MURSYS - Baltic Sea

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MURSYS - Baltic Sea


Algal Blooms in the Baltic Sea in 2001

  Finnish Institute of Marine Research, Helsinki
  Finnish Environment Institute (SYKE)


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Cyanobacterial blooms in the late 1990's

In summer 2001 the blue-green algal blooms (cyanobacterial blooms) in the Gulf of Finland and the Archipelago Sea were less intensive than in years 1997 and 1999. In the southern Baltic Proper, the Arkona Basin and the Belt Sea area the surface accumulations were extensive and dense.

Surface water temperatures stayed relatively low in June and early July and the amount of blue-green algae 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.

Cyanobacterial blooms in the late 1990's:

Gulf of Bothnia
In the Gulf of Bothnia small amounts of blue-green algae were noted locally during the summer. 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. 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 and 1999. That is in accordance with the prognosis made by Finnish Environment Institute in the spring 2000 (The whole prognosis document in Finnish). The prognosis was based on the early spring concentrations of biologically available phosphorus, measured by Finnish Institute of Marine Research, Estonian Marine Institute, Uusimaa Regional Environment Center, Southeast Finland Regional Environment Centre, Southwest Finland Regional Environment Centre and City of Helsinki Environment Center.

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.
In 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.

Other blooms

Phytoplankton spring bloom

Due to the mild winter of 2001 the spring bloom started in March about two weeks earlier than the mean of 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. 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.

Potentially toxic blooms

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 the 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 are previously known in the Baltic Sea. Luminescent blooms make the water glow in dark.

Algal toxins

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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