Chironomidae assemblages in reference condition Mediterranean streams: environmental factors, seasonal variability and ecotypes

Chironomidae spatial and temporal distributions were investigated over two seasons at 31 reference sites in eight Mediterranean river basins in NE Spain. A total of 126 taxa included in 67 genera of chironomid larvae were identified. Canonical correspondence analysis indicated that variation in the assemblage structure of chironomids was primarily explained by temperature, altitude and geology, together with hydrological factors such as discharge and dry period, which play an important role in structuring communities in Mediterranean streams. A final classification based solely on Chironomidae reference assemblages produced four biological groups, and the corresponding indicator taxa were identified. One group is almost exclusively formed by summer samples, showing that seasonal differences are one of the most important driving forces behind differences in Chironomidae assemblage composition in Mediterranean streams. Furthermore we tested the agreement between Chironomidae assemblages and three river ecotypes (WFD approach) obtained after conducting an environmental classification in the Mediterranean region of Spain. Non-metric multidimensional scaling showed a large overlap of chironomid assemblages among ecotypes, although communities belonging to the ecotype Siliceous headwaters and high altitude streams proved to be the most different from the other ecotypes studied. Our results suggest that a top-down classification of streams (using ecotypes) does not necessarily imply exclusive assemblages of chironomids. Thus, classification based on biological data may be more appropriate than environmental classification with subsequent testing using biological data. These findings have important implications for the application of the WFD in Mediterranean streams.