Lowering a navigation groyne increases the frequency of flooding and reduces greenhouse gas evasion from an oxbow lake

Lowering navigation groynes can restore lateral connectivity between regulated rivers and adjacent floodplains. In the Po River, northern Italy, this intervention increased the frequency of hydraulic connection between the river and the Gussola oxbow lake. However, despite the potential effects of reconnection on oxygen conditions, nutrient supply, organic matter processing and biodiversity, little is known about how alternating periods of connection and disconnection regulate greenhouse gas (GHG) emissions from oxbow lakes. Carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) saturation and fluxes, and water physical-chemical parameters, were measured for one year at the Gussola oxbow lake, periodically connected to the Po River. We hypothesized that hydraulic disconnection would result in oxygen depleted stagnant ponds, leading to GHGs accumulation and evasion. GHG saturations and fluxes were measured in situ using portable gas analyzers. Hydrological isolation was associated with water column stratification, dominance of anoxic bottom water, and elevated CO 2 and CH 4 saturation, approximately 2.6 and 1.6 times higher than when the oxbow lake was connected to the river. River floods and hydraulic connection destratified the water column, restored oxic conditions, and resulted in large nutrient and sediment input to the oxbow lake. Unexpectedly, N 2 O saturation remained unchanged across hydrological conditions. This may reflect the combined effects of increased oxygen availability and nitrate supply during river inflow, which can support different N 2 O-producing and consuming pathways. These results shed light on multiple mechanisms regulating GHG dynamics during alternating phases of flooding and hydraulic connection, and low discharge and isolation of oxbow lakes.