Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2005

	Authors: Mika Raateoja and Seija Hällfors, FIMR

Key message

Cyanobacterial blooms in the Northern and Central Baltic Proper in 2005 were amongst the most intense and widespread ever encountered.

The blooms in the Gulf of Finland - the area that often suffers from vigorous blooms - were only average in their intensity in 2005.

The blooms in the Southern Baltic Proper remained below the average in their intensity despite the considerable phosphate reserves there.

Fig. 1. Annual variation of chlorophyll a (mg m^-3) in the Western Gulf of Finland (upper), in the Northern Baltic Proper (middle), and in the Arkona Basin (lower). The green curve represents the average for the years 1992-2004, the red dots the measurements made in 2005. Image: FIMR/Alg@line. 

Relevance of the indicator for describing developments in the environment

Eutrophication is a striking manifestation of the man-made alterations in the Baltic Sea ecosystem, and is still considered one of the most serious threats against the Baltic Sea. Eutrophication is defined as the increasing nutrient levels in the aquatic system, which as a primary effect echoes to increased phytoplankton primary productivity and biomass, and thus, to an increased autoctonic organic load. Chl a level, being a relative measure of phytoplankton biomass, assesses directly the eutrophication-driven alterations of the Baltic Sea. More importantly, it can address with an adequate precision the intensity and occurrence of cyanobacterial blooms that are the most conspiuous manifestation of the advancing eutrophication of the Baltic Sea.

Policy relevance and policy references

Although being a natural phenomenon per se, the algal bloom events have become more frequent, intense, and extensive due to the man-made eutrophication of the Baltic. Since the mid-90’s, the strength of cyanobacterial blooms have increased to levels to raise wide public concern. Currently, noxious and often harmful cyanobacterial blooms disrupt the functioning of the Baltic ecosystem, limit the recreational and economic use of the Baltic, and represent a clear and present health risk for humans and domestic animals. At the moment, there exist no agreed target level for cyanobacterial mass occurrences due to the lack of any reference value describing the situation of a minor level of human impact, but the first goal should be to lower the intensity of the cyanobacterial blooms to the level that took place in the 1980’s. No signs of settling cyanobacterial blooms have been seen yet, nor will emerge in the nearest future.

Background information: strong wintertime mixing in the Baltic Proper

The rather unusual cyanobacterial bloom pattern in the Baltic Sea in 2005 – intensive blooms in the Northern and Central Baltic Proper that overrun the blooms in the Gulf of Finland – was based on the wintertime meteorology in the form of passages of low-pressure systems over the Scandinavia in December 2004 – January 2005. These storms hit most severely to the Northern and Central Baltic Proper mixing surface water masses and eroding the main halocline. In the Gotland deep, for instance, the upper reaches of anoxic water mass declined from 70 m in the summer 2004 to 100 m in January 2005. As this depth interval includes the main density gradient in the Baltic Proper it can be concluded that unusually great stock of nutrients was introduced into the surface layer of the Baltic Proper. As a result, record-breaking surface water concentrations of phosphate were measured in the Southern Baltic Proper throughout the year 2005. In the Gulf of Finland, the wintertime concentrations of phosphate were second in a row of all-time record concentrations preceded only by the previous year 2004, but here the extremely strong vernal bloom effectively consumed the phosphate reserves. As the fate of the vernal bloom is sedimentation with only a minor fraction decomposing in the surface layer, these phosphate stocks were not transported to the summertime algal community. Taken together, the foundation of summertime cyanobacterial bloom dynamics in the Baltic Sea was already laid up to the end of the vernal bloom period in the mid May.

Assessment for the Gulf of Finland

The spring bloom was an average one in terms of its timing and duration, but its magnitude was considerably higher than typically met during the latest decade. The peak of the bloom occurred in the late April with Chl a levels of about 35 mg m^-3 (Fig. 1), but single values as high as 70 to 80 mg m^-3 were observed in the Central Gulf of Finland. The vernal algal community was dominated by the typical vernal taxa: the dinoflagellates Woloszynskia halophila and Peridiniella catenata as well as the diatoms Thalassiosira baltica, T. levanderi, Achnanthes taeniata, Chaetoceros wighami, C. ceratosporus and Skeletonema costatum.

The summer minimum phase took place from the late May to the late June. The dominating phytoplankton taxa were the green alga Monoraphidium contortum and the diatom Skeletonema costatum. The weather during the early June was changeable, but a high pressure weather type established around Midsummer and water temperature started to rise up promptly.

The algal biomasses in the late summer were at a level typical for the latest decade, and the highest Chl a levels (about 5 to 7 mg m^-3) were observed during a long period from the early July to the early August. The algal community was dominated by the cyanobacterial taxa Aphanizomenon flos-aquae, Nodularia spumigena and Anabaena spp. Aphanizomenon was the most dominant during the early stages of the bloom period and in the coastal/archipelago areas. The dinoflagellate Heterocapsa triquetra was also common.

No blue-green algal surface accumulations took place in June, although Aphanizomenon was already present in considerable numbers. The surface blooms emerged in the early July, and the most consistent patches were observed in the eastern parts of the Finnish sea areas in the Gulf of Finland, and in the Åland Sea south of the island Åland. A week later the surface blooms were introduced to the southern reaches of the Archipelago Sea, and eventually they were observed almost throughout the Archipelago Sea. The maximum coverage of the surface accumulations were reached around the 18^th of July. The passing low-pressure systems dispersed the surface patches shortly after this. To summarise, the blue-green algal season in the Gulf of Finland was pronounced in terms of its duration, only the clear peaks in the algal biomasses remained from emerging (Fig. X). The surface patches mainly remained in the open sea areas.

