Regional carbon and CO2 budgets of North Sea tidal estuaries

https://doi.org/10.1016/j.ecss.2016.04.007Get rights and content

Highlights

  • First estuarine carbon budget at regional scale estimated by a RTM approach.

  • Regional estuarine organic carbon filtering capacity is high (i.e. 76%).

  • Regional estuarine total carbon filtering capacity is low (i.e. 15%).

  • Regional OC:IC ratio reflects the drainage of carbonate-rich catchments.

  • Contribution of oversaturated riverine waters to the CO2 dynamics can be large.

Abstract

This study presents the first regional application of the generic estuarine reactive-transport model C-GEM (Carbon-Generic Estuary Model) that is here combined with high-resolution databases to produce a carbon and CO2 budget for all tidal estuaries discharging into the North Sea. Steady-state simulations are performed for yearly-averaged conditions to quantify the carbon processing in the six main tidal estuaries Elbe, Ems, Humber, Scheldt, Thames, and Weser, which show contrasted physical and biogeochemical dynamics and contribute the most to the regional filter. The processing rates derived from these simulations are then extrapolated to the riverine carbon loads of all the other North Sea catchments intercepted by smaller tidal estuarine systems. The Rhine-Meuse estuarine system is also included in the carbon budget and overall, we calculate that the export of organic and inorganic carbon from tidal estuaries to the North sea amounts to 44 and 409 Gmol C yr−1, respectively, while 41 Gmol C are lost annually through CO2 outgassing. The carbon is mostly exported from the estuaries in its inorganic form (>90%), a result that reflects the low organic/inorganic carbon ratio of the riverine waters, as well as the very intense decomposition of organic carbon within the estuarine systems. Our calculations also reveal that with a filtering capacity of 15% for total carbon, the contribution of estuaries to the CO2 outgassing is relatively small. Organic carbon dynamics is dominated by heterotrophic degradation, which also represents the most important contribution to the estuarine CO2 evasion. Nitrification only plays a marginal role in the CO2 dynamics, while the contribution of riverine oversaturated waters to the CO2 outgassing is generally significant and strongly varies across systems.

Introduction

Estuaries are morphologically complex ecosystems that act as important modulators of the carbon and bio-associated element fluxes (N, P, Si) from the land to the ocean (e.g. Gattuso et al., 1998, Mackenzie et al., 2005, Bauer et al., 2013, Regnier et al., 2013a). The most recent global estimates report that 0.15 Pg C are lost from the land-ocean aquatic continuum every year through CO2 outgassing from estuaries (Laruelle et al., 2013) and reveal that such flux may offset the CO2 uptake by continental shelf waters (0.19 ± 0.05 Pg yr−1; Laruelle et al., 2014). To date, global estimates of the estuarine biogeochemical removal of carbon by the estuarine filter are generally estimated by extrapolating local measurements (n ≈ 100 in the most recent studies) to the total estuarine surface (e.g. Frankignoulle et al., 1998, Borges, 2005, Chen and Borges, 2009, Laruelle et al., 2010, Laruelle et al., 2013, Cai, 2011, Chen et al., 2013). Yet, this approach relies on geographically clustered observations, generally biased towards industrialized countries and does not accurately account for the wide variety of systems across the Earth (Bauer et al., 2013, Laruelle et al., 2013, Regnier et al., 2013b). Therefore, in spite of the significant role of the estuarine biogeochemistry in the global carbon budget, global and regional scale quantification remains associated to large uncertainties (Bauer et al., 2013, Regnier et al., 2013a). Because they provide a mechanistic description of energy and matter fluxes, reactive-transport models (RTMs; Steefel et al., 2005) are currently regarded as an efficient tool to resolve and quantify process rates that are often difficult or impossible to measure (Soetaert and Meysman, 2012) and to predict the estuarine responses to future climate and land-uses changes (Regnier et al., 2003, Volta et al., 2016). However, the limited availability of data required to force and validate RTMs, the computational constrains associated to the resolution of the estuarine dynamics at fine spatial and temporal scales and the focus of most RTM application towards specific management issues have limited their use to local studies so far. Therefore, such system-specific models are not suitable to represent the wide diversity of the estuaries and to quantify their biogeochemical role at regional and/or global scales (Regnier et al., 2013b).

