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​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 'State of the Baltic Sea' report 2018, and the results are to be considered as intermediate.

This pre-core indicator evaluates cyanobacterial surface accumulations and cyanobacteria biomass during summer (20th June – 31st August) and covers the assessment period 2011-2015. Currently data is not available for 2016 and thus the indicator evaluation period is only from 2011 to 2015.

Of the 10 assessed open-sea sub-basins, good status was not achieved in any sub-basin based on the cyanobacterial bloom index (CyaBl biomass achieving the defined threshold value, which reflects good conditions). The sub-basins causing greatest concern in the assessment period 2011-2015 were the Northern Baltic Proper and the Gulf of Riga. The sub-basins closest to achieving the threshold value were the Arkona Basin, Bornholm Basin, Eastern Gotland Basin and Western Gotland Basin.

Cyanobacterial bloom index 2018 2.png

Key message figure 1. Status assessment results for 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). See Results section below for details. Click here to access interactive maps at the HELCOM Map and Data Service: Cyanobacterial bloom.


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 Basin and the 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 open sea areas, caused by nutrient enrichment. Phosphorus load in a dominantly nitrogen-limited environment is considered the main anthropogenic pressure affecting the indicator since many cyanobacteria are capable of fixing their own nitrogen from the atmosphere. Human populations, as well as anthropogenic activities such as agriculture and industry, contribute the majority of nutrient inputs 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, resulting in increased turbidity, oxygen depletion, changes in species composition and nuisance blooms of algae (HELCOM, 2013). The indicator also reflects potential changes in the phytoplankton community as cyanobacteria commonly dominate the community during a bloom. 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 linkBaltic 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 linkFavourable 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:  ​


Cite this indicator

HELCOM (2018). Cyanobacteria bloom index. HELCOM pre-core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full indicator report

Cyanobacterial bloom index HELCOM pre-core indicator 2018 (pdf)