Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2004

	Authors: Mika Raateoja, Seija Hällfors and Eija Rantajärvi, FIMR

Key message

Cyanobacterial blooms in the Gulf of Finland and in the Northern Baltic Proper were more intense in 2004 than in 2003 in terms of Chl a. However, the blooms in these areas were even more intense in 2002.

Figure 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 1992-2003, the red dots the measurements made in 2004. Image: FIMR/Alg@line.

 Relevance of the indicator for describing developments in the environment

Eutrophication is considered one of the most serious threats against the Baltic Sea ecosystem and it is a striking manifestation of the man-made alterations in the Baltic Sea system. Eutrophication is defined as the increasing nutrient levels and primary productivity, leading to increased phytoplankton biomass. Thus, Chl a level being a relative measure of phytoplankton biomass describes directly the trends in eutrophication. Recently intensified cyanobacterial blooms are the most conspicuous manifestation of the advancing eutrophication of the Baltic Sea. As the intensity and occurrence of these blooms can be sufficiently accurately estimated by Chl a concentration, Chl a can provide information of this recently-introduced undesired development in the Baltic Sea environment.

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 harmful and toxic cyanobacterial blooms have increased to levels to raise wide public concern. Currently, the noxious blooms annually disrupt the Baltic ecosystem and limit the recreational and economic use of the Baltic, and moreover, 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, but the first goal should be to lower the intensity of the cyanobacterial blooms to the level that was met in the 1980’s. However, no signs of settling cyanobacterial blooms have been seen yet.

Background information: unusually high phosphorus levels in the Gulf of Finland in 2004

The year 2004 was distinctive at least for the extremely high springtime and summertime phosphate levels in the surface layers of the Gulf of Finland. This phenomenon had its starting point in January-February 2003 when there were several inflow events of saline, oxygen-rich water through the Danish Straits to the Baltic Sea. These events pushed the “old” (anoxic, mesosaline, nutrient rich) waters northwards from the deep basins of the Central Baltic Proper. This sequence of events had a pronounced influence to physiochemical conditions of the deep layers of the Gulf of Finland. The incoming old water masses weakened the oxygen conditions in the water-sediment interface thus accelerating the liberation of phosphate into the water already rich in phosphate. The incoming saline waters also strenghtened the vertical density anomaly and hindered the vertical water exchange with the upper and more oxygen-containing waters. However, the strong storm events during late autumn and early winter – at a time when there was little density anomaly due to temperature - mixed the waters of the Gulf of Finland, rose up significant amounts of phosphate, and record-breaking surface water concentrations of phosphate were measured both in the surface layer and in the deeps of the central and western parts of the Gulf of Finland in November-December 2003. The extremely high phosphate levels observed in the surface water masses in the Gulf of Finland from the winter 2003 all the way to the summer 2004 enabled the high growth potential of cyanobacteria.

Assessment for the Gulf of Finland

The wintertime algal community was low in biomass and dominated by the dinoflagellate Scrippsiella hangoei, which occurred in relatively high biomasses in the ice-covered Gulf of Finland between mid February and mid March. This species dominated also the vernal bloom period. 

The spring bloom was an average one in terms of its duration and magnitude, as compared to the latest decade. It commenced and declined somewhat earlier than usually. The peak of the bloom occurred in the mid April with Chl a levels of about 25 mg m^-3 (Fig. 1). In addition to Scrippsiella, the vernal algal community was dominated by the dinoflagellate Peridiniella catenata and the diatoms Thalassiosira baltica, Achnanthes taeniata, and Chaetoceros holsaticus.

The dominating algal species during the summer minimum phase were the green alga Monoraphidium contortum, and the haptophytes Chrysochromulina spp. along with various small flagellates. The weather during June was cold and windy, and water temperature was rising up slower than usually.

The algal biomasses in the late summer were clearly higher than the average during the latest decade, and the highest Chl a levels (about 10 mg m^-3) were observed from the late July to the mid August. The algal community was dominated by mixed communitites of the cyanophytes Nodularia spumigena, Anabaena sp., and Aphanizomenon flos-aquae. The dinoflagellate Heterocapsa triquetra was also common. Although the cyanobacterial biomasses were moderately high already in the late July, the surface accumulations in their widest were observed only in the early August due to lack of long enough calm periods (Fig 2).

