02 Dec 2021

02 Dec 2021

Review status: this preprint is currently under review for the journal BG.

Suspended Particulate Matter drives the spatial segregation of nitrogen turnover along the hyper-turbid Ems estuary

Gesa Schulz1,2, Tina Sanders1, Justus E. E. van Beusekom2,3, Yoana G. Voynova2, Andreas Schöl4, and Kirstin Dähnke2 Gesa Schulz et al.
  • 1Institute of Geology, Center for Earth System Research and Sustainability (CEN), University Hamburg, Hamburg, 20146, Germany
  • 2Institute of Carbon Cycles, Helmholtz-Zentrum Hereon, Geesthacht, 21502, Germany
  • 3Institute of Oceanography, University Hamburg, Hamburg, 20146, Germany
  • 4Department of Microbial Ecology, Federal Institute of Hydrology, Koblenz, 56068, Germany

Abstract. Estuaries are nutrient filters and change riverine nutrient loads before they reach coastal oceans. They have been extensively changed by anthropogenic activities like draining, deepening, and dredging to meet economic and social demand, causing significant regime changes like tidal amplifications and in some cases to hyper-turbid conditions. Furthermore, increased nutrient loads, especially nitrogen, mainly by agriculture cause coastal eutrophication. Estuaries can either act as a sink or as a source of nitrate, depending on environmental and geomorphological conditions. These factors vary along an estuary, and change nitrogen turnover in the system. Here, we investigate the factors controlling nitrogen turnover in the hyper-turbid Ems estuary (Northern Germany) that has been strongly impacted by human activities. During two research cruises in August 2014 and June 2020, we measured water column properties, dissolved inorganic nitrogen, dual stable isotopes of nitrate and dissolved nitrous oxide concentration along the estuary. Overall, the Ems estuary acts as a nitrate sink in both years. However, three distinct biogeochemical zones exist along the estuary. A strong fractionation (~ 26 ‰) of nitrate stable isotopes points towards nitrate removal via water column denitrification in the hyper-turbid Tidal River, driven by anoxic conditions in deeper water layers. In the Middle Reaches of the estuary nitrification gains in importance turning this section into a net nitrate source. The Outer Reaches are dominated by mixing with nitrate uptake in 2020.

We find that the overarching control on biogeochemical nitrogen cycling, zonation and nitrous oxide production in the Ems estuary is exerted by suspended particulate matter concentrations and the linked oxygen deficits.

Gesa Schulz et al.

Status: open (until 03 Feb 2022)

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  • RC1: 'Comment on bg-2021-321', Anonymous Referee #1, 04 Jan 2022 reply

Gesa Schulz et al.

Gesa Schulz et al.


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Short summary
Estuaries can significantly alter nutrient loads before reaching coastal waters. Our study of the heavily managed Ems estuary reveals three zones of nitrogen turnover along the estuary. Overall, the Ems acted as a nitrate sink with water-column denitrification in the most upstream hyper-turbid part, nitrate production in the Middle Reaches and mixing/nitrate uptake in the North Sea. Suspended particulate matter was the overarching control on nitrogen cycling in the hyper-turbid estuary.