Summer time (June-September) water transparency measured as Secchi depth has decreased in all sub basins of the Baltic Sea over the last one hundred years. The decrease is most pronounced in the Baltic Proper (about 35 to 50 %) and the Gulf of Finland (about 40 %) The primary cause is most likely the increase in algal biomass and especially in the Baltic Proper and the Gulf of Finland the increase of cyanobacterial blooms since the 1970’s.
Over the last twenty years water transparency decreased most dramatically in the Northern, Western and Eastern Baltic Proper.
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Figure 1. Water transparency in June-September measured as Secchi depth (m) between years 1903 and 2004 in the different regions of the Baltic Sea. Sampling stations are indicated on the map with different colours for each of the regions. Secchi depth observations (m) are plotted against the year of observation and the curves fitted with non-linear smoothing and shown with 95 % confidence intervals. The number of observations (n) is shown on each figure.
Results and assessment
Relevance of the indicator for describing developments in the environment
Secchi depth gives an indication of water transparency. Water transparency is dependent on the amount of particulate and dissolved material, such as phytoplankton biomass, humic substances and other organic as well as inorganic particles suspended in the water. In the summer period from June to September phytoplankton biomass has the highest influence on Secchi depth transparency. At that time bloom-forming cyanobacteria are abundant in the open sea areas. They are able to fix nitrogen and their biomass is related to the availability of nutrients, especially to phosphate. On the other hand, high water temperatures and calm seas strongly favour the growth of cyanobacteria. Over longer time periods Secchi depth transparency gives an indication of changes in phytoplankton biomass since concentrations of dissolved and suspended matter vary less than that of phytoplankton.
Policy relevance and policy references
Initiatives to reduce loads of phosphate and nitrogen to water are key to reducing phytoplankton biomass. At the present there exist no target values for Secchi depths in the Baltic Sea but the observations from the beginning of the 20th century most likely give an indication of pristine state of the sea.
A decrease in water transparency was observed in all Baltic Sea regions over the last one hundred years. The decrease was most pronounced in the Northern (about 50 %) and Eastern (45 %) Baltic Proper, the Gulf of Finland (40 %). In the Bothnian Sea, Western and Southern Baltic Proper the decrease was in the order of 35 %. The primary cause for the decreased water transparency is most likely the increase in phytoplankton biomass, and especially in the Baltic Proper and the Gulf of Finlad the increase of cyanobacterial blooms. In the Bothnian Bay and Kattegat areas reduction was less, approximately 25 %.
Different areas of the Baltic Sea had different dynamics of water transparency. Secchi depth observations from the first half of the 20th century were in average 10 m or more in all regions except the Gulf of Finland, the Bothnian Bay and the Western and Southern Baltic Proper. In the Bothnian Bay and the Gulf of Finland natural turbidity caused by river run-off is higher than in the other regions causing the water to be less transparent even in the beginning of the 20th century.
The most dramatic reductions in the Secchi depths were observed in the Northern, Western and Estern Baltic Proper since the end of the 1970’s. In the Gulf of Finland, such dramatic reduction over the past 25 years was not observed, instead the trend has been linearly downward since the the measurements were started in 1905. Cyanobacteria blooms have increased especially in these regions and they cause decreased water transparency. Increased cyanobacteria blooms are an indication of increased nutrients, diminished N/P ratios and a sign of progressing eutrophication of the Baltic Sea.
In the Southern Baltic Proper and Kattegat the changes have not been as dramatic as in the northern regions but an anomaly of Secchi depth readings was observed around year 1990. Kattegat is a transitional region between the high salinity North Sea and the brackish water Baltic Sea, which may be related to such variation.
The Bothnian Bay is the only Baltic Sea region where the trend in water transparency is not decreasing. The Bothnian Bay is not prone to cyanobacteria blooms due to its high N/P ratio and high amounts of humic substances play a role in causing natural turbidity.
Finni, T., Kononen, K., Olsonen, R. & Wallström, K. (2001): The history of cyanobacterial blooms in the Baltic Sea. – Ambio 30:172-178.
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.)
Poutanen, E.-L. & Nikkilä, K. (2001): Carotenoid pigments as tracers of cyanobacterial blooms in recent and post-glacial sediments of the Baltic Sea. - Ambio 30:179-183.
Sandén, P. & Håkansson, B. (1996): Long-term trends in Secchi depth in the Baltic Sea. – Limnol. Oceanogr. 41:346-351.
1. Data source: Secchi depth measurements were made on research cruises of the FIMR or corresponding since year 1903. The data produced by FIMR is kept at the database of the FIMR. In addition, observations from the databank at the ICES were used. At the FIMR the contact person is Riitta Olsonen.
2. Description of data: Original unit of measurements is meter. Original purpose of the data was to give an indication of water clarity over long-time periods.
3. Geographical coverage: All regions of the Baltic Sea except for the Gulf of Riga.
4. Temporal coverage: From 1903 to 2004 except for the period from the 1940’s to the beginning of the 1970’s due to the second world war.
5. Methodology and frequency of data collection: Measurements have been made on research cruises taking place irregularly and in association with monitoring programs.
6. Methodology of data manipulation: Months of June, July, August and September were chosen to represent the period of abundant occurrence of cyanobacteria on the basis of monitoring data of Alg@line. Data from the coastal zones (as defined in the Water Framework Directive of the EC) was removed from the data sets in order to have ecologically as homogenic regions as possible. Data from the beginning of the century that was produced using 0.6 m diameter secchi disk and a viewer was corrected according to Launiainen et al. (1989). The Secchi depth data from each geographical area was plotted against the observation year and a non-linear smoothing curve was fitted to the plot. This technique estimates the local fit of the curve. The 95 % confidence intervals of the curve were estimated on the basis of standard error of ± 2 of the curve estimation.
7. Strength and weakness: Secchi depth is one of the few parameters on which there is data from a long time period. Technically Secchi depth measurement is simple, cheap and easy to do. The temporal and spatial coverage of the data is not even, and data is lacking from certain time periods, such as that from the 1940’s to the beginning of the 1970’s. In addition, timing of the measurements in relation to the cyanobacterial biomass maximum may have an effect on the results especially if the amount of data is low.
8. Reliability, accuracy, robustness, uncertainty (at data level): Interpretation of the data ought to be done over long time periods (minimum of twenty years).
9. Further work required (for data level and indicator level): The indicator will be updated annually with data collected from as many temporal and spatial points as possible in each of the regions.