Abstract. Metabolic activities in estuaries, especially these of large rivers, profoundly affect the downstream coastal biogeochemistry. Here, we unravel the impacts of large industrial port facilities, showing that elevated metabolic activity in the Hamburg port (Germany) increases total alkalinity (TA) and dissolved inorganic carbon (DIC) runoff to the North Sea. The imports of particulate inorganic carbon, particulate organic carbon, and particulate organic nitrogen (PIC, POC, and PON) from the upstream Elbe River can fuel up to 90 % of the TA generated in the entire estuary via calcium carbonate (CaCO3) dissolution. The remaining at least 10 % of TA generation can be attributed to anaerobic metabolic processes such as denitrification of remineralized PON or other pathways. The Elbe Estuary as a whole adds approximately 15 % to the overall DIC and TA runoff. Both the magnitude and partitioning among these processes appear to be sensitive to climatic and anthropogenic changes. Thus, with increased TA loads, the coastal ocean (in particular) would act as a stronger CO2 sink, resulting in changes to the overall coastal system's capacity to store CO2.

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<p>The Earth system has entered a new geological epoch, the Anthropocene. The oceans’ capacity to regulate atmospheric carbon dioxide (CO 2 ) at various<br />timescales is amongst the most crucial players to maintain climate on Earth in a habitable range. The biogeochemical property exerting this regulatory mechanism is alkalinity, the oceans’ CO 2 and pH buffer capacity. The proposed project will investigate how the oceans’ alkalinity is impacted firstly by human measures, required by the Paris agreement (COP 21) to mitigate climate change via bioenergy production and its downstream effects on shallow oceans, and secondly by climate change, in particular by increased weathering in the Arctic because of ice retreat.</p>

<p><strong>Abstract: EGU 2020</strong></p><p><strong>Session: BG4.1: Biogeochemistry of coastal seas and continental shelves (Helmuth Thomas)</strong></p><p>Mona Norbisrath<sup>1</sup>, Kirstin Dähnke<sup>1</sup>, Andreas Neumann<sup>1</sup>, Justus van Beusekom<sup>1</sup>, Nele Treblin<sup>1</sup>, Bryce van Dam<sup>1</sup>, Helmuth Thomas<sup>1</sup></p><p><sup>1</sup>Institute for Coastal Research, Helmholtz-Zentrum Geesthacht</p><p>Contact: mona.norbisrath@hzg.de</p><p> </p><p><strong>In-situ investigation of alkalinity - denitrification coupling in the sediment - water column interface</strong></p><p> </p><p>As a shallow shelf sea, the North Sea is very vulnerable to anthropogenic impacts like rising CO<sub>2</sub> concentrations, increasing nutrient inflows and coincident oxygen loss.</p><p>Two important processes that determine the role of the coastal ocean as a net sink for anthropogenic CO<sub>2</sub> are alkalinity and denitrification. Alkalinity, the acid binding capacity of the ocean, buffers natural and anthropogenic changes in the oceans’ CO<sub>2</sub> and pH system. Denitrification, an anaerobic microbial process in which organic matter is respired, uses NO<sub>3</sub><sup>-</sup> instead of O<sub>2</sub> as a terminal electron acceptor. Denitrification reduces NO<sub>3</sub><sup>-</sup> to N<sub>2</sub> and in turn produces alkalinity.</p><p>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 CO<sub>2</sub>. However, the quantitative relationship between denitrification and alkalinity production and its control under changing environmental conditions is yet to be determined.</p><p>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 N<sub>2</sub> isotope species (<sup>28</sup>N<sub>2</sub>, <sup>29</sup>N<sub>2</sub> and <sup>30</sup>N<sub>2</sub>) 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 CO<sub>2</sub> in the atmosphere.</p><p> </p>