​​​​​​​Key Message​​​​​

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 mid-2017 'State of the Baltic Sea' report, and the results are to be considered as intermediate.


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. The indicator is not yet agreed as a HELCOM core indicator but is currently being tested in the HOLAS II project for the Eastern Gotland Basin only. In this assessment unit, good status was just missed, that means the average Dia/Dino index of the years 2011-2015 was below the threshold value of 0.5.​

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).


Relevance of the core indicator​

Phytoplankton is the key primary producer in marine ecosystems, and diatoms and dinoflagellates are dominating groups in spring. They play a decisive role as food for higher trophic levels. Shifts in the diatom/dinoflagellate ratio may have high relevance for the nutrition of zooplankton and the following trophic levels. They influence even the benthos as diatom blooms sink quickly down and contribute more food to zoobenthos than dinoflagellates, which stay longer in the water column. The Dia/Dino index is primarily a descriptive trend indicator for changes in the food web. Moreover it may indicate silicate limitation which is an effect of eutrophication.


Policy relevance of the core indicator

BSAP segment and objective​​s

MSFD descriptor and criteria

Primary link

Eutrophication  and biodiversity segment

Thriving and balanced communities of plants and animals

D4: 4.3.1 Abundance trends of functionally important selected groups/Species (Key trophic groups)

D5: 5.2. Direct effects of nutrients: species shift in floristic composition such as diatom/flagellate ratio

Secondary link

Natural Distribution and occurrence of plants and animals​

1.6. Habitat condition: Condition of the typical species and communities, relative abundance and/or biomass

1.7. Composition and relative proportion of ecosystem components

4.1. Productivity of key species or trophic groups (productivity, biomass)

Other relevant legislation: In some coastal waters also Water Framework Directive

Cite this indicator

HELCOM (2017). Diatom/Dinoflagellate index. HELCOM pre-core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543

Download full indicator report​

HOLAS II component – pre-core indicator report – web-based version July 2017 (pdf)