Nitrogen cycling in the Elbe estuary from a joint 3D-modelling and observational perspective

Abstract

<p><strong>Abstract.</strong> 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.</p>

Interactive impacts of meteorological and hydrological conditions on the physical and biogeochemical structure of a coastal system

“…1) for temperature, salinity and oxygen (the latter only at Deutsche Bucht) were downloaded from the COSYNA (Coastal Observing System for Northern and Arctic Seas) data portal (https://www.cosyna.de, last access: 19 October 2020; see Breitbach et al, 2016) at daily resolution (snapshots at 00:00 local time averaged within an hourly time window). Collection and processing of the semicontinuous data collected by FerryBox platforms at the Cuxhaven and Helgoland monitoring stations and on the M/V Funny Girl ferry operating between Büsum and Helgoland during May-September have been described previously by Petersen et al (2011) and Voynova et al (2017) and are available from the COSYNA data portal as well.…”

Internal Model Variability of the Regional Coupled System Model GCOAST-AHOI

“…In order to investigate the impact of the coupling on the RCM's IV, we use the coupled model setup AHOI (Atmosphere-Hydrology-Ocean-Sea Ice), which has been developed as a part of the coastal model framework GCOAST (Geesthacht Coupled cOAstal model SysTem) [32,33]. GCOAST-AHOI (in short AHOI) comprises a regional atmospheric model, a hydrological discharge model, and a regional ocean model with a sea ice model included.…”

Interactive impacts of meteorological and hydrological conditions on the physical and biogeochemical structure of a coastal system

“…Since the first 3D models of the North Sea (Backhaus, 1985;Dippner, 1993;Schrum, 1997), computational capacity has been significantly improved, which resulted in development of ever finer resolution setups that can resolve meso-scale features 320 such as the coastal freshwater fronts and baroclinic eddies (Holt and James, 2006;Pohlmann, 2006;Staneva et al, 2009;Pätsch et al, 2017) and the smaller-scale dynamics, such as the estuarine mixing Stanev et al, 2019). For large-domain biogeochemical applications that require a costly calculation of transport of dozens of additional state variables, the coarse-resolution models (10-20 km) are being actively used (e.g., Ford et al, 2017;Große et al, 2016;Daewel et al, 2019). With a spatial resolution of 1.5-4.5 km covering the southern North Sea (Fig.…”