Phytoplankton biomass and species succession in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2002

	Authors: Lotta Ruokanen, Seija Hällfors and Eija Rantajärvi Finnish Institute of Marine Research

Key message

In 2002, the cyanobacterial (blue-green algal) blooms were clearly more intense than during the summers 1999 and 2001 and almost as intense as in 1997. The chlorophyll a level in 2002 was clearly higher than the average in 1992-2001 from the beginning of July until the beginning of August in the Gulf of Finland, from the end of June until the end of July in the northern Baltic Proper and the beginning of June until mid August in Arkona Sea.
For the Arkona Sea a clear and for Gulf of Finland and northern Baltic Proper a light long-term (1993-2002) trend was observed when the ratio of two most abundant cyanobacteria species was compared. The relative abundance of the toxic Nodularia spumigena had increased in relation to the non-toxic Aphanizomenon flos-aquae.

Results and assessment

Relevance of the indicator for describing developments in the environment

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

Varying combinations of environmental factors form the basis for phytoplankton species composition. Thus, the annual species succession reflects changes in the marine environment. Also the occurrence of potentially harmful species can be revealed.

 Figure 1. Area covered with algal blooms (km²) in the Baltic Sea during summer 2002 by NOAA, AVHRR satellite images. The intensity of blooms is also represented, the blue curve stands for areal coverage of clouds. Data: SMHI.

Assessment of the Gulf of Finland

The strong stratification built up during a couple of years in the Gulf of Finland broke down during the autumn storms 2001. In the same process phosphate phosphorus, dissolved from bottom sediments to deepwater, was transported up to the surface water. This predicted good growth conditions for cyanobacteria for the summer 2002. The spring bloom was about the average duration and intensity as compared to last ten years. The summer minimum occurred just before mid June and after that the biomass started to increase. The summer maximum was observed around mid July and the phytoplankton biomass was clearly higher than the average of the 1990s. The maximum ceased towards the beginning of August, and phytoplankton biomass returned again to average values in August and September.

In the the spring bloom the phytoplankton species composition was the normal combination of diatoms and dinoflagellates. In the early summer there were still high levels of phosphate phosphorus in surface waters in the basin. Phytoplankton was very sparse in the beginning of June; some diatoms, nanoflagellates, cyanobacteria (blue-green algae) and prymnesiophyceans occurred scatteredly. Amount of non-toxic filamentous cyanobacterium Aphanizomenon flos-aquae started to increase in the second week of June, when also the first visual observations of cyanobacteria, still mixed into water were made. Cyanobacterial growth accelerated towards the end of June but strong winds kept the surface waters mixed and cool. During the second week in July, as the weather calmed and surface waters warmed up, extensive cyanobacterial surface aggregations built up. These aggregations grew to cover most of the offshore basin. The fraction of the toxic species Nodularia spumigena increased in the phytoplankton flora as cyanobacteria grew more abundant. By the third week in July Nodularia was the most abundant species of cyanobacteria in offshore areas of the basin, although the cyanobacterial amount and total phytoplankton biomass decreased. During the last week in July strong winds dispersed finally the surface aggregations.

Figure 2. Annual variation of surface layer phytoplankton biomass (measured as chlorophyll a mg m^-3) in the western and central Gulf of Finland. The green curve represents the average for 1992-2001, the red dots the 2002 measurements.

Calm periods followed and in the beginning of August large, but discontinous, cyanobacterial aggregations were observed again. Aphanizomenon flos-aquae was clearly more abundant than Nodularia spumigena. By mid August cyanobacterial surface aggregations disappeared, phytoplankton biomass had decreased to 1990s average and both numbers of species and cells were poor. Nanoflagellates dominated the species composition and the cyanobacteria were getting sparse, Nodularia spumigena and Anabaena spp. had disappeared completely. In August surface water temperatures in all Finnish offshore areas were several degrees above the usual. The longlasting sunny weather and the decrease in algal concentrations made offshore waters in the basin clearer but at the end of August there were still local blooms observed in nutrient rich and sheltered bays in the coastal waters. During the end of August, some local blooms of nontoxic dinoflagellates coloured the water red-brown. Different from the year 1997, no large surface aggregations were observed after mid August and most of the cyanobacterial aggregations kept offshore in the basin. In September the phytoplankton biomass followed the ten years’ average. Nanoflagellates dominated the species composition, but diatoms appeared in higher numbers indicating the oncoming autumn.

 Figure 3. Ratio of the abundances (the mean abundance value from samples collected in July-August) of bloom forming blue-green algae, toxic Nodularia and nontoxic Aphanizomenon in 1993 – 2002 in the western and central Gulf of Finland. The ranking is sample specific (0 = not detected, 1 = very sparse, 2 = sparse, 3 = scattered, 4 = abundant, 5 = dominant). The detailed definition of the method can be found in Rantajärvi et al., Hydrobiologia 363: 127 – 139, 1998.

Assessment of the northern Baltic Proper

After the spring bloom there was still plenty of phosphate phosphorus in the surface water in the northern Baltic Proper. These conditions favoured the formation of extensive cyanobacterial (blue-green algal) blooms during the hot summer 2002. Until mid June the phytoplankton was sparse and no species occurred in great numbers. Small flagellates (Pyramimonas spp., Chrysochromulina spp.) were dominating, but the non-toxic cyanobacterium Aphanizomenon flos-aquae was also common. Surface water temperature was around 15 degrees centigrade. Chrysochromulina spp. started to dominate in the end of June, but filaments of Aphanizomenon flos-aquae were abundant, while toxic Nodularia spumigena and Anabaena spp. were relatively common. Winds and consequent water mixing prevented the formation of surface accumulations and decreased surface water temperature a few degrees inhibiting the growth of cyanobacteria.

