Spatiotemporal variations of fCO 2 in the North Sea

Abstract. Data from two Voluntary Observing Ship (VOS) (2005–2007) augmented with data subsets from ten cruises (1987–2005) were used to investigate the spatiotemporal variations of the CO2 fugacity in seawater (fCO2sw) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO2sw variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO2 exchange. Over the study period, the seasonal amplitude in fCO2sw induced by SST changes was 0.4–0.75 times those resulting from variations in biology plus mixing. Along a meridional transect, fCO2sw normally decreased northwards (−12 μatm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO2sw in southern parts of the North Sea. Along a zonal transect, a weak gradient (−0.8 μatm per degree longitude) was observed in the annual mean fCO2sw. Annually and averaged over the study area, surface waters of the North Sea were CO2 undersaturated and, thus, a sink of atmospheric CO2. However, during summer, surface waters in the region 55.5–54.5° N were CO2 supersaturated and, hence, a source for atmospheric CO2. Comparison of fCO2sw data acquired within two 1°×1° regions in the northern and southern North Sea during different years (1987, 2001, 2002, and 2005–2007) revealed large interannual variations, especially during spring and summer when year-to-year fCO2sw differences (≈160–200 μatm) approached seasonal changes (≈200–250 μatm). The springtime variations resulted from changes in magnitude and timing of the phytoplankton bloom, whereas changes in SST, wind speed and total alkalinity may have contributed to the summertime interannual fCO2sw differences. The lowest interannual variation (10–50 μatm) was observed during fall and early winter. Comparison with data reported in October 1967 suggests that the fCO2sw growth rate in the central North Sea was similar to that in the atmosphere.

[1]  K. Ruddick,et al.  Net ecosystem production and carbon dioxide fluxes in the Scheldt estuarine plume , 2008, BMC Ecology.

[2]  S. Watanabe,et al.  Is there a “continental shelf pump” for the absorption of atmospheric CO2? , 1999 .

[3]  M. Bell,et al.  The Forecasting Ocean Assimilation Model (Foam) System , 2006 .

[4]  A. Borges,et al.  Time series of the partial pressure of carbon dioxide (2001–2004) and preliminary inorganic carbon budget in the Scheldt plume (Belgian coastal waters) , 2006 .

[5]  A. Borges,et al.  Controls of the surface water partial pressure of CO2 in the North Sea , 2005 .

[6]  J.M.J. Hoppema.  The seasonal behaviour of carbon dioxide and oxygen in the coastal North Sea along The Netherlands , 1991 .

[7]  R. Wanninkhof,et al.  Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods , 1993 .

[8]  J. Kelley.  CARBON DIOXIDE IN THE SURFACE WATERS OF THE NORTH ATLANTIC OCEAN AND THE BARENTS AND KARA SEAS1 , 1970 .

[9]  C. Culberson,et al.  MEASUREMENT OF THE APPARENT DISSOCIATION CONSTANTS OF CARBONIC ACID IN SEAWATER AT ATMOSPHERIC PRESSURE1 , 1973 .

[10]  Andrew J. Watson,et al.  Climatological Mean and Decadal Change in Surface Ocean Pco(2), and Net Sea-Air Co2 Flux Over the Global Oceans (Vol 56, Pg 554, 2009) , 2009 .

[11]  F. Millero,et al.  A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media , 1987 .

[12]  Y. Bozec,et al.  The continental shelf pump for CO2 in the North Sea - evidence from summer observation , 2005 .

[13]  W. K. Johnson,et al.  Coulometric total carbon dioxide analysis for marine studies: maximizing the performance of an automated gas extraction system and coulometric detector , 1993 .

[14]  R. Wanninkhof.  Relationship between wind speed and gas exchange over the ocean , 1992 .

[15]  C. Sweeney,et al.  Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects , 2002 .

[16]  J. Zimmerman,et al.  Review of the physical oceanography of the North Sea , 1990 .

