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

Onur Kerimoglu,Corinna Schrum,Justus E E Van Beusekom,Holger Brix,Richard Hofmeister,Yoana G Voynova,Fatemeh Chegini,Knut Klingbeil,Ulrich Callies

doi:10.5194/bg-17-5097-2020

Abstract

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 biogeochemical states of the system were 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 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 led to reduction within the nutrient-limited outer region, and it caused a widespread 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 to intensification of the thermohaline estuarine-like circulation in the Wadden Sea, both driven by the flood event.

Highlights

Riverine discharges influence the thermohaline stratification, nutrient availability and, as a result, primary production within coastal zones (e.g., Hickey et al, 2010; Cloern et al, 2014; Emeis et al, 2015)
Horizontal density gradients caused by riverine freshwater inputs govern gravitational circulation, where the seaward flow of the lighter water at the surface is counteracted by a landward flow of the saltier, denser waters near the seafloor
In this study, based on the simulations obtained with a coupled physical–biogeochemical model, we examine the physical and biogeochemical structure in the German Bight during July 2013, i.e., following a major flood event (Voynova et al, 2017), in comparison to that in the previous year, to characterize the sensitivity of the hydrodynamical and biogeochemical structure within the German Bight to the meteorological and hydrological conditions

Summary

Introduction

Riverine discharges influence the thermohaline stratification, nutrient availability and, as a result, primary production within coastal zones (e.g., Hickey et al, 2010; Cloern et al, 2014; Emeis et al, 2015). Excess amounts of riverine nutrient input cause coastal eutrophication, associated with a host of problems (Smith and Schindler, 2009), including development of dense and harmful algal blooms (e.g., Garnier et al, 2019), decline in submerged vegetation (e.g., Dolch et al, 2013), and oxygen depletion (see the review by Fennel and Testa, 2019). Horizontal density gradients caused by riverine freshwater inputs govern gravitational circulation (i.e., exchange flow), where the seaward flow of the lighter water at the surface is counteracted by a landward flow of the saltier, denser waters near the seafloor (see Burchard et al, 2018). Destabilizing and stabilizing effects of flood and ebb currents, respectively, may further enhance the gravitational circulation (Burchard and Hetland, 2010)

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