Water transparency in the Baltic Sea between 1903 and 2008
With extention: Transparency of the Baltic Sea during the growth season 2008 according to MERIS Satellite data
Decrease in summer time water transparency was observed in all Baltic subregions 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).
An increase in water transparency during the last 20 years was detected in Bornholm Sea and Arkona Sea.
In Kattegat and the Eastern Gotland Basin the decreasing trend ceased during the past 10 to 15 years and since then the water transparency remained in about the same level.
Results and assessment
Relevance of the indicator for describing developments in the environment
Water transparency is dependent on the amount of particulate matter and dissolved substances in the water. The material in the water may consist of inorganic suspended solids, plankton, humic substances and dissolved coloured substances. During the summer from June to September, the phytoplankton biomass is likely to have a high influence on water transparency, although especially in the Bothnian Bay humic substances are of importance. In summer time, bloom-forming filamentous cyanobacteria are abundant in other open sea areas except in the Gulf of Bothnia. Certain cyanobacteria are able to fix nitrogen and their growth is dependent on phosphate, but irradiance and water temperature also have an effect on their abundance. Changes in water transparency give an indication of changes in summer phytoplankton biomass.
Policy relevance and policy references
Initiatives to reduce loads of phosphate and nitrogen to water are key to reducing phytoplankton biomass. Secchi depth readings from the very beginning of the last century may be considered as reference values.
Decrease in summer time water transparency was observed in all Baltic 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 Gotland Basin, Northern Baltic Proper and the Gulf of Finland. On the other hand, in Kattegat and Eastern Gotland Basin the decreasing trend ceased during the past 10 to 15 years and since then the water transparency remained in about the same level. In Arkona Sea and Bornholm Sea the water transparency has increased during the last two decades.
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 Finland, the increase of cyanobacterial blooms may have contributed to the decrease. Intensified cyanobacterial blooms are a result of increased nutrient concentrations, especially that of phosphorus, and a sign of progressing eutrophication of the Baltic Sea.
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. This has no doubt had a strong effect on the summer time water transparency.
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 in the area is active ditching in the drainage area since the 19th century, causing leaching of organic as well as inorganic substances from land.
In the beginning of the 20th century the 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 that area is frequent, which may be related to variations in water transparency.
Monthly averages from MERIS satellite images show the spatial distribution of water clarity in 2008. (Click link for further information).
Fig. 1. Water transparency in June-September measured as Secchi depth (m) between years 1903 and 2008 in the different 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 and shown with 95 % confidence intervals. The estimation of reference conditions provided by HELCOM Eutro Pro is shown as a red line, the target value with a blue line and the assessment status during 2001-06 as a gray solid line. The number of observations (n) is shown on each figure. Note: Click on the following sub-regions for more detailed images of the respective plots: Bothnian Sea, Bothnian Bay, Gulf of Finland, Eastern Gotland Basin, Northern Baltic Proper, Western Gotland Basin, Arkona Sea, Bornholm Sea, Gulf of Riga and Kattegat.
Fig. 2. Water transparency observations made between 1903 and 2008 in the sub-regions (BB = Bothnian Bay, BS = Bothnian Sea, GOF = Gulf of Finland, NBP = Northern Baltic Proper, GR = Gulf of Riga, KAT = Kattegat, WGB = Western Gotland Basin, EGB = Eastern Gotland Basin, ARK = Arkona Basin and BOR = Bornholm Basin).
Finni, T., Kononen, K., Olsonen, R. & Wallström, K. (2001): The history of cyanobacterial blooms in the Baltic Sea. – Ambio 30:172-178.
Fleming-Lehtinen V, Kaitala S. (2007) Phytoplankton spring bloom biomass in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2007. HELCOM Indicator Fact Sheet
Fleming-Lehtinen V, Laamanen M, Kuosa H, Haahti H, Olsonen R (2008) Long-term development of inorganic nutrients and chlorophyll a in the open northern Baltic Sea. – Ambio 37:86-92.
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 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 as well as datasets provided by the Swedish Meteorological and Hydrological Institute, the Institute of Meteorology and Water Management in Poland, the Center of Marine Research in Lithuania and the Latvian Institute of Aquatic Ecology. At the FIMR the contact person is Vivi Fleming-Lehtinen.
2. 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.
3. Geographical coverage: All regions of the Baltic Sea except for the Gulf of Riga.
4. Temporal coverage: From 1903 to 2008 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.
5. Methodology and frequency of data collection: Measurements have been made on irregular research cruises and during monitoring cruises.
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 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.
The reference conditions for the Baltic sub regions were determined by combining information achieved from the means of the data collected between 1905 and 19010 and the value of the smoothing curve before 1940. The target value was set at a 25% interval from the reference conditions The status in was achieved from the mean of all observations in the sub-region during the period between 2001 and 2006. All values are used as tools for an eutrophication assessment for the Baltic Sea in the Helcom Eutro Pro project.
7. Strength and weaknesses of data: 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.
8. Reliability, accuracy, robustness, uncertainty (at data level): Interpretation of the data ought to be done over long time periods (minimum of ten 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. In order to resolve the significance of Secchi depth as a eutrophication parameter, the roles of the parameters affecting water transparency should be thoroughly investigated in the Baltic.
For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2008. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated: 14 October 2008