A demonstrative cyanobacteria bloom forecast for the Baltic Sea, summer 2002
Mikko Kiirikki
Finnish Environment Institute
Introduction
The blooms of nitrogen fixing cyanobacteria are known to be favoured by high phosphorus
availability and high surface temperatures in the Baltic Sea (Kononen et al. 1994, Kononen &
Leppänen, 1997). The winter concentrations of algal available phosphorus are well studied during
monitoring cruises carried out by research institutes and environment administration of the Baltic
Sea countries. Measurements carried out in January-March represent the starting situation for the
following growing season. In late winter, most of the mineralisation of algal biomass has already
taken place and the water mass is often mixed down to the permanent halocline, which can be
verified in nutrient, temperature and salinity profiles.
The phytoplankton spring bloom starts in the southern Baltic in late March and it sweeps over the
Baltic reaching the northernmost Bothnian Bay at the end of May. The blooming diatoms and
dinoflagellates consume the available nutrients basically according to the Redfield ratio. There may
occur deviations from the Redfield ratio, e.g. luxury uptake of PO4 by diatoms, but its importance
in the Baltic Sea is not yet known. In practice the spring bloom acts as a filter, which consumes
nutrients leaving excess of the less limiting nutrient intact to the surface water. In the case of the
Baltic proper and the Gulf of Finland, the spring bloom is in most cases limited by the availability
of nitrogen, which means that there will often be excess phosphorus left after the bloom. This
phosphorus pool maintains the blooms of nitrogen fixing cyanobacteria during the summer months.
The potential for bloom forming can already be evaluated on the base of the winter nutrient
concentrations by calculating the excess phosphorus in the following way: excess DIP [mg m-3] =
DIP [mg m-3] – (DIN [mg m-3] / 7.2). In addition to the winter concentrations, load from the
drainage area also affects the excess DIP concentration. In the case of estuaries, high DIN load
carried by spring flood acts as a decreasing factor promoting consumption of DIP in water. At least
partly for this reason nitrogen fixing cyanobacteria are often scarce in areas with high freshwater
input. The only way to take into account the effect of nutrient loading and the general flow pattern
of the Baltic Sea, is to utilise mathematical models in the calculation of nutrient concentrations and
algal growth.
The main factor restricting forecasting of algal blooms is the lack of long term, e.g. 3 moths,
weather forecasts. In this situations we can only use past years weather data or artificial weather
scenarios for calculating the potential for bloom formation. The principle of forecasting the
cyanobacteria blooms is presented by Kiirikki et al. 2001.
Material and methods
The winter nutrient data used in the present work has been collected by Finnish Institute of Marine
Research, Finnish Environment Administration, Swedish Meteorological and Hydrological
Institute and the City of Helsinki. Altogether, data have been available from 362 nutrient profiles
presented in Fig. 1A. In the case of the estuaries of the eastern side of the Baltic (Neva, Daugava,
Nemunas and Vistula) long term average winter values have been used to fill the gaps in the present
data set.
The average surface concentrations (0-20m) of NO3, NO2, NH4 and PO4 have been used in
calculating DIN and DIP. In the case of DIP, nucleapore filtered samples have been preferred in
estuarine conditions, where suspended solid concentration has been known to be high. The
measurements have been used to generate starting fields for the 3D-model calculations.
Interpolation has been carried out by using DAS-program developed in the Stockholm University
(http://data.ecology.su.se/Models/index.htm).
A 3D-hydrodynamic model (Koponen et al. 1992, Virtanen et al. 1986) with ke-turbulence
calculation and ecosystem model (Kiirikki et al. 2001) was applied to the whole Baltic Sea, except
the Danish Straits, with horizontal resolution of 5 km and 17 vertical layers. The loading
information follows Helcom PLC3 and atmospheric forcing SMHI real-analysis data. For river
runoff, climatological values has been used. The simulation was started at the beginning of March.
The demonstrative forecast has been calculated with atmospheric forcing of year 1997, which
represents optimal conditions for the growth of cyanobacteria and thus the calculated forecast is
practically the worst case scenario.
Results
The DIP concentration map (Fig. 1C) shows elevated values (30-40 mg m-3) for the whole Gulf of
Finland. These concentrations are 30-50% higher than during the previous winter period (20002001). The main explanation for the dramatic increase is an exceptionally high internal phosphorus
loading, which was estimated to have released ca. 10 000 tons of biologically available phosphorus
from the bottom sediments during summer and autumn 2001. This figure corresponds to three years
land based external loading to the Gulf of Finland (Heikki Pitkänen, pers. comm.). When DIN
concentration data (Fig. 1B) is taken into account in the calculation of the excess DIP, two distinct
areas of high excess DIP concentrations (Fig. 1D) can be detected. In addition to the Gulf of
Finland, excess DIP concentrations exceeding 15 mg m-3 are found on the southern sides of Öland
and Gotland. No excess DIP is available in the Bothnian Bay, Riga Bay and the Bay of Gdansk.
