During 2002 and 2003 major parts of the Baltic Sea were affected by strong surface accumulations of cyanobacteria. Both years had strong blooms but different areas were affected. During 2002 the most intense blooms were observed in the northern Baltic Proper and in the Gulf of Finland. During 2003, blooms were concentrated in the eastern Baltic Proper and the area around Gotland.
Analysis of sediment cores has indicated that cyanobacterial blooms are as old as the brackish phase of the Baltic Sea, starting 7000 years B.P. (Bianchi 2000). It has also been suggested that cyanobacteria pigments in sediment cores have increased in concentration since the 1960s (Poutanen et al 2001). In recent years there has been a debate on whether there has been an increase of the intensity and areal extent of the cyanobacterial bloom due to eutrophication (Karhu 1994, Finni 2001, Bianchi 2000). Monitoring of the areal extent and the intensity of the surface blooms may give clues about possible human impacts and natural variability.
The first cyanobacterial bloom was seen on the 8th of July. The bloom spread around Gotland and the most intense blooms were found in the Northern Baltic Proper and in the Gulf of Finland. The maximal area extent (~110000 km2) was observed on the 13th of July but due to cloudy weather at the end of July the observed areal extent decreased. The last reliable bloom observation was made on the 16th of August.
Figure 1. Summary of 2002. Number of days with cyanobacteria observed in each pixel (from NOAA-AVHRR satellite imagery).
Figure 2. Summary of 2002. Daily areal extent of cyanobacterial blooms in the Baltic Sea. Red bars correspond to certain bloom observations and yellow bars indicate uncertain bloom observations. The blue line represents the integrated cloud cover in percent of the total area over the whole Baltic Sea, excluding the Kattegat, Skagerrak and the Bothnian Bay.
The first observation of surface accumulations of cyanobacteria was made on the 8th of July. Calm, sunny and warm weather conditions, and higher than normal concentrations of phosphorus were favourable for a massive bloom and within a week intense blooms affected a large parts of the Baltic Proper. Maximal areal extent (~110000 km2) was observed on the 24th and 31st July and most observations were made east and south-east of Gotland. The bloom then spread westwards along the Polish and Swedish coast to the Arkona Basin and Belt Sea. No blooms were observed after the 18th of August.
Figure 3. Summary of 2003. Number of days with cyanobacteria observed in each pixel (from NOAA-AVHRR satellite imagery).
Figure 4. Summary of 2003. Daily areal extent of cyanobacterial blooms in the Baltic Sea. Red bars correspond to certain bloom observations and yellow bars indicate uncertain bloom observations. The blue line represents the integrated cloud cover in percent of the total area over the whole Baltic Sea, excluding the Kattegat, Skagerrak and the Bothnian Bay.
Excess phosphorus after the spring bloom and suitable weather conditions favour cyanobacteria blooms in the Baltic Sea. Stagnation periods in the Baltic Sea Deep Water due to reduced Atlantic water inflow since the late 1970s has increased the phosphorus concentration in the near bottom layers. Weakening stratification in the deeper layers permits vertical transport of phosphorus to the euphotic zone. The intrusion of high salinity water from the Skagerrak and Kattegat can also accelerate the vertical transport since the inflowing water displaces the old bottom water (with high phosphorus concentration) further up the water column making it available for cyanobacteria growth. (See indicator report: Hydrography and Oxygene in the Deep Basins, and Water exchange between the Baltic and the North Sea and conditions in the deep basins)
Both 2002 and 2003 showed similar pattern in affected area extent and the time period of visible surface accumulations of cyanobacteria. However, the area with the most intense blooms was different during the two years.
Bianchi T. S., E. Engelhaupt, P. Westman, T. Andren, C. Rolff, R. Elmgren, 2000, Cyanobacterial blooms in the Baltic Sea: Natural or human-induced?, Limnol. Oceanogr., 45(3), 716–726.Finni, T., K. Kononen, R. Olsonen and K. Wallström, 2001, The History of Cyanobacterial Blooms in the Baltic Sea. Ambio Vol. 30 No.4-5.
Kahru, M., U. Horstmann and O. Rud, 1994, Satellite Detection of Increased Cyanobacteria Blooms in the Baltic Sea: Natural Fluctuation or Ecosystem change?, Ambio Vol. 23 No. 8.
Kahru, M., 1997, Using Satellites to Monitor Large-Scale Environmental Change: A case study of the Cyanobacteria Blooms in the Baltic Sea. Monitoring algal blooms: New techniques for detecting large-scale environmental change. Landes Bioscience.
Poutanen, E.L and K. Nikkilä, 2001, Carotenoid pigments and tracers of cyanobacterial blooms in recent and postglacial sediments in the Baltic Sea. Ambio, vol 30, No. 4-5.
The SMHI satellite receiving station in Norrköping collected the NOAA-AVHRR data.