Terrestrial diatoms: a breadcrumb trail for tracking rapid flowpaths connectivity across scales

Headwater streams are known for playing a key role in sustaining hydrological connectivity between uplands, riparian areas and streams, and eventually structuring ecosystems up to the regional scale. In this context, connectivity stands out as a natural backbone onto which a multitude of heterogeneous sub-scale processes, functions or habitats map. However, hillslope connectivity to streams is modulated by riparian zones. Consequently, there is a pressing need to understand the spatio-temporal dynamics of connectivity between uplands and valley bottoms. Here, we present new work that shows how diatoms can be used to increase our understanding of hydrological (water sources and flowpaths) and ecological (dynamics of terrestrial diatoms) connectivity and functioning of watersheds. We examine hillslope-riparian-stream (HRS) systems across nested catchments in the Attert River basin (Luxembourg). Our main objective is to understand the spatio-temporal dynamics of saturated areas, the onset/cessation of their connectedness, and eventually their contribution to the storm hydrographs via rapid flowpaths. Our results show that during storm events, terrestrial diatoms are systematically mobilized and flushed into the streams. Catchments exhibit specific diatom assemblages that are largely controlled by prevailing physiogeographic characteristics (e.g. geology, land cover, soil types, soil moisture, etc.) and that are different from the benthic and planktic diatom assemblages found in streamwater. Drift diatom assemblages in streamflow determined at the Attert basin outlet during storm hydrographs bear the distinct signatures of upstream catchments - eventually translating the onset and cessation of rapid flowpaths connectivity throughout the river network. The large heterogeneity in terrestrial diatom communities within the HRS continuum, their flushing to the stream, as well as the distinct diatom assemblages characterising catchments with specific physiographic characteristics demonstrate the potential for this microscopic biological tracer to address questions of hydrological connectivity.