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Key Message

This core indicator evaluates zooplankton community structure to determine whether it reflects good environmental status. As a rule, good status is achieved when large-bodied zooplankters are abundant in the plankton community. Due to strong environmental gradients and community variations, size distribution and total stock of the zooplankton corresponding to good status vary between the Baltic Sea sub-basins. 


Zooplankton key message.png

Key message figure 1. Evaluation of the status assessment results for zooplankton indicator 'Mean size and total stock' (MSTS). The assessment is carried out using Scale 2 HELCOM assessment units (for more information see the HELCOM Monitoring and Assessment Strategy Annex 4Click here to access interactive maps at the HELCOM Map and Data Service: Zooplankton mean size and total stock.

The indicator-based status evaluation has been completed for the northern Baltic Sea, namely the Gulf of Bothnia, Gulf of Finland, Åland Sea, and Western Gotland Basin, and for the Gdansk Basin in the southern Baltic Sea. For the other basins, work to establish the threshold values needed to carry out the evaluation is still in progress.

Good status during the assessment period 2011-2016 was found in the Bothnian Bay, Bothnian Sea and Gdansk Basin. By contrast, in the Åland Sea, Gulf of Finland and Western Gotland Basin, zooplankton mean size and/or total biomass have declined during the last decades, and MSTS does not reflect a good status during the assessment period 2011-2016. This negative development results from both an increased contribution of small zooplankton species, a probable consequence of eutrophication, and a decreased share of copepods, a probable consequence of increased predation by zooplanktivorous fish. It is also possible, albeit not verified, that altered environmental conditions (e.g. decreased salinity, increased temperature and deep water hypoxia) have contributed to these trends. The detected trends in the mean size and total stocks of zooplankton communities indicate that today's pelagic food web structure is not optimal for energy transfer from primary consumers (phytoplankton) to fish.

The confidence of the indicator evaluation is moderate since the data used cover fairly long time periods for the sub-basins where the evaluation results are completed, but also for the sub-basins where these results are not yet available.

The indicator is applicable in the waters of all the countries bordering the Baltic Sea. However, currently the indicator is only operational in some assessment units, and further development work is needed to make it operational in the remaining assessment units in the future.


Relevance of the core indicator

Zooplankton includes an array of macro and microscopic invertebrates. They play a vital role in the marine food web. The herbivorous zooplankton feed on phytoplankton and in turn constitute prey to animals at higher trophic levels, including fish. Therefore, zooplankton are an essential link in aquatic food webs, influencing energy transfer in the pelagic food webs and recruitment to fish stocks as well as ecosystem productivity, nutrient and carbon cycling. Hence, the evaluation of zooplankton communities is a prerequisite for analysis of pelagic food web structure.

The mean size of a zooplankter in the community is indicative of both fish feeding conditions and grazing pressure from zooplankton on phytoplankton. Large stocks of zooplankton composed of large-bodied organisms have a higher capacity for transfer of primary producers (phytoplankton) to fish, i.e. higher energy transfer efficiency. By contrast, dominance of small-bodied zooplankton is usually associated with lower energy transfer efficiency, due to higher losses.  Thus, a high community biomass of zooplankton with large individual body size represents both favourable fish feeding conditions and a high potential for efficient utilization of primary production. According to ecological theories, this would represent an efficient food web and correspond to a good environmental status. All other combinations of zooplankton stock and individual size would be suboptimal and imply food web limitations in terms of energy transfer through the food web and productivity.


Policy relevance of the core indicator

 BSAP segment and objectivesMSFD Descriptor and criteria
Primary link


  • Thriving and balanced communities of plants and animals

D4 Food-web

D4C3 The size distribution of individuals across the trophic guiled is not adversely affected due to anthropogenic pressures

Secondary link


  • Natural distribution and occurrence of plants and animals

D1 Biodiversity

D1C6 The condition 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.


Cite this indicator

HELCOM (2018) Zooplankton mean size and total stock. HELCOM core indicator report. Online. [Date Viewed], [Web link].

 ISSN 2343-2543

Download full indicator report

Zooplankton mean size and total stock HELCOM core indicator 2018 (pdf)