Finnish Institute of Marine Research
PO. Box 33
Juha-Markku Leppänen, Eija Rantajärvi & Seija Hällfors
PHYTOPLANKTON BLOOMS IN THE BALTIC SEA IN 1994
Introduction and method
Phytoplankton in 1994
the algal bloom monitoring and early warning
Routes of the ferries
Total extent of cyanobacterial surface accumulations in 1994.
Variability in the concentration of chlorophyll a.
Variability in the chlorophyll a concentrations in 24 and 25 July 1994.
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.
Figure 1. Routes of the ferries with the unattended recording and sampling unit for
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.
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.
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 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
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
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.
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
Figure 2. Total extent of cyanobacterial surface accumulations in 1994. The figure
is compiled satellite images provided by Vesa Laine (Finnish Institute of
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
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).
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
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).
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.
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.
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.
Kahru, M., Horstmann, U. &
Rud, O: Increased cyanobacterial blooming in the Baltic Sea detected by satellites:
Natural Fluctuation or ecosystem change? -Submitted to Ambio.
Leppänen, J.-M., Kahru,
M. & Nômmann, S. 1991: Variability in the surface layer in the Gulf of Finland as
detected by repeating continuous transect between Helsinki and Tallinn - a progress
report. -ICES Symposium on patchiness in the Baltic Sea, Mariehamn, Finland, 3-4
June 1992. Paper No. 30 (Mimeo).
Rantajärvi, E., Maunumaa, M., Larinmaa, M. & Pajala, J. 1994: Unattended algal
monitoring system - a high resolution method for detection of phytoplankton blooms in
the Baltic Sea. -Oceans 94 Proceedings, IEEE, New York 1:461-463.
Rantajärvi, E. &
Leppänen, J.-M. 1994: Unattended algal monitoring on merchant ships in the
Baltic Sea. -TemaNord 546:1-60.
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