Cyanobacteria in the Baltic Sea and lakes: symptoms, map and safety

Cyanobacteria can ruin a day at the beach faster than a sudden storm. Green, cloudy water near the shore, foam gathering on the surface and a red flag raised by lifeguards are enough to change the day’s plans. For visitors, the key questions are practical: is it safe to enter the water, where can the current bathing-water status be checked, and what should be done if a child or dog has come into contact with suspicious water?

This guide covers the most important information about cyanobacteria in Poland, the Baltic Sea, lakes, reservoirs and popular destinations abroad. It explains why blooms occur, how to recognise them, which symptoms may appear after swimming or other contact with suspicious water, why children and dogs require particular care, and where to find local notices in Poland and other countries.

Closed bathing areas and one-day beach lists become outdated quickly. In practice, “cyanobacteria 2026” means checking the current status of a specific bathing area rather than relying on a general forecast for the entire coast. This guide helps you assess the risk before and during a trip, whether you are at the beach, by a lake, on a campsite, taking a city break by the water or travelling with a dog.

Contents

• What are cyanobacteria?
• Where do cyanobacteria come from?
• Green vortex over the Baltic Sea: cyanobacteria visible from space
• Cyanobacteria in Poland: the Baltic Sea, lakes and lagoons
• Cyanobacteria abroad: is this only a problem in Poland?
• The Baltic Sea outside Poland: Finland, Sweden, the Baltic countries, Germany and Denmark
• Finland and the Scandinavian lakes
• Lakes of Europe: Balaton, Garda, Trasimeno and inland bathing areas
• USA and Canada: Lake Erie, Lake Okeechobee and the season from spring to fall
• How to check for cyanobacteria abroad?
• What do cyanobacteria look like?
• Where can I check the map of cyanobacteria and the condition of swimming areas?
• Symptoms after contact with cyanobacteria
• Cyanobacteria and dogs: why is exposure especially dangerous?
• What to do after contact with cyanobacteria?
• What to do when a beach is closed because of cyanobacteria?
• Poland, the Baltic Sea and short trips: what to read next
• FAQ: frequently asked questions about cyanobacteria

Cyanobacteria in the Baltic Sea in 2026: where are they now and how to check the map

To find out where cyanobacteria are present in the Baltic Sea in 2026, do not rely on an old list of beaches. Check the current status of bathing areas in Poland’s official GIS Bathing Water Service. Conditions can change from one day to the next, or even faster, depending on temperature, wind, wave action, local blooms and decisions by the sanitary authorities.

The main searches people make before entering the water include: Baltic Sea cyanobacteria 2026, cyanobacteria map, cyanobacteria map 2026, where are cyanobacteria, Baltic Sea cyanobacteria map and the name of a specific town, e.g. Gdańsk, Sopot, Gdynia, Jastrzębia Góra, Władysławowo, Łeba, Ustka, Kołobrzeg, Międzyzdroje or Świnoujście.

The safest approach involves three steps: check the official GIS bathing-water map, read the notices at the beach entrance and assess the water on site. Do not enter water that is green or cloudy, resembles spilled paint, has surface scum or foam, or smells unpleasant, even if no warning was visible online a moment earlier.

Important: A cyanobacteria map is not a weather forecast. It shows the status of bathing areas and whether the water is suitable for swimming. Do not take the risk if a bathing area is marked as closed or swimming is prohibited, particularly when accompanied by children, older adults or a dog.

Cyanobacteria map 2026: colours in the official GIS Bathing Water Service

In practice, the cyanobacteria map for 2026 is the official GIS bathing-water map, which shows the current status of monitored bathing areas in Poland. It is useful not only on the Baltic coast but also at lakes, reservoirs and popular urban bathing areas.

Check the colour and notice shown for the specific bathing area. Green usually means that the water is suitable for swimming. Red or a no-swimming notice means that you must stay out of the water. A grey or inactive marker may indicate that the site is out of season, closed or has no recent update.

When searching for phrases such as “cyanobacteria map”, “Baltic Sea cyanobacteria map”, “cyanobacteria map 2026” or “where are cyanobacteria”, prioritise official data and local notices. News reports become outdated quickly, while beach conditions may change with the wind, temperature and wave action.

What are cyanobacteria?

Cyanobacteria, commonly called blue-green algae, are microscopic organisms that occur naturally in aquatic environments. They are found in seas, lakes, reservoirs, ponds and rivers. Although they are often described as algae, biologically they are photosynthetic bacteria.

