Regional carbon and CO2 budgets of North Sea tidal estuaries
Graphical abstract
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.
References (81)
- et al.
Excess atmospheric carbon dioxide transported by rivers into the Scheldt estuary
Earth Planet. Sci.
(2000) - et al.
Behaviour of organic carbon in nine contrasting European estuaries
Estuar. Coast. Shelf Sci.
(2002) - et al.
Error quantification of a high-resolution coupled hydrodynamic-ecosystem coastal-ocean model: part 2. chlorophyll-a, nutrients and SPM
J. Mar. Syst.
(2007) - et al.
Carbon dynamics in the freshwater part of the Elbe estuary, Germany: implications of improving water quality
Estuar. Coast. Shelf Sci.
(2012) - et al.
Seasonally-resolved nutrient filtering capacities and export fluxes in a macrotidal estuary
J. Mar. Syst.
(2009) - et al.
The carbonate system in the North Sea: sensitivity and model validation
J. Mar. Syst.
(2012) - et al.
pH variability and CO2 induced acidification in the North Sea
J. Mar. Syst.
(2007) - et al.
Carbon dioxide in European coastal waters
Estuar. Coast. Shelf Sci.
(2006) - et al.
On the chemical mass-balance in estuaries
Geochim. Cosmochim. Acta
(1974) - et al.
Reconciling opposing views on carbon cycling in the coastal ocean: continental shelves as sinks and near-shore ecosystem as sources of atmospheric CO2
Deep-Sea Res. II
(2009)
Global CO2-consumption by chemical weathering: what is the contribution of highly active weathering regions?
Glob. Planet. Change
Major ion concentrations and the inorganic carbon chemistry of the Humber rivers
Sci. Total Environ.
The water quality of the river trent: from the lower non-tidal reaches to the freshwater tidal zone
Sci. Total Environ.
Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic – a model study, part II: carbon budget and fluxes
Cont. Shelf Res.
Global nutrient export from WaterSheds 2 (NEWS 2): model development and implementation
Environ. Model. Softw.
Dynamics of mixing in estuaries
Estuar. Coast. Shelf Sci.
Long-term fluxes of reactive species in macrotidal estuaries: estimates from a fully transient, multicomponent reaction-transport model
Mar. Chem.
A discussion of methods for estimating residual fluxes in strong tidal estuaries
Cont. Shelf Res.
A high resolution estimate of the inorganic nitrogen flux from the Scheldt estuary to the coastal North sea during a nitrogen-limited algal bloom, spring 1995
Geochim. Cosmochim. Acta
Nutrient fluxes through the Humber estuary
J. Sea Res.
Reactive transport in aquatic ecosystems: rapid model prototyping in the open source software R
Environ. Model. Softw.
Reactive transport modeling: an essential tool and a new research approach for the Earth sciences
Earth Planet. Sci. Lett.
The fluxes and transformations of suspended particles, carbon and nitrogen in the Humber estuarine system (UK) from 1994 to 1996: results from an integrated observation and modelling study
Sci. Total Environ.
Organic carbon in the Humber rivers
Sci. Total Environ.
Reactive-transport modelling of a river-estuarine-coastal zone system: application to the Scheldt estuary
Mar. Chem.
Inorganic carbon fluxes in the inner Elbe estuary, Germany
Estuaries Coasts
Shallow-water ocean: a source or sink of atmospheric CO2?
Front. Ecol. Environ.
Diatom growth response to physical forcing in a macrotidal estuary: coupling hydrodynamics, sediment transport and biogeochemistry
J. Geophys. Res.
The changing carbon cycle of the coastal ocean
Nature
Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?
Estuaries
Increased mobilization of aged carbon to rivers by human disturbance
Nat. Geosci.
Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration
Annu. Rev. Mar. Sci.
Air-sea exchanges of CO2 in the world's coastal seas
Biogeosciences
Global carbon sequestration in tidal, saline wetland soils
Glob. Biogeochem. Cycles
Lithological composition of the Earth's continental surfaces derived from a new digital map emphasizing riverine material transfer
Glob. Biogeochem. Cycles
World-wide typology of near-shore coastal systems: defining the estuarine filter of river inputs to the oceans
Estuar. Coasts
Carbon dioxide emission from European estuaries
Science
Carbon and carbonate metabolism in coastal aquatic system
Annu. Rev. Ecol. Syst.
Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)
Estuaries
An Interestuarine Comparison for Ecology in TIDE – the Scheldt, Elbe, Humber and Weser. Study in the Framework of the Interreg IVB Project TIDE
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2020, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Therefore, studying the source, fate, and flux of riverine DIC is important for understanding the global and local carbon cycles. Riverine DIC is eventually discharged into the estuary and mixed with marine DIC (Samanta et al., 2015; Volta et al., 2016). In areas outside of the maximum turbidity zone of the estuary, water is clearer and the nutrients carried by the river promote large-scale phytoplankton growth that preferentially uses light carbon in photosynthesis, thereby increasing δ13CDIC in the surface water (Ingrosso et al., 2016).
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2018, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Altogether, oxygen yield by primary production only slightly counteracts the DO depletion in the most critical zone. Opposing evidences on the predominance of heterotrophy in estuaries (Bianchi, 2007; Caffrey, 2004; Cloern et al., 2014; Volta et al., 2016b) may, to some extent, merely reflect the diversity of estuarine systems. Our results reinforce the heterotrophic view, particularly for anthropogenically impacted estuaries.
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2017, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Except for ΔNO3–-N, most biogeochemical additions were at salinities between 4 and 16, suggesting that the associated biogeochemical processes mainly occurred in the above-mentioned salinity front area. Estuaries are not only transport passages of terrestrial material from the continents to oceans, but are also chemical reactors and buffers (Officer, 1979; Frankignoulle et al., 1996; Abril et al., 2003; Regnier et al., 2013; Volta et al., 2016). Before discussing biogeochemical processes and element transport fluxes in the inner Changjiang Estuary, the relevant residence times need to be examined.