Seasonal variability in the sources of particulate organic matter of the Mekong River as discerned by elemental and lignin analyses

Abstract

[1] The Mekong River ranks within the top ten rivers of the world in terms of water discharge and sediment load to the ocean, yet its organic matter (OM) composition remains unstudied. This river is experiencing anthropogenically forced changes due to land use and impoundment, and these changes are expected to intensify in the future. Accordingly, we monitored the composition (including vascular-plant signatures) of Mekong River fine particulate organic matter (FPOM) over a one-year period. Autochthonous production comprises a greater proportion of FPOM during the dry season than in the rainy season, as demonstrated by higher percent organic carbon values (7.9 AE 2.4 versus 2.2 AE 0.4%), lower yields of lignin normalized to carbon (0.40 AE 0.05 versus 1.1 AE 0.3 mg (100 mg OC) À1), and an increase in N:C ratios toward phytoplankton values during the dry season (from 0.06 to 0.12). Changes in the lignin-phenol composition of FPOM suggest that gymnosperms contribute more toward FPOM composition during the dry season, with angiosperms dominating in the wet season. This is supported by calculations of the lignin phenol vegetation index of riverine FPOM, which increases between the dry to wet seasons (dry: 29.4 AE 15.0 versus wet: 74.6 AE 17.3). These changes likely reflect seasonal differences in the proportion of flow that is coming from the Upper and Lower Basin, corresponding to compositional differences between the vegetation of these regions. Therefore, this work provides a baseline understanding of FPOM variability that can be used to assess how future change will affect this river.Citation: Ellis, E. E., R. G. Keil, A. E. Ingalls, J. E. Richey, and S. R. Alin (2012), Seasonal variability in the sources of particulate organic matter of the Mekong River as discerned by elemental and lignin analyses,

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…4) (Reynolds and Descy, 1996), as also observed in other tropical rivers (for example Descy et al, 2017). Phytoplankton development during low water was also reported in the Upper Mekong River (confluence with the Tonle Sap River) by Ellis et al (2012), based on elemental and lignin analyses. The impact of biological activity on CO 2 dynamics in the uppermost freshwater part of the estuary was confirmed by δ 13 C-DIC values that were higher in April 2004 (−8.7 ± 0.4 ‰, n = 5) compared to December 2003 (−10.6 ± 0.6, n = 6 ‰) and October 2004 (−10.9 ± 0.3 ‰, n = 15).…”

“…Reported seasonal cycles of remotely sensed chlorophyll a concentration also indicate higher phytoplankton biomass and primary production in October compared to April and December (Gao et al, 2013;Loisel et al, 2017). The δ 13 C-POC values in the freshwater part of the delta (salinity < 1) from the three sampling campaigns averaged −26.7 ± 0.7 ‰ (n = 34), distinctly higher than the data from Ellis et al (2012), which averaged −29.8 ± 0.9 ‰, but similar to data collected by Martin et al (2013;average −26.4 ‰) at the same site as the Ellis et al (2012) study. These δ 13 C-POC values are consistent with the expected dominance of terrestrial C3 vegetation in the riverine organic carbon load.…”

“…The 13 Cdepleted signature in freshwater DIC results mainly from the degradation of organic matter, which contributes CO 2 with a signature close to that of the source organic carbon, which in the Mekong delta for POC ranged between −28 and −26 ‰, and from the weathering of carbonate and silicate minerals (which are typically driven by reaction with CO 2 derived from organic matter). CH 4 showed very strong seasonal variations in freshwaters of the Mekong delta with (Meybeck, 1982;Ludwig et al, 1996) and in estuaries (Abril et al, 2002) and within the range measured in the Lower Mekong just upstream of the confluence with the Tonle Sap river during an annual cycle by Ellis et al (2012). However, %POC values were distinctly higher (up to ∼ 13 %) in marine waters in October 2004, probably resulting from a phytoplankton bloom, as also testified by low POC : PN ratios (as low as 4.9), high %O 2 (up to 114 %) and δ 13 C-DIC (up to 0 ‰) values, and low pCO 2 (as low as 232 ppm at salinity 12.9) values.…”

Carbon dynamics in the Mekong Delta

“…Phytoplankton development during low water was also reported in the Upper Mekong River (confluence with the Tonle Sap River) by Ellis et al (2012) (Fig. 4), as also shown in other tropical 25 rivers such as the Oubangui .…”

Sources and export of particle-borne organic matter during a monsoon flood in a catchment of northern Laos

“…Masiello and Druffel, 2001;Hilton et al, 2010;Smith et al, 2013) stable isotope fingerprinting methods may be applied to particulate material collected from hillslopes to rivers, either independently or in combination with fallout radionuclides to document variations in sediment sources and pathways across catchments (e.g. Ritchie and McCarty, 2003;Ellis et al, 2012;Schindler Wildhaber et al, 2012;Ben Slimane et al, 2013;Koiter et al, 2013). In addition, complementary information on sediment conveyed to the river by runoff and overland flow can also be inferred from water tracers such as 18 O natural abundance (for a review see Klaus and McDonnell, 2013).…”