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 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 2011-2016 was below the threshold value of 0.5.
Key message figure 1. Preliminary status assessment of the Eastern Gotland Basin based on the ratio of diatoms and dinoflagellate biomass for spring during the assessment period 2011-2016. This current test evaluation is carried out at HELCOM assessment scale 2 (HELCOM Monitoring and Assessment Strategy Attachment 4). Click here to access
interactive maps at the HELCOM Map and Data Service: Diatom-Dinoflagellate index.
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).
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.
Eutrophication and biodiversity segment
Thriving and balanced communities of plants and animals
D4 - Food webs
D4C1 The diversity (species composition and their relative abundance) of the trophic guild is not adversely affected due to anthropogenic pressures.
D1 - Food webs
D1C6 The conditions of the habitat type, including its biotic and abiotic structure and its functions (e.g. its typical species composition and their relative abundance, absence of particularly sensitive or fragile species or species providing a key function, size structure of species), is not adversely affected due to anthropogenic pressures.
HELCOM (2018). Diatom/Dinoflagellate index. HELCOM pre-core indicator report. Online. [Date Viewed], [Web link].
Diatom-Dinoflagellate index HELCOM pre-core indicator 2018 (pdf)