Under normal conditions, cyanobacteria form part of the ecosystem and are not always a problem. Risk increases when they reproduce rapidly and form a bloom. The water may change colour, become cloudy or resemble green soup or spilled paint. Surface scum, foam, streaks and deposits may collect near the shore.

Visitors cannot determine from appearance alone whether a particular bloom is toxic. Some cyanobacteria produce cyanotoxins that can irritate the skin, eyes, digestive system and respiratory tract. In more serious cases they may affect the liver or nervous system. When a bloom is suspected, staying out of the water is the safest decision.

Where do cyanobacteria come from?

Cyanobacteria do not appear simply because a beach is “dirty”. Blooms result from a combination of environmental factors, most commonly warm water, prolonged heat, low wave action, weak winds, sheltered bays, shallow water and high nutrient levels.

Nitrogen and phosphorus compounds are the main fuel for blooms. They reach the water through agricultural runoff, fertilisers, wastewater, leaking septic tanks, stormwater drains and polluted rivers. Combined with high temperatures and calm water, these nutrients can cause cyanobacteria to multiply rapidly.

This is why cyanobacteria become a recurring issue in summer. Heatwaves, calm weather and warm water create conditions in which blooms become visible. During periods of very high temperatures, check the status of bathing areas every day. Our guide to coolcation, or holidays away from extreme heat.

Green vortex over the Baltic Sea: cyanobacteria visible from space

Updated: July 11, 2026. On the Internet, next to new observations of the Baltic Sea from 2026, an impressive photo of a green spiral taken by a NASA satellite a few years earlier is often shared. The phenomenon itself is real, but it is worth separating archival images from the latest data and explaining what a photo from space cannot confirm.

NASA and Copernicus photos: what do they show in the Baltic Sea?

The best-known image of the Baltic Sea’s green vortex was captured by Landsat 8 on July 18, 2018. It shows phytoplankton forming a spiral along the edge of a water vortex. NASA reported that three decades of satellite observations and in-water measurements pointed mainly to cyanobacteria. Some of the green colour may also have come from other chlorophyll-rich organisms, including diatoms.

Landsat 9 recorded another extensive bloom on July 20, 2025, south of the Swedish island of Gotland and southeast of Stockholm. NASA stressed that a satellite image alone cannot reliably identify the type of phytoplankton. Concurrent observations by Sweden’s SMHI nevertheless confirmed the presence of cyanobacteria.

The latest observation comes from the Copernicus programme. On June 22, 2026, Sentinel-3 photographed green streaks and spirals on the sea between Sweden and Latvia. Copernicus described the patterns as consistent with an extensive algal and phytoplankton bloom. An independent SMHI report dated June 25, 2026, also confirmed a strong cyanobacteria bloom at stations in the open Baltic Sea, where cyanobacteria dominated the phytoplankton community and chlorophyll concentrations were elevated.

Green bloom in the Baltic Sea: where do streaks and swirls come from?

The green spiral is not one giant organism or a dangerous vortex sucking in water. It is created by clusters of microscopic phytoplankton floating in the upper layer of the sea. Wind, currents, temperature differences and local circulations move organisms across the surface, arranging them in bands, swirls and spirals. The phytoplankton then act as a natural marker showing water movement.

The higher the concentration of chlorophyll-containing organisms, the more intense the green or yellow-green colour may be. A satellite image nevertheless covers a large area at one particular moment. It does not show automatically what conditions will be like a few hours later at the shore in Gdańsk, Sopot, Gdynia, Hel or another local bathing area.

Does every green swirl in the Baltic Sea mean cyanobacteria?

No. Green colouring may be caused by cyanobacteria, other phytoplankton, diatoms, suspended sediment or pollen floating on the water. Satellites show changes in colour and the extent of a bloom very well, but reliable identification requires samples, microscopic analysis and measurements by environmental monitoring bodies.

Not every cyanobacteria bloom produces toxins. Visitors cannot assess toxicity from the water’s appearance, however. Green or heavily clouded water, foam, surface scum, dense streaks or an unpleasant smell should be treated as a reason not to swim.

How do cyanobacterial blooms deepen dead zones in the Baltic Sea?