The vertically integrated biomass of nitrogen fixing cyanobacteria [g m-2] peaks at the turn of July
and August (Fig. 2A). The highest biomasses are detected in the Gulf of Finland, Northern Baltic
Proper SW of Åland, and as separate area between Öland, Gotland and Bornholm. In warm
summer conditions the first floating accumulations can be detected at the beginning of July, and the
peak biomasses be reached between mid July and early august. In the case of suboptimal weather
conditions the bloom can be easily delayed by one month. In that case the peak biomasses will be
lower.
The modelled biomasses can be roughly interpreted as a visible abundance of floating cyanobacteria
blooms. This interpretation, the final forecast, is presented in Fig. 2B. The forecast can be compared
with similar abundance maps based on observations from several sources describing the Gulf of
Finland situation in 1997-2001 (Fig 3). If the weather conditions will be favourable for the growth
of cyanobacteria, it seems that the records of summer 1997 can be reached in summer 2002.
References
Kiirikki, M., Inkala, A., Kuosa, H., Kuusisto, M. & Sarkkula, J. 2001. Evaluating the effects of
nutrient load reductions on the biomass of toxic nitrogen-fixing cyanobacteria in the Gulf of
Finland, the Baltic Sea. Boreal Environment Research 6: 131–146.
Kononen, K., Lahdes, E.O. & Grönlund, L. 1994. Physiological and community responses of
summer plankton to nutrient manipulation in the Gulf of Finland (Baltic Sea) with special reference
to phosphorus. Sarsia 78: 243-253.
Kononen, K. & Leppänen, J.-M. 1997. Patchiness, scales and controlling mechanisms of
cyanobacterial blooms in the Baltic Sea: Application of a multiscale research strategy. In:
Monitoring algal blooms: New techniques for detecting large-scale environmental change. (Kahru,
M. & Brown, C.W., eds.) Landes Bioscience. Austin, pp. 63-84.
Koponen, J., Alasaarela, E., Lehtinen, K., Sarkkula, J., Simbierowicz, P., Vepsä, H. & Virtanen, M.
1992. Modelling dynamics of large sea area. Publications of the Water and Environment Research
Institute 7: 1-91.
Virtanen, M., Koponen, J., Dahlbro, K. & Sarkkula, J. 1986. Three-dimensional water-qualitytransport model compared with field observations. Ecological Modelling 31: 185-199.
A
B
Sampling stations
Dissolved inorganic nitrogen
Data collected by FIMR,
SMHI, Finnish Environment
Administration and the
City of Helsinki in
January-March 2002
DIN mg m-3
300
270
240
210
180
150
120
90
60
30
0
C
D
Dissolved inorganic phosphorus
Excess dissolved inorganic phosphorus
DIP mg m-3
DIP mg m-3
40
25
35
20
30
15
25
10
20
5
15
0
10
<0
5
0
Figure 1. Sampling points, surface water nutrient contrentartations and calculate excess DIP in the
Baltic Sea, January-March 2002. Data is provided by Finnish Institute of Marine Research, Finnish
Environment Administration, Swedish Meteorological and Hydrological Institute and the City of
Helsinki.
A
B
Model, August 1st
Forecast for summer 2002
Biomass of nitrogen
fixing cyanobacteria
g m-2
Risk for cyanobacteria
blooms
very high
>40
high
30-40
moderate
20-30
low
10-20
0-10
Figure 2. A. Ecosystem model simulation showing the maximum biomass of nitrogen fixing
cyanobacteria for the summer 2002. The simulation is carried out by using weather forcing of the
year 1997 known to be favourable for the growth of cyanobacteria. B. A simplified forecast image
presenting the worst case scenario for the summer 2002.
Summer 2001
Abundance of cyanobacteria
Summer 2000
Abundance of cyanobacteria
low
moderate
high
very high
low
moderate
high
very high
Summer 1999
Summer 1998
Abundance of cyanobacteria
low
moderate
high
very high
Abundance of cyanobacteria
low
moderate
high
very high
Summer 1997
Abundance of cyanobacteria
low
moderate
high
very high
Figure 3. Simplified maps showing the abundance of floating cyanobacteria accumulations in the
Gulf of Finland, northern Baltic Proper and Bothnian Sea during summers of 1997-2001.