This core indicator evaluates water clarity based on average Secchi depth during summer (June – September) during the assessment period 2011-2016.
In open sea areas, good status (Secchi depth above a defined threshold value, which reflects 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 coast of Sweden, Poland, Latvia and Estonia (Key message figure 1).
Since the 1970s there has been a general decrease in summer-time water clarity in all Baltic Sea areas, though during the last two decades, water clarity has increased in some southern Baltic Sea sub-basins (Great Belt and Kattegat, Results figure 3).
Key message figure 1. Status assessment 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). Click here to access interactive maps at the HELCOM Map and Data Service: Water clarity.
The confidence in the presented water clarity status evaluation are based on the availability of monitoring data and the confidence of the target-setting procedures. In the Quark, Åland Sea, Gulf of Finland and Gulf of Riga open sea assessment units the indicator confidence was determined to be low. High confidence was found in the southwestern parts of the Baltic (Kattegat, Great Belt, The Sound, Kiel Bay, Bay of Mecklenburg, Arkona Basin, Bornholm Basin and the Eastern Gotland Basin). 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.
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 (2018). Water clarity. HELCOM core indicator report. Online. [Date Viewed], [Web link].
Water clarity HELCOM core indicator 2018 (pdf)