Benthic condition in low salinity areas of the Mira estuary (Portugal): Lessons learnt from freshwater and marine assessment tools

The assessment of the ecological integrity of aquatic ecosystems based on benthic macroinvertebrate communities has progressed in recent years, due to the implementation of water quality regulations worldwide, including the European Water Framework Directive (WFD). Several biotic indices have been developed in order to evaluate the ecological status of freshwater and marine ecosystems but the assessment of estuarine condition is much less developed because of its transitional characteristics. Some adaptations of marine indices have been proposed for estuarine areas, but very few include specific thresholds to be used in low salinity areas, where freshwater species can also be found. This study focused on the tidal freshwater and brackishwater sections of the Mira estuary, where salinities lower than 0.5 were registered at least during a certain period. Since no reference conditions are defined for benthic macroinvertebrate communities of Portuguese transitional waters, physicochemical variables recognized as indicative of human stress (organic and nutrient enrichment and oxygen levels) were used to classify the study area. Metrics commonly used to assess benthic condition in marine and freshwater systems were calculated and a modification of the AMBI and IBMWP indices was proposed. Two different ecotopes based on salinity were defined across the tidal freshwater–estuarine interface of the Mira estuary, a tidal freshwater and a brackish water ecotope. Only AMBI, IBMWP and the proposed modified versions of those indices responded predictably to human pressure indicative variables, but the ecological quality status classifications indicated that ecotope specific reference conditions need to be defined in order to improve the performance of those indices.