Spatial variations in sedimentary N-transformation rates in the North Sea (German Bight)

Abstract

<p><strong>Abstract.</strong> In this study, we investigate the role of sedimentary N cycling in the Southern North Sea. We present a budget of ammonification, nitrification and sedimentary NO<sub>3</sub><sup>−</sup> consumption/denitrification in contrasting sediment types of the German Bight (Southern North Sea), including novel net ammonification rates. Dissolved inorganic nitrogen concentration (nitrate, nitrite and ammonium) in the water column showed low levels between 0.2 to 3.2 µmol L<sup>−1</sup>. We incubated sediment cores with labeled nitrate and ammonium to calculate net and gross N transformation rates. The results show that impermeable sediments are the main site of ammonification (on average 10.2 ± 1.2 mmol m<sup>−2</sup> d<sup>−1</sup>) and that they are an important source for primary producers in the water column, contributing ~ 17 to 61 % of reactive nitrogen in the water column. Ammonification and oxygen penetration depth are the main drivers of sedimentary nitrification. One third of freshly produced nitrate in impermeable sediment and two-thirds in permeable sediment were reduced to N<sub>2</sub>. The semi-permeable and permeable sediments are responsible for ~ 80 % of the total benthic N<sub>2</sub> production rates (~ 890 t N d<sup>−1</sup>) in the southern North Sea. We conclude that impermeable sediments are important sources of reactive N and that semi-permeable and permeable sediments are the main sinks of reactive N, counteracting eutrophication in the southern North Sea (German Bight).</p>