Figure 4. Annual variation of surface layer phytoplankton biomass (measured as chlorophyll a mg m^-3) in the northern Baltic Proper. The green curve represents the average for 1992-2001, the red dots the 2002 measurements.

In the beginning of July surface water temperature had risen to 17 – 19 ^oC and the cyanobacteria dominated the phytoplankton species. The bundles of A. flos-aquae and small chroococcalean colonies were abundant, while N. spumigena and Anabaena spp. were only relatively common. The amount of cyanobacteria increased considerably after the first week of July in the whole basin. Extensive cyanobacterial surface accumulations built up as the weather calmed and surface waters warmed up after the first week of July. Around mid July blooms were extremely strong, and spread out to cover most of the basin. The situation fluctuated between weak but wide, and very dense distinct local blooms until the end of July. The high surface water temperature 21 – 23 ^oC in the basin favoured the growth of cyanobacteria. By the third week in July N. spumigena turned to be the most abundant cyanobacterium even if the total amount of cyanobacteria decreased. However, already in mid July amount of A. flos-aquae and Anabaena spp. had clearly decreased. Nanoflagellates dominated the species composition, eventhough the partly empty trichomes of N. spumigena were still relatively abundant. By the end of July the cyanobacteria N. spumigena and A. flos-aquae were sparse and Anabaena spp. absent. In the end of July strong winds mixed the surface accumulations to water column in the area. While the weather became calm some blooms formed again.

In the beginning of August wide but discontinuous cyanobacterial accumulations appeared in different parts of the basin. A. flos-aquae was clearly more abundant than N. spumigena. By mid-August blooms disappeared. In mid August nanoflagellates dominated the species composition, the cyanobacterium N. spumigena was sparse and A. flos-aquae very sparse. In the end of August the phytoplankton flora of the basin consisted mainly of small flagellates and diatoms. By mid September diatoms slightly increased in abundance, the diatom Cylindrotheca closterium was common, but nanoflagellates dominated still the species composition.

Figure 5. Ratio of the abundances (the mean abundance value from samples collected in July-August) of bloom forming blue-green algae, toxic Nodularia and nontoxic Aphanizomenon in 1993 – 2002 in the northern Baltic Proper. The ranking is sample specific (0 = not detected, 1 = very sparse, 2 = sparse, 3 = scattered, 4 = abundant, 5 = dominant). The detailed definition of the method can be found in Rantajärvi et al., Hydrobiologia 363: 127 – 139, 1998.

Assessment of the Arkona Sea

In the Arkona Basin nanoplankton flagellates (e.g. Pyramimonas spp., Chrysochromulina spp.) dominated the phytoplankton species in May and June. In the end of June also filamentous blue-green algae or cyanobacteria (non-toxic Aphanizomenon flos-aquae and toxic Nodularia spumigena) and small colonial species (Cyanodictyon spp.) were sparsely found. A weak cyanobacterial bloom was observed in the second week of July in the northern basin (SMHI). In mid July only diatoms and the potentially toxic dinoflagellate Prorocentrum minimum were common, but cyanobacteria were still sparse. In the turn of the months July and August the non-toxic cyanobacterium Aphanizomenon flos-aquae was abundant in the basin, while the toxic Nodularia spumigena was much more sparse and most of the trichomes of the N. spumigena were dead. At the same time there was an apparent bloom of the dinoflagellate Prorocentrum minimum (Länsstyrelsen i Stockholms län).

Figure 6. Annual variation of surface layer phytoplankton biomass (measured as chlorophyll a mg m^-3) in the Arkona Sea. The green curve represents the average for 1992-2001, the red dots the 2002 measurements.

In the beginning of August in Køgebukt south of Kopenhagen just out of basin there were also surface accumulations of cyanobacteria (Länsstyrelsen i Stockholms län).

Phytoplankton continued to be poor both in number of species and cells by mid August, Chrysochromulina spp. were moderately common as well as the dinoflagellate Prorocentrum minimum. In mid August there was a small scattered cyanobacterial bloom of the latter species in the basin (SMHI). In the southern Baltic Proper the abundance of cyanobacteria increased in the German coast waters after mid August and moderate cyanobacterial blooms were observed in the coastal waters of Denmark. Also the dinoflagellate Prorocentrum minimum bloomed and coloured water red in Germany. After mid August there were upwellings along the German and Polish coast that cooled the surface water temperatures (SMHI).

Figure 7. Ratio of the abundances (the mean abundance value from samples collected in July-August) of bloom forming blue-green algae, toxic Nodularia and nontoxic Aphanizomenon in 1993 – 2002 in the Arkona Sea. The ranking is sample specific (0 = not detected, 1 = very sparse, 2 = sparse, 3 = scattered, 4 = abundant, 5 = dominant). The detailed definition of the method can be found in Rantajärvi et al., Hydrobiologia 363: 127 – 139, 1998.

Metadata

Technical information

1. Source: Finnish Institute of Marine Research, contact persons Lotta Ruokanen and Seija 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: 1992-2002.

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 March-October.

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.