In this study, we use a generic framework for a regional-scale application of the Carbon-Generic Estuary Model (C-GEM), a computationally efficient RTM designed to disentangle and quantify estuarine biogeochemical processes at the regional and global scales (Volta et al., 2014, Volta et al., 2016). More specifically, C-GEM is used here following the generic modeling approach proposed by Volta et al. (2016) for tidal estuaries in temperate regions. The latter combines the model with an idealized representation of the estuarine geometry, a generic set of model parameters and high-resolution environmental databases and is applied here to quantify the carbon budget and associated air-water CO2 exchange fluxes in tidal estuaries flowing into the North Sea. These tidal systems (Type 2 in the estuarine classification proposed by Dürr et al., 2011) have a potentially important role on the carbon dynamics along the land-ocean continuum owing to their relatively long residence time and to their resulting strong biogeochemical processing (Wollast, 2003). Moreover, they represent the most common near-shore environment along the coasts of the North Sea (Fig. 1) and the limited understanding of their biogeochemical role at the regional scale has been identified as an important limitation towards establishing an accurate carbon budget for the North Sea continuum (e.g. Allen et al., 2007, Artioli et al., 2012).

Previous estimates of carbon and nutrient input fluxes to the North Sea were estimated from the widely-used apparent zero end-member approach (e.g. Thomas et al., 2005, Blackford and Gilbert, 2007, Artioli et al., 2012). This method, similar to the LOICZ budgeting procedure (Gordon et al., 1996), assumes that the distribution of any dissolved element is determined by dilution of riverine water with seawater and ignores the spatial and temporal variability of biogeochemical processing in estuaries (Regnier et al., 2013b). Hence, it does not allow identifying and quantifying the complex mechanisms underlying the estuarine biogeochemical functioning and may introduce large errors in the estimation of fluxes to the coastal zone (e.g. Boyle et al., 1974, Officer and Lynch, 1981, Regnier and Steefel, 1999, Webster et al., 2000, Gazeau et al., 2005). Furthermore, as a salinity-based technique, it overlooks the high biogeochemical processing occurring in the freshwater tidal river zone (Arndt et al., 2007, Arndt et al., 2009, Vanderborght et al., 2007, Amann et al., 2012, Amann et al., 2014). The overall goal of this study is thus to provide a modeling approach able to identify the controlling mechanisms of the estuarine biogeochemical functioning by explicitly resolving the overall estuarine dynamics and testing different environmental scenarios. Ultimately, this approach should be suitable for coupling with regionalized models in order to reduce the uncertainty associated to the quantification of the biogeochemical role of estuaries. The generic modeling strategy used here, as well as the C-GEM modeling platform and model setups are described in Sect. 2 and are followed by model validation and sensitivity analysis in Sect. 3. The regional estuarine carbon budget is then presented and discussed in Sect. 4.

Section snippets

Strategy

Our strategy consists in using the generic estuarine model C-GEM to quantify the carbon processing taking place in the major tidal estuaries flowing into the North Sea. The processing rates derived from simulations are then applied to the riverine carbon loads of the North Sea catchments that are intercepted by smaller tidal estuarine systems, which are not explicitly represented here. Around the North Sea, six tidal estuaries dominate the overall budget as they account for about 40% of total

Summer CO2 dynamics: comparison with field data and analysis

Results from simulation set 1 (Sect. 2.3.1) are compared to field measurements collected during the summer period in the Scheldt and Elbe estuaries (Fig. 4, Fig. 5). In addition, the complex process interplay that drivers their CO2 dynamics is analyzed (Fig. 6) and simulated volume-integrated CO2 exchange fluxes across the air-water interface are compared to ranges of values reported in the literature for both systems.

C-GEM's ability at predicting hydrodynamics, transport and organic carbon

Carbon fluxes and budget

The NEM and FCO2 fluxes simulated using the generic parameter set and constant estuarine depth in the six main tidal estuaries discharging into the North Sea are summarized and compared to values reported in previous studies in Table 3a. The literature only reports previous quantifications of the NEM for the Scheldt (Gazeau et al., 2005, Arndt et al., 2009, Volta et al., 2014) and the Ems estuary (van Es, 1977). For both systems, the simulated NEM is about half of previous estimates, which is

Conclusions

In this study, the first regional carbon and CO2 budget for tidal estuaries bordering the North Sea was estimated using a RTM approach. Our budget calculations indicate that, in this region, estuaries filter about 15% of the total carbon inputs from rivers on a yearly basis and export about 5.3 Tg C (450·109 mol C) every year to the sea. This delivery to the shelf occurs mainly as DIC, reflecting the drainage of carbonate-rich catchments and the strong processing of organic carbon (>70%

Acknowledgements

The authors thank Dr. Ronny Lauerwald for his help with defining boundary conditions for our simulations and Dr. Thorben Amann for providing the Elbe monitoring data. The authors would also like to thank the four anonymous reviewers for their positive and constructive comments. G. G. Laruelle is Chargé de recherches du F.R.S.-FNRS at the Université Libre de Bruxelles.

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