Biogeochemical dynamics of organic carbon fluxes in intermittent spring catchments

Springs represent a direct interface between groundwater and surface water. They can be classified differently with the main distinctions being the spring type and discharge, which mainly influence their biogeochemistry. However, the investigation of organic matter dynamics in springs previously have been neglected due to the assumption of stable conditions, especially in perennial springs. Contrarily, in intermittent springs higher organic carbon (OC) concentrations are expected due to the temporal interruption of the flow regime and therefore longer accumulation rates and residence times of organic matter in the adjacent soil substrate. In the course of climate change, intermittent springs will become more frequent as a consequence of decreasing groundwater levels during dry periods. Dry falling of springs during the year will therefore affect the quantity and quality of OC exports to the adjacent headwater streams.

Here we investigate 44 springs in four different study areas in Germany (Sauerland, Rhenish Slate Mountains, Ore Mountains, Black Forest) along a gradient of geology and vegetation type. We complement long-term hydrological instrumentation with quarterly biogeochemical and event-based sampling campaigns. Dissolved and particulate organic carbon concentrations (DOC and POC), composition (via absorbance and fluorescence measurements), stable water isotopes (δ²H, δ¹⁸O) and nutrient concentrations (PO₄, NO₃, NH₄) of spring samples and additionally of precipitation, soil water and groundwater samples are analyzed.

We aim to unravel seasonal biogeochemical changes, identify drivers of spatial-temporal variability of OC fluxes and to quantify OC export fluxes of springs to the adjacent headwater streams. The results of the first seasonal sampling campaigns point to discharge impacting DOC concentrations and high spatial variability in DOC concentration and composition between the 44 spring sites within the four study catchments.