Assessment for the Northern Baltic Proper

The spring bloom period in the Northern Baltic Proper was quite normal in terms of timing, duration and magnitude, as compared to the latest decade. The peak of the bloom occurred in the mid April with Chl a level of about 10 mg m^-3 (Fig. 1) with highest individual values of 12 to 14 mg m^-3. This level was about one third of the biomass in the Gulf of Finland. The line of anomaly took place around the 23ºE meridian, where the vernal biomasses changed from the level of the Northern Baltic Proper to the one of the Gulf of Finland. The vernal algal community was dominated by the same representatives of phytoplanktonic taxa as in the Gulf of Finland: the dinoflagellates Woloszynskia halophila and Peridiniella catenata as well as the diatoms Thalassiosira baltica, T. levanderi, Achnanthes taeniata, Chaetoceros wighami, C. ceratosporus and Skeletonema costatum.

Fig. 2. Left: the relative index describing the timing of cyanobacterial bloom observations for 2005 and for 1998-2004. Image: SYKE.

The summer minimum phase took place from the mid May to the mid June. Small flagellates, such as the haptophytes Chrysochromulina spp. and the prasinophytes Pyramimonas spp. dominated the algal community with the dinoflagellates Heterocapsa rotundata ja Gymnodinium/Glenodinium spp. The weather during the early June was changeable, but high pressure weather type established itself around Midsummer and water temperature started to rise up promptly.

The Baltic Proper experienced probably the most hectic cyanobacterial blooms ever in terms of intensity and spreadiness. The highest Chl a levels (about 5 to 7 mg m^-3) were observed during a long period from the early July to the early August. At their largest, the surface blooms covered the entire Northern and Central Baltic Proper from the southern reaches of the Åland archipelago in the north to the line: the island Oland − the Bay of Gdansk in the south. The blooms decreased in their intensity rather abruptly southwards from the line. The most violent blooms took place in the waters around the island Gotland in the Central Baltic Proper. Blue-green surface accumulations were also observed in the Bay of Riga. The blooms started to develop vigorously during the first days of July, and on the 6^th of July the Northern and Central Baltic Proper were blooming (Fig. 2). The maximum coverage of the surface accumulations was reached from the 10^th to 15^th of July (Fig. 3). The passing low-pressure systems dispersed the surface patches a week later. The blooms consisted mainly of the toxic taxon Nodularia spumigena with Anabaena spp. occurring in significant numbers particularly in the Northern Baltic Proper. By coming ashore, the blooms caused a recreational disaster and major economic losses in Sweden.

Fig. 3. In 2005, the extent of the cyanobacterial surface blooms in the Baltic Proper in July 2005. Image: FIMR/Alg@line.

Assessment for the Arkona Sea

The magnitude and the timing of the spring bloom in the Arkona Sea was approximately a typical one as compared to the latest decade, but the duration was somewhat longer: the vernal biomasses returned to the summertime levels only in the early May. The peak Chl a level was  about 6 mg m^-3 (Fig. 1) in the early April with individual highest values of 8 to 9 mg m^-3.  The vernal algal community was dominated by diatoms Skeletonema costatum and Chaetoceros spp.

The summer minimum phase did not exist in terms of Chl a concentration, as the level during the late summer never deviated from its summer minimum level. Small flagellate taxa, such as the haptophytes Chrysochromulina spp., the prasinophytes Pyramimonas spp. and the chrysophytes Pseudopedinella spp. dominated.  The dinoflagellates Gymnodinium/Glenodinium spp. were also common.

The algal biomasses in the late summer were clearly lower than typically during the latest decade with Chl a levels of 2 mg l^-1. This is the 2^nd subsequent year when this happens. The small colonial cyanophytes Cyanodictyon balticum and Aphanothece parallelliformis were abundant, whereas filamentous cyanophytes were sparse. The diatoms Cyclotella choctawhatcheeana and Chaetoceros impressus were also abundant. At the time of the most extensive surface accumulations in the northern parts of the Baltic Proper, no consistent surface blooms were observed in the Arkona Sea.

Metadata

Technical information

1. Data provider (source)

Finnish Institute of Marine Research (FIMR)

Contact persons Mika Raateoja and Seija Hällfors

2. Description of data

Original unit of measure: mg chl a m^-3

Semiquantitative phytoplankton analysis are based on the ranks 1 to 5 describing relative sample-based abundance of an algal species. In the cyanobacterial bloom map, visual observations are included.

Original data in WGS84-coordinates

Original purpose of the data: Phytoplankton monitoring of FIMR, Alg@line project

3. Geographical coverage

Gulf of Finland, Archipelago and Åland Sea, Baltic Proper, Arkona Sea

4. Temporal coverage

1992-2005

5. Methodology and frequency of data collection

The data has been collected using an automated flow-through sampling system on merchant ships, sampling depth ca. 5 m, weekly sampling during the period February/March-October/November in each year. Detection device Jasco 750 spectrofluorometer

6. Methodology of data manipulation

No data manipulation

Quality information

1. Strength and weakness (at data level)

Strength: very high both temporal and spatial sampling frequency

Weakness: -

2. Reliability, accuracy, precision, robustness (at data level)

Filtration and extraction of Chl a from samples according to accredited method SFS-EN ISO/IEC 17025. Procedure uncertainty: 5%.

3. Further work required (for data level and indicator level)

More sophisticated statistical analysis

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/. 

Last updated 25 Nov 2005.