Assessment Protocol

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.

The 'Cyanobacterial bloom index' indicator evaluates the increase in cyanobacterial blooms, taking into account different aspects of the bloom phenomenon. As a multiparametric index, it has the advantage of taking into account different types of data, e.g. observed from different platforms: remotely (satellites) or in-situ (measurements in the sea), and measured using varying techniques.

All applied parameters must fulfill the following requirements: they 1) describe a relevant aspect of cyanobacteria accumulations not already considered by the other parameters, 2) have sub-basin specific threshold values and 3) be updatable by status values for the sub-basin division and status period defined for the existing indicator.

This HELCOM pre-core indicator (CyaBI) consists of two parameters: 1) cyanobacterial surface accumulations (CSA), combining information of volume, length of bloom period and severity of surface accumulations estimated from remote sensing observations and 2) the cyanobacteria biomass in the water column analyzed from in-situ observations. The parameters are normalized, to allow combined use of the different parameters in the index.

The CSA (parameter 1) relies on high-frequency data, and is optimal for describing the bloom formation at the surface. However, this parameter is strongly influenced, not only by eutrophication, but also by climate-related variation including wind conditions. The cyanobacteria biomass (parameter 2) supplements CSA by providing information of the actual amount of cyanobacteria in the water column. Due to less frequent monitoring, neither the status evaluation nor the threshold values of cyanobacteria biomass have sufficient confidence to stand alone as a HELCOM core indicator. Combining the two parameters, allows for deriving more reliable status estimates: an indicator expressing the consequence of increased cyanobacteria (the blooms) with high confidence; yet related strongly to changes in the actual amount of cyanobacteria, and subsequently, to eutrophication.

The 'Cyanobacterial bloom index' indicator responds negatively to increasing eutrophication, i.e. low values indicate increased eutrophication.


Parameter 1: Cyanobacterial surface accumulations (CSA)

The main data source used to develop the indicator was satellite data derived from the daily algal bloom product of the Finnish Environment Institute (SYKE), which is in turn based on chlorophyll-a and turbidity products. The observations were interpreted to estimate the potentiality of surface algae accumulations in four classes [0-3 i.e. no, potential, likely and evident] ( The spatial aggregation of daily Earth Observation (EO) observations from the assessment units was conducted by calculating an algae barometer value. The algae barometer (AB) value is a weighted sum of the proportion of positive algae observations in the different classes in an assessment area (Eq. 1; Rapala et al. 2012).

Eq. 1

where ntot is the total number of algae observations, and n#cl1, n#cl2, and n#cl3 are the number of algae observations in classes 1-3.

Seasonal bloom characteristics were estimated using an empirical cumulative distribution function (ECDF) drawn from seasonal observations of daily algae barometer values from each assessment area. ECDF gives the cumulative proportion of the seasonal algae barometer values. The bloom characteristics (i.e. the indicative variables of CSA) were defined for each assessment unit as follows: 1) seasonal volume, i.e. the areal coverage above the ECDF functions, 2) length of algal surface accumulation period, i.e. the percentage of observations with algae barometer values above zero, and 3) bloom severity, i.e. the 90-percentile of the algae barometer observations. The CSA index time series was derived by taking an average from the normalized time series of the indicative variables and grouping all the three EO-based parameters together.


Assessment protocol figure 1. An example (Gulf of Finland) of grouping the normalized EO-based parameters (from top to bottom) A) seasonal bloom volume, B) length of bloom period and C) Severity of blooms, and D) a combined cyanobacterial surface accumulation (CSA) parameter. As the indicator responds negatively to increased eutrophication, 1 represents the best conditions and 0 the worst. Black dashed horizontal line indicates the parameter-specific target condition and the red dashed line indicates the estimate for 2011-2014.


Parameter 2: Cyanobacteria biomass

Cyanobacteria biomass was analyzed using microscopy techniques, analyzing water samples. The data used in the indicator parameter originated from Estonian, Finnish, German, Latvian, Lithuanian, Polish and Swedish national monitoring data. The data was collated by the HELCOM phytoplankton expert group (PEG), in order to produce a HELCOM Baltic Sea Environmental Fact Sheet, updated annually (Wasmund et al. 2015). The main sampling locations are presented in Assessment protocol figure 2.



Assessment protocol figure 2. Map of the regularly sampled stations, containing one graph on diazotrophic cyanobacteria biomass per area (seasonal mean biomass µg/L). Names of some Finnish coastal stations are abbreviated. Stations in Bothnian Bay, Kiel Bay and Kattegat have been tested but the results are not presented.


The data included biomass analyses (wet weight in µg/L) of integrated water samples (0-10 m, less at some shallower coastal stations; 0-20 m at the Landsort Deep; surface = 0-1 m in Bay of Mecklenburg). Sampling at the Finnish high-frequency coastal stations "Hailuodon ed int.asema", "Suomenl Huovari Kyvy-8A", "UUS-23 Längden" and "Vav-11 V-4" reached from surface to the depth of 2x Secchi depth (usually 0-8m); they could be integrated into the existing data series without problems. Genera included in the index include: Nodularia, Aphanizomenon and Dolichospermum (previously Anabaena).

The information is based on national monitoring samples analyzed and identified by phytoplankton experts, using the mandatory HELCOM methods (HELCOM 2014). Additional explanations on the counting procedure in size classes is given by Olenina et al. (2006). Sampling frequency was variable and dependent on national monitoring cruises. At least one sample per month has to be available to allow the calculation of the seasonal average. This precondition could also be fulfilled by pooling nearby stations. Only with a few exceptional cases are data presented despite missing data from one month out of three.

Monthly means were calculated from the single data, which served as a basis for calculation of seasonal mean values.


Assessment units

The pre-core indicator is applicable in 10 open sea assessment units (at least one nautical mile seawards from the baseline).

The indicator is applicable in the following open sea assessment units: Bothnian Sea, Gulf of Finland, Northern Baltic Proper, Gulf of Riga, Western Gotland Basin, Eastern Gotland Basin, Bornholm Sea, Gdansk Basin, Arkona Sea and Bay of Mecklenburg. The indicator is currently not relevant in the Kattegat, the Sound areas, the Bothnian Bay and the Quark due to the absence of cyanobacterial bloom formations, and in its present form, not applicable in Åland Sea or coastal areas. It is also not used in the Kiel Bay, as the relevance of the indicator remains un-certified.

The indicator is assessed within the geographical assessment unit level 4 proposed by HELCOM: open sea sub-basin areas and coastal waters WFD coastal types and bodies. The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.