In-situ investigation of alkalinity - denitrification coupling in the sediment - water column interface

Abstract: EGU 2020

Session: BG4.1: Biogeochemistry of coastal seas and continental shelves (Helmuth Thomas)

Mona Norbisrath¹, Kirstin Dähnke¹, Andreas Neumann¹, Justus van Beusekom¹, Nele Treblin¹, Bryce van Dam¹, Helmuth Thomas¹

¹Institute for Coastal Research, Helmholtz-Zentrum Geesthacht

Contact: mona.norbisrath@hzg.de

 

In-situ investigation of alkalinity - denitrification coupling in the sediment - water column interface

 

As a shallow shelf sea, the North Sea is very vulnerable to anthropogenic impacts like rising CO₂ concentrations, increasing nutrient inflows and coincident oxygen loss.

Two important processes that determine the role of the coastal ocean as a net sink for anthropogenic CO₂ are alkalinity and denitrification. Alkalinity, the acid binding capacity of the ocean, buffers natural and anthropogenic changes in the oceans’ CO₂ and pH system. Denitrification, an anaerobic microbial process in which organic matter is respired, uses NO₃^- instead of O₂ as a terminal electron acceptor. Denitrification reduces NO₃^- to N₂ and in turn produces alkalinity.

Eutrophication, caused by leaching of excess fertilizer nutrients into coastal seas, leads to enhanced denitrification and therefore to enhanced alkalinity as well as an increased uptake of CO₂. However, the quantitative relationship between denitrification and alkalinity production and its control under changing environmental conditions is yet to be determined.

In the German Bight, denitrification is usually restricted to anoxic sediments. In this study, we therefore focus on in-situ experiments in the sediment - water column interface. Batch core incubations in combination with the isotope pairing technique (IPT) and labelled nitrate additions were used to detect denitrification and gauge its effect on alkalinity production during a cruise on RV Heincke (HE541) in September 2019 in the German Bight. To quantify denitrification, the production of all three N₂ isotope species (²⁸N₂, ²⁹N₂ and ³⁰N₂) is measured using a membrane inlet mass spectrometer (MIMS). We expect an increase of denitrification rates with nitrate concentrations and incubation times, and we will quantify benthic denitrification. We will further evaluate the assumption of concurrent increases in alkalinity production and will investigate the benthic-pelagic coupling of these processes. Investigating the in-situ interaction of metabolic alkalinity and denitrification will give an estimation of the alkalinity impact on the reduction of anthropogenic CO₂ in the atmosphere.