Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2006
Cyanobacterial blooms in the northern Baltic Proper in 2006 were below average.
The blooms in the Gulf of Finland - the area that often suffers from vigorous blooms - were below average in their intensity.
The blooms in the Arkona Basin were vigorous and above average.
Results and assessment
Fig. 1. Annual variation of chlorophyll a (mg m-3) in the Western Gulf of Finland (top), the Northern Baltic Proper (middle) and the Southern Baltic Proper (including Arkona Basin, bottom). The blue curve shows the weekly mean and the red curve shows the standard deviation for the years 1992-2005. The black diamonds represent the weekly means in 2006. Image: FIMR/Alg@line.
Fig. 2. Left: the relative index describing the timing of cyanobacterial bloom observations for 2006 and for 1998-2005. Image: SYKE.
Fig. 3. The extent of the cyanobacterial surface blooms in the Baltic Proper in July 2006 (left) and August 1st to August 7th 2006 (right), data achieved from MODIS-satellite images validated by Algaline ship-of-opportunity chlorophyll a observations. Image: FIMR/Alg@line. Click images to enlarge!
Relevance of the indicator for describing developments in the environment
Eutrophication is considered one of the most serious threats against the Baltic Sea. It is defined as an increase in the rate of supply of organic matter to an ecosystem, and is most commonly caused by nutrient enrichment. Chlorophyll a concentration, representing phytoplankton biomass, assesses the eutrophication-driven alterations of the Baltic Sea. More importantly, it can address with an adequate precision the intensity and occurrence of cyanobacterial blooms. It must be kept in mind, however, that although highly responsive to changes in surface nutrient concentrations, chlorophyll a is also a product of parameters not related to eutrophication, namely other biological factors, hydrography and climate.
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 eutrophication of the Baltic Sea. 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 sea, and represent a clear and present health risk for humans and domestic animals. No signs of decrease of cyanobacterial blooms have been seen yet.
Background information: low wintertime phosphate concentrations in northern Baltic
The early spring of 2006 surface phosphate concentrations were lowest in ten years in the northern Baltic Proper and the Gulf of Finland, due to lack of vertical mixing. High levels of phosphorus were measured in the central and southern part of the Baltic during the same period. This caused high risk for cyanobacterial blooms in the southern and central Baltic areas.
Assessment for the Gulf of Finland
The spring bloom started in the first week of April, almost two weeks later than the previous year and the ten-year average. Both the peak and duration of the bloom were below average (Fig. 1, Fleming & Kaitala 2006). The algal community was dominated before the bloom peak by the diatom Skeletonema costatum, later also by other diatoms (Chaetoceros spp., Achnanthes taeniata, Thalassiosira spp.). When the bloom biomass began to decrease, dinoflagellates (Woloszynskia halophila, Scrippsiella hangoei and Peridiniella catenata) and the prasinophycean Pyramimonas spp. became abundant as the diatoms formed resting spores.
The summer minimum phase lasted throughout June (Fig. 1). 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 (between 5 and 117 mg m-3) were observed in late august. The algal community was dominated by the cyanobacteria Aphanizomenon flos-aquae and Anabaena spp.. The dinoflagellate Heterocapsa triquetra was also common.
The first cyanobacteria surface accumulations were found at the mouth of the Gulf of Finland on 7th July, and on the following week surface accumulations were also found in the eastern Gulf of Finland. During the heat wave of late July and early August blue-green algae concentrations increased on the Finnish coast, although they remained mixed in the surface layer thanks to the prevailing windy weather. During the second week of August surface accumulations increased in the Finnish coast as well in the open sea areas. After mid-August the situation calmed down in open sea areas, partly following strong winds. Algal concentrations remained high along the Finnish coast. The algae, which had been mixed up in the surface layer, rose to the surface during calm weather. Very few surface accumulations were observed after the end of August, although locally algal concentrations remained moderate in the water. To summarize, the blue-green algal season started later and was below average in intensity in the Gulf of Finland.
Assessment for the Northern Baltic Proper
The spring bloom started on April 6th, which is later than average, and lasted about three weeks (Fig. 1, Fleming & Kaitala 2006). All in all, the spring bloom was less intense than the previous year and the average, when taking into account both peak and duration of bloom. The spring bloom started with diatoms, mainly Skeletonema costatum, dominating. Later also dinoflagellates began to increase with Woloszynskia halophila/Scrippsiella hangoei and Peridiniella catenata abundant.
The summer minimum lasted from the end of May to the first week of June (Fig. 1). During this period the dinoflagellate Peridiniella catenata was usual and the golden alga Dinobryon balticum also relatively common.
The first cyanobacterial blooms were observed during the second week of July. The northern Baltic Proper did not suffer of extensive blooms as during the previous summer. The algal community was dominated by the cyanobacteria Aphanizomenon flos-aquae and Anabaena spp.. The dinoflagellate Heterocapsa triquetra was also common.
Assessment for the Arkona Sea
As during the previous year, the spring bloom was negligible. A slight rise in the chlorophyll a level reaching about 4 µg/l could be detected from the middle of March to the middle of April (Fig. 1, Fleming & Kaitala 2006). The diatoms dominated the early spring community, with Skeletonema costatum common. Small flagellates, such as Chattonella aff.vVerruculosa, Apedinella radians and Dinobryon balticum, increased in April.
A distinct summer minimum could not be detected in the chlorophyll a concentration, since the concentrations had great variability after the spring period (Fig. 1). Small haptophytes (Chrysochromulina spp.) were dominant in the early summer community, with colonial cyanobacteria (Chroococcales spp.) also abundant.
The most intensive blue-green algal blooms occurred in the southern Baltic Proper and the Arkona Basin. The first blooms occurred in the beginning of July, and massive blooms covered the area throughout the second half of July . Nodularia spumigena was the most common cyanobacteria species, with small colonial cyanobacteria (Chroococcales spp.) also present.
Fleming, V. & Kaitala, S. 2006. Phytoplankton spring bloom biomass in the Gulf of Finland, northern Baltic Proper and Arkona Basin in 2006. Helcom Indicator Fact Sheet.
1. Data provider (source): Finnish Institute of Marine Research (FIMR)
Contact persons: Vivi Fleming-Lehtinen, Seppo Kaitala 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: 1992-2006
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
1. Strength and weakness (at data level)
Strength: Very high both temporal and spatial sampling frequency
Weakness: Satellite images are achieved only on clear weather. Ship-of-opportunity –measurements are restricted to the ships route, and dependant on it’s schedule; diurnal changes of data are not taken into account.
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 2006. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated 14.3.2007