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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  31 Jan 2020

31 Jan 2020

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This preprint is currently under review for the journal BG.

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

Onur Kerimoglu1,2, Yoana G. Voynova1, Fatemeh Chegini3, Holger Brix1, Ulrich Callies1, Richard Hofmeister1, Knut Klingbeil3, Corinna Schrum1, and Justus E. E. van Beusekom1 Onur Kerimoglu et al.
  • 1Institute for Coastal Research, Helmholtz-Zentrum Geesthacht, Germany
  • 2Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, Germany
  • 3Leibniz Institute for Baltic Sea Research, Warnemünde, Germany

Abstract. The German Bight was exposed to record high riverine discharges in June 2013, as a result of flooding of the Elbe and Weser rivers. Several anomalous observations suggested that the hydrodynamical and the biogeochemical state of the system was impacted by this event. In this study, we developed a biogeochemical model and coupled it with a previously introduced high resolution hydrodynamical model of the southern North Sea, in order to better characterize these impacts, and gain insight into the underlying processes. Performance of the model was assessed using an extensive set of in-situ measurements for the period 2011–2014. We first improved the realism of the hydrodynamic model with regard to the representation of cross-shore gradients, mainly through inclusion of flow-dependent horizontal mixing. Among other characteristic features of the system, the coupled model system can reproduce the low salinities, high nutrient concentrations and low oxygen concentrations in the bottom layers observed within the German Bight following the flood event. Through a scenario analysis, we examined the sensitivity of the patterns observed during July 2013 to the hydrological and meteorological forcing in isolation. Within the region of freshwater influence (ROFI) of the Elbe-Weser rivers, the flood event clearly dominated the changes in salinity and nutrient concentrations, as expected. However, our findings point out to the relevance of the peculiarities in the meteorological conditions in 2013 as well: a combination of low wind speeds, warm air temperatures and cold bottom water temperatures resulted in a strong thermal stratification in the outer regions, and limited vertical nutrient transport to the surface layers. Within the central region, the thermal and haline dynamics interactively resulted in an intense density stratification. This intense stratification, in turn, led to enhanced primary production within the central region enriched by nutrients due to the flood, but reduction within the nutrient-limited outer region, and it caused a wide-spread oxygen depletion in bottom waters. Our results further point to the enhancement of the current velocities at the surface as a result of haline stratification, and intensification of the thermohaline estuarine-like circulation at the Wadden Sea, both driven by the flood event.

Onur Kerimoglu et al.

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Onur Kerimoglu et al.

Onur Kerimoglu et al.

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Short summary
In this study, using extensive field observations and a numerical model, we analyzed the physical and biogeochemical structure of a coastal system following an extreme flood event. Our results suggest that a number of anomalous observations were driven by a co-occurrence of peculiar meteorological conditions and increased riverine discharges. Our results call for attention to the combined effects of hydrological and meteorological extremes that are anticipated to increase in frequency.
In this study, using extensive field observations and a numerical model, we analyzed the...
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