Status: this preprint was under review for the journal BG but the revision was not accepted.
Nitrogen cycling in the Elbe estuary from a joint 3D-modelling and observational perspective
Johannes Pein,Annika Eisele,Richard Hofmeister,Tina Sanders,Ute Daewel,Emil V. Stanev,Justus van Beusekom,Joanna Staneva,and Corinna Schrum
Abstract. The study addresses the nitrogen cycling in Elbe estuary. Observations of salinity, nutrients and oxygen from moored stations, ship casts and helicopter surveys are presented. Observations are complemented by simulations obtained from a coupled physical-biogeochemical 3D unstructured model, applied for the first time to the estuarine environment. Model simulations reproduce the temporal variability of nutrients and oxygen along the estuarine salinity gradient. Both, observations and model results, demonstrate mostly conservative mixing of nitrate and non-conservative behavior of ammonium. Model hind-casts of the years 2012 and 2013 provide a detailed reconstruction of nitrogen recycling with ammonium appearing as the key species of the remineralisation process. Estuarine turnover processes are fueled by inputs of diatoms and organic nitrogen at the tidal weir with intense primary production manifest in the shallow river section downstream of the weir. The harbor area is the hot spot of heterotrophic decay associated with growth of meso-zooplankton, sedimentation of degradable material, remineralisation, oxygen depletion, denitrification and ammonium production. In the harbor, biochemistry shows strong vertical gradients while hydrodynamics demonstrate connectivity between the main channel and the harbor. At the estuary bed nitrogen is deposited during spring and early summer. Resuspension leads to nearly closed budget by the end of the year. During the Elbe flood in June 2013, estuarine biogeochemistry is significantly disturbed with the harbor being deactivated as hot spot of heterotrophic decay. Plankton and organic matter are flushed towards the outer estuary which in consequence sees high abundance of grazers, oxygen depletion and elevated release of ammonium.
How to cite. Pein, J., Eisele, A., Hofmeister, R., Sanders, T., Daewel, U., Stanev, E. V., van Beusekom, J., Staneva, J., and Schrum, C.: Nitrogen cycling in the Elbe estuary from a joint 3D-modelling and observational perspective, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2019-265, 2019.
The Elbe estuary is subject to vigorous tidal forcing from the sea side and considerable biological inputs from the land side. Our 3D numerical coupled physical-biogeochemical integrates these forcing signals and provides highly realistic hindcasts of the associated dynamics. Model simulations show that the freshwater part of Elbe estuary is inhabited by plankton. According to simulations these organism play a key role in converting organic inputs into nitrate, the major inorganic nutrient.
The Elbe estuary is subject to vigorous tidal forcing from the sea side and considerable...