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According to the United...
Large rafts of potentially toxic blue-green algae have discouraged swimmers at some of Helsinki's beaches. The algae situation has been accelerated by recent warm weather conditions.
At the worst times, largish rafts of algae have been visible offshore, though the wind can easily blow them farther from the beach. Munkkiniemi has so far been the worst affected, but Hietaranta - Helsinki's largest public beach - has had warning signs up since mid-July. The beach has not been closed to sunbathers at any point. A lifeguard at Hietaranta noted that the situation had at times been worse, and that the relative quiet of recent days had been as much due to cloudy skies as anything else.
The City of Helsinki's Environment Center monitors water quality and algae at the city's beaches constantly throughout the summer season. A researcher warned that even after the visible rafts of blooming blue-green algae had drifted away, the toxicity levels in the water could remain for some time. A good many different species of blue-green algae are found in the Gulf of Finland; some of them produce neurotoxins and others hepatic toxins. According to the Finnish Environment Institute, the most common poisonous blue-green algae are Nodularia. This is found in the open sea, and its blooms form filamentous balls in the water which are visible to the naked eye as yellow-green, oatmeal-like masses. Blue-green algae blooms are difficult to predict. After blooming, the toxins dissolve and disappear from the water. The algae are a symptom of the eutrophied state of the Gulf of Finland.