Seasonal variation and controls of nitrous oxide and methane concentrations at the entrance to the Elbe Estuary

Inland water bodies and rivers can be important sources of the greenhouse gases (GHG) nitrous oxide and methane, which are particularly high in managed water bodies receiving high nutrient loads. GHG emissions derived from such input are an important subset to national greenhouse gas inventories, but are difficult to parameterize. Within the national monitoring network ITMS, we thus investigated the seasonal variability of GHG concentration in the Elbe River. To support the parametrization of GHG fluxes and emissions, we measured nitrous oxide and methane concentrations along with key biogeochemical properties in the water column at a sampling station at the entrance to the Elbe Estuary in Geesthacht, Germany, in 2024.

Methane and nitrous oxide concentrations appear to be governed by surface water input and organic matter decomposition. Nitrous oxide remains close the equilibrium for most of the study period, with little autochthonous production in the river, and only increases during a flood event driven by elevated discharge and soil water inflow. In contrast, the limnic Elbe shows intense methane production, with strongly increasing concentrations over the course of the vegetation period, fuelled by organic matter turnover in the riverine water column and sediments. High chlorophyll and high methane concentrations with low nutrient concentrations and long residence times at high temperatures suggest intense internal recycling in the water column over the summer. Especially in summer, we also see a strong inverse correlation of water discharge and methane concentration in the river.

Our data show the interplay of water sources, discharge patterns and biological productivity in river and catchment on GHG concentration and underscore the complex interplay of processes that make the eutrophic Elbe River an important source of GHG under global change.