Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2003

	Authors: Ruokanen Lotta, Hällfors Heidi, Rantajärvi Eija, FIMR

Key message

Wide spread cyanobacterial surface accumulations were observed in July-August in the northern Baltic Proper and in the Gulf of Finland, though the cyanobacterial blooms in 2003 were less intense than during 2002 and 2001.

Figure 1. The occurrence of cyanobacterial surface accumulations in the Baltic Sea during 2003 expressed as the number of days. The data is based on satellite images. Source: SMHI.

Results and assessment

Environmental context

Chlorophyll a concentration is a relative measure of phytoplankton biomass in the water. Since high nutrient concentrations increase phytoplankton growth and subsequently the intensity and frequency of blooms, chlorophyll a can be used as an indicator of the eutrophication level in a sea basin.

The intensity of the spring bloom reflects the scale of the nutrient reserves. The spring bloom species of diatoms and dinoflagellates consume most of the phosphorus and nitrogen nutrients that were built up in the water mass during the previous winter.

The excess of phosphorus left over by the spring bloom species triggers the growth of cyanobacteria. Consequently, the cyanobacterial blooms can be a highly visible phenomenon in the open sea areas during the summer from July to August.

Policy relevance

The availability of nutrients regulates the primary production and biomass of planktonic algae and the nutrient ratio of the main nutrients nitrogen and phosphorus largely determines which species can proliferate. The eutrophication of the Baltic Sea is still accelerated by the diffuse loading of nutrients from the whole drainage area from scattered housing, agriculture, aquaculture and from traffic.

The mass occurrences of planktonic algae have become more frequent and intense due to the eutrophication of the Baltic Sea. Since the last decade harmful and toxic algal blooms have occurred annually disrupting the marine ecosystems and limiting the recreational and economic uses of marine resources. Toxic algal blooms represent a real health risk for humans and (domestic) animals.

Background information for the assessment: the saltwater inflow in 2003

In January-February 2003 there were several inflow events of saline, oxygen-rich water through the Danish Straits to the Baltic Sea. These events pushed northwards the old anoxic and nutrient rich waters from the deep basins of the central Baltic Sea. In the Gulf of Finland elevated surface water concentrations of excess phosphate were measured after the spring bloom - possibly connected to the inflow event.
For more information on the inflow evenets, see a specific indicator report on the subject.

 Figure 2. The strength of cyanobacterial blooms observed in the Gulf of Finland and in the Archipelago Sea in the summer 2003 (1 June - 31 August). The strength is based on the frequency and intensity (visible, moderate, dense) of the blooms. The data is based mainly on the visual observations made by the pilots of the Finnish Frontier Guard and the observed area is marked with dashed line. (FIMR/Joona Lehtimäki)

Figure 3. Annual variation of surface layer phytoplankton biomass (measured as chlorophyll a mg m^-3) in the western and central Gulf of Finland, the northern Baltic Proper and the Arkona Sea. The green curve represents the average for the years 1992-2002, the red dots the measurements made in 2003.

Figure 4. Area covered by algal blooms in the Baltic Sea during the summer of 2003 as shown by NOAA/AVHRR satellite images. The intensity of the blooms is also presented, the blue curve stands for the areal coverage of clouds. Source: SMHI.

Assessment of the Gulf of Finland

In the autumn of 2002 the stratification of the Gulf of Finland started to strengthen and the oxygen content of the near-bottom water layer decreased rapidly. In December the near-bottom layer of saline inflow water from the Baltic Proper reached the eastern Gulf at the depth of 60 m. The reason for this may be the high atmospheric pressure that persisted over southern Finland for half a year and caused exceptionally low water levels in the Gulf. Consequently, the stratification was very strong in the deep parts of the Gulf of Finland already at the end of January and the oxygen concentrations were low. In January-March 2003 the phosphate levels in the surface layer were clearly lower than during the same period in 2002 or 2001, also the nitrate concentrations were lower. The stratification was not expected to break down and bring nurient rich water from the near-bottom layer to the surface. Thus the summer cyanobacterial situation was expected to be less extensive than in 2002. However, possibly due to the inflow to the Baltic Sea in the begging of the year, the phosphate concentrations of surface waters remained clearly higher in April than in the last two years. Based on that the risk of cyanobacterial blooms during summer 2003 was expected to be significant.

	The duration of the spring bloom was of about average, but the intensity was increased compared to the previous ten years. The summer minimum occurred just before mid-June as normal, and after that the algal biomass started slowly to increase and remained quite constant for the whole of August. There was no clear summer maximum and the phytoplankton biomass was noticeably lower than the average of the years 1992-2002. (See Figure 3.)

During the spring bloom the phytoplankton species composition was made up of the usual combination of diatoms and dinoflagellates. Amounts of these started to increase at the turn of March-April. Nitrate concentrations started to decrease, but phosphate levels remained high until the spring bloom peak at the end of April. The surface water temperatures were several degrees lower than in 2002 in March-May.

