Contribution from a eutrophic temperate estuary to the landscape flux of nitrous oxide.

For mitigation of climate change, all sources and sinks of greenhouse gases from the environment must be quantified and their driving factors identified. Nitrous oxide (NO) is a strong greenhouse gas, and the contribution of aquatic systems to the global NO budget remains poorly constrained. In this study, we measured NO concentrations in a eutrophic coastal system, Roskilde Fjord (Denmark), and combined measurements with statistical modeling to quantify the NO fluxes and budget in the system over a period of six months. To do so, we collected water at 15 sampling points and measured NO concentrations along with physico-chemical water quality parameters, e.g. temperature, salinity, dissolved inorganic nitrogen and phosphorus, and silicon. We used mixed-effect regression models to predict NO concentrations in the water from water quality parameters. We then derived NO fluxes using well-established equations of NO solubility and water-atmosphere exchanges. These fluxes were then put in perspective with those measured at the landscape scale by eddy-covariance at a 96 m nearby tall tower, and to those estimated from the agricultural land next to the fjord using Intergovernmental Panel on Climate Change (IPCC) guidelines. NO concentrations in the Roskilde Fjord ranged between 2.40 and 8.05 nmol l. The best fitting model between water parameters and NO concentrations in water included phosphorus and temperature. We estimated that (i) Roskilde Fjord was a sink of NO, with a median inward flux of -0.04 nmol m s, (ii) while the surrounding median agricultural flux was 0.13-0.18 nmol m s, and (iii) the median landscape flux was 0.07 nmol m s. All estimates of NO fluxes were of the same magnitude and consistent with each other. These preliminary results need to be consolidated by further research.