Planktonic algae are an important part of marine ecosystems, and algal blooms at sea are naturally occuring phenomena. However, these mass occurences of microscopic algae have become more frequent and intense due to the eutrophication of the sea, which is caused by various types of human activity. Harmful and toxic algal blooms have occured annually in the Baltic Sea in recent years.
There are essentially two annual blooms in the Baltic Sea. The spring bloom takes place in early March-May, depending on the area and year. The blue-green algal blooms consisting of cyanobacteria occur mainly in late summer, in July to September. In addition there can be regional algal blooms in summer, depending on the weather and the nutrients available in the water.
Summer 2002 - Cyanobacterial blooms almost as intense as in the record summer 1997
Cyanobacterial blooms this summer were much like those during the record year 1997. High levels of phosphate phosphor in surface water during both summers made possible strong cyanobacterial growth and the hot and sunny weather with little wind collected the algae to large surface accumulations. Different from the year 1997, no large surface aggregations were observed after mid-August, most of the aggregations kept offshore and little cyanobacteria were observed in the Bothnian Sea this year.
The strong stratification built up during a couple of years in the Gulf of Finland disappeared during the autumn storms 2001. In the same process phosphate phosphor, dissolved from bottom sediments to deepwater, was transported up to the surface. This predicted good growth conditions for cyanobacteria during summer 2002.
After the spring bloom 2002 there were still high levels of phosphate phosphorus in surface waters, especially in the Gulf of Finland. Cyanobacteria mixed in the water were observed around Åland islands, in the Gulf of Finland, Bothnian Sea and Bay of Bothnia during the second week in June. Cyanobacterial concentrations were still low and the dominant species was the nontoxic Aphanizomenon. Cyanobacterial growth accelerated towards the end of June but strong winds kept the surface waters mixed and cool. More nutrients were brought to the Gulf of Finland surface waters by these same winds through upwelling along the northern coast. During the second week in July, as the weather calmed and surface waters warmed up, extensive cyanobacterial surface aggregations built up. These aggregations grew to cover most of the offshore Gulf of Finland. From mid-July and onwards surface aggregations were observed in the Archipelago Sea.
The fraction of the toxic species Nodularia increased in the phytoplankton flora as cyanobacteria grew more abundant. By the third week in July Nodularia was the most abundant species of cyanobacteria in offshore areas, even if the total amount of cyanobacteria decreased. The nontoxic Aphanizomenon grew more abundant in the Gulf of Finland towards the end of July. Strong winds during the last week in July mixed finally the surface aggregations in the Archipelago Sea and Gulf of Finland but calm periods followed and some aggregations formed again.
The variation in the amount of phytoplankton (chlorophyll-a mg m-3) in the western Gulf of Finland according to measurements by Alg@line. The green line are the monthly mean values for 1992-2001 and the red dots are the values for 2002.
In the beginning of August large, but discontinous, cyanobacterial aggregations were observed in the Gulf of Finland, Archipelago Sea and south of the Archipelago Sea. In the Gulf of Finland, Aphanizomenon grew clearly more abundant than Nodularia. By mid-August cyanobacterial surface aggregations disappeared from the the Gulf of Finland and the Bothnian Sea but some local blooms remained in the Archipelago Sea.
In the Bothnian Sea, the first surface aggregations were observed on the last day of July and again during the second week of August. The dominant species in these Bothnian blooms was the toxic Nodularia. Some rests of these blooms were observed aboard R/V Aranda during a cruise at the end of August. As in preceding summers no surface aggregations were observed in the Bothnian Bay but local blooms were seen outside Kemi. Surface water temperatures in all Finnish offshore areas were several degrees above the usual. The longlasting sunny weather and the decrease in algal concentrations made offshore waters in the Gulf of Finland more clear but at the end of August there were still observed blooms in nutrient rich and sheltered bays in the Gulf of Finland coast as well as the Archipelago Sea. During the end of August, local blooms of nontoxic dinoflagellates coloured the water red-brown in the Archipelago Sea.
