Seasonal variability of nitrous oxide concentrations and emissions in a temperate estuary

Abstract

Abstract. Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and its emissions along the Elbe Estuary (Germany), a well-mixed temperate estuary with high nutrient loading from agriculture. During nine research cruises performed between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrient and oxygen concentrations along the estuary, and calculated N2O saturations, flux densities, and emissions. We found that the estuary was a year-round source of N2O, with the highest emissions in winter when dissolved inorganic nitrogen (DIN) loads and wind speeds are high. However, in spring and summer, N2O saturations and emissions did not decrease alongside lower riverine nitrogen loads, suggesting that estuarine in situ N2O production is an important source of N2O. We identified two hotspot areas of N2O production: the Port of Hamburg, a major port region, and the mesohaline estuary near the maximum turbidity zone (MTZ). N2O production was fueled by the decomposition of riverine organic matter in the Hamburg Port and by marine organic matter in the MTZ. A comparison with previous measurements in the Elbe Estuary revealed that N2O saturation did not decrease alongside the decrease in DIN concentrations after a significant improvement of water quality in the 1990s that allowed for phytoplankton growth to re-establish in the river and estuary. The overarching control of phytoplankton growth on organic matter and, subsequently, on N2O production highlights the fact that eutrophication and elevated agricultural nutrient input can increase N2O emissions in estuaries.

Dissolved Nitric Oxide in the Lower Elbe Estuary and the Hamburg Port Area

“…Recently, Gong et al (2023) noted nitrification as the major contributor to microbial production of NO in the coastal seas of the Yellow and East China Seas. Prior studies suggest nitrification as the main production pathway of N2O in the Elbe Estuary (Brase et al, 2017;Schulz et al, 2023). In this study, we noted that nitrification occurs in the entire stretch of the study site based on the plots of AOU and excess N2O (Fig.…”

“…The N2O concentrations (Fig. 3a) ranged from 9.1 nM (Elbe-km 737) to 38.0 nM (Elbe-km 628.04), with a mean ± SD concentration of 18.0 ± 6.5 nM and a median concentration of 16.6 nM (see also Schulz et al, 2023). Enhanced N2O concentrations (mean: 23.9 ± 7.1 nM) were measured in the Hamburg Port area (Fig.…”

“…Calculations for apparent oxygen utilization (AOU), N2O saturation, N2O sea(water)-to-air flux density, and excess N2O (ΔN2O) are discussed in Schulz et al (2023). The data in this study were visualized using Origin 10.5.117 and MATLAB…”

“…Additionally, in the Hamburg Port area, the decomposition of phytoplankton from the Elbe River may contribute to increased NO2 − and NH4 + concentrations due to remineralization. This could subsequently lead to oxygen depletion, especially during the warmer summer months, coinciding with increased microbial activity that could intensify nitrogen transformation processes (Schulz et al, 2023;Sanders et al, 2018). No comprehensive study to date has examined the seasonal dynamics of NO concentrations in such contexts.…”

“…Inc., San Jose, CA, USA) coupled with a (sea)water/gas equilibrator. The details of the N2O measurements are described in Schulz et al (2023).…”