Indicator species index for assessing benthic ecological quality in marine waters of Norway
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- NIVA-rapport 
An indicator species index (ISI) for assessing ecological benthic quality is presented. Development of the index was based on data from Norwegian soft-bottom fauna stations. Different species react differently to detrimental environmental conditions. Diversity values at the stations were used as indicators of stress levels endured by the species occurring at that stations, thus establishing specific sensitivities. Sensitivity values were determined for 200 common taxa. The occurrence or absence of such indicator species in a sample can be used for calculating an indicator species index value (biotic index) of the sample. This value is used as an indicator of ecological quality. The indicator species index value (ISI) of a sample is defined as the average of the sensitivity values of the species occurring in the sample. Only presence/abscence of indicator species, not their abundance, is considered. Species which occur in the sample, but having no sensitivity values assigned to them, are ignored in the calculation of ISI. Examples of application of the index are shown for the Lillesand, Tvedestrand and Hvaler area, Norway.