Algae bloom lingers for months off Australia 

Today’s Image of the Day from the European Space Agency features a persistent algae bloom along the coastline of South Australia near Adelaide. For more than five months, the waters have been marked by the unusual phenomenon. 

A bloom of microscopic organisms, almost invisible to the naked eye, has painted the sea with hints of yellow-green, signaling an ecological shift that scientists have been watching closely. 

Persistent bloom near Australia

The bloom began in mid-March around the Fleurieu Peninsula and has yet to fade away. Its persistence has raised alarms for both marine ecosystems and people living along the coast.

The culprit behind this event is a dinoflagellate called Karenia mikimotoi. Although this algae is not classified as toxic to humans, it can still cause skin and eye irritation, along with respiratory discomfort for those who spend time near the shore. 

For marine animals, however, the impact is far more serious. The bloom can damage fish gills, suffocating entire populations and threatening local fisheries.

Heatwaves and unstable seas

The Government of South Australia has linked the prolonged bloom to an equally persistent marine heatwave. Since September 2024, southern waters have been unusually warm, setting the stage for algal growth on an unprecedented scale. 

Warm conditions, paired with calm and nutrient-rich waters, create an ideal environment for Karenia mikimotoi to thrive.

The species flourishes in stratified waters. When the ocean layers remain stable, sunlight penetrates the surface while nutrients accumulate in the shallows. 

That balance, while favorable for algae, disrupts the natural cycles that coastal ecosystems depend on. Once blooms like this take hold, they can be extremely difficult to break.

A closer look from space

While the bloom has unfolded along South Australia’s coast, the story is being told from far above. 

Satellites orbiting Earth now act as essential sentinels, offering wide-ranging views that scientists on the ground could never achieve alone. 

The Copernicus Sentinel-3 mission, equipped with its Ocean and Land Colour Instrument, has been tracking this bloom with remarkable precision. By measuring chlorophyll-a, which is a pigment that signals the presence of algae, researchers have been able to map the bloom’s reach and intensity.

Satellite data has become more than just a monitoring tool. It provides decision-makers with early warnings and helps governments respond faster to environmental changes that might otherwise go unnoticed. 

In this case, the extended bloom highlights the importance of pairing ground-based studies with large-scale observations from orbit.

Why long algae blooms matter

Algal blooms are not unusual in coastal waters, but their duration and scale often dictate their impact. Short-lived blooms may come and go without causing much disruption. 

Extended events like the one unfolding near Adelaide are different. They alter food webs, reduce oxygen levels, and can trigger cascading effects across entire marine ecosystems.

Fisheries are among the first sectors to feel the strain. When fish die in large numbers, not only do local communities face economic losses, but the region’s biodiversity also suffers. Recovery can take years, especially when multiple species are affected.

An uncertain future

The South Australian bloom is part of a larger story unfolding worldwide. As oceans warm and marine heatwaves become more frequent, conditions that favor harmful algae are expected to expand. 

Blooms that once occurred sporadically may become seasonal features. For coastal communities, this means preparing for more frequent disruptions to fisheries, tourism, and public health.

Even as scientists work to understand the mechanics of Karenia mikimotoi blooms, the bigger picture points to climate change as a driving force. 

Marine systems are highly dynamic, and shifts in temperature, currents, and nutrient flow create openings for species that thrive in disturbance. 

The bloom near Adelaide may be a local event, but it represents global challenges that are becoming harder to ignore.

Image Credit: Copernicus Sentinel data (2025), processed by ESA

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