Sensitivity of macroinvertebrate indicator taxa to metal gradients in mining areas in Northern Spain

The study of macroinvertebrate communities using a Reference Condition Approach (RCA) is a powerful tool for detecting the impacts of mining activities due to heavy metal contamination in rivers. However, studies on this topic based on an RCA fully satisfying the Water Framework Directive (WFD) criteria for reference site selection are still scarce. Following a WFD-compliant RCA design, a total of 14 reference and 15 test sites were intensively sampled during July 2015 in the Nalon River basin (Asturias, Northern Spain). Macroinvertebrate samples, water physicochemistry, metal concentrations in sediments and habitat quality were evaluated at each selected site. To determine whether increasing heavy metal levels in sediments have significant ecological impacts in the structure of macroinvertebrate communities in the study area, we applied a combination of non-parametric multivariate and univariate procedures, as well as gradient analysis and Threshold Indicator Taxa ANalysis (TITAN) over the biotic and environmental datasets. We found significant alterations in the macroinvertebrate community structure with respect to the reference condition (REF group) for those test sites within mercury (HG group) and gold (AU group) mining areas, with Ephemeroptera, Plecoptera and Trichoptera (EPT) richness and abundance being the community descriptors showing the strongest alterations in relation to mining pressures in the studied area. Metal concentrations in sediments were significantly correlated to the metrics of community structure related to abundance, richness and biodiversity, with As and Hg being the two metals showing higher biological effects on macro-invertebrate assemblages. TITAN also allowed us to establish a set of macro invertebrate families specifically sensitive to metal concentrations in sediments, most of which corresponded to EPT families. Our results prove that determined sensitive taxa could be considered as reliable indicators of metal pollution in the mining areas of Northern Spain. From a WFD perspective, our study clearly indicates that the responsive community descriptors found in this study are actually key parameters in the evaluation of the ecological status in the rivers of Northern Spain. These data are highly relevant for the future development of environmental quality standards for river sediments in Spain.