High temporal coverage of carbon dioxide measurements in the Southern Bight of the North Sea

doi:10.1016/j.marchem.2007.01.001

Marine Chemistry

Volume 106, Issues 1–2, July 2007, Pages 161-173

https://doi.org/10.1016/j.marchem.2007.01.001 Get rights and content

Abstract

A monthly survey of the partial pressure of CO₂ (pCO₂) was carried in the Southern Bight of the North Sea (SBNS) from June 2003 to May 2004. The spatial variability of the surface distribution of the pCO₂ was relatively small (within a range of 10–70 μatm) compared to the amplitude in the seasonal signal (∼ 260 μatm). On an annual scale, the pCO₂ dynamics appeared to be controlled by biological processes (primary production in springtime and respiratory processes in summer), rather than temperature (in summer). The comparison with measurements carried out in 2001 and 2002 (13 cruises) shows that the inter-annual variability of pCO₂ was close to the range of the spatial variability and mostly observed in spring, associated to biological processes (primary production). Net ecosystem production estimated from dissolved inorganic carbon (DIC) temporal variations showed that the SBNS is autotrophic, at an annual rate of 6.3 mol C m^− 2 yr^− 1. The decoupling in time between autotrophy in spring and heterotrophy in summer, associated to the relatively rapid flushing time of the water mass in the area (∼ 70 days), might allow the export of a fraction of the springtime synthesized organic matter to the adjacent areas of the North Sea. The SBNS was on a yearly basis a sink of atmospheric CO₂ at a rate of − 0.7 mol C m^− 2 yr^− 1.

Introduction

The representation of the present-day CO₂ fluxes and the understanding of their biogeochemical drivers are essential requirements to reliably validate numerical models, as well as to strengthen their predictive capabilities on future increase of atmospheric CO₂. It is well established that open oceanic waters are a major sink for anthropogenic CO₂, absorbing about 48% of the CO₂ emissions related fossil fuels burning and cement production (Sabine et al., 2004). However, coastal ecosystems remain grossly under-represented in global present-day CO₂ budgets. Estimates of the global CO₂ sink over marginal seas based on scaled CO₂ fluxes (F_CO₂) computed from field measurements of the partial pressure of CO₂ (pCO₂) or from carbon mass balances range between − 0.2 and − 1.0 PgC yr^− 1 (Tsunogai et al., 1999, Chen, 2004, Thomas et al., 2004, Borges, 2005, Borges et al., 2005). This wide range of estimates reflects to some extent the latitudinal and ecosystem variability of coastal environments, but also to a large extent the insufficient data coverage in these heterogeneous and dynamic environments. Nevertheless, such values are comparable to the open ocean sink of atmospheric CO₂ estimated to − 1.6 PgC yr^− 1 (Takahashi et al., 2002).

The Southern Bight of the North Sea (SBNS from the Dover Strait to 53°N, Fig. 1) is a permanently well-mixed coastal zone that receives high inputs of nutrients and organic and inorganic carbon from the Scheldt, Rhine/Meuse and Thames estuaries, and indirectly from the Seine through inputs from the English Channel. Previous studies in the SBNS reported measurements of pCO₂ along transects or survey approaches at seasonal or annual scales (Kempe and Pegler, 1991, Hoppema, 1991, Bakker et al., 1996, Frankignoulle and Borges, 2001, Borges and Frankignoulle, 2003, Thomas et al., 2004). These studies based on different temporal and spatial coverages of pCO₂ in surface waters of the SBNS report a broad range of pCO₂ and F_CO₂ values. Indeed, in dynamic and heterogeneous coastal ecosystems, the two sampling approaches (high spatial coverage with low temporal resolution versus high temporal resolution at a fixed station) can lead to uncertainties regarding the pCO₂ dynamics and the direction and amplitude of F_CO₂ (e.g. Schiettecatte et al., 2006).

In the present study, the spatial distribution and temporal variability of surface water pCO₂ in the SBNS are investigated based on 23 cruises from 2001 to 2004, with a monthly dataset from June 2003 to May 2004. We attempt to address the biogeochemical drivers of the seasonal pCO₂ dynamics and some of the factors involved in the inter-annual pCO₂ variability. Net ecosystem production (NEP) is estimated from the temporal variation of dissolved inorganic carbon (DIC), and F_CO₂. The latter estimates allow us to characterize the SBNS as a source or a sink for atmospheric CO₂.

