A high-resolution carbon balance in a small temperate catchment: Insights from the Schwabach River, Germany

Abstract

The present study is one of only few to address the stable carbon isotope (δ13C) and concentration dynamics of DIC, DOC, and POC over an entire hydrologic year, using a dataset of high sampling resolution (4–11 samples retrieved per month). This research was performed in the catchment of the Schwabach River, a typical mid-latitude small headwater stream in Germany emplaced mainly in karstic bedrock. The DIC data indicated the dominance of mineral weathering as a DIC source, with a noticeable dilution effect during periods of high river flow. A weakly negative relationship between discharge and δ13CDIC hints at a greater importance of plant-derived organic matter during flooding events, likely transported to river waters via overland runoff and intermediate flow. DOC inputs appeared to be enhanced during periods of high discharge, indicating a greater importance of overland runoff as a DOC source. POC concentrations seem unaffected by changes in discharge, although a slight negative correlation between δ13CPOC and discharge may be derived from increased inputs of C4 plant material.Estimated CO2 concentrations were in excess of ambient atmospheric values throughout the year, confirming that the surface waters of the Schwabach River are a net CO2 source. The total riverine carbon flux was dominated by DIC (70%), followed by CO2 outgassing (21%), DOC (7%), and POC (2%). While the selection of a bi-monthly sampling scheme yielded a broadly similar carbon flux estimate to that utilizing the entire dataset, the use of a monthly sampling interval differed by as much as 19% from values using the high-resolution data set. This discrepancy is due to the inability of a monthly sampling scheme to capture sudden and large variations in river discharge and associated changes in dissolved/particulate carbon concentrations, such as those observed during flooding. We suggest that bi-monthly sampling is the minimum timeframe required to achieve an acceptable degree of accuracy in the calculation of carbon fluxes, at least during periods of high runoff. The application of high sampling frequencies and comprehensive DIC, DOC, and POC studies in future research would greatly reduce uncertainties in local riverine carbon budgets, and help clarify the role of smaller streams in the global carbon cycle.