Air–sea exchanges of CO<sub>2</sub> in the world's coastal seas

Abstract

Abstract. The air-sea exchanges of CO 2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO 2 (39 ± 56 mol C m −2 yr −1 , positive flux indicates that the water is losing CO 2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m −2 yr −1 ) and lower estuaries with salinities of more than 25 are weak sources . Mixing with low-pCO 2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO 2 in estuaries, but wind speed, seldom discussed, is critical to controlling the air-water exchanges of CO 2 . The total annual release of CO 2 from the world's estuaries is now estimated to be 0.10 Pg C yr −1 , which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high pCO 2 , are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where pCO 2 is lower but wind speeds are much higher, such that the CO 2 fluxes are higher than in Asia. Newly emerged CO 2 flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO 2 .Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO 2 and absorb CO 2 from the atmosphere in all seasons. Shelves between 0 and 23.5 • S are on average a weak source and have a small flux per unit area of CO 2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO 2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO 2 values are low, the air-to-sea fluxes are high in regions above 50 • N and below 50 • S. At low latitudes, the winds tend to be weak, so the sea-to-air CO 2 flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr −1 from the atmosphere.

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)

“…This efflux compares favorably with that of the Delaware Estuary (2.4 ± 4.8 mol C m −2 yr −1 ; Joesoef et al, 2015), another large estuarine system with a long water residence time, but is significantly lower than estimates in the marinedominated Sapelo Sound and Doboy Sound estuaries (10.5 to 10.7 mol C m −2 yr −1 ; Jiang et al, 2008). From a compilation of 165 estuaries worldwide, almost all systems, with the exception of those in the Arctic (−1.1 mol C m −2 yr −1 ), serve as sources of CO 2 to the atmosphere (Chen et al, 2013). Chen et al (2013) concluded that the world's upper estuaries Table 2.…”

“…From a compilation of 165 estuaries worldwide, almost all systems, with the exception of those in the Arctic (−1.1 mol C m −2 yr −1 ), serve as sources of CO 2 to the atmosphere (Chen et al, 2013). Chen et al (2013) concluded that the world's upper estuaries Table 2. Mean, standard deviation, and range of pCO 2 , k W-14 , k R&C-01 , F W-14 , and F R&C-01 in the surface waters of the St. Lawrence River, USLE, LSLE, and GSL during all sampling months.…”

“…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.…”

“…North American estuaries rank first in terms of global estuarine surface area (41 %) but account for the lowest numerically averaged CO 2 flux per unit area (12 %) among all continents (Chen et al, 2013). These estimates are subject to large uncertainties due to data paucity.…”

“…A recent synthesis by Regnier et al (2013) highlighted the meagre spatial coverage of estuarine pCO 2 measurements, particularly along the eastern Canadian seaboard. Ironically, the Estuary and Gulf of St. Lawrence (EGSL) in eastern Canada is the largest semi-enclosed estuarine system in the world and is among the world's most intensively studied estuaries (El-Sabh and , but was left unmentioned in recent global (Cai, 2011;Chen et al, 2012Chen et al, , 2013 and regional (Laruelle et al, 2015) data compilations. Furthermore, previous estuarine CO 2 studies have focused on riverdominated estuaries, whereas there has been limited research on CO 2 dynamics in large estuaries and bays (Joesoef et al, 2015), i.e., marine-dominated systems, including the areas of mixing at sea (outer estuaries or river plumes) .…”