Figure 2. The relative index describing the timing of cyanobacterial bloom observations for 2004 and for 1998-2003. Image: SYKE.

In July, limited surface accumulations consisting mainly of Aphanizomenon were observed throughout the Finnish side of the Gulf of Finland, but the most consistent occurrences were formed in the eastern Gulf of Finland. The toxic species Nodularia increased in its numbers towards the end of the month as the water temperature rose up. The calm and warm weather type occurring in the early August increased the water temperature and allowed surface accumulations to emerge in their fullest. Extensive surface accumulations took place all around in the Gulf of Finland and in the Northern Baltic Proper. The maximum coverage of the surface accumulations were reached around the 5^th-8^th of August (Fig. 3). Shortly afterwards the accumulations were dispersed by the winds. The areal coverage did not make real justice to the actually high cyanobacterial biomasses in the Gulf of Finland this year because of the lack of long enough calm periods. As a consequence, day-to-day variation in the occurrence of the surface accumulations was high.

Figure 3. The cyanobacterial surface blooms in the Gulf of Finland were at their widest in the late July – early August 2004. Image: FIMR/Alg@line

Assessment for the Northern Baltic Proper

The spring bloom period in the Northern Baltic Proper was slightly below the average one both in terms of its duration and magnitude, but approximately the average by its timing, 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). The vernal algal community was dominated by the dinoflagellates Scrippsiella hangoei and Peridiniella catenata and by the diatoms Chaetoceros wighamii, Achnantes taeniata and Thalassiosira baltica

The summer minimum phase was short as expressed by the dip in Chl a in the early June. The haptophyte Chrysochromulina spp. dominated the algal community with dinoflagellates of the genus Gymnodinium. The weather during June was cold and windy, and water temperature was rising up slower than usually.

The algal biomasses in the late summer were slightly lower than in the average during the latest decade, and the highest Chl a levels (about 4 mg m^-3) were observed already in the beginning of July. Of the cyanophytes, Anabaena spp. and Pseudanabaena sp. were relatively abundant, but Nodularia spumigena and Aphanizomenon flos-aquae not so. Limited local cyanobacterial surface accumulations were first observed in the Northern Baltic Proper in the early July. The areal coverage of the surface accumulations were restricted by strong winds, and day-to-day variation in the occurrence of the surface accumulations was high. More extensive surface accumulations formed in the whole Baltic Proper at the end of July. Extensive cyanobacterial blooms covered most parts of the Baltic Proper at the 6^th-10^th of August (Fig. 4). A week afterwards the accumulations were dispersed by the winds.

Figure 4. The cyanobacterial surface blooms at their widest in the Baltic Proper in the early August 2004. Image: SMHI.

Assessment for the Arkona Sea

The magnitude of the spring bloom in the Arkona Sea was clearly lower than the average one during the latest decade: the peak Chl a levels were only about 5 mg m^-3 (Fig. 1).  Its timing was approximately the average, though. The vernal algal community was dominated by diatoms Skeletonema costatum and Thalassiosira levanderi.

The summer minimum phase did not exist when Chl a level was used as a criterion. Nanoflagellates (Heterocapsa rotundata, Chrysochromulina spp., Pyramimonas spp. and Plagioselmis prolonga) and the green alga Planctonema lauterbornii were dominating in the time of the minimum phase.

The algal biomasses in the late summer were clearly lower than in average during the latest decade with Chl a levels (1 to 2 mg l^-1). Nanoflagellates (Pyramimonas, Chrysochromulina, Plagioselmis, Teleaulax spp., Gymnodinium spp.) and the green alga Planctonema were most abundant, whereas cyanophytes were sparse. Consisting surface accumulations of cyanobacteria were first observed only at the end of July. At the time of the most extensive surface accumulations in the Baltic Proper at the early August, no consistent surface blooms were observed in the Arkona Sea, probably due to strong winds.

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
(http://www.fimr.fi/en/palvelut/levatiedotus.html)

3. Geographical coverage: Gulf of Finland, Archipelago and Åland Sea, Baltic Proper, Arkona Sea

4. Temporal coverage: 1992-2004

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.