Limnetica 37

This study included the main hydrogeomorphic categories of the lower Ebro River and aimed to detect spatial and temporal patterns of macroinvertebrates and diatoms communities inhabiting downstream a reservoir system. We hypothesized that biological communities will reflect a gradient of alteration related to the distance from the impact (i.e., dams). Surveys conducted along five river sections 2 km long integrating different years and seasons were analyzed. Non-metrical Multidimensional Scaling (MDS), Similarity Percentage Analysis (SIMPER) and 1-way Analysis of Similarities (ANOSIM) were performed to assess spatial and temporal differences in community structure. The relationship between biological and environmental data was investigated with BIOENV routine and a Principal Components Analysis (PCA) was also carried out. Significant differences in the composition of the macroinvertebrate community were found between the section closest to reservoirs (E5) and the rest of the study sections; the present community was also very different of that found in previous studies carried out in the 80’s and 90’s. For the case of diatoms, significant differences in community composition were only found between seasons but not between study sections. As well, indices for ecological status assessment based on both macroinvertebrates (IBMWP) and diatoms (IPS) showed lowest scores at section E5, likely caused by the impact of dams. Macroinvertebrates and diatoms responded rather differently to anthropogenic stressors in the lower Ebro River; macroinvertebrates were more sensitive to hydromorphological impacts, while diatoms were more sensitive to water quality alteration