In the early summer the phosphate phosphorus levels of the surface water were clearly higher than in 2002 or 2001. Phytoplankton was very sparse at the end of May, some nanoflagellates, chrysophyceans and diatoms occurred scatteredly. The weather was cool and windy in June. The first visual observations of cyanobacteria still mixed into the water occurred in the eastern Gulf during the last week of June. The amount of phytoplankton in the water was still low and small flagellates were the dominant species, cyanobacteria were still very sparse. During the first week of July the cyanobacterial abundances rose slightly and scattered algal aggregates mixed into the water were observed also in the western Gulf of Finland.

In mid-July the surface water temperature had risen to 17-19^oC and the first wider surface accumulations forming long trails were observed in the central and eastern Gulf, and there were also several local blooms in the coastal areas. The abundance of cyanobacteria doubled in one week, but the toxic species Nodularia spumigena occurred only in small amounts. Cyanobacteria benefited from the rise of temperature up to c. 20°C towards the end of July and blooms were observed along the coastal waters of the Gulf. However, the surface accumulations of the open sea areas stayed mostly to the south of the Archipelago Sea in the northern Baltic Proper. For a couple of days at the turn of the month there were wider surface accumulations also in the open Gulf. The most abundant cyanobacterial species at the entrance of the Gulf were the toxic N. spumigena and the potentially toxic Anabaena spp., in the western Gulf the non-toxic A. flos-aquae and the potentially toxic Anabaena spp. Non-toxic species were dominant in the sea area off Helsinki.

In the beginning of August winds dispersed the existing blooms and only some scattered surface accumulations were observed in the eastern Gulf. At the entrance to the Gulf the most abundant bloom-forming cyanobacteria was the toxic N. spumigena, in the western Gulf and off Helsinki the most abundant species was the non-toxic A. flos-aquae.In mid-August northern winds transported warm surface water towards the coast of Estonia and upwelling brought cold near-bottom water to the costal area of Finland. The surface water temperature declined nearly ten degrees in one day. In some places near the coast the temperature decreased to about 12^oC, but in the open Gulf the surface water temperature remained at about 22^oC. At the entrance to the Gulf there were very small amounts of bloom-forming cyanobacteria. In the western and central Gulf the amounts were slightly greater, but not visible. Of bloom-forming cyanobacteria the most abundant was the non-toxic A. flos-aquae and off Helsinki also the potentially toxic genus Anabaena. At the end of August there were still some local blooms in coastal waters, but the amount of bloom-forming cyanobacteria was very low. In the the western and central Gulf the dominating bloom-forming cyanobacteria were the potentially toxic Anabaena spp. The surface water temperature had cooled to 17-18^oC. In the western and central Gulf the amounts of bloom-forming cyanobacteria were very small, the most abundant of these being the non-toxic A. flos-aquae and the potentially toxic Anabaena spp. (See Figures 2 and 3.)

In the beginning of September there were still some visible cyanobacterial aggregates mixed into the water in scattered locations in the Gulf. The temperature had decreased to 15-16^oC and the phytoplankton species composition consisted of e.g. cyanobacteria, small flagellates, diatoms and dinoflagellates, indicating the oncoming autumn.

Assessment of the northern Baltic Proper

In the beginning of March the surface water of the northern Baltic Proper was a few degrees centigrade colder than at the same time in 2002. Concentrations of phosphate and nitrate were lower than during the previous winter, but not as much lower as in the Gulf of Finland. In the beginning of June the winter’s saltwater pulses had reached the Gotland Deep, and the near-bottom waters there were oxygenated.

	The spring bloom was of about the average duration and intensity. The summer minimum occurred in mid-June and algal biomass stayed lower than the average during the period 1992-2002 until the end of July, when it increased a little and a slight summer maximum could be observed. (See Figure 3.)

The chlorophyll concentrations started to increase after mid-March with a subsequent decrease in the dissolved nitrate concentrations, which was totally used up after mid-April. During the second week of April the spring algae were blooming, the diatom Achnanthes taeniata being the most abundant species. During the last week of the month the spring bloom more or less ceased. In May after the spring bloom there was clearly more phosphate phosphorus left in the surface water than in 2002 and the amounts of algae were slightly high for the time of the year.

During June phytoplankton was quite sparse and chrysophyceans like Dinobryon balticum were abundant, and no visible cyanobacterial blooms were detected. The surface water temperature was 10-15oC. At the turn of the months June-July small flagellates like the prymnesiophycean Chrysochromulina spp. started to dominate the phytoplankton community, while the filamentous blue-green alga A. flos-aquae occurred sparsely and the toxic N. spumigena only in very small amounts. In this way 2003 differs from the year 2002, when A. flos-aquae was abundant and toxic N. spumigena and Anabaena spp. were relatively common already of the end of June.