Extensive blooms are one of the symptoms of eutrophication, i.e. excessive enrichment of water with nitrogen and phosphorus compounds. When large amounts of phytoplankton die, fall and are broken down by bacteria, this process consumes oxygen. In deeper layers of the sea, this may deepen hypoxia and create very difficult conditions for fish and organisms living at the bottom.

This does not mean that every green spot visible from space immediately creates a new dead zone or announces the closure of beaches. However, it is a clear signal of a larger problem in the Baltic Sea: a large supply of nutrients, warm and stratified water and oxygen deficiencies, which may reinforce each other.

Does the green vortex mean a ban on swimming in the Baltic Sea?

Not automatically. Satellite images may cover tens or hundreds of kilometres and cannot replace an assessment of a specific bathing area. Check current notices from the sanitary authorities, the status in the official GIS Bathing Water Service, local signs, information from lifeguards and the appearance of the water near the shore.

Practical conclusion: Satellite images help show the scale of blooms, but before entering the water check the current cyanobacteria map and bathing-water status. If the water is green, cloudy or covered with foam or surface scum, stay out even when a broad satellite image does not show a clear bloom near the Polish coast.

Sources: NASA Earth Observatory – Landsat 8, July 18, 2018, NASA Earth Observatory – Landsat 9, July 20, 2025, Copernicus Sentinel-3 – bloom observed on June 22, 2026 and SMHI – June 2026 cyanobacteria bloom report.

Cyanobacteria in Poland: the Baltic Sea, lakes and lagoons

In Poland, cyanobacteria are usually associated with the Baltic Sea, but this is not only a coastal problem. Blooms may also occur in lakes, reservoirs, ponds, retention basins and inland bathing areas. Check the water status not only before visiting Gdańsk, Gdynia, Sopot, Hel or the Bay of Puck, but also before a weekend at a local lake.

The Baltic Sea is particularly susceptible to blooms because it is a shallow, brackish sea and highly dependent on river inflows. Quieter parts of the coast, bays and places where water mixes more slowly are particularly vulnerable. Therefore, reports about cyanobacteria often concern the Bay of Gdańsk, the Bay of Puck, the Tricity and sheltered swimming areas, although the problem may move with wind and currents.

Cyanobacteria may also appear seasonally in lakes and reservoirs. Risk rises after several hot days when the water is calm and streaks, foam or greenish deposits begin to collect near the shore. When planning a short break by the water, also see ideas for a long weekend and a short holiday in Poland, because when the swimming area is closed, it is worth having an alternative plan.

Cyanobacteria abroad: is the problem limited to Poland?

Cyanobacteria are found worldwide, not only in Poland or the Baltic Sea. They thrive particularly in warm, shallow and slowly mixing water and may occur in lakes, reservoirs, sheltered bays and lower-salinity seas, especially where nutrient levels are high.

For travellers, this means that checking the weather and water temperature is not enough. Learn what cyanobacteria are called locally and where warnings are published. Useful English search terms include: blue-green algae, cyanobacteria, harmful algal bloom, algal bloom advisory or bathing water warning.

The Baltic Sea outside Poland: Finland, Sweden, the Baltic countries, Germany and Denmark

The Baltic Sea does not end at the Polish coast. Blooms of cyanobacteria also occur in Finland, Sweden, Estonia, Latvia, Lithuania, northern Germany and Denmark. Quiet bays, shallow sections of the shore and places where the water is heated and poorly mixed are particularly vulnerable.

The highest risk in the Baltic region usually occurs during summer, from late June through July and August and sometimes into early September. Hot, windless weather and long periods without strong wave action increase the risk. When visiting another Baltic country, check local bathing-water notices, beach flags and information boards at access points.

Finland and the Scandinavian lakes

Finland, Sweden and other Nordic countries are known for pristine nature, but cyanobacteria can occur there too, especially in lakes, shallow bays, urban bathing areas and places where water remains calm for long periods. The most typical season is June to August, with more reports during the second half of summer.

When camping, kayaking, paddleboarding or travelling with a dog in Scandinavia, treat cyanobacteria as seriously as you would in Poland. Do not let a dog drink from a suspicious lake, and do not use the water for washing dishes, in a sauna, rinsing fruit or preparing food.

Lakes of Europe: Balaton, Garda, Trasimeno and inland bathing areas

Cyanobacteria may also appear in popular tourist lakes in Europe. The Hungarian Lake Balaton, the northern and central Italian lakes, alpine lakes, municipal bathing areas and artificial recreational reservoirs are monitored precisely because blooms can occur locally and seasonally.