In 2001, the blue-green algal blooms in the Gulf of Finland and the Archipelago Sea were less intensive than in the years 1997 and 1999, but in the southern Baltic Proper, the Arkona Basin and the Belt Sea area the surface accumulations were extensive and dense. Due to the mild winter of 2001 the spring bloom started about two weeks earlier than average in the last ten years. The spring bloom was about the average duration and intensity in the Gulf of Finland, but in the northern Baltic Proper and the Arkona Basin the maximum exceeded average intensity of the last ten years.
Blue-green algae in the Gulf of Finland during the summers 1998, 1999, 2000 and 2001
For more information, contact Mr Mikko Kiirikki, senior research scientist, Finnish Environment Institute, tel. + 358 9 40300 214; Mobile: +358 40 8678366.
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Phytoplankton spring bloom
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Due to the mild winter in 2001, the spring bloom started in March about two weeks earlier than has been average in the last ten years. Its duration and intensity in the Gulf of Finland was about average, but in the northern Baltic Proper and the Arkona Basin the spring bloom's maximum exceeded the average intensity of the last ten years. The vernal species composition was the normal combination of arctic cold-water diatoms and dinoflagellates.
Gulf of Finland and Archipelago Sea
The spring bloom started in mid March, reached its maximum in mid April and ended in late May in the Gulf of Finland. Diatoms such as Thalassiosira spp., Achnanthes taeniata and Chaetoceros spp. dominated in the western Gulf, and the dinoflagellates Scrippsiella hangoei and Peridiniella catenata were most abundant in the central Gulf.
Baltic Proper, Arkona Basin and Belt Sea
In the Arkona Basin and the northern Baltic Proper, the spring bloom started around mid March. Bloom maximum was reached in the Arkona Basin in the end of March and in the northern Baltic Proper before mid April. The spring bloom terminated in the southern Baltic Proper in early April and in the northern Baltic Proper about one month later.
The dominant diatom and dinoflagellate species during the spring bloom in the Baltic Proper and the Arkona Basin were largely the same as in the Gulf of Finland. In the Gulf of Riga algae started to increase in late March, and the spring maximum was reached during mid and late May. The dinoflagellate Peridiniella catenata and the diatoms Achnanthes taeniata, Thalassiosira baltica and Chaetoceros wighamii formed the spring bloom in the Gulf of Riga, but small flagellates were also common.
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Potentially toxic blooms
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Baltic Proper, Arkona Basin and Belt Sea
In the Belt Sea the potentially toxic dinoflagellates Dinophysis acuminata and D. norvegica and a potentially toxic flagellate Chattonella sp. were very abundant in the turn of March and April. Occurrences of toxic dinoflagellates cause annually temporary closing of aquacultures, since algal toxins accumulate into mussels. In 1998, Chattonella sp. caused high mortality of fish in Danish and Norwegian waters, but in 2001 no toxic effects were observed. Chattonella sp. occurred in the Baltic Sea region for the first time in 1998.
In late August, dense reddish blooms of the toxic dinoflagellate Alexandrium ostenfeldii were formed in the Gulf of Gdansk. No previous records of so intensive and luminescent blooms of A. ostenfeldii have previously been known in the Baltic Sea. Luminescent blooms make the water glow in dark.
Gulf of Finland and the Baltic Proper
In early August moderate concentrations of the blue-green algal toxin nodularin were measured in the Gulf of Finland and the northern and central Baltic Proper. Nodularin concentrations were at least two times higher in the Gulf of Finland than in the Baltic Proper. Nodularin is a hepatotoxin, which damages liver. The toxin is produced by the blue-green alga Nodularia spumigena, which is a common bloom-forming species in the Baltic Sea.
