Flowering of the sea or algae bloom is a natural phenomenon that occurs in warmer months. Discover why it occurs, how long it lasts, and whether it is safe for tourists to swim.

As the summers get warmer, you often contact us when you notice this phenomenon in the sea, when you see larger or smaller areas of the sea covered with a usually green mass floating near or even close to the shore. The water becomes slightly viscous, slimy. Sometimes these are small areas and sometimes they are slightly larger, and tourists then choose another beach, not knowing if it is safe to swim in such water.

Natural blooming of the sea in Greece - when and where does it occur?

“Sea bloom” or “algae bloom” is a natural and common phenomenon in the sea, lakes, and rivers that usually occurs with the arrival of warmer weather in late spring and early summer, as the sea warms up. Under these conditions, there is a sudden increase in the number of phytoplankton - microscopic algae that live in the water - as well as other microscopic and macroscopic algae.

This phenomenon is not common in Greece. It most often occurs in the Thermaikos Gulf because it is close to a city with a million inhabitants. The last major sea bloom was recorded in 2017 (the photo above shows how it looked). However, the city has invested in water purification, thus reducing the frequency of these events on a large scale.
We do not have data on all cases, but our followers reported that at the beginning of the 2024 season, they saw a small floating surface of these algae not far from Thassos. The waves and currents carried it further out to sea the next day.
Also in 2024, in the northern Adriatic, there was a case of a larger area affected, from Pula to Trieste. Fortunately, it did not last long (one of the photos shows how it looked). At the same time, the “green tide” was in the Black Sea in Bulgaria, as can be seen in the photo above.
In 2023, a large patch of phytoplankton and dirt came into the Aegean Sea directly from the Bosporus. It was a mini ecological disaster. Fortunately, it did not reach the Greek islands or Halkidiki, but dispersed somewhere in the center of the Aegean.

Why does sea blooming occur?

Several factors influence the growth of algae:

  • increased amount of substances entering the sea from agriculture, sewage, or industrial sources (e.g. nitrates and phosphates), which algae use for growth;
  • warmer water temperature;
  • calm or stagnant water;
  • sufficient sunlight for photosynthesis;

“Eutrophication due to increased nutrient inputs from agriculture, sewage and industry is one of the main drivers of harmful algal blooms in the Mediterranean.”
(Tsikoti & Genitsaris, Diversity, 2021 – link)

It is not always the same species that blooms, so the appearance and color of the water can vary.
Examples of different types of blooms

  • “Red tide” occurs when dinoflagellates rapidly increase and can produce toxins.
  • “Green tide” is often associated with cyanobacteria (blue-green algae) that can also produce harmful substances.

“Monitoring of microalgae in the main fish and shellfish farming areas of Greece (2000–2004) showed the presence of both non-toxic species that cause water discoloration and toxin-producing species.”
(Nikolaidis et al., Journal of Biological Research, 2005 – link)

Watch a video of what it looks like when this phenomenon spreads HERE.

Consequences of algal bloom on the environment, economy, and health

Environmental impact

When algae die and decompose, the amount of oxygen in the water decreases, which can lead to “dead zones” where fish and other marine organisms struggle to survive. This phenomenon can disrupt ecosystems and cause the death of marine organisms.

Economic effects

Algal bloom can affect fishing, tourism, and recreational activities. Fish and shellfish catches may decrease, and tourists may be reluctant to visit beaches where the water is unpleasant in appearance or smell.

Health risks

Most algal blooms are not dangerous for humans if the water is bacteriologically safe. However, some algae species produce toxins that can be harmful to humans and animals. Contact with toxic algae can cause skin irritations, and consuming contaminated seafood can lead to serious health problems.

Is it safe to swim during algal bloom in Greece?

Although the phenomenon is entirely natural, it is best to avoid swimming until experts take samples and determine that there are no dangerous chemical substances or pollution. Algae themselves are not toxic and harmful to humans, but the pollution that may accompany them is.

Although algal bloom is not usually dangerous, swimming in such water is often not pleasant due to the large amount of biomass - organic matter breaking down. The duration of such events depends on many factors: the composition of the algae community can change within a week, algae can be decomposed or carried away by currents, and additional organic matter may appear on the surface later on. Therefore, it is difficult to predict how long the bloom will last without sampling and on-site analysis.

How to prevent and control algal bloom?

Reducing pollution, less use of fertilizers, and better waste management can help reduce the nutrients that enter the sea. It is also important to regularly monitor water quality and research algal dynamics.

In some countries, such as Bulgaria, green algae are removed from beaches. Lithuania and Latvia have had projects to harvest algae and use them in agriculture as fertilizer, for biofuels, food, cosmetics, and medicines (the “Algae Service” project).
Useful texts that present examples of good practice for utilizing algae:
Useful texts presenting examples of good practice for algae exploitation:
AlgaeRevive (Interreg NEXT – Crno more)
This project, organized by the Greek NGO “Dimitra”, aims to create a sustainable ecosystem for research and industrial application of algae. The focus is on pollution reduction and the application of artificial intelligence to monitor algae blooms.
Functional Prototype in Athens – CO₂ valorization
Developed as part of the Life CircforBio project, it is located in Athens (NTUA – Lavrion), and uses autotrophic algae to exploit CO₂ from biofuels. The pilot includes waste recovery and the production of biodiesel, bioethanol and biogas.
BIOALGAFOOD
Coordinated by YIOTIS S.A., this project develops a sustainable national cultivation of macroalgae from Greek seas for the food industry.
Other examples:
How LIFE uses algae to create valuable products
How LIFE uses algae to create valuable products - describes the AlgaeService for LIFE project that collects algae and cyanobacteria from eutrophic waters in Lithuania and Poland and uses them to produce biogas, fertilizers, natural dyes (phycocyanin), cosmetics, etc. The project also uses prototype vessels, drones and satellites for monitoring. ( European Climate, Infrastructure and Environment Executive Agency, January 20, 2023)
Life Sunalgae - Description of the LIFE project that aimed to use diatomaceous earth (ALGICA® material) as an additive to increase the efficiency of solar panels. The material was developed in an environmentally friendly process with recycled nutrients and is also used in cosmetics and batteries. ( Swedish Algae Factory\, LIFE project 2018–2023)
SeaFilm - develops an edible and biodegradable algae-based film for packaging frozen fish\, thereby reducing the use of plastic and extending the shelf life of products. ( SeaFilm\, EU initiative)

Frequently Asked Questions (FAQ) about sea blooming and algae in Greece - answers to common doubts

Is sea blooming dangerous for humans?
Not all algae are dangerous, but the problem arises only if the water is additionally polluted.
Is it safe to swim during sea blooming?
It is best to avoid swimming in such water, although most blooms are not dangerous if the water is bacteriologically safe. During the tourist season, experts will likely perform an analysis, but this process takes time. If it is toxic and harmful for swimming, the beach will be closed. However, until the analysis is done, it is better to avoid swimming in that water and go to another beach.
How long does sea blooming last?
It depends on many factors - the composition of the algae community can change within a week, currents can carry away algae or organic matter can persist longer. Without on-site analysis, the duration cannot be accurately predicted.
Why does sea blooming occur?
Warmer water, more nutrients for algae (nitrates and phosphates from industrial and agricultural wastewater), calm conditions, and sunlight favor algae reproduction.
Can algae be removed?
In some countries, such as Bulgaria, Lithuania, and Latvia, there are projects for removing and utilizing algae as fertilizer, biofuel, food, cosmetics, and medicines.

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