At Lake Balaton, the greatest risk usually occurs in summer, especially after a sudden warming up and when the water is calm. This does not mean that the lake is “always dangerous”, but before swimming it is worth checking local announcements and the appearance of the water near the shore.

In Italy, a similar rule applies to lakes such as Garda, Trasimeno, Como, Maggiore and Iseo. Most popular swimming areas have regular monitoring, but local blooms may occur especially during hot, sunny and windless summers, at lower water levels and in shallower bays.

USA and Canada: Lake Erie, Lake Okeechobee and the season from spring to fall

In North America, cyanobacteria are an important public-health issue at lakes, rivers and recreational reservoirs. One of the best-known examples is Lake Erie, where bloom season usually runs from July to October. Florida is also affected, including Lake Okeechobee, where warm weather, nutrients and calm water can support extensive blooms.

Many Canadian provinces issue cyanobacteria warnings from late spring to autumn, most commonly between May and October. When travelling by camper van, staying by a lake, kayaking or holidaying with a dog, check local public-health alerts rather than relying only on the weather forecast.

How to check for cyanobacteria abroad?

When abroad, check local bathing-water services, public-health authority websites, national park notices, signs at beaches and warnings published by cities or regions. In English-speaking countries, search for the place name together with phrases such as: blue-green algae, cyanobacteria, harmful algal bloom, lake advisory or bathing water quality.

The main rule is the same everywhere: stay out if the water resembles green paint or soup, has foam or surface scum, or smells unpleasant, even when no current warning appears online.

What do cyanobacteria look like?

Cyanobacteria are most often recognised by changes in the water’s appearance. It may turn green, blue-green, grey-green, brown or yellowish. Instead of a clear surface, the water becomes cloudy. It may resemble spilled paint, green soup, green jelly or dust floating near the surface.

Streaks, flakes, surface scum and foam may gather near the shore. This matters because children often play there and dogs may drink the water. Even if the centre of the lake or reservoir looks clearer, the shoreline can be the most affected area.

Water affected by a bloom may also smell unpleasant. Visitors describe musty, earthy, muddy, decaying or ammonia-like odours. Smell alone is not decisive, but combined with foam, streaks and cloudy green water it is a warning sign.

Where to check the cyanobacteria map and bathing-water status

In Poland, the most important official place for tourists is Official GIS Bathing Water Service. This official map is maintained by Poland’s Chief Sanitary Inspectorate. It shows whether a bathing area is open, whether the water is suitable for swimming and whether a swimming ban is in force.

Bathing areas are colour-coded on the map. Green means the water is suitable for swimming, while red means that swimming is prohibited. A grey or inactive marker may indicate that the site is out of season or has no recent update. Select a bathing area to see details, including test dates and information about the location.

In other countries, equivalent information is provided by local water-quality services, municipal bathing-area websites, regional health warnings and signs at beaches. Search for the name of the lake or beach together with “blue-green algae”, “cyanobacteria” or “bathing water quality”.

In practice, a combination of three things works best: checking the official map or local announcement before going out, observing the flags on the beach and assessing the water once you arrive. Nature can change the situation faster than updates in the system.

Symptoms after contact with cyanobacteria

Symptoms depend on how exposure occurred, how much water was swallowed, how long the person was in the water and individual sensitivity. Exposure may occur through the skin or eyes, by accidentally swallowing water or by inhaling water droplets near the shore.

After contact with skin, the following may occur: itching, burning, redness, rash, hives, skin irritation, burning eyes or conjunctivitis. After swallowing water, the following may occur: abdominal pain, nausea, vomiting, diarrhea, fever, chills, muscle pain, headaches and dizziness.

Inhaling water spray may cause coughing, throat irritation, runny nose, shortness of breath or chest tightness. Children, the elderly, pregnant women, allergy sufferers, people with chronic diseases and people with respiratory problems should be especially careful.

Seek medical advice if contact with suspicious water is followed by severe vomiting, diarrhoea, breathing difficulties, fever, loss of balance, a widespread rash or concerning symptoms in a child.

Cyanobacteria and dogs: why is exposure especially dangerous?

For dogs, cyanobacteria are among the most serious summer hazards around water. A dog may be poisoned by drinking from a lake, sea or reservoir, playing in the water, retrieving a ball or stick, licking wet fur after swimming, or swallowing foam and deposits near the shore.

