Spatial and seasonal variation of dissolved nitrous oxide along the Elbe estuary

Nitrous oxide (N₂O) is a greenhouse gas contributing to global warming. Estuaries are a potential source for N₂O.  We aimed to identify seasonal and spatial variations of N₂O production and emission along the Elbe estuary in Germany.

Between 2015 and 2021, we performed nine research cruises along the Elbe estuary. Most of the cruises took place in growing seasons (April – September), while one cruise was conducted in winter (early March). We continuously measured the dissolved N₂O dry mole fraction 2 m below the surface using a laser-based analyzer coupled with an equilibrator. Based on these profiles, we calculated N₂O concentration, saturation and emissions.

During all cruises, the Elbe estuary was supersaturated in N₂O. Highest N₂O concentration occurred in the Hamburg port region, a hotspot of N₂O production by nitrification in the water column and denitrification in the sediments. The maximum concentration in this region was 158 nmol L^‑1 in March 2021. Nitrification in the maximum turbidity zone (MTZ) produced a second local N₂O maximum.  Average N₂O emissions were 0.19 Gg a^‑1(0.52 Mg d^-1­) during the growing season. The N₂O emission was highest in winter with 0.64 Gg a^-1 (1.76 Mg d^-1).

During growing seasons emissions were strongly correlated with pH (R² = 0.73) and suspended particulate matter concentration (R² = 0.55). A trend toward higher N₂O saturations and emissions during cruises in summer is evident. We presume that N₂O saturation and emission were likely driven by temperature-dependent turnover processes in high turbidity areas of the Elbe estuary, such as nitrification and denitrification.  

However, the maximum N₂O concentrations in winter (March 2021) cannot be explained that way, because water temperature was low. N₂O production may be driven by the dissolved inorganic nutrient (DIN) load, which is more than doubled in comparison to all other cruises. Two other possible explanations come to mind: First, N₂O production in this case may be less sensitive to water temperature, possibly due to sedimentary sources. Second, a sink for N₂O in the water column may exist, which is more active during higher temperatures. These two scenarios may both apply and might interact over the course of the year.  

Overall, seasonality affects N₂O production in the Hamburg port region more than in the maximum turbidity zone. In late spring/summer, N₂O production is driven mainly by enhanced microbial productivity. High N₂O concentrations in colder seasons may result from high DIN concentration, but further research on the controls on N₂O production, and possibly consumption, is clearly needed.