Divergent drivers of the spatial variation in greenhouse gas concentrations and fluxes along the Rhine River and the Mittelland Canal in Germany

Lotic ecosystems are sources of greenhouse gases (GHGs) to the atmosphere, but their emissions are uncertain due to longitudinal GHG heterogeneities associated with point source pollution from anthropogenic activities. In this study, we quantified summer concentrations and fluxes of carbon dioxide (CO$_2$), methane (CH$_4$), nitrous oxide (N$_2$O), and dinitrogen (N$_2$), 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 and fluxes along the two ecosystems and to determine the main driving factors responsible for their longitudinal GHG heterogeneities. The results indicated that the two ecosystems were sources of GHG fluxes to the atmosphere, with the Mittelland Canal being a hotspot for CH$_4$ and N$_2$O fluxes. We also found significant longitudinal GHG flux discontinuities along the mainstems of both ecosystems, which were mainly driven by divergent drivers. Along the Mittelland Canal, peak CO$_2$ and CH$_4$ fluxes coincided with point pollution sources such as a joining river tributary or the presence of harbors, while harbors and in-situ biogeochemical processes such as methanogenesis and respiration mainly explained CH$_4$ and CO$_2$ hotspots along the Rhine River. ... mehrIn contrast to CO$_2$ and CH$_4$ fluxes, N$_2$O longitudinal trends along the two lotic ecosystems were better predicted by in-situ parameters such as chlorophyll-a concentrations and N$_2$ fluxes. Based on a positive relationship with N$_2$ fluxes, we hypothesized that in-situ denitrification was driving N$_2$O hotspots in the Canal, while a negative relationship with N$_2$ in the Rhine River suggested that coupled biological N$_2$ fixation and nitrification accounted for N$_2$O hotspots. These findings stress the need to include N$_2$ flux estimates in GHG studies, as it can potentially improve our understanding of whether nitrogen is fixed through N$_2$ fixation or lost through denitrification.