Divergent drivers of the spatial variabilities in CO2, CH4, N2O, and N2 concentrations along the Rhine river and the Mittelland canal in Germany

Abstract

Lotic ecosystems transversing mixed land-use landscapes are sources of GHGs to the atmosphere, but their emissions are uncertain due to longitudinal GHG heterogeneities. In this study, we quantified summer CO2, CH4, N2O, and N2 concentrations, as well as several water quality parameters along the Rhine river and the Mittelland canal, two critical inland waterways in Germany. Our main objectives were to compare GHG concentrations along the two ecosystems and to determine the main driving factors responsible for their longitudinal heterogeneities. The results indicated that GHGs in the two ecosystems were up to three orders of magnitude oversaturated relative to equilibrium concentrations, particularly in the Mittelland canal, a hotspot for CH4 and N2O concentrations. We also found significant longitudinal variabilities in % GHG saturations along the mainstems of both ecosystems (CV = 26 – 98 %), with the highest variability recorded for CH4 concentrations in the Mittelland canal, suggesting that single GHG measurements along large lotic ecosystems are unrepresentative of entire reaches. However, these significant longitudinal GHG heterogeneities were driven by divergent drivers between the two lotic ecosystems. Within the Canal, longitudinal CO2 and CH4 hotspots were linked to external inflows of the GHGs from surrounding WWTPs. Contrastingly, harbors and in-situ biogeochemical processes such as methanogenesis and respiration explained CH4 and CO2 hotspots along the Rhine river. In contrast, N2O was strongly linked to N2 concentrations, with a negative relationship in the Rhine river and a positive relationship in the Mittelland canal. Based on these N2 relationships, we hypothesized that denitrification drove N2O hotspots in the Canal, while coupled N-fixation and nitrification accounted for N2O hotspots in the Rhine. This finding stresses the need to include N2 concentration measurements in GHG sampling campaigns, as it has the potential to determine whether nitrogen is fixed through N-fixation or lost through denitrification.