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.

Riverine regime shifts through reservoir dams reveal options for ecological management.

Worldwide, dams are a main threat reducing river ecological functioning and biodiversity by severely altering water temperature, flow, and sediment regimes up- and downstream. Sustainable dam management therefore has a key role in achieving ecological targets. Here, we present an analysis of the effects of reservoir dams and resulting regime shifts on community structure and function of lotic macroinvertebrates. Our study derived management options to improve ecological integrity of affected streams. To do this, we contrasted time series data for water temperature (15-min intervals over one year), discharge (daily means over 10 yr), and records of deposited fine sediments against macroinvertebrate samples from pairs of river reaches downstream of dams and of comparable tributaries not affected by dams in the German low mountain range. We observed a decline in the density and diversity of disturbance-sensitive macroinvertebrates (Ephemeroptera, Plecoptera, and Trichoptera) and a correlation between hydrologic metrics and macroinvertebrate deterioration downstream of the dams. Typical "rhithral" (flow-adapted) species changed to "littoral" (flow-avoiding) species below dams, thus indicating a hydrologic regime shift. Increased fine sediment accumulations and deficits of pebbles and small cobbles below dams indicated a severe habitat loss below dams. Additional comparison with undisturbed reference streams allowed us to derive management options that could mitigate the negative impact of hydrologic alterations and accumulations of fine sediments downstream of dams. These options are conditional on the season and in particular address the frequency and duration of low and high flow events.