Limnetica 32

The macroinvertebrate communities of two mountain streams in the Sierra Cebollera Natural Park (La Rioja, Northern Spain) were studied to ascertain the changes in these communities and their ecological status, considering the two main pressures that exist in the area, i.e., the Pajares Reservoir and various wastewater discharges. While the alterations in the macroinvertebrate communities were minimal downstream of the wastewater discharges, significant changes were detected in the composition of the community in the stream reaches located downstream of the Pajares Reservoir. This major disturbance downstream of the dam was especially intense in those reaches affected by a massive growth of the invasive algae Didymosphenia geminata, which might be related to dam operation. Despite a small decrease in the different biotic indices used, especially IBMWP and IMMi-T, downstream of the Pajares Reservoir, no large changes in the biological quality levels were detected (from high to good conditions). We conclude that the biotic indices did not completely reflect the changes produced by the disturbances present in the studied streams, especially the effects produced by the presence of a large dam. This study highlights the importance of considering measures other than the biotic indices to study the ecological status of headwater streams and rivers using macroinvertebrates, such as multivariate analysis, when the main disturbance is the alteration of the physical habitat rather than organic pollution.