​Key Message

This pre-core indicator and its threshold values are yet to be commonly agreed in HELCOM.
The indictor is included as a test indicator for the purposes of the mid-2017 'State of the Baltic Sea' report, and the results are to be considered as intermediate.


This pre-core indicator evaluates cyanobacterial surface accumulations and cyanobacteria biomass during the summer period (20th June – 31st August) and covers the assessment period 2011-2015.

Of the 10 assessed open-sea sub-basins, good status for according to the cyanobacteria bloom index (CyaBl biomass below defined threshold value, which reflects good conditions) was not achieved in any sub-basin.

The sub-basins causing greatest concern regarding status in the assessment period 2011-2015 were Northern Baltic Proper and Gulf of Riga. The sub-basins closest to good status were Arkona Sea, Bornholm Sea, Eastern Gotland Basin and Western Gotland Basin.


Key message figure 1: Status assessment results based on the evaluation of the indicator 'Cyanobacterial Bloom Index. 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 cyanobacteria status estimate was high (ES-score 100) for the 7 basins where both Earth Observation (EO-) and biomass-estimates could be applied. The threshold confidence was moderate (ET-Score 50).

The indicator is applicable in the waters of all countries bordering the Baltic Sea. The indicator is operational in the following open sea assessment units: The Bothnian Sea, Gulf of Finland, Northern Baltic Proper, Gulf of Riga, Western Gotland Basin, Eastern Gotland Basin, Bornholm Basin, Gdansk Basin, Arkona Sea and Bay of Mecklenburg. The indicator in its current form is not relevant in the Kattegat, Great Belt, the Sound, Kiel Bay, the Quark or in the Bothnian Bay. The indicator is not applicable in the Åland Sea.


Relevance of the core indicator

The indicator describes the symptoms of eutrophication in the sea areas caused by nutrient enrichment. Especially phosphorus load in a dominantly nitrogen-limited environment is considered the main anthropogenic pressure affecting the indicator. Human populations as well as anthropogenic activities such as agriculture and industry contribute the majority of nutrient input to the Baltic Sea. Eutrophication is driven by a surplus of the nutrients nitrogen and phosphorus in the sea. Nutrient over-enrichment causes elevated levels of algal and plant growth, increased turbidity, oxygen depletion, changes in species composition and nuisance blooms of algae (HELCOM, 2013). The indicator reflects also changes in the phytoplankton community. These are related to the changes in nutrient composition and climate, and have direct impact on sea-use and ecosystem service. Extensive cyanobacterial blooms have a potentially negative impact on the biodiversity of marine ecosystems as well as on its socio-economic value.


Policy relevance of the core indicator

  BSAP Segment and ObjectivesMSFD Descriptors and Criteria
Primary link

Baltic Sea unaffected by eutrophication

D5  Human-induced eutrophication

- D5C3 The number, spatial extent and duration of harmful algal bloom events are not at levels that indicate adverse effects of nutrient enrichment

Secondary link

Favourable status of Baltic Sea biodiversity



D1 Biodiversity
- D1C6 The condition of the habitat type, including its biotic and abiotic structure and its functions (e.g. its typical species composition and their relative abundance, absence of particularly sensitive or fragile species or species providing a key function, size structure of species), is not adversely affected due to anthropogenic pressures.​

Other relevant legislation: ​ ​

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Cite this indicator

HELCOM (2017). Cyanobacteria bloom index. HELCOM pre-core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full indicator report

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