Seawater does not always have the familiar blue hue expected along the coast of South Africa. Sometimes the water takes on a rusty-brown, murky-red, or even pink color, causing beach visitors to question what has happened to their favorite stretch of coastline.
Although the color change may look alarming, it is not necessarily a sign of pollution. One possible cause is algal bloom—a natural phenomenon where microscopic organisms rapidly multiply in the water. Some such blooms are harmless, while others can negatively affect marine life, cause unpleasant odors, and, in certain cases, pose a danger to humans and animals.
Understanding the factors causing these changes will help better interpret what is seen during the next trip to the coast and determine when to stay away.
The color of seawater is determined by several elements, including the organisms living in it, suspended particles, and how sunlight interacts with these components. When the sea suddenly turns reddish or brownish, the culprit may be an algal bloom.
Algae is a diverse group of organisms that live in water and use sunlight to obtain energy. Many of them are microscopic and form an integral part of the marine food chain, providing food for small animals, which are in turn eaten by fish and other marine creatures.
However, under suitable conditions, certain types of algae can reproduce rapidly, forming a dense concentration of cells in the water. This state is known as an algal bloom. The color depends on the type of organisms involved, the pigments in their cells, and the density of their aggregation. Some blooms give the water a red or orange appearance, while others create shades of brown, green, or yellow; in some places, the sea may appear pink.
Not all color changes are caused by algae. A brown or discolored appearance of coastal waters can also be the result of sediment uplift by strong waves, river runoff after heavy rain, or decomposing organic matter.
You may have heard the term 'red tide,' which is used to describe reddish water near the coast. It refers to specific algal blooms that change the color of the seawater, although this name can be misleading. It is important to note that not every red tide is toxic, and not every harmful algal bloom colors the water red. Some blooms are brown or green, and others are almost unnoticeable. Their appearance alone does not provide information about water safety.
The West Coast of South Africa is particularly familiar with such phenomena. Nutrient-rich Benguela Current systems support an abundance of microscopic marine life, including algae, which can sometimes accumulate in large quantities.
When a bloom reaches the shore, the water color can change abruptly. In some cases, this is accompanied by an unpleasant odor, foam, or the washing ashore of marine animals. Nevertheless, the absence of these signs does not guarantee that the bloom is harmless.
Algal blooms develop when conditions allow microscopic organisms to reproduce or accumulate rapidly in one area. Several contributing factors include:
- Nutrients in the water: Nitrogen and phosphorus promote algal growth. These substances occur naturally in the marine environment but can enter coastal waters due to agricultural runoff, sewage discharge, and other sources.
- Sunlight and temperature: Many algae thrive with sufficient light and when water conditions are suitable for their specific species.
- Ocean currents and wind: Water movement can concentrate algae near the shore or carry the bloom into a bay.
- Changes in environmental conditions: Shifts in water circulation, weather patterns, and nutrient availability can influence when and where blooms occur.
There is no single explanation for every bloom. Different species react to different conditions, and scientists continue to study why some events become particularly extensive or dangerous.
Fortunately, algae are a natural and vital part of marine ecosystems, and most species are not dangerous. Microscopic algae form the basis of a significant portion of the oceanic food chain, supporting small animals and other organisms upon which larger marine life depends. Some blooms are simply a visible sign of this productivity; for example, certain diatoms can give the surf zone a brown color without making the water unsafe for swimming. The city of Cape Town notes that such blooms are observed in False Bay.
However, other species can produce toxins that affect humans, fish, mollusks, seabirds, and marine mammals. Even non-toxic blooms can cause problems if a large amount of algae dies and begins to decompose, reducing the oxygen available to marine life.
The important distinction is that a change in water color serves as a warning sign requiring verification, not proof that the water is safe or dangerous. The consequences depend on the type and size of the bloom.
Some harmful algal blooms release toxins that can accumulate in shellfish, making them unsafe to eat. Other blooms can irritate skin or eyes, or cause respiratory symptoms when toxins enter the air near the shore. Marine animals can also suffer: toxins can directly harm fish and other wildlife, and dense blooms and decomposing algae can disrupt oxygen levels in the water, sometimes leading to fish kills or mass stranding of marine animals.
South Africa has faced serious consequences from such phenomena. In December 2025, the Department of Forestry, Fisheries and the Environment warned the public against consuming white mussels and other shellfish washed ashore around St Helena Bay and Eland's Bay following suspected harmful algal bloom activity. The Department cautioned that shellfish may contain dangerous toxins, even if the bloom itself was not visible in the water.
This serves as a reminder that risks are not always obvious from the beach. While a discolored sea can be a fascinating natural sight, precautions should be taken before entering the water.
If you notice an unusual color change, a strong unpleasant odor, surface foam, frothiness, or dead fish and other marine animals, avoid swimming or wading in the affected water until receiving the latest local advice. Several sensible precautions can help make your coastal trip safe:
- Check local beach warnings. Before swimming, look for updates from your municipality, environmental agencies, or the Department of Forestry, Fisheries and the Environment.
- Do not collect shellfish for consumption without checking official notifications. Toxins can accumulate in shellfish even in clear water. Cooking does not always make all marine biotoxins safe.
- Keep children and pets away from suspicious water and washed-up marine animals. Do not allow them to touch or eat dead fish, shellfish, or other animals.
- Watch for symptoms. If you experience irritation, difficulty breathing, or other symptoms after contact with a suspected harmful bloom, move away from the affected area and seek appropriate medical attention. Severe breathing difficulties require urgent medical care.
- Observe official closures. If authorities issue a warning or restrict access, do not assume that an adjacent section of the beach is safe just because it looks unaffected.
Remember that not all harmful blooms are visible, and not every unusual color change is caused by algae. Local monitoring and official advice are more reliable than appearance.
For Cape Town beaches, the City provides up-to-date water quality information through its Beach Alerts website. In other parts of South Africa, relevant municipal and national environmental agencies should be checked for local updates.
