Toxic blue-green algal blooms are appearing earlier and covering more water across Europe. In August 2026, health authorities in Rostock, Germany, confirmed toxin-producing cyanobacteria in the Baltic Sea off Warnemünde and warned swimmers away from affected beach sections. Satellite imagery showed a related bloom stretching across the Arkona Basin, between southern Sweden, Denmark, and Germany. Local authorities issued the swimming advisory within days of the first sighting, and warned that wind and weather made the bloom’s spread hard to predict.
This was not an isolated event. Cyanobacteria return to the Baltic Sea most summers, and recent blooms have grown larger and lasted longer. The pattern raises a question that applies well beyond one sea: how do you know if a body of water is safe, and what does testing tell you?
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What Causes a Harmful Algal Bloom
Cyanobacteria, commonly called blue-green algae, occur naturally in fresh and brackish water. Under the right conditions, they multiply fast and form dense mats on the surface. Three factors drive this growth:
- Warm water temperatures, which speed up reproduction
- Calm, sunny weather, which allows blooms to build at the surface
- Excess nutrients, mainly nitrogen and phosphorus from agricultural runoff, wastewater discharge, and fertiliser use
The Baltic Sea sits at the centre of this last factor. Decades of nutrient input from surrounding farmland and cities have left the sea in an over-fertilised state. Sweden’s meteorological institute, SMHI, confirmed a strong cyanobacteria bloom at open Baltic Sea monitoring stations in June 2026. Cyanobacteria dominated the phytoplankton community at the time of sampling.
Why a Bloom Cannot Be Judged by Sight
Not every bloom produces toxins, and toxin levels change even within a single bloom. This makes visual inspection an unreliable way to judge risk.
Cyanotoxins, the toxins some cyanobacteria produce, are linked to a range of health effects in humans and animals:
- Skin irritation, swelling, and allergic reactions on contact
- Nausea, vomiting, and stomach upset if swallowed
- Liver damage, in the case of microcystins, the most common cyanotoxin group
- Neurological effects, in the case of rarer toxin types
The Leibniz Institute for Baltic Sea Research notes that not every type of blue-green algae produces toxins. Low concentrations pose little risk to healthy people. Within a dense, fast-forming bloom, though, toxin levels can climb high enough to pose a real risk. The institute has recorded repeated cases of cattle, dogs, and ducks dying after drinking contaminated water.
Germany’s own bathing water record shows how often this happens. According to the German Environment Agency, cyanobacteria were behind almost half of all lake closures during last year’s bathing season.
Boiling water does not remove cyanotoxins. In some cases, boiling raises the toxin concentration rather than destroying it, because water evaporates while the toxin remains behind.
Testing methods are still catching up with the science. In 2024, researchers at Queen’s University Belfast identified a class of toxins called anabaenopeptins in an Irish water body for the first time. Because these compounds had not been detected in Ireland before, standard water testing did not screen for them. This is a useful reminder: routine testing only finds what it is designed to look for.
The Same Conditions Exist Beyond the Baltic
The Baltic Sea offers a clear, well-documented example of nutrient-driven algal blooms. But the underlying conditions are not unique to it. Warm water, nutrient enrichment, and slow-moving or standing water combine to create bloom risk in lakes, reservoirs, and rivers across Europe, including Ireland.
Any freshwater body that receives runoff from farmland, septic systems, or wastewater discharge carries some risk of a bloom during warm summer months. For farms, food businesses, and landowners who draw water from lakes, rivers, or private supplies, this creates a real safety and compliance question, especially where that water is used for livestock, irrigation, or food production.
Testing Is the Only Reliable Answer
Green or blue-green discolouration, surface scum, and an unusual smell can suggest a bloom is present. None of these signs confirms whether toxins are present, or at what level. Laboratory analysis is the only reliable way to answer that question.
Germany’s public bathing sites are tested for bacteria every two to four weeks throughout the bathing season, with results published for swimmers to check before they get in the water. Private and agricultural water supplies rarely get that same routine oversight, which puts more weight on the property owner to arrange testing themselves.
At Southern Scientific, we support farms, food businesses, and landowners across Ireland with chemical and environmental water testing covering nutrient levels and other contaminants that contribute to algal bloom risk. Understanding what is in your water is the first step in moving from reactive crisis management to proactive protection.
