Oxygen (O$_2$) deficiency, i.e., dissolved O$_2$ concentrations below 6\,mg\,O$_2$\,L$^{-1}$, is a common feature in the southern North Sea. Its evolution is governed mainly by the presence of seasonal stratification and production of organic matter, which is subsequently degraded under O$_2$ consumption. The latter is strongly influenced by riverine nutrient loads, i.e., nitrogen (N) and phosphorus (P). As riverine P loads have been reduced significantly over the past decades, this study aims for the quantification of the influence of riverine and non-riverine N inputs on the O$_2$ dynamics in the southern North Sea. For this purpose, we present an approach to expand a nutrient-tagging technique for physical-biogeochemical models – often referred to as ‘trans-boundary nutrient transports’ (TBNT) – by introducing a direct link to the O$_2$ dynamics. We apply the expanded TBNT to the physical-biogeochemical model system HAMSOM-ECOHAM and focus our analysis on N-related O$_2$ consumption in the southern North Sea during 2000–2014. The analysis reveals that near-bottom O$_2$ consumption in the southern North Sea is strongly influenced by the N supply from the North Atlantic across the northern shelf edge. However, riverine N sources — especially the Dutch, German and British rivers — as well as the atmosphere also play an important role. In the region with lowest simulated O2 concentrations (around 56\,$^\circ$N, 6.5\,$^\circ$E), riverine N on average contributes 39\% to overall near-bottom O$_2$ consumption during seasonal stratification. Here, the German and the large Dutch rivers constitute the highest riverine contributions (11\% and 10\%, respectively). At a site in the Oyster Grounds (around 54.5\,$^\circ$N, 4\,$^\circ$E), the average riverine contribution adds up to 41\%, even exceeding that of the North Atlantic. Here, highest riverine contributions can be attributed to the Dutch and British rivers adding up to almost 28\% on average. The atmospheric contribution results in 13\%. Our results emphasize the importance of anthropogenic N inputs and seasonal stratification for the O$_2$ conditions in the southern North Sea. They further suggest that reductions in the riverine and atmospheric N inputs may have a relevant positive effect on the O$_2$ levels in this region.

@article{ANMATQTION17,
	author	 = {Fabian Große and Markus Kreus and Hermann Lenhart and Johannes Pätsch and Thomas Pohlmann},
	title	 = {{A Novel Modeling Approach to Quantify the Influence of Nitrogen Inputs on the Oxygen Dynamics of the North Sea}},
	year	 = {2017},
	editor	 = {Christophe Rabouille},
	publisher	 = {Frontiers},
	address	 = {Avenue du Tribunal Fédéral 34, CH-1005 Lausanne, Switzerland},
	journal	 = {Frontiers in Marine Science},
	series	 = {4},
	pages	 = {383},
	doi	 = {http://dx.doi.org/10.3389/fmars.2017.00383},
	abstract	 = {Oxygen (O$_2$) deficiency, i.e., dissolved O$_2$ concentrations below 6\,mg\,O$_2$\,L$^{-1}$, is a common feature in the southern North Sea. Its evolution is governed mainly by the presence of seasonal stratification and production of organic matter, which is subsequently degraded under O$_2$ consumption. The latter is strongly influenced by riverine nutrient loads, i.e., nitrogen (N) and phosphorus (P). As riverine P loads have been reduced significantly over the past decades, this study aims for the quantification of the influence of riverine and non-riverine N inputs on the O$_2$ dynamics in the southern North Sea. For this purpose, we present an approach to expand a nutrient-tagging technique for physical-biogeochemical models -- often referred to as ‘trans-boundary nutrient transports’ (TBNT) -- by introducing a direct link to the O$_2$ dynamics. We apply the expanded TBNT to the physical-biogeochemical model system HAMSOM-ECOHAM and focus our analysis on N-related O$_2$ consumption in the southern North Sea during 2000--2014. The analysis reveals that near-bottom O$_2$ consumption in the southern North Sea is strongly influenced by the N supply from the North Atlantic across the northern shelf edge. However, riverine N sources — especially the Dutch, German and British rivers — as well as the atmosphere also play an important role. In the region with lowest simulated O2 concentrations (around 56\,$^\circ$N, 6.5\,$^\circ$E), riverine N on average contributes 39\\% to overall near-bottom O$_2$ consumption during seasonal stratification. Here, the German and the large Dutch rivers constitute the highest riverine contributions (11\\% and 10\\%, respectively). At a site in the Oyster Grounds (around 54.5\,$^\circ$N, 4\,$^\circ$E), the average riverine contribution adds up to 41\\%, even exceeding that of the North Atlantic. Here, highest riverine contributions can be attributed to the Dutch and British rivers adding up to almost 28\\% on average. The atmospheric contribution results in 13\\%. Our results emphasize the importance of anthropogenic N inputs and seasonal stratification for the O$_2$ conditions in the southern North Sea. They further suggest that reductions in the riverine and atmospheric N inputs may have a relevant positive effect on the O$_2$ levels in this region.},
	url	 = {https://www.frontiersin.org/articles/10.3389/fmars.2017.00383/pdf},
}