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Key Message

This core indicator evaluates the average chlorophyll-a concentration in surface water (0 – 10 m) during summer (June – September) during the assessment period 2011-2016.

In open sea areas, good status of chlorophyll-a was only achieved in the Kattegat. In the remaining 16 sub-basins, the status was not good. In coastal waters, good status is found in some areas for Sweden, Denmark, Germany, Poland, Lithuania and Estonia (Key message figure 1).



Key message figure 1. Status assessment results evaluation for the indicator 'chlorophyll a'. The assessment is carried out using Scale 4 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4). Note the open sea area of The Sound is red (fails the threshold value). Click here to access interactive map at the HELCOM Map and Data Service: Chlorophyll a.


In many sub-basins the summer-time chlorophyll-a increased until the 1990s (e.g. Arkona Basin, Kattegat) or early 2000s (e.g. Bothnian Bay, Northern Baltic Proper, Gulf of Riga, and Western Gotland Basin), but concentrations have generally decrease thereafter. Only in the Gulf of Finland, Bothnian Sea and Eastern Gotland Basin, has the increase in concentrations continued after the early 2000s.

The confidence of the presented chlorophyll-a status estimate is high in all open sea assessment unit except Kattegat, where the confidence is moderate.

The indicator is applicable in the waters of all countries bordering the Baltic Sea.


Relevance of the core indicator

Phytoplankton abundance and biomass increases due to increased eutrophication, directly as a result of increased nutrient concentrations. Chlorophyll-a concentration is used as a proxy of phytoplankton biomass.


Policy relevance of the core indicator

  BSAP Segment and ObjectivesMSFD Descriptors and Criteria
Primary linkBaltic Sea unaffected by eutrophication

D5  Human-induced eutrophication

- D5C2 Chlorophyll a concentrations are not at levels that indicate adverse effects of nutrient enrichment

Secondary link  
Other relevant legislation: EU Water Framework Directive


Cite this indicator

HELCOM (2018). Chlorophyll a. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full indicator report

Chlorophyll a HELCOM core indicator 2018 (pdf)