Sandy streambeds can be mobilized at base flow and sediments are transported as bedload, more specifically as migrating ripples. Within migrating ripples, microbial communities experience an erosion-resting cycle of sediment grains. Besides, small changes in discharge can results in frequent transitions between migrating ripples and no-transport. Despite the ubiquity of both migrating ripples and sediment transport transition, their effect on streambed functioning and microbial community composition remain unclear. We performed a microcosm experiment mimicking two sediment transport conditions, namely ripple and no transport (i.e., stable), and their transition to observe the response of sediment community function and composition. Both net community production (NCP) and community respiration (CR) were suppressed in ripple sediments compared to stable sediments. In ripples, a combination of mechanic stress, advective supply and light limitation likely hampered microbial metabolism. Sediment stability likely facilitated an active community of autotrophs, mainly diatoms, as indicated by high NCP, high rates of DOC release and Si-SiO2 retention. Retention of nitrate and the high DIN : SRP ratio indicated efficient resource utilization in stable sediments. After the transition, microbial communities from each treatment responded differently to sediment transport, most likely as a result of the interaction between their previous environmental conditions and functional status in response to the new conditions. Our data indicate that sediment transport in the form of migrating ripples at low flow can strongly modulate streambed metabolism, and discharge oscillations (transitions) will result in a mosaic of metabolism and communities that will emerge at larger scales determining reach scale metabolism.
In the course of a cooperative project between the University of Zurich, ETH, WSL and the University of Freiburg, a first prototype of an information platform for droughts will be developed. Thus, it is necessary to evaluate different hydrological regimes to derive the sensitivity of catchments to droughts. In the scope of this subproject a wide range of hydrological and meteorological drought indices are tested for approximately 50 mesoscale catchments in Switzerland. These indices are based on precipitation, temperature and/or runoff. The aim is to identify more (or less) appropriate indices as a description of catchment behavior. The indices will also be related to various catchment characteristics and the correlation among the indices themselves will be studied. Finally the catchments are classified in terms of their sensitivity to droughts by using a cluster analysis.