Estuaries are the main pathways for the transfer of particulate and dissolved matter from land to the ocean (through rivers). Particulate and dissolved matter undergo strong transformations, as estuaries are sites of intense biogeochemical processing (for example Bianchi, 2006) that in most cases leads to substantial emissions of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) (for example Borges and Abril, 2011). Most estuarine environments are net heterotrophic ecosystems (Gattuso et al., 1998; Testa et al., 2012), leading to the production and emission to the atmosphere of CO2 and CH4. The production of CO2 and CH4 is modulated by various physical features resulting from estuarine geomorphology such as water residence time (Borges et al., 2006; Joesoef et al., 2017), tidal amplitude and vertical stratification (Borges, 2005; Koné et al., 2009; Crosswell et al., 2012; Joesoef et al., 2015), and connectivity with tidal flats and salt marshes (Middelburg et al., 2002; Cai, 2011). Highly eutrophic (Cotovicz Jr. et al., 2015) or strongly stratified estuarine systems (Koné et al., 2009) can exceptionally act as sinks of CO2 due to high carbon sequestration, although high organic matter sedimentation can concomitantly lead to high CH4 production and emission to the atmosphere (Koné et al., 2010; Borges and Abril, 2011).
The global CO2 emissions from estuaries have been estimated by several studies (Abril and Borges, 2004; Borges, 2005; Borges et al., 2005; Chen and Borges, 2009; Laruelle et al., 2010, 2013; Cai, 2011; Chen et al., 2012, 2013) and range from 0.1 to 0.6 PgC yr−1, equivalent in magnitude to 5-30 % of the oceanic CO2 sink of ∼ 2 PgC yr−1 (Le Quéré et al., 2016). These values were derived from the scaling of air-water CO2 flux intensities (per surface area) compiled from published data that were extrapolated to estimates of the global surface of estuaries. The most recent estimates are lower than the older ones, reflecting the increase by an order of magnitude in the availability of data on air-water CO2 fluxes and more precise estimates of surface areas of estuaries structured by types (for example Dürr et al., 2011). The global estimates of CH4 emissions from estuaries are also relatively variable, ranging between 1 and 7 TgCH4 yr−1 (Bange et al., 1994; Upstill-Goddard et al., 2000; Middelburg et al., 2002; Borges and Abril, 2011) and are modest compared to other natural (220-350 TgCH4 yr−1) and anthropogenic (330-335 TgCH4 yr−1) CH4 emissions (Kirschke et al., 2013). Unlike CO2, the most recent global estimate of estuarine CH4 emissions is the highest because it accounts for the direct emissions of CH4 from sediment to atmosphere (when intertidal areas are exposed) (Borges and Abril, 2011). However, published estuarine CH4 emissions are most probably underestimated because they do not account for CH4 ebullition and gas flaring, although emissions to the atmosphere of CH4 originating from gassy sediments in coastal environments have been shown to be intense (Borges et al., 2016, 2017). Reported CO2 and CH4 emissions from rivers are also highly uncertain and the proposed values also span a considerable range. Global riverine CO2 emission estimates range between 0.1 PgC yr−1 (Liu et al., 2010) and 1.8 PgC yr−1 (Raymond et al., 2013), while riverine CH4 emission estimates range between 2 TgCH4 yr−1 (Bastviken et al., 2011) and 27 TgCH4 yr−1 (Stanley et al., 2016). Both CO2 and CH4 riverine emissions mainly occur in tropical areas (Borges et al., 2015a, b).
The first studies of CO2 and CH4 dynamics and emissions from estuaries were carried out during the late 1990s in Europe (Frankignoulle et al., 1996, 1998; Middelburg et al., 2002) and the US (Cai and Wang, 1998). Since then, CO2 data coverage has tremendously increased with additional studies at subtropical and tropical latitudes (for example Sarma et al., 2012; Chen et al., 2012; Rao and Sarma, 2016) and in the large river-estuarine systems such as the Amazon (Lefèvre et al., 2017), the Mississippi (Huang et al., 2015), the Yangtze (Changjiang) (Zhai et al., 2007; Zhang et al., 2008), and the Pearl (Guo et al., 2009; Zhou et al., 2009). The number of studies on CH4 in estuarine and coastal environments has not increased in recent years as spectacularly as those concerning CO2, attracting less research efforts because the marine source of CH4 to the atmosphere (0.4-1.8 TgCH4 yr−1; Bates et al., 1996; Rhee et al., 2009) is very modest compared to other natural and anthropogenic CH4 emissions (Kirschke et al., 2013); however, continental shelves and estuaries are more intense sources of CH4 to the atmosphere than the open ocean, in particular shallow and permanently well-mixed coastal zones (Borges et al., 2016, 2017). However, numerous large river-estuarine systems, such as the Mekong although it is the world’s 10th largest river in water discharge (470 km3 yr−1), 12th largest in length (4800 km), and 21st largest in drainage area (795 000 km2) (Li and Bush, 2015), remain totally uncharted with respect to CO2 and CH4 data.
As a contribution to the special issue in Biogeosciences on “Human impacts on carbon fluxes in Asian river systems”, we report a data set obtained in the three branches (Mỹ Tho, Hàm Luông, Co^´ Chiên) of the Mekong delta (Fig. 1) in December 2003, April 2004, and October 2004 of biogeochemical variables related to carbon cycling: pH, total alkalinity (TA), O2, calculated partial pressure of CO2 (pCO2), dissolved CH4 concentration, particulate (POC) and dissolved (DOC) organic carbon concentration and stable isotopic composition, particulate nitrogen (PN), dissolved inorganic carbon (DIC) stable isotopic composition, and total suspended matter (TSM). The aim of the paper is to give a general description of carbon cycling with an emphasis on CO2 and CH4 dynamics in the Mekong delta estuarine system that can be used as a reference state to evaluate future changes in response to modifications in hydrology related the construction of planned large dams (leading to water abstraction and sediment retention), eutrophication, shoreline erosion, and sea level rise.