Sensitivity of indicators matters when using aggregation methods to assess marine environmental status.

The sensitivity of the 15 indicators included in a complex aggregation method (Nested Environmental status Assessment Tool), applied to a case study in the Caspian Sea in Iran, has been studied to discriminate between areas impacted and non-impacted by bathing activities. Two methods were used: (i) the 15 indicators were grouped into four groups (physicochemical, bacteria, plankton, and benthos) and each group was investigated separately (one-way sensitivity analysis), calculating NEAT values after omitting each group independently; and (ii) indicators were selected randomly, using 1000 Monte Carlo iterations, and removing from 1 to 14 indicators at each iteration. The results revealed that the abundance of Pontogammarus was the single indicator that made the difference in assessing the status among locations, differentiating bathing and non-bathing areas. Hence, this indicator is regarded as a monitoring element detecting the impacts produced by a management measure (beach nourishment) taken by the authorities to maintain the bathing activity.