This 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, and the results are to be considered as intermediate.
The data required for this indicator are attained by quantitative phytoplankton analysis (cf. HELCOM 2014). The input data required is wet weight biomasses of major functional or dominating phytoplankton groups over a sampling year. Sampling frequency should be at least once per month. The selection of groups may differ between sub-basins or assessment units of the Baltic Sea, and expert judgement based on long-term monitoring data is required to identify the correct and most suitable candidate groups. In all test areas cyanobacteria, auto- and mixo-trophic dinoflagellates, diatoms and the autotrophic ciliate Mesodinium rubrum were selected. In the Eastern Gotland Basin Lithuanian Coastal Waters green algae were included in the analysis as an extra component. As chlorophytes are important in coastal waters of low salinity and their biovolume is also one of the metrics of the German WFD phytoplankton index PPIcw, the use of chlorophytes should be investigated for potential inclusion in assessment purpose (Sagert et al., 2008).
The process of establishing phytoplankton group reference growth curves for marine water bodies was originally described by Devlin et al. (2007). Type- or site-specific seasonal growth curves can be designed for each dominating phytoplankton group:
1) Skewed data is accounted for by the transformation of phytoplankton biomass (x) on a natural log scale (ln x+1);
2) Overall and monthly means and standard deviations are calculated for each functional group over a reference period;
3) Monthly Z scores are calculated as follows:
A positive z-score implies that the observed type and site specific growth curve for a certain month is greater than the mean. And this in turn indicates that the phytoplankton group has grown more in that month than average. A negative score indicates that the observation is less than the mean and the phytoplankton group is missing or constitutes only minor part of biomass in the whole community.
4) Acceptable deviations for monthly means (reference envelopes) are calculated (zmonth±0.5).
The indicator value is calculated:
The indicator value is based on the number of data points from the test area which fall within the acceptable deviation range that has been set for each monthly point of the reference growth curve. Percentage-based thresholds are established for each dominating group to determine index values for the assessment of the ecological status:
An R-script has been developed for calculating the indicator; however it has not yet been operationalised.
Currently this indicator is tested in a selection of assessment units. The indicator has the potential to be applied for the entire Baltic Sea, though clear definition of the phytoplankton groups in each assessment unit is required. The set of dominating phytoplankton groups can vary between different sub-basins, for example cyanobacteria do not generally occur among the dominant groups in high salinity areas.
The underlying characteristics vital to the function of this indicator differ between areas of the Baltic Sea due to seasonal and environmental factors, thus derivation of assessment unit specific reference conditions and threshold values is critical. The indicator values may also differ between the coastal and open sea zone within the same sub-basin. Further testing of the indicator will determine the most appropriate assessment scale and currently the aim is to use known characteristics of individual waterbodies to assess status on the largest possible scale.
Tentatively, HELCOM assessment unit Level 3, dividing the Baltic Sea into 17 sub-basins and further into coastal areas is used for testing this indicator.
Data for the open sea units are aggregated from three to eight stations with most regular monitoring covering the whole vegetation period. The number of stations in coastal water units ranges from one (Northern Baltic Proper Swedish Coastal Waters) to eight (Gulf of Riga Latvian Coastal Waters). Due to different hydrological conditions, mainly salinity (5–7 vs. 3–5 PSU), Estonian coastal sea area in the Gulf of Finland is divided into two separate assessment units (western and eastern part). Most of selected stations belong to the current monitoring programs.
The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.