author(s)
Luis Quevedo, Carles Ibáñez, Nuno Caiola and David Mateu

abstract
The influence of a thermal discharge caused by the cooling system of a nuclear power station on benthic macroinvertebrate communities was assessed at the lower Ebro River (in Spain). Surveys collected from natural and artificial substrata and conducted at sites before and after the effluent were analyzed and in order to assess changes in community structure, Non-metrical Multidimensional Scaling (NMDS), Similarity Percentage Analysis (SIMPER) and 1-way Analysis of Similarities (ANOSIM) were performed. The relationship between macroinvertebrate assemblages and environmental variables was assessed with a multivariate distance-based linear regression model (DISTLM) and the model was visualized through a redundancy analysis (dbRDA). NMDS ordination was obtained with a stress of 0.12 and 0.11 for natural and artificial substrates, respectively. ANOSIM showed significant differences between Control and Impacted sites (p<0.05). Simper analysis showed that the mean dissimilarity between Control and Impacted sites was of 68.37 % for natural substrate and of 39.97 % for artificial substrate. DISTLM selected a set of explanatory variables (temperature difference, depth, total nitrogen and conductivity) with an 82.58 % of fitted variation for natural substrate, while for artificial substrate, the set of variables selected were temperature difference, total phosphorus and chlorophyll with an 83.13 % of fitted variation. Macroinvertebrates assemblages showed sensitivity to thermal changes both in natural and artificial substrata, even though warming did not exceed 3 ^oC. Factors that seemed to influence benthic macroinvertebrate assemblages the most were the thermal increase caused by the nuclear power station and seasonal variation in nutrients and conductivity.