Abstract
Abstract. 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 NO3- consumption and denitrification in contrasting sediment types of the German Bight (southern North Sea), including novel net ammonification rates. We incubated sediment cores from four representative locations in the German Bight (permeable, semi-permeable and impermeable sediments) with labeled nitrate and ammonium to calculate benthic fluxes of nitrate and ammonium and gross rates of ammonification and nitrification. Ammonium fluxes generally suggest oxic degradation of organic matter, but elevated fluxes at one sampling site point towards the importance of bioirrigation or short-term accumulation of organic matter. Sedimentary fluxes of dissolved inorganic nitrogen are an important source for primary producers in the water column, supporting ∼7 % to 59 % of the average annual primary production, depending on water depth. We find that ammonification and oxygen penetration depth are the main drivers of sedimentary nitrification, but this nitrification is closely linked to denitrification. One-third of freshly produced nitrate in impermeable sediment and two-thirds in permeable sediment were reduced to N2. The semi-permeable and permeable sediments are responsible for ∼68 % of the total benthic N2 production rates, which, based solely on our data, amounts to ∼1030 t N d−1 in the southern North Sea. Thus, we conclude that semi-permeable and permeable sediments are the main sinks of reactive N, counteracting eutrophication in the southern North Sea (German Bight).
Highlights
The continental shelves and coastal margins make up for < 9 % of the total area of ocean surface but are responsible for vast majority of the biogeochemical cycling both in the water column and in the sediments (Jorgensen, 1983)
The highest net ammonium flux and gross ammonification rates were measured in the impermeable, organic-rich sediment at the station North Sea Observation and Assessment of Habitats (NOAH)-C (6.6 ± 1.4 and 9.5 mmol N m2 d−1 for net flux and gross ammonification, respectively)
The lowest net ammonium fluxes were measured in the semi-impermeable sediment at the station NOAH-D (0.5 ± 0.1 mmol N m−2 d−1)
Summary
The German Bight is part of the southern North Sea, is bordered by densely populated and industrialized countries, and receives large amounts of nutrients via river discharge (e.g., Rhine, Maas, Elbe, Weser, Ems) (Los et al, 2014). This caused clear eutrophication symptoms such as phytoplankton blooms, oxygen deficiencies and macrobenthos kills especially during the 1980s (Hickel et al, 1993; von Westernhagen et al, 1986) in the North Sea. In the adjacent Wadden Sea intense phytoplankton blooms, a possible decrease in seagrass and massive blooms of opportunistic macroalgae were attributed to eutrophication (e.g., Cadée and Hegemann, 2002). Since the mid-1980s, the nitrogen (N) loads into the German Bight have been decreasing, but the entire SE North Sea is still flagged as an eutrophication problem area (OSPAR, 2010)
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