Thresholds and Status evaluation

This core indicator employs zooplankton mean size and total stock (MSTS) to evaluate pelagic food web structure, with particular focus on lower webs. MSTS evaluates whether good status is achieved using two threshold values, one for mean size and one for total standing stock (abundance or biomass) of zooplankton (Thresholds figure 1). An area is evaluated as having achieved good status using the MSTS indicator when both mean size and total stock are achieve their specific threshold values.

Thresholds figure 1.png

Thresholds figure 1. Schematic illustration of the core indicator applying two threshold values.

Due to strong environmental gradients in the pelagic communities in the sub-basins of the Baltic Sea, the exact threshold values are specific for each assessment unit (Thresholds table 1).


Thresholds table 1. Assessment unit specific threshold values.

Assessment unit
Threshold value 
mean size(µg wet weight ind-1) / total stock (mg m-3)
Gdansk Basin
Western Gotland Basin
5.0 / 220
Gulf of Finland
8.6 / 125
Åland Sea
10.3 / 55
Bothnian Sea
8.5 / 84
Bothnian Bay
23.7 / 161


The threshold values are set using a reference period which defines a status when the food web structure was not measurably affected by eutrophication and represents good fish feeding conditions within the time series of existing data. Thus, the reference periods for MSTS reflects a time period when effects of eutrophication (defined as 'acceptable' chlorophyll a concentration) are low, whereas nutrition of zooplanktivorous fish is adequate for optimal growth. Hence, these are the periods when eutrophication and overfishing related food web changes are negligible. In some cases, reference periods can be adopted from neighbouring areas, for which longer datasets are available.

As the indicator evaluates the structural- and functional integrity of the food web, the threshold values are conceptually achieved when:

  • there is a high proportion of large-sized individuals (usually copepods) in the zooplankton community that efficiently graze on phytoplankton and provide good-quality food for zooplanktivorous fish, and
  • the abundance (biomass) of zooplankton is at an adequate level to support fish growth and exert control over phytoplankton production.

Two alternative strategies for setting reference conditions are possible.

    1. The first approach should be used when the data series are very short. Conceptually this approach is similar to using a trend as a threshold value. When using this approach, the long-term mean and corresponding variance (95% confidence interval, CI) for both the mean size and the total stock parameter are calculated based on the entire available dataset. The lower bound of 95%-CI is then used as threshold value to evaluate deviations in the current observations. This approach is possible; however, it was not used in the MSTS-based evaluation in 2011-2016.
    2. The second approach is based on (i) specific reference conditions for chlorophyll a concentrations (RefConChl) that have been defined for the different sub-basins of the Baltic Sea (either observed in the past or based on models), and (ii) reference data on clupeid fish (young herring and sprat) that are used to identify the reference time periods (RefConFish) when both the fish growth (i.e. weight-at-age, WAA, or other body condition indices, such as fat content) and fish stocks were relatively high in the relevant ICES subdivisions. Once the reference time periods have been identified based on chlorophyll a and fish time series, the threshold values for both mean size and total stock were defined as the lower bound of the 99%-CI for the respective mean values calculated for zooplankton time series during the reference time period. This approach was used for the 2011-2016 assessment period (Thresholds figure 2).


Thresholds figure 2.png

Thresholds figure 2. The MSTS concept (left) and a data example (right) to illustrate the use of the indicator. The green area on the left panel represents good status conditions, orange areas represent not good status where only one of the two parameters is adequate and the red area represents not good status where both parameters fail to meet the threshold value. On the right panel, an example of long-term zooplankton data for mean size and total biomass (station B1, Askö, Western Gotland Basin) were analysed. The corresponding thresholds are shown as red and blue lines, respectively. The years in green were classified as in good status and those in red as not in good status. Generally, all years located in the right upper quadrant (green area in panel A) reflect good status. However, some years (e.g., 1979, 1985, 1994, etc.) are classified as reflecting good status, although they are placed outside of the green area. For these years, even though the absolute values for the indicator components (MeanSzie and biomass) are below the threshold value, the deviation is not significant as determined by CuSum. To achieve a significantly not good status value, the change must be persistent and cumulative negative change must exceed 5σ difference from the threshold value. Similarly, some years (e.g., 2007) are classified as not good status, although they are placed in the green area; during these years the observed values were above the thresholds, however this has not resulted in a significant shift in any of the MSTS component that was sufficiently persistent to return the MSTS values in the reference state. See the Assessment protocol for details.