Thresholds and Status evaluation

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


The threshold value is established based on the monitoring data using a target setting approach with percentiles of the normal distribution as the threshold value as recommended by ICES for biological effect indicators (Davies and Vethaak 2012). The not good status is defined as a significant increase in embryo malformation frequency or a significant increase in frequency of females carrying more than one malformed embryo compared to the background level. The threshold values do not change over time, and the same values are proposed to be applied throughout the Baltic Sea in all assessment units because no difference in the baseline values for embryo malformation rate was detected between the basins for which the monitoring data are available.

In various Baltic Sea areas, the baseline embryo malformation levels in Monoporeia affinis were found to be similar to those found for the monitoring sites, implying that the indicator is not sensitive to variations in salinity, depth and food availability. At the same time, populations inhabiting areas with elevated concentrations of contaminants in the sediments or living close to known point sources showed a higher malformation rate, implying that the indicator is responsive to xenobiotics and represents a relevant measure of biological effects of environmental contaminants.

There are certain assumptions in the target setting approach when the latter is based on the percentiles of the normal distribution. One such assumption is that reference areas used to analyse statistical distribution of the malformation rate are not negatively affected and represent pristine areas. However, in the Baltic Sea such pristine areas are difficult to find. Due to the lack of unquestionably pristine reference sites, some uncertainty in the accuracy of the threshold value remains, but is not deemed to be very substantial, due to the large amount of data available for target setting and broad coverage of the area (i.e., large number of stations) within each basin.


Primary threshold values

The embryo malformation indicator for amphipods is a multimetric indicator based on two variables measured in the sampled population, namely

(1) the proportion of malformed embryos and

(2) the proportion of females with more than one malformed embryo.

These two variables are measured using the same pool of field-collected gravid females; to achieve good status for an area, both variables must be below or equal their respective threshold value.

Thresholds figure 1.png

Thresholds figure 1. Schematic representation of the threshold value, which indicates good status when both the proportion of malformed embryos and the proportion of females with more than one malformed embryo are below their respective threshold values.


For the amphipod Monoporeia affinis, data from the Swedish National Marine Monitoring Program (SNMMP) running from 1994 to 2011 were used to determine the baselines (as mean percentage in the population) and the thresholds for the percentage of malformed embryos and the percentage of females with more than one malformed embryo (Thresholds table 1 and figure 2). Data are based on gravid females collected at fourteen stations; in total, 8,600 gravid females and 230,000 embryos were included in the analysis (Thresholds figure 2). Background assessment criteria (BAC) have been derived from median-values of 90th percentiles of data selected from areas regarded as less polluted reference areas in the Baltic Sea, and lower bound of environmental assessment criteria (EAC) was set at the 90th percentile value for each variable (Davies and Vethaak 2012). Bootstrapping (100 000 runs) was used to derive mean, median and the 90th percentile values. All types of malformations (embryos with malformed limbs, eyes and midgut, membrane damaged embryos and embryos with arrested development) were included to the category malformed embryos, because all these developmental aberrations are lethal and all of them were found to be associated with various contaminants (Löf et al. 2016).


Thresholds table 1. Threshold value for the amphipod Monoporeia affinis. Background assessment criteria (BAC) and environmental assessment criteria (EAC) are adopted from Davies and Vethaak 2012.

Assessment criteriaMeanBACEACThreshold value
Proportion of malformed embryos0.041<0.059>0.0590.059
Proportion of females with >1 malformed embryo0.23<0.3>0.30.3

Thresholds figure 2.png

Thresholds figure 2. Defining thresholds for (A) Proportion of malformed embryos and (B) Proportion of females with > 1 malformed embryo in the population of the amphipod Monoporeia affinis. The solid green line indicates the background level (mean value in the population), the dotted green line denotes the median, and the orange line denotes the threshold value (90th percentile), beyond which malformation rate is significantly higher than the background levels.


Secondary threshold values – Gammarids

In areas where M. affinis does not occur naturally or is found sporadically and/or at low abundances, other amphipods with a similar life cycle and reproduction biology can be used to derive the embryo malformation indicator. For example, in the Gulf of Finland and the Gdansk Bay, where M. affinis is relatively rare, secondary thresholds were established for other amphipod species belonging to gammarids (Thresholds table 2). As with M. affinis, these thresholds involve two values: one for the percentage of malformed embryos and another for the percentage of females carrying more than one malformed embryo.


Thresholds table 2. Secondary thresholds for the gammaridean amphipods Gmelinoides fasciatus, Pontogammarus robustoides and Gammarus tigrinus (based on Gulf of Finland monitoring data, Russia).

Assessment criteriaMeanBACEACThreshold value
Proportion of malformed embryos0.02<0.05>0.050.05
Proportion of females with >1 malformed embryo0.15<0.2>0.20.2