Abstract: The present study analyses the relationships of a set of 19 macroinvertebrate community metrics in relation to hydromorphological and water quality stress gradients in the Pas river basin, Northern Spain. Moreover, this study also aims to identify the most sensible season for biomonitoring this temperate Atlantic basin by means of macroinvertebrate communities. Thus, macroinvertebrate communities were analysed, along with the physicochemical characteristics of water in January, April, August and November 2005, in 9 sites. Hydromorphological conditions were evaluated using the Hydrogeomorphologic Index and by quantifying the percentage of reinforced banks, the number of low-head dams and their distance to each study site. Hydromorphological and water quality stress gradients were determined by means of two independent Principal Component Analysis. Thus, we obtained two water quality stress gradients, indicating organic pollution and watershed runoff, respectively, and a hydromorphological stress gradient. Macroinvertebrate metrics correlated better with the hydromorphological and the organic pollution stress gradients during the stable flow season, probably because of the homogenisation of water and hydraulic characteristics in the high flow season. On the other hand, relationships between macroinvertebrate metrics and watershed runoff stress gradient seem to be more dominated by hydraulic conditions than by water quality characteristics. LIFE index and the ICMi were the macroinvertebrate metrics that best correlated with the hydromorphological stress gradient, while EPTt, EPt, IBMWP, ASPT, LIFE, AsMet, Rhet, NoIns% and MACh% were well correlated with the organic pollution stress gradient in the low flow period. EPTt, EPt, ASPT, Rhet, and AsMet did not show significant seasonal differences and, thus, they are recommended as appropriate metrics to integrate biomonitoring results from different seasons in temperate European Atlantic rivers. Finally, autumn seems to be the best season to sample macroinvertebrate communities in this temperate Atlantic basin for biomonitoring purposes, and might be also the case for other similar European rivers.
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