Methane in the Changjiang (Yangtze River) Estuary and its adjacent marine area: riverine input, sediment release and atmospheric fluxes

Abstract

Dissolved methane (CH 4 ) was measured in the waters of the Changjiang (Yangtze River) Estuary and its adjacent marine area during five surveys from 2002 to 2006. Dissolved CH 4 concentrations ranged from 2.71 to 89.2 nM and had seasonal variation with the highest values occurring in summer and lowest in autumn. The horizontal distribution of dissolved CH 4 decreased along the freshwater plume from the river mouth to the open sea. Dissolved CH 4 in surface waters of the Changjiang was observed monthly at the most downstream main channel station Xuliujing (12182 0 E, 31846 0 N), which ranged from 16.2 to 126.2 nM with an average of 71.6 ± 36.3 nM. The average annual input of CH 4 from the Changjiang to the Estuary and its adjacent area was estimated to be 2.24 mol s -1 equal to 70.6 9 10 6 mol year -1 . Mean CH 4 emission rate from the sediments of the Changjiang Estuary in spring was 1.97 lmol m -2 day -1 , but it may be higher in summer due to hypoxia in the bottom waters and higher temperatures. The annual sea to air CH 4 fluxes from the Changjiang Estuary and its adjacent marine area were estimated to be 61.4 ± 22.6 and 16.0 ± 6.1 lmol m -2 day -1 , respectively, using three different gas exchange models. Hence the Changjiang Estuary and its adjacent marine area are net sources of atmospheric CH 4 .

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

“…The average F CH 4 value in the inner estuarine branches of the Mekong delta (118 µmol m −2 d −1 ) is within the range of values in European estuaries (17-1352 µmol m −2 d −1 ) compiled by Upstill-Goddard and Barnes (2016) but distinctly higher than the range of values for Indian estuaries (7-15 µmol m −2 d −1 ) reported by Rao and Sarma (2016). The F CH 4 in the Yangtze and Pearl river estuaries reported by Zhang et al (2008) and Zhou et al (2009) of 61 and 64 µmol m −2 d −1 , respectively, are also higher than the range of F CH 4 in Indian estuaries. The F CH 4 in the Mekong delta inner estuarine branches was higher than the value in the Yangtze River and Pearl River estuaries probably because of the lower intrusion of seawater into the Mekong delta (see above).…”

“…Since then, CO 2 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 , 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 CH 4 in estuarine and coastal environments has not increased in recent years as spectacularly as those concerning CO 2 , attracting less research efforts because the marine source of CH 4 to the atmosphere (0.4-1.8 TgCH 4 yr −1 ; Bates et al, 1996;Rhee et al, 2009) is very modest compared to other natural and anthropogenic CH 4 emissions (Kirschke et al, 2013); however, continental shelves and estuaries are more intense sources of CH 4 to the atmosphere than the open ocean, in particular shallow and permanently well-mixed coastal zones (Borges et al, 2016.…”

Enhanced methane emissions from oil and gas exploration areas to the atmosphere – The central Bohai Sea

“…e Air-sea gas transfer velocity was estimated using Sweeney et al (2007) equation . East Siberian Arctic Shelf. In the northern East China Sea, CH 4 -rich effluents from the Yangtze River support the very high surface water [CH 4 ] (Zhang et al, 2008). In the Black Sea shelf, anaerobic metabolism together with numerous methane-emitting bottom features, such as seeps, methane clathrate hydrates, and mud volcanoes, sustain the high sea surface [CH 4 ] in this, the world's largest anoxic basin (Amouroux et al, 2002).…”

“…In waters overlying continental shelves, however, the organic-rich sediment serves as a major source of CH 4 in the water column compared to net production in the mixed layer (Martens and Berner, 1974;Martens and Klump, 1980;Reeburgh, 2007). The shallow-water [CH 4 ] can also be elevated due to gas seepages and/or oil spills (Bernard et al, 1976;Reed and Kaplan, 1977;Rehder et al, 1998Rehder et al, , 2002Kessler et al, 2011;Gülzow et al, 2013) and advection of CH 4 -rich freshwaters (Sackett and Brooks, 1975;Cline et al, 1986;Zhang et al, 2008). Since CH 4 is oxidized to dissolved inorganic carbon via microbial oxidation processes (Sieburth et al, 1987;Jones, 1991;Reeburgh, 2007) and/or lost via sea-air gas exchange (Crutzen, 1991;Bange et al, 1994), determining [CH 4 ] in surface water with high spatiotemporal data coverage is critical in adding to our understanding of the marine CH 4 cycle and furthermore of the marine carbon cycle.…”

Seasonal Variation of Dissolved Methane Concentration and Flux in the Nakdong Estuary

“…Estuary의 경우 산화와 대기 방출로 인해 메탄 농도가 빠르게 감소 하였지만, Changjiang Estuary와 Great Bay의 경우 전반적으로 농 도가 감소하는 중간에 주변 염습지나 퇴적물로부터 메탄이 공급되 어 농도가 급증하는 패턴이 보여 지기도 했다 (Sansone et al, 1999;Middelburg et al, 2002;Zhang et al, 2008).…”