Multiple stressors and the role of hydrology on benthic invertebrates in mountainous streams.

Hydrological alteration of rivers is recognised as a major threat to lotic biodiversity acting at broad spatial scales, however, the effect size and pathways of hydrology are rarely quantified in comparison with other stressors such as land use and physico-chemistry. Here we present a multiple stressor study that aims to disentangle the effect sizes and pathways of hydrological alteration on benthic invertebrate community structure and functional metrics. Therefore, we analyse the following four multiple stressor groups: land use, hydrology, physical habitat structure, and physico-chemistry at 51 sites including 72 surveys in the German mountain range. Stressor data were contrasted to benthic invertebrate data using partial canonical correspondence analysis to quantify the community-level response and path analysis to investigate the cause-effect pathway structure of single stressors affecting benthic invertebrate metrics either directly or indirectly (i.e. mediated by other stressors). Hydrological stressors showed a strong impact on community structure, with its unique effects being more dominant than those of any other stressor group. Path analysis confirmed strong direct effects of hydrological stressors on biological metrics but revealed land use to be the most influential stressor group in terms of the sum of direct and indirect effects on biology. Notably, indirect land use effects are mediated by hydrology. Our findings suggest a key role of hydrological stressors in lotic ecosystem assessment, which, however, are rarely addressed in operational river monitoring and management. In light of the wide-spread availability of hydrological data from gauging stations throughout Europe, we plea for a better involvement of hydrological data in river basin management.