Finnish Institute of Marine Research
PO. Box 33
FIN-00931 Helsinki
Juha-Markku Leppänen, Eija Rantajärvi & Seija Hällfors
e-mail: leppanen@fimr.fi

PHYTOPLANKTON BLOOMS IN THE BALTIC SEA IN 1994

*Introduction and method
*Phytoplankton in 1994
*Spring bloom
*Summer minimum
*Cyanobacterial blooms
*Other bloom observations
*Future of the algal bloom monitoring and early warning
*References

FIGURES

*Fig. 1. Routes of the ferries
*Fig. 2. Total extent of cyanobacterial surface accumulations in 1994.
*Fig. 3. Variability in the concentration of chlorophyll a.
*Fig. 4. Variability in the chlorophyll a concentrations in 24 and 25 July 1994.

*Introduction and method

The Finnish Institute of Marine Research has monitored the variability in the surface concentrations of chlorophyll a and phytoplankton species composition since 1992 using unattended recording and sampling on passenger ferries crossing the Baltic Sea (Leppänen et al. 1991, 1994). On the ferries chlorophyll a fluorescence, temperature and salinity are recorded quasi-continuously with spatial resolution of 100- 300 m while the ferries are moving. Concurrently, water samples are taken for the microscopic analysis of phytoplankton species composition and for the quantitative determination of chlorophyll a and nutrients. The chlorophyll a data is used to convert the fluorescence readings to chlorophyll a concentrations. For details of the method and data processing see Rantajärvi and Leppänen (1994). Recordings are carried out on 'Finnjet' that is crossing the whole Baltic Proper from Helsinki to Travemünde, on 'Konstantin Simonov' between Helsinki and Sankt Petersburg in the Gulf of Finland and on 'Fennia' in the Quark region in the Gulf of Bothnia during the ice-free period (Fig. 1). On 'Konstantin Simonov' the recordings started in June.

* * * [Route Map] * * *
Figure 1. Routes of the ferries with the unattended recording and sampling unit for phytoplankton monitoring.

At present, the phytoplankton species composition is determined in ca. 500 samples. Regular reports on the plankton bloom situation in the Baltic Sea have been compiled on the basis of these ferry recordings but information from satellite images and from other sources has been used as well.
The project has been financed by the Finnish Institute of Marine Research and the Nordic Council of Ministers. The 'Fennia' project is carried out by Sinikka Jokela and Olle Siren (the Kokkola Water and Environment District). Ove Rud (Stockholm University) and Vesa Laine (Finnish Institute of Meteorology) has provided useful satellite information.

* Phytoplankton in 1994

No exceptional phytoplankton blooms were observed in 1994 in the Baltic Sea. High phytoplankton biomass values were recorded during the spring bloom in the whole Gulf of Finland and the vigorous blooms continued in the easternmost area of the Gulf during the whole growth period. Cyanobacterial blooms covered the whole Baltic Sea proper in June-July. The extent of the bloom was largest since the beginning of 1980's.

* Spring bloom

The temporal succession of the spring bloom followed the typical pattern: It started in the beginning of March almost at the same time in the southern and northern regions of the 'Finnjet' transect. The peak of the bloom was reached earlier in the southern regions, the highest values were measured in the Gulf of Finland and chlorophyll a concentrations stayed low in the middle of the transect.
The spring bloom phytoplankton population was dominated by dinoflagellates and diatoms in the whole Baltic Sea area (Peridiniella catenata, Peridinium hangoei, Achnanthes taeniata, Skeletonema costatum, Thalassiosira levanderi, Chaetoceros spp.).

* Summer minimum

In the mid-summer period (June and beginning of July) the phytoplankton biomass in the Baltic Proper and in the western and middle parts of the Gulf of Finland was low and the species composition was predominated by small flagellates (e.g. Chrysochromulina spp. and Pyramimonas spp.). Some potentially toxic phytoplankton species (mostly Chrysochromulina polylepis) were present but they formed no blooms.

* Cyanobacterial blooms

The cyanobacterial blooms in July-August extended over the whole Baltic Proper. The total extent of the surface accumulations (Fig. 2) was comparable to that in the beginning of 1980s (Kahru et al. subm.). The surface accumulations were mostly dominated by the toxic species Nodularia spumigena.

