Diatom tracing at the watershed scale : A new approach for assessing the geographic sources of water

Understanding of the time- and geographic sources of water at catchment scale has been derived largely from geochemical and isotope tracers. While helpful, these techniques suffer from a number of assumptions and limitations, including : unstable end-member solutions, temporally varying input concentrations and the need for unrealistic mixing assumptions. As a result, continued advancement of our conceptual understanding of water mixing, source apportionment and connectivity at the watershed scale is currently tracer limited. Here we present new work in the experimental Attert watershed in the Grand-Duchy of Luxembourg (Europe) that focuses on possible new tracers that might help to answer questions of the geographic source of water. These preliminary investigations have been focusing on the mobilisation of living organisms in the watersheds, and more specifically on the potential for diatoms as tracers of water source and hydrological connectivity during rainfall-runoff events. Diatoms are unicellular algae and one of the most common algal groups in freshwaters and marine ecosystems. Since diatoms are very sensitive to environmental variables, such as light, moisture conditions or temperature, they are often used in geological, archaeological and water quality research. Diatom taxonomy and systematics rely on species-specific silica cell-wall ornamentations. With most species measuring between 10 and 200 μm, they are easily transported by flowing water and thus have the potential for tracing the geographic source of water at the watershed scale. Their occurrence is not only limited to aquatic ecosystems, since they also live in moist terrestrial habitats, such as soils, rock surfaces or epiphytes. Especially the so-called drift diatoms thus have the potential to link the terrestrial and aquatic environments with a view to trace water sources and hydrological connectivity at the watershed scale. Our preliminary investigations in the Attert watershed suggest that diatoms can help to detect the onset/cessation of surface runoff connected to the river and thus represent a new way to quantify geographical sources of surface runoff. Ultimately, this new type of information can be used by hydrologists to confirm or reject the existence of a surface runoff component in total runoff and eventually to constrain assumptions on a potential surface runoff component in conventional tracer based hydrograph separations.