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Summary of summer 2007: Blue-green algae season was short but intensive

Press release 2007-09-13 at 12:00

Released by the Finnish Environment Institute, the Finnish Institute of Marine Research and the regional environment centres

Again, weather conditions played a major role in the regulation of the blue-green algae populations. Compared with long-time mean values, the blue-green algae conditions this summer were average in the lakes but worse than usual in the sea. The chilly and windy weathers in the middle of the summer curbed the growth of the algae and the formation of algae mats in the sea. As a consequence, the large bluegreen algae mats occurred during an unusually short period. 

In lakes, the quantities of bluegreen algae reached a peak during the first week of July and again during the warm period that started in the beginning of August. During the peak periods, bluegreen algae were seen at almost one third of the monitoring sites and most of the occurrences were small. In the sea, the quantities of bluegreen algae reached the first peak in June and the second and higher peak in August. During the latter peak period there were algae mats in the open sea as well as along the coast. The blooms in June were larger than what was predicted.

Sea areas

The first observations on bluegreen algae were made already in mid-June in the eastern part of the Gulf of Finland and in the Baltic Sea south of the Åland Islands where the pilots of the Frontier Guard paid attention to strip formations on the sea surface. Measurements confirmed that the strips were not made of pollen. They were indeed blooms of bluegreen algae, even if the water temperature was only 14 centigrades. The blooms were comprised of the non-toxic bluegreen algae genus Aphanizomenon.

After the middle of June the surface strips were dissipated by winds. The following Aphanizomenon formations were observed around Midsummer in the western and eastern parts of the Gulf of Finland. In July the weather was chilly and windy. Only occasional and scattered blooms of bluegreen algae were observed in the Gulf of Finland, in the Archipelago Sea, in the Åland Sea, and along the coasts.

In the beginning of August the bluegreen algae situation in the sea areas was considerably better than in the average summer. Large surface mats were formed in the Gulf of Finland, in the Åland Sea, in the Archipelago Sea and even in the Gulf of Bothnia where the upwelling in July contributed to the process together with the warm weather in August. Moreover, there were numerous blooms along the coast. When the surface water had warmed up, the proportion of the toxic species Nodularia spumigena increased in the surface blooms. The surface mats were soon scattered by winds, but there were algae mixed with water until the end of August. 

The algae season 2007 in the sea areas can be characterized as short but intensive. The quantities of blooms were below the average of the previous years in July but above the average in June and considerably above the average in August. The quantities of bluegreen algae depend mostly on the weather conditions. Other factors are the quantities of nutrients brought to the surface by the winter storms, the heat periods in the summer, and the upwelling of deep water to the surface.

According to the bluegreen algae prognosis published in the beginning of June by the Finnish Institute of Marine Research and the Finnish Environment Institute, the bloom risk in the sea areas of Finland would be highest in the western and central parts of the Gulf of Finland and in the southern parts of the Archipelago Sea. In the eastern parts of the Gulf of Finland and in the Åland Sea the prognosis suggested an average bloom risk while the Gulf of Bothnia was regarded as a low-risk region. The largest blooms were formed in the Archipelago Sea and in the western parts of the Gulf of Finland, as it was predicted, but also in the eastern Gulf of Finland and in the Åland Sea. As in the previous years, algae mats were seen in the Gulf of Bothnia too.

Lakes

During the first half of June there were only small quantities of bluegreen algae in lakes. The number of observed occurrences increased considerably during the first week of July when algae were seen at one fifth of the monitoring sites. The chilly and windy weathers curbed the growth of algae during the rest of July.

A warmer period started in the beginning of August and the quantities of bluegreen algae increased considerably. The largest amounts of algae were observed during week 32 when algae were seen at every third observation site. 14 % of the sites had large or very large quantities of bluegreen algae. The large occurrences were mostly found in southern and southwestern Finland and in Kainuu.

The species observed during the monitoring were familiar ones. Most of them belonged to one of the genera Aphanizomenon, Microcystis and Anabaena,  and some of them are potentially toxic.

The purpose of the monitoring is to give an overall view of the algae situation in various types of lakes. There can be marked differences in the quantities of algae in different parts of a lake, depending on local conditions and such factors as the wind direction. If no algae are seen at the observation site, there may nevertheless be large amounts of algae in some other parts of the lake, or vice versa.

The nationwide algae report season is over

The algae report service of this summer is now closed. The reports were produced by the Finnish Environment Institute, the Finnish Institute of Marine Research, and the regional environment centres. The observations were made by environment and health authorities, Frontier Guard pilots, and voluntary citizens. The observations are visual estimates and they are made once a week. The number of sites amounts to 260 in lakes and rivers, and 63 in the sea. Additional information on the sea areas is obtained from the observations made by the pilots of the Frontier Guard, by the research vessels Aranda and Muikku and by commercial and patrol ships, and from satellite images.


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