* * * [Satellite image] * * *
Figure 2. Total extent of cyanobacterial surface accumulations in 1994. The figure is compiled satellite images provided by Vesa Laine (Finnish Institute of Meteorology.

In the beginning of July the first surface accumulations were observed in the Gotland Basin and soon after that in the southern Baltic Proper north from Bornholm. Some small floats were also detected in the western and eastern parts of the Gulf of Finland.
In the middle of the July large surface accumulations were observed in the Estonian coast off Tallinn.
In the end of July the intensive surface blooms covered the whole Baltic Proper and western Gulf of Finland and were drifted to the Swedish coast.
In the beginning of August the intensity of the cyanobacterial blooms started to decrease in the Baltic Proper although surface accumulations still occurred in large areas. During this time in the southern and southwestern Aland accumulations occurred near the coast as well as some local blooms were observed in the Archipelago Sea in the southwest Finland.
In the end of August strong wind dispersed all the surface accumulations in the Baltic Proper and in the Gulf of Finland.
In the Bothnian Sea scattered surface accumulations were detected at the end of August and at the same time in the middle of the Bothnian Bay. No samples for species determination were collected.
The cyanobacterial blooms were detected by the flow-through ferry measurement well before the accumulation into the surface when they become first visible in the satellite images (Fig. 3).

* * * [Satellite image] * * *
Figure 3. Variability in the concentration of chlorophyll a (mg m-3 ) in the surface layer of the northern Baltic Proper and in the western Gulf of Finland in 25 June 1994 as recorded on 'Finnjet' as compared to a satellite image in 31 June 1994 showing the surface accumulations of the cyanobacteria. The ship transect is marked in the satellite image by a broken line. The phytoplankton composition in the westernmost and easternmost areas of the transect is indicated in the upper part of the figure. The satellite image is provided by Vesa Laine (Finnish Institute of Meteorology).

* Other bloom observations

In the easternmost Gulf of Finland and in the Neva Estuary intensive blooms dominated by the cyanobacteria (e.g. Planktothrix agardhii and Anabaena spp.) were recorded during the whole summer period (Fig. 4.). These blooms did not form surface accumulations and were thus not visible in the satellite images.

* * * [Satellite image] * * *
Figure 4. Variability in the chlorophyll a concentrations in the Baltic Sea as recorded on the ferries 'Finnjet' and 'Konstantin Simonov' in 24 and 25 July 1994. The intensive bloom off St. Petersburg is clearly seen in the high chlorophyll concentrations. Elevated concentrations in the Gotland Sea, in the Northern Baltic Proper as well as at the entrance to the Gulf of Finland are caused by Nodularia blooms.

In the end of July a potentially toxic dinoflagellate Alexandrium sp. was detected in relatively high numbers in the Mecklenburg Bight. South from Aland dinoflagellate Heterocapsa triquetra formed an intensive bloom, which colored the water locally red. The species is not known to be toxic.
In the beginning of August benthic cyanobacteria (e.g. Lyngbya sp., Oscillatoria limosa) detached from the bottom and drifted to the shore in the eastern part of Finnish coast in the Gulf of Finland. They caused skin symptoms and nausea for the persons dealt with the algal assemblage.
In the end of August and in September the diatom Rhizosolenia fragilissima and the potentially toxic dinoflagellate Prorocentrum minimum formed high biomass values t in the Mecklenburg Bight.
Intensive Chlamydomonas bloom in May and a diatom bloom in June-July were detected in the coast off Kokkola in the Bothnian Bay. Local Chlamydomonas blooms were observed in the Archipelago Sea and in the southern coast of Finland in the end of April.

* Future of the algal bloom monitoring and early warning

The monitoring and early warning of potentially harmful algal blooms in the Baltic Sea are carried out in several countries. The most extensive programmes are in Germany, Sweden and Finland. However, only the Finnish one covers most of the Baltic Sea area by the regular high resolution ferry observations. The system provides rapid and reliable information on the development of the algal biomass to the environmental authorities around the Baltic Sea. Additionally the system provides extensive information on the plankton species with special emphasis on potentially harmful ones. A highly experienced taxonomist is taking care of the microscopic species determination and if necessary, electron microscopy can be used e.g. for the determination of the potentially toxic Chrysochromulina species. A strict quality assurance programme is developed for the data collection and processing. The project including the development of the analyzer system and the data dissemination has been predominately financed by the Nordic Council of Ministers. This financing will end this year.

* References


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