Technical data

Metadata

Technical information 

Data source: The HELCOM countries have provided status concentrations for 2003-2007. The open sea Secchi depth measurements were made during research and monitoring cruises of the HELCOM Contracting Parties. The data is kept at the database of the Finnish Environment Institute SYKE, where the contact person is Vivi Fleming-Lehtinen.

Description of data: The measurement unit is meter. The 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.

Figure. Water transparency observations made between 1903 and 2009 in the sub-regions (BB = Bothnian Bay, BS = Bothnian Sea, GoF = Gulf of Finland, NBP = Northern Baltic Proper, GR = Gulf of Riga, WGB = Western Gotland Basin, EGB = Eastern Gotland Basin, Ark = Arkona Basin and Bor = Bornholm Basin).

Temporal coverage: From 1903 to 2007 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: Data were mainly collected for the HELCOM COMBINE and national monitoring programmes. Measurements have been made on irregular research cruises and during monitoring cruises.

Methodology of data analyses: The status of the Baltic Sea according to the described indicator has been classified using the multi-metric indicator-based HELCOM Eutrophication Assessment Tool (HEAT). Each area was assessed using information on reference conditions (RefCon) and acceptable deviation from reference condition (AcDev) combined with national monitoring data from the period 2003–2007. The basic assessment principle is RefCon ± AcDev = EutroQO, where the latter is a "eutrophication quality objective" (or target) corresponding to the boundary between good and moderate ecological status. When the actual status data (average for 2003-2007) exceeds the EutroQO or target, the areas in question is regarded as affected by eutrophication.

The Ecological Quality Ratio (EQR) is a dimensionless measure of the observed value (AcStat) of an indicator compared with the reference value (RefCon). The ratio is equal to 1.00 if actual status is better than or equal to reference conditions and approaches 0.00 as deviation from reference conditions becomes large. The value of EQR is used to assign a quality class to the observed status. The classes in descending order of quality are RefCon, High, Good, Moderate, Poor, Bad. The central definition of the quality classes is given by the value of acceptable deviation (AcDev).

The RefCons and AcDev values for the water transparency assessment were first defined by a group of national experts from the HELCOM Contracting Parties for the HELCOM thematic assessment on eutrophication (HELCOM 2009a). The first assessment was based on identifying the status for the period 2001-2006, including data from coastal areas. This assessment covers the period 2003-2007.

For a complete explanation of the methodology used, please see Andersen et al (2010) and integrated thematic assessment on eutrophication of the Baltic Sea (HELCOM 2009).

Months of June, July, August and September were chosen to represent the period of abundant occurrence of cyanobacteria. 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.

Reference conditions for the Baltic sub regions were obtained by combining information achieved from the means of the data collected between 1905 and 1910 and the value of the smoothing curve before 1940. The target value was set at a 25% deviation from the reference conditions as calculated for the HELCOM integrated thematic assessment on eutrophication of the Baltic Sea (HELCOM 2009).

The eutrophication status maps were produced by spatially interpolating the values for the areas listed in the data table. ArcGIS 9.3.1 was used to interpolate the open and coastal areas. The coastal areas interpolation was delimited by a 6 nautical miles buffer along the coastline. The result was then joined to the open sea areas and the final map was processed to add a smoother transition between coast and open sea areas.

Strength and weaknesses of data: Secchi depth is one of the few parameters for 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 1970s. 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 five 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 sub-areas. In order to resolve the significance of Secchi depth as a eutrophication parameter, the roles of the parameters affecting water transparency are be thoroughly investigated.

References

Andersen, J.H., P. Axe, H. Backer, J. Carstensen, U. Claussen, V. Fleming-Lehtinen, M. Järvinen, H. Kaartokallio, S. Knuuttila, S. Korpinen, M. Laamanen, E. Lysiak-Pastuszak, G. Martin, F. Møhlenberg, C. Murray, G. Nausch, A. Norkko, & A. Villnäs. 2010. Getting the measure of eutrophication in the Baltic Sea: towards improved assessment principles and methods. Biogeochemistry. DOI: 10.1007/s10533-010-9508-4.

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)

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Last updated: 26 May 2010