Organic carbon fluxes in intermittent springs

Springs link the terrestrial and the aquatic ecosystem and connect groundwater and surface water. They are distinguished primarily by their type and discharge, whereby the latter can influence the biogeochemistry. Perennial springs, characterized by continuous spring discharge, show stable conditions and relatively low organic carbon contents. Although previous studies have investigated the sources of dissolved organic carbon (DOC) in streams considering mainly the riparian zone, the hyporheic zone and the hillslopes, our current understanding of springs as sources of organic carbon (OC) is still limited. Thus, our study focuses on intermittent springs, which are particularly vulnerable to climate change induced decreased groundwater levels. Additionally, changing groundwater levels may further increase the frequency of springs with an interrupted discharge during dry periods. Intermittent springs with a temporarily loss of the connectivity to the groundwater, impacting the quantity and quality of received OC and consequently the in-stream respiration, may lead to changed OC quantity and quality transported to downstream ecosystems.

The aim of this investigation is to quantify the export fluxes of OC and to analyze their origin and composition in intermittent springs. For this purpose, 40 springs at four study sites in different low mountain range regions in Germany (Sauerland, Ore Mountains, Hesse Mountains and Black Forest) with different geology and vegetation types will be instrumented with hydrological on-site-measurements for discharge and electrical conductivity. Continuous quarterly biogeochemical sampling campaigns will be carried out and event-based sampling with an autosampler during 4 rainfall events at one spring per site is intended. Additionally, analyses of groundwater, soil water and precipitation samples as well as fDOM and CO₂ measurements are implemented. Stable water isotopes (δ²H, δ¹⁸O) and nutrient concentrations (PO₄, NO₃, NH₄) will also be determined to enable flux modelling. In this project the combination of continuous measurements and frequent sampling campaigns will be used to gather long-term data with high temporal resolution. Thus, the seasonal dynamics and spatio-temporal variability of OC export fluxes as well as event-based changes in OC and nutrient status and further the influence of spring OC on the following headwater streams will be studied in the next three years. First results show that there are great spatial variabilities in DOC concentration between the 40 intermittent springs in the four study catchments. This underscores that intermittent springs differ substantially from perennial springs in their export behavior.