Relevance of the Indicator

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.

Eutrophication assessment

The status of eutrophication is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of cyanobacterial blooms, this indicator also contributes descriptively to the overall eutrophication assessment, along with the other eutrophication core indicators. The integrated eutrophication assessments of the open sea areas is based on an integration of several core indicator evaluations, though this indicator is currently not included. Since cyanobacterial blooms are also affected by non-eutrophication related changes (see chapter 'Role of cyanobacterial blooms in the ecosystem'), the indicator should receive a low weight in any integrated assessment, unless unusually high relationship to eutrophication is shown.


Policy relevance

Eutrophication is one of the four thematic segments of the HELCOM Baltic Sea Action Plan (BSAP) with the strategic goal of having a Baltic Sea unaffected by eutrophication (HELCOM 2007). Eutrophication is defined in the BSAP as a condition in an aquatic ecosystem where high nutrient concentrations stimulate the growth of algae, which leads to imbalanced functioning of the system. The goal for eutrophication is broken down into five ecological objectives, of which one is "natural levels of algal blooms".

The EU Marine Strategy Framework Directive (Anonymous 2008) requires that "human-induced eutrophication is minimized, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algal blooms and oxygen deficiency in bottom waters" (Descriptor 5). The Commission Decision on GES (2017) defines 'Harmful algal blooms (e.g. cyanobacteria) in the watercolumn' as the criteria element to be assessed using the criteria D5C3 'The number, spatial extent and duration of harmful algal bloom events are not at levels that indicate adverse effects of nutrient enrichment'.


Role of cyanobacterial blooms in the ecosystem

Surface blooms of nitrogen-fixing cyanobacteria, though considered to be a natural phenomenon (Bianchi et al. 2000), have become extensive and frequent in many parts of the Baltic Sea since the 1990s (Finni et al. 2001). The blooms partly consist of the toxic species Nodularia spumigena, which has been reported to have negative effects on grazing zooplankton (Engström et al. 2000, Sellner et al. 1994, Sopanen et al. 2009). Cyanobacteria have been shown to have allelopathic effects on other phytoplankton groups and increasing effects on bacteria (Suikkanen et al. 2004, 2005). Since a major part of cyanobacterial biomass generated during bloom events eventually settles to the bottom of the sea, it potentially increases oxygen depletion in stratified areas (Vahtera et al. 2007a). Thus, extensive cyanobacterial blooms potentially have a negative impact on the biodiversity of both the pelagic and benthic communities.

The increase of cyanobacterial blooms is partly caused by anthropogenic nutrient enrichment, especially the proportional increase of dissolved phosphorus. Also other, non-eutrophication related, causes have been suggested to have an effect, such as: hydrographic changes - increased temperature, decreased salinity or more frequent vertical mixing, changes in micronutrients or trace metals, as well as changes in the interaction between phyto-zooplankton species (Kahru et al. 1994).


Human pressures linked to the indicator

  General MSFD Annex III, Table 2a
Strong link

Substances, litter and energy

- Input of nutrients – diffuse sources, point sources, atmospheric deposition

Weak link ​