The core indicator evaluates water clarity by average Secchi depth during summer (June – September) during 2011-2015.
In open sea areas, good status (Secchi depth above defined threshold value, which reflect good conditions) for water clarity has been achieved in the Kattegat and The Sound. In coastal waters, good status has only been achieved in some water bodies along the coasts of Sweden, Poland, Latvia and Estonia.
During the last century, summer-time water clarity has decreased in all Baltic Sea areas. During the last two decades, water clarity has increased in the southern Baltic Sea sub-basins.
Key message figure 1: Status assessment results based evaluation of the indicator 'Water clarity'. The assessment is carried out using Scale 4 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4).
The confidence of the presented water clarity status estimate was based on the availability of monitoring data and the confidence of the target-setting procedures. The areas of greatest concern are the Quark and Åland Sea, where indicator confidence was determined low. High confidence was found in the Bothnian Bay and the Northern Baltic Proper. In the remaining open-sea basins, the indicator confidence was moderate.
The indicator is applicable in the waters of all countries bordering the Baltic Sea.
Eutrophication is caused by excessive inputs of nutrients (nitrogen and phosphorus) resulting from various human activities. High concentrations of nutrients and their ratios form the preconditions for algal blooms, reduced water clarity and increased oxygen consumption. Water clarity is affected by the light attenuation of the media, caused mainly by water itself, planktonic organisms, especially phytoplankton, suspended particulate matter, colored dissolved organic matter (CDOM) and inorganic compounds. Phytoplankton is the dominating optical constituent in most oceanic waters, whereas in the Baltic Sea, especially the north-eastern parts, a considerable share of the attenuation is caused by CDOM, which is to a large extent not related to increased nutrient loading. Though water clarity responds strongly to eutrophication, it may in some areas express a non-eutrophication-related signal. Long-term nutrient data are key parameters for quantifying the effects of anthropogenic activities and evaluating the success of measures undertaken.
Baltic Sea unaffected by eutrophication
D5 Human-induced eutrophication
D5C4 The photic limit (transparency) of the water column is not reduced, due to increases in suspended algae, to a level that indicates adverse effects of nutrient enrichment
HELCOM (2017). Water clarity. HELCOM core indicator report. Online. [Date Viewed], [Web link].
II component - Core indicator report – web-based version July 2017 (pdf)