​​​​​​​​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 sized zooplankters occur abundantly in the plankton community. Due to strong environmental gradients, size distribution and total stock of the zooplankton corresponding to good status vary between the Baltic Sea sub-basins.


Zooplankton MSTS Key message figure 1.png

Key message figure 1: Status a​​ssessment results based evaluation of the indicator 'zooplankton mean size and total stock'. The assessment is carried out using Scale 2 HELCOM assessment units (for more information see the HELCOM Monitoring and Assessment Strategy Annex 4. Click to enlarge.

The indicator-based status evaluation has been completed for the northern Baltic Sea, namely the Gulf of Bothnia, Gulf of Finland, Åland Sea, and Northern Baltic Proper. 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 2011-2105 was found in the Bothnian Bay and the Bothnian Sea. By contrast, in the Åland Sea, Gulf of Finland and Northern Baltic Proper, 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 of 2011-2015. This negative development results from both an increased contribution of small zooplankton species as a consequence of eutrophication and a decreased share of copepods as a consequence of higher 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 the final 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 webs. The herbivorous zooplankton feed on phytoplankton and in turn constitute prey to animals at higher trophic levels, including fish. Therefore, zooplankton is 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-sized zooplankton in the plankton communities is usually associated with lower energy transfer efficiency, due to higher losses.  Thus, a high community biomass of zooplankton with high individual body size represents both favourable fish feeding conditions and a high grazing potential. According to ecological theories, this would represent an efficient food web, i.e. 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 and productivity.

Policy relevance of the core indicator

  BSAP segment and objectives MSFD 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 (2017) Zooplankton mean size and total stock. HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543​

Download full indicator report

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