Seasonal changes of organic carbon and nutrient fluxes in intermittent spring catchments

Comprehensive, high-resolution data on intermittent natural springs with low discharge are still rare, although they represent an important interface between terrestrial and aquatic environments, and form the basis of our water systems. Due to their connection to groundwater, springs have been considered quite stable in terms of both hydro-biogeochemistry and water quality. However, with climate change, spring systems are subject to significant hydrological dynamics, partly due to changes in water availability. Currently, spring discharges are decreasing or drying up during more frequent droughts. The amount of nutrients exported to headwater streams is closely linked to hydrological processes. For intermittent springs, a significant change in biogeochemistry with increased nutrient export can be expected due to the temporary cessation of groundwater inflow combined with longer residence times of organic matter in the surrounding soil substrate. However, little is known about the role of intermittent springs in carbon cycling and their role in downstream carbon and nutrient export.

In order to fill this research gap, this study aims to asses and quantify the seasonal variability of carbon and nutrient composition and fluxes of intermittent or highly variable discharge springs as a function of climatic, site and biogeochemical parameters. We investigate a range of spring areas (44 springs) spread across the German low mountain ranges of the Ore Mountains, Sauerland, Black Forest and Rhenish Slate Mountains.

We measure the export of organic carbon based on high resolution data in selected springs, and complement these measurements with nutrient (nitrogen and phosphorus) samples on a seasonal basis. In addition, we investigate the composition of dissolved organic matter (DOM) to identify contributing carbon sources.

First results show that the spring flow regime determines carbon and nutrient concentrations, modulated by the characteristics of the spring type. Our study emphasizes the sensitivity of springs to hydrological shifts, particularly in the balance between groundwater and surface water contributions. A shift favoring surface water inputs, can increase nutrient exports, likely due to enhanced surface runoff carrying nutrients from the surrounding landscape. Climatic changes, with extreme rainfall events are becoming more frequent and intense, may alter the balance between groundwater inputs and surface water runoff in springs may result in higher carbon and nutrient fluxes into receiving waterbodies.