Dogs face greater risk because they often drink exactly where foam and deposits are most concentrated. After leaving the water, they may also lick toxins from their fur. Symptoms can develop quickly, so do not wait for them to pass on their own.

Warning signs in a dog include vomiting, diarrhoea, drooling, foaming at the mouth, weakness, staggering, difficulty walking, muscle tremors, seizures, breathing difficulties, sudden lethargy or loss of consciousness. If symptoms appear after contact with suspicious water, take the dog to a veterinarian immediately.

Prevention is simple: do not allow a dog into cloudy, green or foul-smelling water or water with foam near the shore. Bring fresh drinking water and do not let the dog drink from a lake, reservoir or sea when a bloom is suspected.

What to do after contact with cyanobacteria?

If you, a child or a dog have entered suspicious water, act immediately. Leave the water and avoid further contact with foam, deposits and wet clothing. Do not remain in a wet swimsuit or allow a child to continue playing near the shore.

• Rinse the skin with clean running water.
• Remove and wash your swimsuit.
• Rinse water shoes, toys, a paddleboard, kayak and other equipment with clean water.
• If water gets into your eyes, rinse them with clean water.
• After swallowing water, monitor your stomach symptoms and general well-being.
• If symptoms are severe, contact your doctor.
• For a dog, rinse the fur with clean water and prevent the dog from licking it.
• If your dog has symptoms, go to the vet immediately.

Do not boil water if you suspect cyanobacteria and do not use it to prepare food. Some cyanobacterial toxins may be resistant to high temperatures, so cooking is not a safe way to neutralize the risk.

What to do when a beach is closed because of cyanobacteria?

A closed beach does not have to mean the end of your vacation. It is best to treat this as a signal to change your plan: check other swimming areas, choose a walk, a bicycle route, a viewpoint, a lighthouse, a museum, a landscape park or a trip to a nearby city.

If you are at the seaside but the swimming area is closed, you can plan a day without entering the water: a walk along the shore without contact with foam, sunset, local food, attractions for children or a short trip away from the most crowded beach. In the case of domestic trips, it is good to have a backup plan before the trip – especially in July and August.

When planning your next trip, also check OndaTravel travel guides and ideas for city breaks from Gdańsk. If heat is the main concern, our guide to the coolcation.

Poland, the Baltic Sea and short trips: what to read next

Cyanobacteria are a practical concern because they may determine whether it is safe to enter the water or whether plans need to change. The related OndaTravel guides below can help you plan a trip in Poland, a short break, an alternative during a heatwave or a journey from Gdańsk.

POLAND

GUIDE

Where to go for a long weekend?

Calendar dates, ideas and short trips — a useful plan B when a beach is closed because of cyanobacteria.

See the guide

WEATHER

Water safety

Where is the storm? Live storm and precipitation radar

Storm map, precipitation radar and a quick check of where the rain is falling or if a storm is heading your way.

Check the storm radar

HEAT

TREND

Coolcation: a vacation without the heat

A guide to traveling in milder climates, when heat waves and warm water spoil your holiday.

Explore coolcation

GDANSK

CITY BREAK

City break from Gdańsk

Short trips from Gdańsk when you want to replace sunbathing with an alternative weekend plan.

Explore destinations

TOURISM

HUB

OndaTravel travel guides

The main gateway to travel guides, routes, trends and inspiration for future trips.

Go to the travel hub

BALTIC SEA

FUN FACT

Baltic Anomaly

The Baltic Sea has more stories than beaches and bathing areas. Discover one of its best-known mysteries.

Read on

FAQ: frequently asked questions about cyanobacteria

Can cyanobacteria be seen from space?

Yes. Extensive surface blooms may appear in satellite images as green, turquoise or yellow-green streaks and spirals. However, the image itself is not always sufficient to confirm the type of cyanobacteria or the presence of toxins.

Does a green vortex in a satellite photo mean swimming is prohibited?

Not automatically. A satellite image shows a large area of the sea. Check the safety of a specific beach through notices from the sanitary authorities, the official GIS Bathing Water Service and signs at the entrance to the bathing area.

Was the NASA image widely shared by the media taken in 2026?

The best-known Landsat 8 image of a spiral green bloom was taken on July 18, 2018. Copernicus Sentinel-3 recorded a newer Baltic Sea bloom on June 22, 2026, while an SMHI report published that month confirmed strong cyanobacteria growth in the open Baltic Sea.