Decrease in summer time water transparency was observed in all Baltic Sea sub-regions over the last one hundred years. The decrease was most pronounced in the Northern Baltic Proper (from almost 9 m to 4 m) and the Gulf of Finland (from 8 m to 4 m).
In the Kattegat, Southern and Eastern Baltic Proper, the Bothnian Sea and the Bothnian Bay the decreasing trend has ceased during the recent 10 to 15 years and since then the water transparency has remained in about the same level.
Figure 1. Water transparency in June-September measured as Secchi depth (m) between years 1903 and 2005 in the different sub-regions of the Baltic Sea. Observation sites 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. Click image to enlarge!
Results and assessment
Relevance of the indicator for describing developments in the environment
Secchi depth gives a measure of water transparency. Water transparency is dependent on the amount of particulate matter and dissolved substances in the water. The matter in the water may consist of suspended solids, plankton, humic substances and dissolved coloured substances. In summer from June to September, phytoplankton biomass is likely to have a high influence on Secchi depth transparency, although especially in the Bothnian Bay humic substances are of importance. In summer time, bloom-forming filamentous cyanobacteria are often abundant in other open sea areas except the Gulf of Bothnia. Cyanobacteria can fix nitrogen and their abundance is related to phosphate phosphorus, irradiance and water temperature. Changes in Secchi depth transparency give an indication of changes in summer phytoplankton biomass.
Policy relevance and policy references
Initiatives to reduce loads of phosphorus and nitrogen to water are key to reducing phytoplankton biomass. Secchi depth readings from the very beginning of the last century can be considered as reference values.
Decrease in summer time water transparency was observed in all Baltic Sea sub-regions over the last one hundred years. The decrease was most pronounced in the Northern Baltic Proper (from almost 9 m to 4 m) and the Gulf of Finland (from 8 m to 4 m). More recent decrease – that over the past 25 years – was most pronounced in the Western and Northern Baltic Proper as well as in the Gulf of Finland. On the other hand, in the Kattegat, Southern and Eastern Baltic Proper, the Bothnian Sea and the Bothnian Bay the decreasing trend ceased during the past 10 to 15 years and since then the water transparency remained in about the same level.
The primary cause for the decreased summer time water transparency in the Baltic Proper and the Gulf of Finland is most likely to be the increase in phytoplankton biomass. In the Baltic Proper and the Gulf of Finlad, the increase of cyanobacterial blooms may have contributed to the decrease. Intensified cyanobacteria blooms are a result of increased nutrient concentrations, especially that of phosphorus, and a sign of progressing eutrophication of the Baltic Sea.
In the Bothnian Bay and the Gulf of Finland the water was less transparent even in the beginning of the 20th century. That is due to higher natural turbidity and colouration caused by leaching of substaces from the drainage area. However, the decrease in water transparency over the past century was notable (from 8 to 6 m) also in the Bothnian Bay. In the open sea areas of the Bothnian Bay, phytoplankton growth is limited by low concentrations of phosphorus and cyanobacteria blooms are not common. One of the reasons for the decreased water transparency there is that in the drainage area of the Bothnian Bay active ditching was carried out since the 19th century causing leaching of organic as well as inorganic substances from land. New ditchings are not opened any longer, which may be one of the reasons to the recent ceasing of the decrease of the water transparency in the Bothnian Bay.
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 part of the transitional area between the high salinity North Sea and the brackish water Baltic Sea and water exchange in that area is frequent, which may be related to such variation.
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.
Data source: Secchi depth measurements were made during the research and monitoring cruises of the FIMR since year 1903. The data produced by FIMR is kept at the database of the FIMR. In addition, observations from the database of the ICES were used. At the FIMR the contact person is Riitta Olsonen.
Description of data: The unit of the measurements is meter. Original purpose of the data was to give an indication of water clarity over long-time periods.
Geographical coverage: All regions of the Baltic Sea except for the Gulf of Riga.
Temporal coverage: From 1903 to 2005 except for the period from the 1940’s to the beginning of the 1970’s when observations are scarce due to the second world war.
Methodology and frequency of data collection: Measurements have been made on research cruises taking place irregularly and during monitoring cruises.
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 originating from the coastal zones 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 a 0.6 m diameter secchi disk and a water viewer was corrected according to Launiainen et al. (1989). The Secchi depth data from each sub-region 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
Strength and weakness: Secchi depth is one of the few parameters on which there is data from a long time period. Practically the method is unchanged. 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.
Reliability, accuracy, robustness, uncertainty (at data level): Interpretation of the data ought to be done over long time periods (minimum of ten years).
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.
For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated 25.11.2005.