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 results of preliminary evaluation are presented for the Bay of Mecklenburg, Arkona, Bornholm, Eastern Gotland and Northern Baltic Proper basins and the Gulf of Riga. The following coastal water units are included – Swedish part of the Northern Baltic Proper, Estonian part of the Gulf of Finland, Estonian and Latvian parts of the Gulf of Riga and Lithunian part of the Eastern Gotland Basin. Data are also provided for the Gdansk Basin Polish coastal waters and German coastal waters, but not yet assessed due to undefined reference period or too short time series, respectively. Finland has announced that reference data for setting threshold values are missing and this indicator cannot be used in Finnish coastal waters.
The results of preliminary evaluation are summarized in Results table 1. The reference period is defined separately for each sea area and depends on the availability of data. Time-series usually start from the beginning of the 1990s, while disturbances in the environment were evident already in the 1960s (Andersen et al., 2015). To start evaluating environmental status and to create reference growth curves, regular data set of at least 10 years with monthly sampling is needed to comprise all natural variability. These are the reasons why tentative reference periods in some units extend until recent years and partly overlap with assessment periods. The test period includes six years (2011−2016). An example of reference growth curves and indicator values within the given assessment period is represented in Results figure 1.
Results table 1. Test results for the period 2011–2016 from different assessment units of the Baltic Sea. Indicator value lies between 0 and 1 and is the proportion of data points within the frame of seasonal reference growth curves and acceptable deviations. Data point is the average of all observations in a month of certain year. For overall assessment, indicator values of individual dominant groups are averaged.
Results figure 1. Reference growth curves with monthly averaged normalized biomass values (Zmonth), acceptable deviations (Zmonth±0.5) and data points from the test period (2012–2016) in the Eastern Gotland Basin.
Spatio-temporal coverage differs between the assessment units. For most of the test areas, the confidence of indicator status is moderate to high according to temporal and moderate according to spatial resolution. Confidence level depends on the length of the time-series and regularity of phytoplankton sampling during the growth period. Once the reference growth curves have been established, some compromises in the frequency of sampling and total number of samples used in the assessment are possible. The indicator value is the proportion of biomass values fitting into the reference growth envelope (region of acceptable deviation) and the values for individual months are independent. It means that if some data points for some months during the assessment period are missing, the evaluation of status is still feasible.
On the other hand, methodological confidence of monitoring data used for this indicator is rather high since all laboratories providing data follow the same guidelines. The quality of data is substantially improved after implementing a standardised species list with fixed size-classes and biovolumes (Olenina et al., 2006).