Carbon dioxide in European coastal waters

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…Strongly tidal estuaries also tend to exhibit lower levels of photosynthetic activity (Monbet, 1992) and carry greater suspended particulate matter loads within their highturbidity regions (Uncles et al, 2002;Middelburg and Herman, 2007) wherein suspended particles and organic-rich aggregates serve as hot spots of microbial recycling (Statham, 2012). Field measurements suggest that 10 % of the total CO 2 emissions from the inner estuary of macrotidal systems is sustained by the ventilation of riverine CO 2 , whereas 90 % is due to local net heterotrophy (Borges et al, 2006) fueled by inputs of terrestrial-and riverine-algae-derived (planktonic) detritus and, in populated areas, sewage (Chen and Borges, 2009). In estuaries with long freshwater residence times, the riverine CO 2 will be fully ventilated to the atmosphere within the estuary, and the total CO 2 emissions can be attributed to net heterotrophy (Borges and Abril, 2011).…”

“…With an estimated global efflux of 0.10-0.15 Pg C yr −1 (Chen et al, 2013;Laruelle et al, 2013), estuarine CO 2 degassing is thought to counterbalance CO 2 uptake on the continental shelves (Chen and Borges, 2009;Laruelle et al, 2010;Cai, 2011). Almost every estuary on Earth, for which data are available, is generally supersaturated with CO 2 with respect to the atmosphere (Cai and Wang, 1998;Frankignoulle et al, 1998;Borges, 2005;Borges et al, 2005Borges et al, , 2006Chen and Borges, 2009;Laruelle et al, 2010;Cai, 2011;Chen et al, 2012;Bauer et al, 2013;Chen et al, 2013;Regnier et al, 2013), with CO 2 partial pressures (pCO 2 ) ranging from 400 to 10 000 µatm (in contrast, the atmospheric pCO 2 in coastal zones was approximately 360-385 µatm in the year 2000) (Cai, 2011). Although estuaries are generally net sources of CO 2 , there is considerable variability and uncertainty in estimates of their CO 2 emissions, reflecting the limited spatial and temporal coverage of pCO 2 measurements in estuaries as well as their heterogeneous nature (hydrological and geomorphological differences, differences in magnitude and stoichiometry of 3222 A.…”

“…In strongly tidal (macrotidal) systems, long water and particle residence times (on the order of weeks to months; Middelburg and Herman, 2007) allow for the extensive modification and degradation of particulate organic carbon during estuarine transport (Borges et al, 2006;Chen and Borges, 2009). In the absence of seasonal or permanent water stratification, the decoupling between production and degradation of organic matter at and below the surface, respectively, does not occur, resulting in less efficient export of DIC (Borges, 2005).…”

Biogeochemistry and carbon mass balance of a coccolithophore bloom in the northern Bay of Biscay (June 2006)

“…Furthermore, air-sea CO 2 fluxes are an integrated signal of the water mass biogeochemical history because of the very long equilibration time of surface waters with respect to atmospheric CO 2 due to the buffering capacity of seawater. Also, CO 2 dynamics integrate both physical (e.g., vertical mixing, advection, and water residence time) and purely thermodynamic (mainly water temperature change) effects, as discussed at length by e.g., Borges et al (2006). This appears to be the most likely explanation for the net sink of atmospheric CO 2 computed from the pCO 2 measurements for all stations, while the balance of community metabolic rates leads to a net release of CO 2 in the photic layer during the maturing and declining bloom phases.…”

“…However, the trophic status of a community does not necessarily imply that it is a source or a sink for atmospheric CO 2 , since the direction of the air-sea CO 2 fluxes results from a combination of several biogeochemical and physical processes acting at different time scales (e.g., Borges et al, 2006). The pCO 2 values in surface waters during the cruise ranged from 265 to 353 matm and indicate that the area was undersaturated with respect to atmospheric equilibrium ( $ 376 matm) (Table 2), acting as a sink for atmospheric CO 2 ranging between À8.5 and À 17.8 mmol C m À 2 d À 1 (Table 3; Suykens et al, 2010a).…”