Abundance, stable isotopic composition, and export fluxes of DOC, POC, and DIC from the Lower Mississippi River during 2006–2008

Abstract

Sources, abundance, isotopic compositions, and export fluxes of dissolved inorganic carbon (DIC), dissolved and colloidal organic carbon (DOC and COC), and particulate organic carbon (POC), and their response to hydrologic regimes were examined through monthly sampling from the Lower Mississippi River during was the most abundant carbon species, followed by POC and DOC. Concentration and δ 13 C of DIC decreased with increasing river discharge, while those of DOC remained fairly stable. COC comprised 61 ± 3% of the bulk DOC with similar δ 13 C abundances but higher percentages of hydrophobic organic acids than DOC, suggesting its aromatic and diagenetically younger status. POC showed peak concentrations during medium flooding events and at the rising limb of large flooding events. While δ 13 C-POC increased, δ 15 N of particulate nitrogen decreased with increasing discharge. Overall, the differences in δ 13 C between DOC or DIC and POC show an inverse correlation with river discharge. The higher input of soil organic matter and respired CO 2 during wet seasons was likely the main driver for the convergence of δ 13 C between DIC and DOC or POC, whereas enhanced in situ primary production and respiration during dry seasons might be responsible for their isotopic divergence. Carbon export fluxes from the Mississippi River were estimated to be 13.6 Tg C yr À1 for DIC, 1.88 Tg C yr À1 for DOC, and 2.30 Tg C yr À1 for POC during 2006-2008. The discharge-normalized DIC yield decreased during wet seasons, while those of POC and DOC increased and remained constant, respectively, implying variable responses in carbon export to the increasing discharge.As one of the world's largest rivers, the Mississippi River drains 41% of the contiguous United States with an increasing trend of water discharge over the past century from 494 to 578 km 3 yr À1 [Milliman, 1991;Raymond et al., 2008]. Water chemistry in the Mississippi River has been heavily influenced by agricultural activities (58% cropland coverage in the entire basin) and the presence of hydraulic infrastructures (dams and flood control levees) on the primary tributaries and the main river channel. Along with the enormous annual river discharge, the Mississippi River delivers large amounts of terrestrially derived materials, including nutrients and carbon, to the Gulf of Mexico [Bianchi et al.

Variation pattern of particulate organic carbon and nitrogen in oceans and inland waters

“…Coastal water with relatively high POC and PON levels has a low POC/PON ratio in the northern and south hemisphere (Table S4). This is inconsistent with 195 previous studies that show coastal water has a higher POC/PON ratio than offshore water (Sterner et al, 2008;Kaiser et al, 2014;Watanabe and Kuwae, 2015) due to the discharge of terrestrial organic matter (Hilton et al, 2015;Cai, Y.H. et al, 2015).…”

Sources and fate of organic carbon and nitrogen from land to ocean: Identified by coupling stable isotopes with C/N ratio

“…The carbon and nitrogen accumulated in the coastal sediments are heterogeneous and complex mixture of organic materials with different characteristics and from different sources (marine and freshwater phytoplankton, soil, leaf debris, wastewater, kerogen etc.) (Cloern et al, 2002;Lamb et al, 2006;Schreiner et al, 2013;Cai et al, 2015). Soils from different land types are the main source of riverine organic matter, contributing >90% of the buried organic carbon in the marine sediments occurring in "terrigenous-deltaic" regions near river mouths (Berner, 1989;Raymond and Bauer, 2001;Tao et al, 2015).…”

“…Due to the complex nature of carbon and nitrogen in the coastal zone, stable isotope (d 13 C and d 15 N) techniques and C/N ratio are robust tools that have been frequently used to elucidate sources, mixing and transformations of carbon and nitrogen in the terrestrial, estuarine and coastal regions, based on the different signatures of the sources (Zhang et al, 2007;Ramaswamy et al, 2008;Cai et al, 2015). In general, terrestrial organic matter has depleted d 13 C and d…”

“…15 N) along with the carbon and nitrogen ratios can be applied to improve the accuracy of source identification (Middelburg and Herman, 2007;Cai et al, 2015). Moreover, geographical mapping of isotope ratios over a large area can also contribute to identifying the specific sources in different regions by using the spatial variability of the isotopic signatures (Hu et al, 2006;Liu et al, 2015).…”

Do invasive quagga mussels alter CO2 dynamics in the Laurentian Great Lakes?

“…5). Collectively, up to (7.7 ± 1.0) × 10 12 g-C/yr or 7.7 ± 1.0 g-C/yr in the form of CO 2 can be released to the atmosphere from the Laurentian Great Lakes, which is comparable to the annual DIC flux to the ocean from the Mississippi River and many other world rivers5455. This annual CO 2 emission flux from the Laurentian Great Lakes is also higher than the annual organic carbon burial rate in all inland seas1656, and makes up 1% of the annual CO 2 emission from global deforestation57.…”