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, and the results are to be considered as intermediate.
The Working Group on Good Environmental Status (European Commission, 2015) recommended a common approach, based on the reference condition plus acceptable deviation, for determining environmental status. According to them, "reference state can be defined using a variety of methods, including historic conditions, based on various evidence about conditions before there was significant anthropogenic activity."
The Dia/Dino index belongs to the few indicators that can be calculated already for the early 20th century, when anthropogenic impact was low. Quantitative phytoplankton analyses date back to that time and were compiled by Wasmund (2017). The derivation of threshold values from historical data was explained by Wasmund et al. (2017). A deviation of 20% from the historical Dia/Dino index was allowed and therefore suggested as threshold. In that paper, a threshold value for the Eastern Gotland Basin of 0.5 was suggested, which marks just the value of balanced (1:1) diatom and dinoflagellate biomass. Suggestions for thresholds values in other assessment units of the southern Baltic Sea were already made by Wasmund et al. (2016). These threshold values have already been accepted by HELCOM (2016).
Thresholds figure 1. Schematic representation of the threshold value for the indictor "Dia/Dino index", that is just 0.5 in the Eastern Gotland Basin.
Diatom dominance over dinoflagellate dominance is typical in the spring blooms of the Baltic Sea according to historical data (Wasmund 2017). Therefore, diatom dominance, i.e. standard Dia/Dino index >0.5, is typical for a non-impacted ecosystem and considered as good status.
The derivation of the threshold value for the alternative Dia/Dino index is described by Wasmund et al. (2017).