, such as Waitpinga and Parsons Beach, were temporarily closed to the public to protect human health.

The main culprit: A record marine heatwave

Experts from the Government of South Australia almost unanimously point the finger at the main driver of this ecological disaster: a prolonged marine heatwave that has been affecting the southern coasts of Australia since September 2024. Like heatwaves on land, marine heatwaves are periods of extremely high sea temperatures that last for an extended time. In this case, sea temperatures in the region were up to 2.5 °C higher than the long-term average for this time of year. Such conditions create the perfect environment for the proliferation of the alga Karenia mikimotoi. Warmer water, along with calm weather conditions and stratification of the water column, allows the alga to reproduce unimpeded, creating the dense accumulations we see as a bloom. This event serves as a dramatic reminder of how climate change and global warming directly affect marine ecosystems, creating conditions for more frequent and intense extreme events like this one.

Eye in the sky: The role of satellite technology

The marine ecosystem is an extremely complex and dynamic system, and monitoring phenomena like algal blooms from the surface can be very difficult and limited. This is where modern technology plays a key role, especially satellites orbiting high above the Earth. In this case, invaluable data is provided by the European Space Agency's Copernicus Sentinel-3 mission. Equipped with an advanced instrument called the Ocean and Land Colour Instrument (OLCI), this satellite can measure the color of the ocean with exceptional precision. By analyzing the spectrum of light reflected from the sea surface, OLCI can detect and quantify the concentration of chlorophyll-a, a pigment directly related to the amount of algae in the water. The data collected in this way allows scientists and authorities to get a broader picture, track the movement and spread of the bloom in real time, and better understand the dynamics of this complex event. Satellite monitoring has become an indispensable tool in managing crisis situations and making informed decisions to protect the environment and people.

Economic blow and government response

In addition to the ecological damage, such a prolonged and widespread algal bloom also causes significant economic damage to local communities that depend on the sea. Fisheries and aquaculture are the first to be hit. Due to fish kills and the risk of contamination, fishermen are facing a drastic drop in catches and income. Some have been forced to completely suspend their activities. Tourism operators are also at risk, including boat rental companies, diving excursions, and surf schools, as water quality warnings and beach closures have deterred visitors. Recognizing the seriousness of the situation, the federal and state governments have launched a joint assistance package worth 28 million Australian dollars. These funds are intended to provide direct financial support to affected businesses, including relief for fishing license payments, grants for small businesses that can prove a drop in turnover, and other forms of assistance to help them overcome the crisis and mitigate the economic consequences.