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Blue-green algal blooms
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In 2001, surface water temperatures stayed relatively low in June and early July and the amount of blue-green algae - cyanobacteria - remained small. The first observations of minor surface accumulations were made in the late June. In the second half of July, warm and calm weather favoured the growth of blue-green algae. Dense blue-green algal blooms developed in almost the entire Baltic Sea. In August the major surface accumulations disappeared, because strong winds mixed the algae into the water column. Some small, local surface accumulations remained in the northern Baltic Sea in September and even in October.
Gulf of Bothnia
In the Gulf of Bothnia small amounts of blue-green algae were noted locally during the summer 2001. Some local surface accumulations were observed along the Swedish coast as far north as the Quark in late September.
Gulf of Finland and Archipelago Sea
Low amounts of phosphorus in the surface water together with the cool weather slowed down the growth of algae in the beginning of summer 2001. The first surface accumulations of blue-green algae were observed in the Gulf of Finland and the Archipelago Sea in the end of June. The blooms consisted mainly of Aphanizomenon sp. This species is not known to be toxic in the Baltic Sea.
In July blue-green algae increased in abundance, due to warm and calm weather. In the second half of the month dense surface accumulations occurred in the Archipelago Sea and the Gulf of Finland. The densest accumulations occurred on the Finnish side of the Gulf, while the blooms were less intensive in Estonian waters. The most common blue-green algae in the Gulf of were Aphanizomenon sp. and Nodularia spumigena. Mass occurrences of Nodularia are always toxic. Potentially toxic Anabaena species were also fairly common in the central Gulf of Finland.
In August low nutrient concentration and strong winds reduced the density of the surface blooms. Small but dense accumulations were locally observed in archipelago on the southern and southwestern coast of Finland during calm weather. Aphanizomenon sp. and Nodularia spumigena were also in August the most common blue-green algae in the observed accumulations.
Even though blue-green algal blooms were observed in the Gulf of Finland and Archipelago Sea, their intensity remained lower than in the summers 1997, 1999 and 2001. Cyanobacterial blooms in the Finnish Sea areas".
Baltic Proper, Arkona Basin and Belt Sea
Surface accumulations of blue-green algae occurred in the central and southern parts of the Baltic Proper from the second half of July onwards. During the last weeks of July, dense surface accumulations were observed in the northern part of the Baltic Proper, between Gotland and Åland, and on the west coast of Estonia. Surface accumulations were also reported from several places around the southern coast of Sweden. Dense accumulations were observed in Danish coastal areas and around Bornholm. In addition, there were large amounts of blue-green algae in the Arkona Basin. Abundant blue-green algal blooms were also observed off the German coast. The water was coloured greenish by blue-green algae also in the coastal area of Poland. In the northern parts of the Baltic Proper, Aphanizomenon sp. was the most common blue-green alga, while Nodularia spumigena was dominating in the southern parts of the Baltic Sea.
At the beginning of August, strong winds mixed the seawater and most surface accumulations were dispersed into the water column. There were, however, still considerable amounts of blue-green algae in the water. Small blooms were noted locally in south of Copenhagen in Denmark and on the coast of Germany.
SeaWiFS satellite image mosaic from 27 - 29 July 2001 shows the extent of the surface accumulations of blue-green algae (yellow and red areas) in the Gulf of Finland and Baltic proper
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References
The microscope photographs have been kindly provided by Alg@line/Seija Hällfors.
The algal report has been compiled by the Finnish Environment Institute (SYKE) and the Finnish Institute of Marine Research (FIMR) using information from:
Swedish Meteorological and Hydrological Institute, Algaware
Federal Maritime and Hydrographic Agency of Germany
National Environmental Research Institute, Denmark (DMU) In Danish: Alger i danske havområder
Estonian Marine Institute, University of Tartu:
Stockholms Marina Forskningcentrum
Ulf Larsson and Lars Andersson, 2001: Närinsmänden ökar syrebrist orsaken, (English summary: Nutrient increase in the Baltic) - Miljötillståndet I egentliga Östersjön, Årsrapport 2000, pp. 51-54. Stockholms Marina Forskningscentrum 2001.