​Relevance of the Indicator

Biodiversity assessment

The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the mean size and total stock of zooplankton, this indicator contributes to the overall biodiversity assessment in 2018 along with the other biodiversity core indicators.


Policy relevance

The indicator on zooplankton mean size and total stock addresses the Baltic Sea Action Plan (BSAP) Biodiversity and nature conservation segment's ecological objective 'Thriving and balanced communities of plants and animals', which has a direct connection to the food web structure. The background document to the Biodiversity segment of the BSAP describes a target for this ecological objective as 'By 2021 all elements of the marine food webs, to the extent that they are known, occur at natural and robust abundance and diversity'.

The core indicator also addresses the following qualitative descriptors of the MSFD for determining good environmental status (European Commission 2008):

Descriptor 1: 'Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions';

Descriptor 4: 'All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity'.

and the following criteria of the Commission Decision (European Commission 2010):

  • Criterion 1.6 (habitat condition)
  • Criterion 4.3 (abundance/distribution of key trophic species)

This core indicator is among the few indicators able to evaluate the structure of the Baltic Sea food web with known links to lower and higher trophic levels.


The role of zooplankton in the ecosystem

Zooplankton play an important role transferring primary production to zooplanktivorous fish. However, different zooplankton taxa often have different preferences for trophic state of the ecosystem and are of different value as prey for zooplanktivores, because of the variations in size, escape response, and biochemical composition. In the Baltic Sea, alterations in fish stocks and regime shifts received particular attention as driving forces behind changes in zooplankton (Casini et al. 2009). With the position that zooplankton has in the food web – sandwiched between phytoplankton and fish (between eutrophication and overfishing) – data and understanding of zooplankton are a prerequisite for an ecosystem approach to management.

With respect to the eutrophication-driven alterations in food web structure, it has been suggested that with increasing nutrient enrichment of water bodies, total zooplankton abundance or biomass increases (Hanson & Peters 1984), mean size decreases (Pace 1986), and relative abundance of large-bodied zooplankters (e.g. calanoids) generally decrease, while small-bodied forms (e.g. small cladocerans, rotifers, copepod nauplii, and ciliates) increase (Pace & Orcutt 1981).


Total zooplankton abundance and biomass

In lakes and estuaries, herbivorous zooplankton stocks have been reported to correlate with chlorophyll a and phytoplankton biomass (Pace 1986; Nowaczyk et al. 2011; Hsieh et al. 2011), but also with total phosphorus (Pace 1986). In general, total zooplankton stocks increase with increasing eutrophication, which in most cases is a result of the increase in small herbivores (Gliwicz 1969; Pace 1986; Hsieh et al. 2011). Both parameters have been recommended as primary 'bottom-up' indicators (Jeppesen et al. 2011).

In most areas of the Baltic Sea, copepods contribute substantially to the diet of zooplanktivorous fish (e.g. sprat and young herring), and fish body condition and weight-at-age (WAA) have been reported to correlate positively to abundance/biomass of copepods (Cardinale et al. 2002; Rönkkönen et al. 2004). In coastal areas of the northern and central Baltic Sea, WAA has been suggested to be used as a proxy for zooplankton food availability and related fish feeding conditions to fish recruitment (Ljunggren et al. 2010).

Herbivorous zooplankton biomass is indirectly impacted by eutrophication via changes in primary productivity and phytoplankton composition, whereas direct impacts are expected mostly from predation, and to a lesser extent, from introduction of synthetic compounds (at point sources) and invasive species (via predation). The latter can also be indirect if invasive species are changing trophic guilds, which may affect zooplankton species. Finally, zooplankton abundance and biomass are affected – both positively and negatively – by climatic changes and natural fluctuations in thermal regime and salinity.


Mean zooplankter size

Evidence is accumulating that a shift in zooplankton body size can dramatically affect water clarity, rates of nutrient regeneration and fish abundances (Moore & Folt 1993). Although these shifts can be caused by a variety of factors, such as increased temperatures (Moore & Folt 1993; Brucet et al. 2010), eutrophication (Yan et al. 2008; Jeppesen et al. 2000), fish predation (Mills et al. 1987; Yan et al. 2008, Brucet et al. 2010), and pollution (Moore & Folt 1993), the resulting change implies a community that is well adapted to eutrophic conditions and provides a poor food base for fish. It has been recommended to use zooplankton size as an index of predator-prey balance, with mean zooplankton size decreasing as the abundance of zooplanktivorous fish increase and increasing when the abundance of piscivores increase (Mills et al. 1987). 


Human pressures linked to the indicator

  General MSFD Annex III, Table 2a
Strong link

Fishery-induced mortality of larger zooplankters. 

Eutrophication leading to dominance of small-sized phytoplankton.


- Extraction of, or mortality/injury to, wild species (by commercial and recreational fishing and other actitivies).

Substances, litter and energy

- Input of nutrients – diffuse sources, point sources, atmospheric deposition.

Weak link

Higher salinity favouring larger zooplankters.

Higher temperature favouring smaller zooplankters.

Changes to oxygen concentration.

Environmental contaminants.

Invasive species.


The core indicator responds to fishing and eutrophication but also other pressures causing changes in the food web, such as salinity and temperature that are particularly relevant in the context of the Baltic Sea. Other pressures that might be involved are environmental contaminants and bottom hypoxia. The effects of fishery activities and eutrophication, although potentially co-occurring, would have different outcomes:

  • Increased fishery that leads to increase in zooplanktivorous fish stocks affects both mean size and total zooplankton biomass negatively. Hence, declining trend in mean size and total stock (MSTS).
  • Increased eutrophication and dominance of bacterio- and picoplankton leads to selective advantage for small-sized zooplankton. Hence, declining trend in mean size, but not total stock (Relevance figure 1) are likely to occur. In moderately eutrophied systems, an increase in abundance and/or biomass can be observed. The regression analysis conducted during the evaluation procedure, confirmed that all metrics in questions (MeanSize, total zooplankton abundance and total biomass) change significantly when both chlorophyll a and WAA values are outside of their reference conditions.

 Relevance figure 1.png

Relevance figure 1. MSTS for two coastal stations (B1 and H4) in the Western Gotland Basin/northern Baltic Proper (years 1976-2010). Data are non-transformed mean values for summer (June-September) and circle size indicates average biovolume of filamentous cyanobacteria during the same period. In the Baltic Sea, the extensive cyanobacteria blooms are commonly considered a sign of eutrophication. Therefore, lower mean size observed during years with particularly strong blooms suggests negative effects of eutrophication primarily on mean size. By contrast, no clear effect on the total stock is apparent. Thick lines show threshold values and the green area corresponds to good status conditions. 


In aquatic ecosystems, a hierarchical response across trophic levels is commonly observed; that is, higher trophic levels may show a more delayed response or a weaker response to eutrophication than lower ones (Hsieh et al. 2011). Therefore, alterations in planktonic primary producers and primary consumers have been considered among the most sensitive ecosystem responses to anthropogenic stress, including eutrophication (Schindler 1987; Stemberger & Lazorchak 1994).