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

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.

On Using Lagrangian Drift Simulations to Aid Interpretation of in situ Monitoring Data

“…The depth of a FerryBox intake, 3-5 m (Petersen and Colijn, 2017), would capture the signature of buoyant plumes, which extend down to 10 m (Garvine and Monk, 1974;Whitney and Garvine, 2005;Cahill et al, 2008;Li et al, 2018). In 2013, the Elbe river estuary buoyant outflow extended to at least 10 m depth and was spread out over the German Bight for at least 2 months (Voynova et al, 2017;Kerimoglu et al, 2020).…”

Evaluating Uncertainties in Reconstructing the Pre-eutrophic State of the North Sea

“…For a better representation of the steep gradients both in the hydrodynamic (e.g., salinity) as well as for the biogeochemical (e.g., nutrients) conditions in the nearshore regions, we use a second model, which offers a fine-scale grid for the SNS and German Bight. This SNS setup includes GETM as hydrodynamical component (Burchard and Bolding, 2002) to which the newly established biogeochemical "Generalized Plankton Model, " GPM (Kerimoglu et al, 2020) is coupled. GPM consists of a flexible generic plankton module and an ECOHAM-based geochemical component.…”

“…The setup is established on an irregular grid with a horizontal grid cell resolution of 1.5 km in the coastal region and up to 4.5 km toward the outer boundary (Figure 1 right; Kerimoglu et al, 2017). Along the open ocean boundaries, we used clamp boundary conditions for temperature, salinity, oxygen and all variables bound to DIM, DOM and detritus pools (see Kerimoglu et al, 2020) from ECOHAM, whereas we assume zero-gradient boundary conditions for the variables bound to phytoplankton and zooplankton variables. Therefore, this nested setup is capable of taking into account changes in the river loads outside the SNS-domain and consider them in the representation of GPM.…”

“…In ECOHAM, we derived CHL from phytoplankton carbon concentration using a fixed CHL:C ratio of 50 mg Chl (g C) −1 . GPM estimates CHL prognostically, based on a photoacclimation scheme (Kerimoglu et al, 2020). DIN and DIP concentrations are calculated as average winter values from December to February and CHL as average concentration over the growing season (shortly "summer" hereafter) from March to September following OSPAR definitions.…”

Long‐term intercomparison of two pCO2 instruments based on ship‐of‐opportunity measurements in a dynamic shelf sea environment

“…The deeper northern part of the North Sea experiences seasonal stratification as summertime warming of the surface establishes a strong pycnocline (Wakelin et al 2012), while the shallower southern part is typically permanently mixed due to strong tidal forcing (Bozec et al 2005). More recent studies have defined biome boundaries on an even finer scale depending on the stratification regime and on the freshwater influence on biogeochemical processes showcasing the high spatial variability in the North Sea (van Leeuwen et al 2015;Kerimoglu et al 2020). While the Lysbris FerryBox-MBS pCO 2 measurements are available over a large area of the North Sea, in order to check and validate the long-term MBS pCO 2 record, we focus on the small restricted box in the Skagerrak where many crossovers with the SHS system on Nuka Arctica were identified.…”