This paper examines the use of biomass data versus density data to estimate spatiotemporal variations in species diversity during ecological investigations assessing environmental impacts on aquatic communities. The community of benthic macroinvertebrates was selected to achieve this goal. Field studies were conducted along the recovery gradient of the regulated Rio Duraton (northern Spain, Durero basin) receiving an industrial effluent. Diversity values were calculated using the Shannon-Wiener's function (H'). Two separate diversities, biomass diversity (H'b) and density diversity (H'd), were estimated. Hypolimnial releases from the hydropower impoundment caused short-term flow fluctuations and low concentrations of dissolved oxygen at S-2, S-3, S-4 and S-5 sampling sites (0.2, 0.4, 2.5 and 7.6 km below the dam, respectively) compared with the upstream reference station (S-1). The industrial effluent caused an increase in fluoride concentration at S-3, S-4 and S-5 stations. S-1 exhibited the highest value of H'd, S (species richness), B (total biomass), and N (total density), followed in decreasing order by S-5, S-4, S-2 and S-3. Conversely, H'b values did not show this spatial pattern. As a consequence, S, B and N values had lower positive correlations with H'b than with H'd. In addition, coefficients of variation within and between macrobenthic surveys were higher for H'b than for H'd values. According to the results obtained, it may be concluded that the use of density data seems to be more appropriate than the use of biomass data to assess environmental impacts on aquatic communities. Yet this conclusion, though significant, cannot serve as the basis for global generalizations since it is based on one data set from a short reach of one river.
