The Western Gulf of Finland became more eutrophied in the 70s and the 80s in terms of phytoplankton biomasses (Chlorophyll a). This trend has not continued in the 90s, however.
The Baltic Sea has become more eutrophied during the latest decades. However, how this phenomenon has actually advanced in the Gulf Of Finland is not easy to assess due to the shortage of the long enough data sets of the productivity parameters. There exists a 30-yr chlorophyll a dataset measured off Hanko archipelago. When the whole data set covering all the seasonal data from April to October was included, this data set shows that the trophic state of this area showed an increasing trend in the 1970s and the 1980s. This trend did not continue in the 1990s.
Vernal (April-May) phytoplankton biomasses have reduced since the 80s, and the late-summer (July-August) phytoplankton biomasses started to increase in the early 90s. The vernal phytoplankton bloom, being mainly dependent on nitrogen, has been affected by both the decrease in total anthropogenic nitrogen inputs to the Gulf of Finland, and by denitrification taking place in the anoxic/hypoxic conditions in the deeper parts of the water column of the Gulf of Finland.
The increased late-summer phytoplankton blooms are largely due to increased amounts of unused phosphorus after the vernal exhaustion of nitrogen. This phosphorus pool is the result either of an increased proportion of phosphorus not used during the spring or of the inorganic internal loading of phosphorus in the Gulf of Finland that has increased in the 90s. The increase in Chlorophyll \a\ during late-summer (July-September) has also been observed between the two periods of 1991-96 and 1997-2001 (Pitkänen 2004), in Finnish, English abstract.
This article is based on the forthcoming article in the article series Ambio: "Mika Raateoja, Jukka Seppälä, Harri Kuosa and Kai Myrberg: Recent changes in trophic state of the Baltic Sea along SW coast of Finland".
More information: Mika Raateoja.