[17]  Grégory Beaugrand,et al.  The North Sea regime shift: Evidence, causes, mechanisms and consequences , 2004 .

[18]  Nicolas Gruber,et al.  The Oceanic Sink for Anthropogenic CO2 , 2004, Science.

[19]  A. Borges.  Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean? , 2005 .

[20]  M. Fasham,et al.  An Examination of the “Continental shelf pump” in an open ocean general circulation model , 2001 .

[21]  Corinne Le Quéré,et al.  Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks , 2007, Proceedings of the National Academy of Sciences.

[22]  Andrew J. Watson,et al.  Corrigendum to Climatological mean and decadal change in surface ocean pCO2, and net sea―air CO2 flux over the global oceans , 2009 .

[23]  P. C. Reid,et al.  Ocean climate anomalies and the ecology of the North Sea , 2002 .

[24]  Taro Takahashi,et al.  Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: A comparative study , 1993 .

[25]  A. Borges,et al.  Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years , 2009 .

[26]  W. Cai,et al.  Air‐sea exchange of carbon dioxide in ocean margins: A province‐based synthesis , 2006 .

[27]  S. Kempe,et al.  Sinks and sources of CO2 in coastal seas - The North Sea , 1991 .

[28]  M. Chierici,et al.  Sea-surface CO 2 fugacity in the subpolar North Atlantic , 2008 .

[29]  A. Borges,et al.  High temporal coverage of carbon dioxide measurements in the Southern Bight of the North Sea , 2007 .

[30]  M. Edwards,et al.  A long‐term chlorophyll dataset reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient levels , 2007 .

[31]  R. Weiss.  Carbon dioxide in water and seawater: the solubility of a non-ideal gas , 1974 .

[32]  Alberto Borges,et al.  Reconciling opposing views on carbon cycling in the coastal ocean: Continental shelves as sinks and near-shore ecosystems as sources of atmospheric CO2 , 2009 .

[33]  Yann Bozec,et al.  Response to Comment on "Enhanced Open Ocean Storage of CO2 from Shelf Sea Pumping" , 2004, Science.

[34]  J.M.J. Hoppema.  The distribution and seasonal variation of alkalinity in the southern bight of the North sea and in the western Wadden Sea , 1990 .

[35]  R. Weiss,et al.  Nitrous oxide solubility in water and seawater , 1980 .

[36]  A. Borges,et al.  Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments , 2008 .

[37]  A. Borges,et al.  Distribution and air-water exchange of carbon dioxide in the Scheldt plume off the Belgian coast , 2002 .

[38]  J. Duinker,et al.  At-sea intercomparison of two newly designed underway pCO2 systems — encouraging results , 1996 .

[39]  A. Watson,et al.  A decrease in the sink for atmospheric CO2 in the North Atlantic , 2004 .

[40]  S. Doney,et al.  Rapid decline of the CO2 buffering capacity in the North Sea and implications for the North Atlantic Ocean , 2007 .

[41]  B. Delille,et al.  Budgeting sinks and sources of CO2 in the coastal ocean: Diversity of ecosystems counts , 2005 .

[42]  A. Borges,et al.  European continental shelf as a significant sink for atmospheric carbon dioxide , 2001 .

[43]  Andrew J. Watson,et al.  Corrigendum to "Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans" [Deep Sea Res. II 56 (2009) 554-577] , 2009 .

[44]  R. Seifert,et al.  The influence of intertidal mudflats on the dissolved inorganic carbon and total alkalinity distribution in the German Bight, southeastern North Sea , 1999 .

[45]  Alberto Borges,et al.  Assessment of the processes controlling the seasonal variations of dissolved inorganic carbon in the North Sea , 2006 .

[46]  R. Feely,et al.  A new automated underway system for making high precision pCO2 measurements onboard research ships , 1998 .

[47]  A. Borges,et al.  Daily and seasonal variations of the partial pressure of CO2 in surface seawater along Belgian and southern Dutch coastal areas , 1999 .