A multi-year observation of nitrous oxide at the Boknis Eck Time Series Station in the Eckernförde Bay (southwestern Baltic Sea)

Xiao Ma,Hermann W Bange,Sinikka T Lennartz

doi:10.5194/bg-16-4097-2019

Abstract

Abstract. Nitrous oxide (N2O) is a potent greenhouse gas, and it is involved in stratospheric ozone depletion. Its oceanic production is mainly influenced by dissolved nutrient and oxygen (O2) concentrations in the water column. Here we examined the seasonal and annual variations in dissolved N2O at the Boknis Eck (BE) Time Series Station located in Eckernförde Bay (southwestern Baltic Sea). Monthly measurements of N2O started in July 2005. We found a pronounced seasonal pattern for N2O with high concentrations (supersaturations) in winter and early spring and low concentrations (undersaturations) in autumn when hypoxic or anoxic conditions prevail. Unusually low N2O concentrations were observed during October 2016–April 2017, which was presumably a result of prolonged anoxia and the subsequent nutrient deficiency. Unusually high N2O concentrations were found in November 2017 and this event was linked to the occurrence of upwelling which interrupted N2O consumption via denitrification and potentially promoted ammonium oxidation (nitrification) at the oxic–anoxic interface. Nutrient concentrations (such as nitrate, nitrite and phosphate) at BE have been decreasing since the 1980s, but oxygen concentrations in the water column are still decreasing. Our results indicate a close coupling of N2O anomalies to O2 concentration, nutrients, and stratification. Given the long-term trends of declining nutrient and oxygen concentrations at BE, a decrease in N2O concentration, and thus emissions, seems likely due to an increasing number of events with low N2O concentrations.

Highlights

Long-term observation with regular measurement intervals can be an effective way to monitor seasonal and inter-annual variabilities as well as to decipher short- and long-term trends of an ecosystem, which are required to make projections of the future ecosystem development
The density gradient of the water column in September 2017 was much lower than in other years. These results indicate the occurrence of an upwelling event at Boknis Eck (BE) Time Series Station in autumn 2017, which might be a result of the saline water inflow from the North Sea considering the change of salinity in the water column (Fig. S1)
The seasonal and inter-annual N2O variations at the BE Time Series Station from July 2005 to December 2017 were driven by the prevailing O2 regime and nutrient availability

Summary

Introduction

Long-term observation with regular measurement intervals can be an effective way to monitor seasonal and inter-annual variabilities as well as to decipher short- and long-term trends of an ecosystem, which are required to make projections of the future ecosystem development (e.g. see Ducklow et al, 2009). Multi-year time series measurements of nitrous oxide (N2O), a potent greenhouse gas and a major threat to ozone depletion (IPCC, 2013; Ravishankara et al, 2009), have been reported from the coastal upwelling areas off central Chile (Farías et al, 2015), off Goa (Naqvi et al, 2010), in the North Pacific Subtropical Gyre (Wilson et al, 2017), and in Saanich Inlet (Capelle et al, 2018). N2O could be further consumed via denitrification to dinitrogen; this process is inhibited with the presence of O2 because of the low O2 tolerance of the enzyme involved (Bonin et al, 1989). This incomplete pathway is called partial denitrification and can lead to N2O accumulation (e.g. Naqvi et al, 2000; Farías et al, 2009)

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