The Diatom/Dinoflagellate index (Dia/Dino index) reflects the dominance patterns in the phytoplankton spring bloom. It has high relevance for the pathway of the food into the pelagic or benthic food web. In principle, the indicator is applicable in all coastal and open sea assessment units, except lagoons, large river plumes and the Bothnian Bay; however, thresholds for the good status are defined only for the southern and central Baltic Sea up to now. In this assessment unit, the average Dia/Dino index of the years 2016-2021 was just below the threshold value of 0.5 for good status.

The confidence of the indicator evaluation depends on the data frequency. The data have to represent the diatom and dinoflagellate blooms adequately. If the diatom bloom is not sufficiently represented in the data, an alternative Dia/Dino index may be applied, based on silicate consumption data. This indicator is robust; its calculation is simple and traceable. The phytoplankton monitoring is operational in the whole Baltic Sea using methods prescribed in the COMBINE manual. Currently, a high quality of the data is assured by the experts of the HELCOM Phytoplankton Expert Group (PEG).

The basic data are phytoplankton biomass data, determined as explained in the HELCOM COMBINE manual, originating from the depth of 0 – 10 m. Also silicate data were used that were taken parallel to the phytoplankton sampling.

Diatom-dinoflagellate ratio dataset displays the result of the indicator in HELCOM Assessment Scale 2 (Division of the Baltic Sea into 17 sub-basins).

Attribute information:

"HELCOM_ID" = ID of the HELCOM scale 2 assessment unit

"level_2" = Name of the HELCOM scale 2 assessment unit

"Indicator_" = Name of the indicator

"Period" = Interval of year of the sampling

"AnnualSamp" = Assessment average sampling value

"Unit" = Unit of the indicator

"Type" = Type of indicator (1=monotonic, 2=unimodal, optimal range)

"Threshold_" = Threshold value of indicator

"Indicator1" = Result of the indicator

"Status" = Status of the indicator (“Achieve”, “Fail” or “Not assessed”)