Moving Beyond Whole-stream Tracer Injections to Understand the Role of Flow and Geomorphic Variability in Stream and River Ecosystems

Flow in aquatic ecosystems affects ecological processes by influencing how sediments and nutrients are stored and transformed. Decades of tracer-addition experiments in streams have been central in revealing the key physical-biological linkages. The averaging of heterogeneous processes made possible by injecting tracers during steady baseflow conditions has allowed the individual roles of transport, storage, and biogeochemical reactions that influence stream ecological health to be clearly separated. However, fluvial systems are inherently unsteady, with flow and sediment transport continually readjusting to one another. Also, very few investigators have addressed effects of temporal variability in flow or interactions that occur between hydrologic or geomorphic processes. Thus, whole-stream tracer addition experiments often end up having limited transferability beyond the very specific flow and geomorphic conditions under which the experiments were conducted. Furthermore, there is increasing recognition that, no matter what measurement technique is used (e.g. hydraulic or tracer-based) or what model is employed, the results are almost always limited by a "window of detection" that is determined by measurement spacing and frequency, sensitivity, and by experiment duration. To counter these challenges, field investigators are increasingly supplementing whole-stream injections with additional measurements that help address different spatial and temporal scales. Furthermore they are often using multi-scale models to more fully evaluate of the full spectrum of water fluxes and biogeochemical reaction rates involved. Often the goal is to identify the combinations of flow and geomorphic conditions which enhance a particular biogeochemical reaction (e.g. dentrification, removal of toxic metals, etc.), or to rank by importance the extent of reactions occurring in different sub-environments. Examples of studies in streams, wetlands, and floodplains range in spatial scale from centimeters to tens of kilometers and temporal scales ranging from seconds (e.g. storage-exchange in streambed periphyton) to weeks in deeper hyporheic flow paths and on floodplains. One attribute of successful studies appears to be use of modeling approaches and other data analysis tools that are simple enough to be implemented based on commonly collected field measurements, while also remaining faithful enough to the underlying physical and chemical principles to ensure that results are transferable. Examples demonstrate how a multi-scale analysis can be used to help solve practical problems, such as improving the scientific basis for prioritization and design of ecological restorations.