Abstract
Oxygen (O2) deficiency , i.e., dissolved O2 concentrations below 6mgO2L-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 O2 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 O2 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 O2 dynamics. We apply the expanded TBNT to the physical-biogeochemical model system HAMSOM-ECOHAM and focus our analysis on N-related O2 consumption in the southern North Sea during 2000–2014. The analysis reveals that near-bottom O2 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°N, 6.5°E), riverine N on average contributes 39% to overall near-bottom O2 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°N, 4°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 ans seasonal stratification for the O2 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 O2 levels in this region.
Highlights
The availability of dissolved oxygen (O2) in marine waters is crucial for the metabolism of many marine species (e.g., Rosenberg et al, 1991; Díaz and Rosenberg, 1995)
This section first provides a brief validation of simulated volume transports as well as spatial distributions of TN from different sources and minimum O2 concentrations, simulated by the applied model and trans-boundary nutrient transports’ (TBNT) framework
We demonstrate that the applied setup reproduces the main features of the North Sea ecosystem relevant to this study
Summary
The availability of dissolved oxygen (O2) in marine waters is crucial for the metabolism of many marine species (e.g., Rosenberg et al, 1991; Díaz and Rosenberg, 1995). The 2nd International Conference on the Protection of the North Sea declared to “aim to achieve a substantial reduction (of the order of 50%) in inputs of phosphorus (P) and nitrogen (N) to these areas between 1985 and 1995” (ICNS-2, 1988), in order to mitigate the effects of eutrophication in the North Sea. Despite significant reductions in riverine nutrients, especially for P but not for N (Artioli et al, 2008; Claussen et al, 2009; OSPAR, 2013), seasonal O2 deficiency (i.e., dissolved O2 concentration < 6 mg O2 L−1; OSPAR, 2003) still occurs regularly in the southeastern North Sea (Topcu and Brockmann, 2015). O2 deficiency remains a persistent problem in the North Sea and the investigation of its causes is essential for the ecological management
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