Spatial and seasonal variation of dissolved nitrous oxide along the Elbe estuary
Authors: Schulz Gesa , Sanders Tina , Dähnke Kirstin 
Published: March 2022

&lt;p&gt;Nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) is a greenhouse gas contributing to global warming. Estuaries are a potential source for N&lt;sub&gt;2&lt;/sub&gt;O.&amp;#160; We aimed to identify seasonal and spatial variations of N&lt;sub&gt;2&lt;/sub&gt;O production and emission along the Elbe estuary in Germany.&lt;/p&gt;&lt;p&gt;Between 2015 and 2021, we performed nine research cruises along the Elbe estuary. Most of the cruises took place in growing seasons (April &amp;#8211; September), while one cruise was conducted in winter (early March). We continuously measured the dissolved N&lt;sub&gt;2&lt;/sub&gt;O dry mole fraction 2&amp;#160;m below the surface using a laser-based analyzer coupled with an equilibrator. Based on these profiles, we calculated N&lt;sub&gt;2&lt;/sub&gt;O concentration, saturation and emissions.&lt;/p&gt;&lt;p&gt;During all cruises, the Elbe estuary was supersaturated in N&lt;sub&gt;2&lt;/sub&gt;O. Highest N&lt;sub&gt;2&lt;/sub&gt;O concentration occurred in the Hamburg port region, a hotspot of N&lt;sub&gt;2&lt;/sub&gt;O production by nitrification in the water column and denitrification in the sediments. The maximum concentration in this region was 158 nmol&amp;#160;L&lt;sup&gt;&amp;#8209;1&lt;/sup&gt; in March 2021. Nitrification in the maximum turbidity zone (MTZ) produced a second local N&lt;sub&gt;2&lt;/sub&gt;O maximum. &amp;#160;Average N&lt;sub&gt;2&lt;/sub&gt;O emissions were 0.19&amp;#160;Gg&amp;#160;a&lt;sup&gt;&amp;#8209;1&lt;/sup&gt;(0.52 Mg d&lt;sup&gt;-1&amp;#173;&lt;/sup&gt;) during the growing season. The N&lt;sub&gt;2&lt;/sub&gt;O emission was highest in winter with 0.64 Gg a&lt;sup&gt;-1&lt;/sup&gt; (1.76 Mg d&lt;sup&gt;-1&lt;/sup&gt;).&lt;/p&gt;&lt;p&gt;During growing seasons emissions were strongly correlated with pH (R&lt;sup&gt;2&lt;/sup&gt; = 0.73) and suspended particulate matter concentration (R&lt;sup&gt;2&lt;/sup&gt; = 0.55). A trend toward higher N&lt;sub&gt;2&lt;/sub&gt;O saturations and emissions during cruises in summer is evident. We presume that N&lt;sub&gt;2&lt;/sub&gt;O saturation and emission were likely driven by temperature-dependent turnover processes in high turbidity areas of the Elbe estuary, such as nitrification and denitrification. &amp;#160;&lt;/p&gt;&lt;p&gt;However, the maximum N&lt;sub&gt;2&lt;/sub&gt;O concentrations in winter (March 2021) cannot be explained that way, because water temperature was low. N&lt;sub&gt;2&lt;/sub&gt;O 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, N&lt;sub&gt;2&lt;/sub&gt;O production in this case may be less sensitive to water temperature, possibly due to sedimentary sources. Second, a sink for N&lt;sub&gt;2&lt;/sub&gt;O 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. &amp;#160;&lt;/p&gt;&lt;p&gt;Overall, seasonality affects N&lt;sub&gt;2&lt;/sub&gt;O production in the Hamburg port region more than in the maximum turbidity zone. In late spring/summer, N&lt;sub&gt;2&lt;/sub&gt;O production is driven mainly by enhanced microbial productivity. High N&lt;sub&gt;2&lt;/sub&gt;O concentrations in colder seasons may result from high DIN concentration, but further research on the controls on N&lt;sub&gt;2&lt;/sub&gt;O production, and possibly consumption, is clearly needed.&lt;/p&gt;