Temporal development of phytoplankton concentration in the Baltic Sea since the 1970s

Figure: Phytoplankton concentration measured in June-September measured as chlorophyll a (µg/l) between years 1974 and 2007 in the open-sea sub-regions of the Baltic Sea. Chlorophyll a observations are plotted against the year of observation and the curves fitted with non-linear smoothing (green line) and shown with 95 % confidence intervals (light green area). The level above the target value set by the HELCOM thematic assessment on eutrophication (HELCOM 2009) is coloured light gray. The number of observations (n) is shown on each figure.

Assessment

An increase in summer phytoplankton concentration was evident in most of the Baltic sub-areas from the 1970s to the present. Only in the south western areas, the Kattegat and Arkona Sea, was the increase virtually non-existant. In the Bornholm Sea even a decrease in summer phytoplankton concentration could be observed. It should be born in mind, however, that the impact of man-made loading was present already in the 1970s, and thus the target value for phytoplankton concentration has not been reached even in these areas.

The increase in phytoplankton concentration was most pronounced in the northern areas: the Gulf of Finland, the Northern Gotland Basin, the Bothnian Bay, the Bothnian Sea, the Eastern Gotland Basin and the Gulf of Riga. In some of these areas, especially the Gulf of Riga, the concentration has turned to a decrease.

The increase in phytoplankton concentration is to a great extent a result of increased nutrient concentrations in the water.

References

Fleming-Lehtinen, V., Laamanen, M., Kuosa, H., Haahti, H. and Olsonen, R. 2008. Long-term development of inorganic nutrients and chlorophyll a in the open northern Baltic Sea. Ambio 37:86-92.

HELCOM 2007. HELCOM Baltic Sea Action Plan. http://www.helcom.fi/BSAP/en_GB/intro/

HELCOM 2009. Eutrophication in the Baltic Sea – An integrated thematic assessment of the effects of nutrient enrichment in the Baltic Sea region. Helsinki Commission. Baltic Sea Environment Proceeding No. 115B.

Schernewski, G. and Neumann, T. 2005. The trophic state of the Baltic Sea a century ago: a model simulation study. Journal of Marine Systems 53:109-124.

Suikkanen, S., Laamanen, M & Huttunen, M. 2007. Long-term changes in summer phytoplankton communities of the open northern Baltic Sea. Estuarine, Coastal and Shelf Science 71:580-592.

Wasmund, N., Uhlig, S. 2003. Phytoplankton trends in the Baltic Sea. ICES Journal of Marine Science 60:177-186.

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