Greenhouse gas fluxes in an oxbow lake and its exposed sediments during periods of hydraulic connection and disconnection

After decades of systematic simplification of river ecosystems through hydraulic interventions that have reduced the interactions between the river and its floodplain, projects are now underway to reconnect rivers with riparian areas and lateral canals. The navigation groynes separating the Po River from the Gussola oxbow lake (Cremona, northern Italy) underwent a requalification intervention in March 2023, which consisted of lowering them in order to increase the frequency of flooding and, consequently, the interaction between the river and the oxbow lake. Before and after the requalification, the oxbow lake saturation and fluxes of CO₂, CH₄ and N₂O were measured to analyse the effects of connectivity and rewetting on the sink or source role for greenhouse gases. Other parameters included temperature, conductivity, pH, dissolved oxygen, nutrients and chlorophyll. We hypothesised that the disconnection of the Gussola oxbow lake from the Po River would result in isolated, stagnant ponds prone to algal blooms, reducing CO₂ concentrations and fluxes, and bottom hypoxia/anoxia, leading to anaerobic pathways in sediments and the accumulation and evasion of CH₄ and N₂O. We also hypothesized that evaporation would set to zero CH₄ and N₂O fluxes to the atmosphere through exposed sediments, but increase those of CO₂ due to increased air and oxygen penetration.

All greenhouse gas fluxes were carried out on a seasonal basis along 2023 and 2024, and intensified during hydrological extremes. Measurements were made in situ, using portable analysers connected to floating or benthic chambers when fluxes were the target, or to 1 L glass bottles half-filled with in situ water when saturations were the target.

The results suggest that when the Gussola oxbow lake is isolated, it rapidly stratifies, the bottom water becomes anoxic and releases large amounts of CH₄. Evaporation leads to fragmentation of the oxbow lake into ponds, where algal blooms can be replaced by macrophytes, if hydraulic disconnection persists. Macrophyte meadows, especially those with emergent plants, are CO₂ sink hotspots. Floods set to zero primary producer communities, destratify the water column, restore oxic conditions and result in large nutrient and sediment input to the oxbow lake. Floods reduce CH₄ saturation and evasion, but large nitrate inputs stimulate denitrification in the oxbow lake and N₂O evasion. In exposed sediments, water saturation gradients regulate CO₂ and N₂O emissions (peaking in unsaturated sediments) and CH₄ emissions (peaking in saturated sediments). In the case of sediments, N₂O is likely to be released as a consequence of increased nitrification rates.

The results of this study shed light on the multiple mechanisms regulating greenhouse gas dynamics, which are activated or deactivated during periods of flooding and hydraulic connectivity, and during period of low discharge and isolation of lateral canals as oxbow lakes.