​​​​​​​​​​​​KEY MESSAGE

​The core indicator evaluates the average chlorophyll-a concentration in the surface water (0 – 10 m) during summer (June – September) during the assessment period 2011-2015.

In open sea areas, good status of chlorophyll-a was 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 along coasts of Sweden, Denmark, Germany, Poland, Lithuania and Estonia.​


Key message figure 1: Status assessment results based evaluation of 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). Use +/- key to zoom.


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

The confidence of the presented chlorophyll-a status estimate is moderate in all open sea assessment unit except the Quark and Åland Sea, where the confidence is low.

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


Relevance of the core ​​​indicator

Phytoplankton increases along with increased eutrophication 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 link

Baltic 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 (2017). Chlorophyll a. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full​ indicator report

HOLAS II component - Core indicator report – web-based version July 2017 (pdf)