Temporal development of water clarity of the Baltic Sea since the beginning of the 20th century

Figure: Water transparency in June-September measured as Secchi depth (m) between years 1903 and 2009 in the open-sea sub-regions of the Baltic Sea. Secchi depth observations (m) are plotted against the year of observation and the curves fitted with non-linear smoothing (blue line) and shown with 95 % confidence intervals (light blue area). The level below 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

Decrease in summer time water transparency has been observed in all Baltic sub-regions over the last one hundred years. The decrease has been most pronounced in the northern Baltic Proper (from 9 m to 5 m) and the Gulf of Finland (from 8 m to 4 m). More recent decreases – over the past 25 years – have been most pronounced in the Western Gotland Basin, Northern Baltic Proper and the Gulf of Finland. On the other hand, in Kattegat and Eastern Gotland Basin the decreasing trend has ceased during the past 20 years and since then the water transparency has remained at about the same level. In the Arkona Sea and Bornholm Sea the water transparency has increased slightly during the last two decades.

The decreased summer time water transparency in the Baltic Proper and the Gulf of Finland is at least partly a result of the increase in phytoplankton biomass. In the Gulf of Finland, Bothnian Sea, Northern Baltic Proper, Gulf of Riga, Western Gotland Basin, Northern Gotland Basin, Bornholm Sea and Arkona Sea, summertime cyanobacterial blooms have become common during the last few decades.

Another reason for the decreased water transparency is no doubt the change in the input of organic as well as inorganic substances from land. This is probably the main reason in the Bothnian Bay, where phytoplankton growth is limited by low concentrations of phosphorus and heterotrophic production is of importance. However, it is evident also in the Bothnian Sea and the Gulf of Finland and possibly in other areas.

In the beginning of the 20th century water transparency was considerably lower in the Gulf of Riga than in the other sub-regions investigated.

Kattegat is part of the transitional area between the high salinity North Sea and the brackish water Baltic Sea and water exchange in the area is frequent, which may be related to variations in water transparency.

References

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.

Launiainen, J., Vainio, J., Voipio, A., Pokki, J. & Niemimaa, J. (1989): Näkösyvyyden vaihteluista ja muuttumisesta pohjoisella Itämerellä (Long-term changes in the secchi depth in the northern Baltic Sea). – XIV Geofysiikan päivät. Geofysiikan seura. Helsinki, 117-121. (In Finnish, English summary)

Sandén, P. & Håkansson, B. (1996): Long-term trends in Secchi depth in the Baltic Sea. – Limnol. Oceanogr. 41:346-351.

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