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

• Main Authors: X. Ma, S. T. Lennartz, H. W. Bange
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-10-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/4097/2019/bg-16-4097-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p>Nitrous oxide (<span class="inline-formula">N₂O</span>) is a potent greenhouse gas, and it is involved in stratospheric ozone depletion. Its oceanic production is mainly influenced by dissolved nutrient and oxygen (<span class="inline-formula">O₂</span>) concentrations in the water column. Here we examined the seasonal and annual variations in dissolved <span class="inline-formula">N₂O</span> at the Boknis Eck (BE) Time Series Station located in Eckernförde Bay (southwestern Baltic Sea). Monthly measurements of <span class="inline-formula">N₂O</span> started in July 2005. We found a pronounced seasonal pattern for <span class="inline-formula">N₂O</span> with high concentrations (supersaturations) in winter and early spring and low concentrations (undersaturations) in autumn when hypoxic or anoxic conditions prevail. Unusually low <span class="inline-formula">N₂O</span> concentrations were observed during October 2016–April 2017, which was presumably a result of prolonged anoxia and the subsequent nutrient deficiency. Unusually high <span class="inline-formula">N₂O</span> concentrations were found in November 2017 and this event was linked to the occurrence of upwelling which interrupted <span class="inline-formula">N₂O</span> 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 <span class="inline-formula">N₂O</span> anomalies to <span class="inline-formula">O₂</span> concentration, nutrients, and stratification. Given the long-term trends of declining nutrient and oxygen concentrations at BE, a decrease in <span class="inline-formula">N₂O</span> concentration, and thus emissions, seems likely due to an increasing number of events with low <span class="inline-formula">N₂O</span> concentrations.</p>