Section snippets

Materials and methods

Data were obtained from 2001 to 2004 from 23 cruises on board of the R.V. Belgica. The ship tracks are shown in Fig. 2, Fig. 3. Cruises always started from Zeebrugge harbour, close to the mouth of the Scheldt estuary (3.18°E 51.3°N), and data were collected to a longitude of 0.5°E and to latitude of 53°N, when possible (Fig. 1). Underway parameters (salinity, temperature and pCO₂) were sampled with a 1 min frequency from the surface seawater supply of the ship (pump inlet at a depth of 2.5 m).

Spatial distribution and seasonal cycle of pCO₂

Fig. 2 shows for the annual cycle 2003–2004 the seasonal variations of the spatial distribution of pCO₂ in the surface waters of the SBNS and adjacent areas (eastern English Channel), while Fig. 4 show the monthly averages of pCO₂, %O₂, TA and DIC for the SBNS excluding the eastern English Channel. For a given cruise, the pCO₂ distribution in the SBNS was quite homogeneous, as the amplitude of spatial variations ranged between 10 and 70 μatm (at most 50 μatm when estuarine plumes are excluded,

Summary and conclusions

A monthly survey from June 2003 to May 2004 in the SBNS shows that the seasonal variations of the surface distribution of pCO₂ are mainly controlled by biological activity rather than by temperature. Comparison with data obtained in 2001 and 2002 shows that a relatively important inter-annual variability of pCO₂ occurs in the SBNS. This inter-annual variability is partly due to temperature but also to biological activity, in particular during the periods of the phytoplankton blooms in April and

Acknowledgments

We thank the crew of the R.V. Belgica for their assistance during the cruises, Management Unit of the North Sea Mathematical Models (MUMM) for thermosalinograph data, Martine Leermaekers, Anna Lourantou, Teodora Ortega Diaz, François Paquay and Marianna Ribas Ribas, for their help in sampling, Frédéric Gazeau for comments during the elaboration of the manuscript, Bruno Delille and Fabian Lenartz for help with remote sensing data. This research was supported by the European Union in the

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High temporal coverage of carbon dioxide measurements in the Southern Bight of the North Sea

Abstract

Introduction

Section snippets

Materials and methods

Spatial distribution and seasonal cycle of pCO2

Summary and conclusions

Acknowledgments

Marine Chemistry

Marine Chemistry

Journal of Marine Systems

Marine Chemistry

Estuarine Coastal and Shelf Science

Deep-Sea Research. Part II—Topical Studies in Oceanography

Deep-sea research. Part A, Oceanographic Research Papers

Water Research

Progress in Oceanography

Estuarine Coastal and Shelf Science

Journal of Sea Research

Netherlands Journal of Sea Research

Netherlands Journal of Sea Research

Netherlands Journal of Sea Research

Marine Chemistry

Deep-Sea Research. Part II—Topical Studies in Oceanography

Netherlands Journal of Sea Research

Marine Chemistry

The influence of short-term wind variability on air–sea CO2 exchange

Geophysical Research Letters

The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere

Limnology and Oceanography

Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?

Estuaries

Distribution and air–water exchange of carbon dioxide in the Scheldt plume off the Belgian coast

Biogeochemistry

Distribution of surface carbon dioxide and air–sea exchange in the English Channel and adjacent areas

Journal of Geophysical Research-Oceans

Budgeting sinks and sources of CO2 in the coastal ocean: diversity of ecosystems counts

Geophysical Research Letters

The continental shelf pump for CO2 in the North Sea — evidence from summer observations

Marine Chemistry

Spatial distribution and seasonal cycle of pCO₂

The influence of short-term wind variability on air–sea CO₂ exchange

Do we have enough pieces of the jigsaw to integrate CO₂ fluxes in the coastal ocean?

Budgeting sinks and sources of CO₂ in the coastal ocean: diversity of ecosystems counts

The continental shelf pump for CO₂ in the North Sea — evidence from summer observations