Assessment Protocol

This indicator is a HELCOM supplementary indicator and is applicable in assessment units shared by Finland and Sweden.


To decrease statistical uncertainty of the analysis, a high number of sampling stations per assessment unit is recommended. Since sample size (i.e., number of gravid females available for the analysis) may have high temporal (year-to-year) and spatial (stations within assessment unit) variability, bootstrapping can be applied to control for the sample size. For each station, a sample size of 50 gravid females and about 1,500 embryos is the recommended sample size within the Swedish National Marine Monitoring Program for evaluating the proportion of malformed embryos in the population. Depending on the area of the assessment unit, distribution of sustained populations of the species in question, and heterogeneity of hydrography, several survey regions should be included, with several stations per region (Assessment Protocol figure 1).


Assessment figure 1.png

Assessment Protocol figure 1. Sampling design employed in SNMMP for biological effect monitoring of the amphipod Monoporeia affinis. Red dots denote sampling stations and ellipses are the survey regions (R1 to R10) along the Swedish coast. Basins are abbreviated as in Results figure 1. All stations are located in areas free from pollution point sources and, therefore, the indicators values are expected to be within BAC. For the assessment, a >50%-rule is applied.


Evaluating whether an area is in good status using amphipods is done by:

  • Detecting the number of malformed embryos in each brood by microscopic analysis and the number of females that carry more than 1 malformed embryo; see Table 2 in Löf et al. 2015 for the description of embryo malformations and the criteria for classification of embryo pathologies;
  • Calculating the percentage of malformed embryos and the number of females that carry more than one malformed embryo in the metapopulation (station); to keep the sample size (i.e. number of the females examined) constant, bootstrapping should be used;
  • Combining the assessment outcome  for the region if several stations per region are used; otherwise, proceed with the station-specific values;
  • Comparing the detected malformation rates for the station/region to the threshold values and concluding whether both percentage of malformed embryos and the percentage of females that carry more than one malformed embryo are below their respective threshold values.
    Primary threshold values for Monoporeia affinis has been calculated using a subset of assessment units where monitoring data for this species were available (Assessment Protocol figure 1). As no geographic difference in embryo malformation rate for the background values has been detected, the same threshold values should be applied throughout the Baltic Sea. In areas where M. affinis does not occur in sufficient numbers, such as the Gulf of Finland, Gdansk Basin and Arkona Basin, other amphipod species can be used, with corresponding secondary thresholds.
    The indicator evaluation is done at HELCOM assessment unit Level 2, because no difference in the background values for the malformation rate between the coastal and the open sea areas was found. To evaluate the status of an assessment unit, a >50%-rule is applied, meaning that assessment unit is considered as being in good status when >50% stations with a region and >50% regions within a basin and >50% years within the assessment period are in good status. It should however be noted, that the contaminant load in sediments may vary significantly over relatively short distances and that the one-out-all-out approach may evaluate a large assessment unit as not in good status based on a station with heavily contaminated sediments. To provide a more balanced approach for such deviations and to account for a spatial variability, a region-based sampling design is recommended (Assessment Protocol figure 1); however, it is also possible that an assessment unit is represented by a single region (e.g., the Quark) sharping this case, deviations from the BAC for a single stations would be less likely to affect the assessment outcome. As some natural variation in the malformation frequencies from year to year is expected, it is recommended that the status evaluation is conducted using data for at least 3-4 years.
    As pollution pressure is distinctly different in different sub-basins, evaluations made by grouping neighbouring assessment units is not deemed possible. For example, the Bothnian Bay area cannot be evaluated using monitoring data from the Bothnian Sea; hence, as no monitoring is currently carried out in the Bothnian Bay, this sub-basin cannot be evaluated.
    The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.