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Summary of the summer 2006: Average blue-green algae conditions this summer despite warm periods

Press release 2006-09-13 at 12:00

Finnish Institute of Marine Research (FIMR) and Finnish Environment Institute (SYKE)

The blue-green algae conditions during the summer 2006 were rather normal in the open parts of the Gulf of Finland. At the coast of southern Finland, in the Archipelago Sea and in the Åland Sea there were in some areas more blue-green algae than on average. The occurrences were often of short duration. Some blue-green algae mats were also seen in the Gulf of Bothnia, but theyt were not as many as last summer. The occurrence of surface blue-green algae mats followed in general outline the prognosis published in May.

In lakes, the blue-green algae conditions of this summer can be considered average despite the high water temperatures. This fact can perhaps at least partly been explained by the scarcity of rain. Unusually small amounts of water ran into the water bodies and the amounts of downwashed algae-available nutrients were therefore unusually low. The most abundant occurrences were, as usual, seen in late July and early August. Due to the warm weather, the blue-green algae season has continued into September.

Sea areas

Despite the warm period in early summer the amounts of blue-green algae didn't begin to grow until the end of June. This fact was due to the slow warming of surface water and the low amounts of phosphorus. In early July the strong and variable winds caused at the coast of the Gulf of Finland an upwelling of cold and nutrient-rich bottom-near water up to the surface.

In the first week of July, mats of blue-green algae appeared in the central parts of the Baltic Sea, and they spread into wide areas in the entire southern part of the Baltic Sea. Within the Finnish territorial waters, the first discontinuous blooms were at the same time seen in the Åland Sea and in the Archipelago Sea, but they were very soon scattered by strong winds. At the mouth of the Gulf of Finland, the first surface mats of blue-green algae were observed in the second week of July. At the same time the blue-green algae were also abundant along the eastern part of the coast of the Gulf of Finland and in the open sea areas.

The warm weather continued, and in late July and early August the amounts of blue-green algae increased at the coast of the Gulf of Finland, in the Archipelago Sea and in the Åland Sea. However, the windy weather kept the algae mixed with the surface water. In the second week of August the surface blooms grew in these areas and in the open parts of the Gulf of Finland. There were also mats of blue-green algae in the Gulf of Bothnia. After mid-August the situation settled down in the open sea areas, partly due to strong winds. At the coast of the provinces Uusimaa and Varsinais-Suomi, and in the Archipelago Sea, blue-green algae were in some places abundant until the end of August. The algae, which had been mixed with water, rose to the surface when the weather was calm. In September, almost no surface mats have been observed, but at some sites there have still been considerable amounts of algae in water.

In the Gulf of Finland, the blue-green algae community was throughout the summer dominated by the toxic species Aphanizomenon flos-aquae, in late summer also by several species of the Anabaena genus. There were also small Chroococcales communities. The toxic Nodularia species were only found in small amounts in the Gulf of Finland, but they were rather abundant in the dying blooms of late August in the Åland Sea. Nodularia was also fairly common in the blooms of the central and southern parts of the Baltic Sea.

According to the blue-green algae prognosis published in May by the Finnish Institute of Marine Research and the Finnish Environment Institute, the risk of blue-green algae blooms was highest in the southern part of the Baltic Sea. The risk was moderate in the western and central parts of the Gulf of Finland and in the Archipelago Sea, and low in the eastern Gulf of Finland and in the Gulf of Bothnia. This prognosis came true fairly well. The heaviest blooms occurred in the southern and central parts of the Baltic Sea.

Inland water bodies

The nation-wide algae monitoring started in the second week of June. At that time, blue-green algae were seen in only one of the monitored lakes. The amounts of blue-green algae grew steadily until the end of July when the amounts were slightly higher than on the average. Blue-green algae were seen at some 20 % of the observation sites, but only 6 % of these occurrences were abundant or very abundant. At other sites the amounts of blue-green algae were low.

From late July to late August the number of blue-green algae occurrences and their abundance remained fairly constant. As in the previous years, abundant occurrences were mostly found in southern and southwestern Finland but also in Kainuu. In some lakes the first occurrences of blue-green algae were noticed as late as in September, and some of these occurrences were abundant. In most cases, the abundant occurrences were caused by the Anabaena blue-green algae which may be toxic. The blue-green algae season may continue long into the autumn if the weather is warm.

Only some selected lakes of Finland are included in the monitoring, and the observations give information about the local situation in a certain part of the lake at a certain time, not about the whole lake. The amounts of algae may vary between lakes and within lakes, depending on the local conditions and wind directions. The purpose of the monitoring is to give a general idea of the algae situation in various kinds of lakes.

The blue-green algae monitoring is over

The blue-green algae monitoring is a common project of the Finnish Environment Institute, the Finnish Institute of Marine research, the regional environment centres and the Frontier Guard. The monitoring campaign of this summer has now been finished. It was carried out by health and environment officials, frontier guard pilots, and voluntary citizens. Observations, based on visual impressions, have been made weekly at 260 sites in lakes and other inland water bodies, and at 63 sites in the sea. The information from the sea areas is also based on observations by frontier guard pilots and patrol boats and by sea scouts, on automatic measurements aboard commercial vessels, and on satellite images.


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