In the beginning of July there was no phosphate left in the surface waters of the basin. The surface water temperature was 15-17^oC during the first half of the month, that is several degrees less than in 2002. During the first week of July A. flos-aquae and Anabaena spp. were relatively abundant, while N. spumigena was encountered considerably less frequently. During the second week of July some weak surface accumulations of cyanobacteria were detected (SMHI’s satellite monitoring). Towards the end of the month the weather turned hot and the temperature of the surface waters rose to 21-24^oC, which benefited the cyanobacterial growth. Strong blooms covered the Baltic Proper from mid-July until the end of the month, however the most extensive blooms remained in the central Baltic Proper (SMHI and SYKE satellite monitoring). The accumulations were also detected in the phytoplankton biomass of the northern Baltic Proper (See Figure 2.) Nevertheless, even at the end of the month the phytoplankton community in the area was still fairly sparse and dominated by other than bloom-forming filamentous blue-green algae (Oscillatoriales spp.), colonial blue-green algae (Chroococcales spp.), the prymnesiophycean Chrysochromulina spp. and the diatom Nitzschia paleacea. Also N. spumigena was fairly common. On the contrary, in 2002 A. flos-aquae dominated the phytoplankton species already in the beginning of July and N. spumigena after mid-July, and the amount still increased considerably.

In the beginning of August the visible surface accumulations of cyanobacteria weakened clearly and disappeared in a few days, partly due to strong winds. Also biomass measured as chlorophyll decreased again. The phytoplankton community was clearly dominated by thin filamentous blue-green algae (Oscillatoriales spp.), and potentially bloom-forming cyanobacteria were encountered only sparsely (A. flos-aquae) or very sparsely (Anabaena spp. and N. spumigena). In mid-August the surface water temperature had decreased to ca. 20^oC and at the end of the month to 17-18^oC. At the time the phytoplankton community was dominated by the diatom Chaetoceros minimus although cyanobacteria (A. flos-aquae, Oscillatoriales spp. and Anabaena spp.) still occurred fairly commonly. (See Figure 1.)

In the beginning of September the surface water temperature of the area was 15-16^oC and alongside cyanobacteria present were also small flagellates, dinoflagellates and diatoms. During the first week of the month some surface accumulations were detected again south of the Archipelago Sea.

Assessment of the Arkona Sea

	In 2003 the spring bloom started in the Arkona Sea earlier than during 1992-2002, the maximum occurred already before mid-March and the spring bloom was over at the end of March. Usually the spring maximum has been at the turn of March and April. Also the intensity of the spring bloom biomass measured as chlorophyll was much higher than the average values. The summer minimum occurred already in mid-April and after that the phytoplankton biomass fluctuated slightly, but no clear summer maximum occurred. (Figure 3)

The concentrations of phosphate available to phytoplankton were during winter somewhat lower than in 2001-2002, but the nitrate levels were about the same. The amount of chlorophyll started to increase and nitrate and phosphate clearly to decrease already at the end of February. In the beginning of March diatoms were abundant and dinoflagellates very sparse. After mid-March the amounts of algae started to decrease and there was no nitrate left at the end of the month. The surface waters in January-April were a few degrees colder than in 2002 during the same period, varying in April between 5 and 7^oC.

In the beginning of March diatoms dominated the spring bloom and Skeletonema costatum was a very abundant species. The surface water temperature increased to over 10oC only at the end of May, but after mid-June the temperatue had increased to 15–17^oC.

By the end of June the phytoplankton community was dominated by the haptophyte Chrysochromulina spp., the diatom Chaetoceros throndsenii occuring rather abundantly also. The colonial cyanobacteria Chroococcales spp. were fairly common, but filamentous A. flos-aquae was observed only in very low numbers.
After mid-July until the end of the month and again at the end of the second week of August there were several weak or uncertain cyanobacterial surface accumulations in the basin (SMHI), at the time the surface water temperature was 19-21^oC. At the end of July relatively few species were present, and none of the species occurred in more than sparse numbers, although N. spumigena was commonly encountered. (See Figure 1.)

At the end of August the phytoplankton community was co-dominated by the prymnesiophycean Chrysochromulina spp. and the chrysophycean Pseudopedinella tricostata, but neither occurred in more than small amounts. A. flos-aquae occurred only sparsely. The surface water temperature was in mid-August still 20-21^oC and declined only in the beginning of September to 16-17^oC.

Metadata

Technical information

1. Source: Finnish Institute of Marine Research, contact persons Lotta Ruokanen and Heidi Hällfors.

2. Description of data:
Original unit of measure: mg chl a m^-3
Semiquantitative phytoplankton analysis are based on the relative abundance (1-5) of the species. In the cyanobacterial bloom map, visual observations are included.
Original purpose of the data: Phytoplankton monitoring of FIMR, Alg@line project

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

4. Temporal coverage: 1992-2003.

5. Methodology and frequency of data collection:
Automated flow-through sampling system on merchant ships, sampling depth ca. 5 m, weekly sampling during the period February/March-October/November.

6. Methodology of data manipulation: None.

Quality information

7. Strength and weakness (at data level): Strength: Very high both temporal and spatial sampling frequency. Weakness: -

8. Reliability, accuracy, precision, robustness (at data level): Measurement uncertainty: Chl a: 0.5 mg m^-3 if the concentration < 5.0 mg m^-3, 1.0 mg m^-3 if the concentration > 5.0 mg m^-3.

9. Further work required (for data level and indicator level): Sophisticated statistical analysis.

Last update 1 October 2003