Distribution, Fluxes, Production And Transformation Of CH₄and N₂O In Representative Rivers And Estuaries

CH₄and N₂O are important greenhouse gases, which play significant roles inglobal warming and atmospheric chemistry. There is a relatively high concentration ofC and N in rivers and estuaries, where more CH₄and N₂O is produced and released tothe atmosphere due to human activities. Hence studies on the dissolved CH₄and N2Oin the rivers and estuaries are becoming more important, and it may help to quantifythe emission from different rivers and estuaries in order to understand their influenceon the future climate changes.In this dissertation, distributions, seasonal variations, fluxes and relatedinfluencing factors of dissolved CH₄and N₂O in the Yangtze River, Yellow river andtheir estuaries were studied. To explore the production mechanism of N₂O and CH₄,slurry incubation experiments aiming at estimating the potential CH₄production andoxidation rate, the potential nitrification and denitrification rate, were carried out atKenli station. The main results are as follows:（1）The seasonal variation of dissolved CH₄and N₂O was studied during surveys atStation Kenli of the Yellow River from March2010to December2011. The resultsindicated that the average dissolved CH₄and N₂O concentrations were （20.14±14.68）nmol/L and （21.89±14.12） nmol/L, respectively. Dissolved CH₄at station Kenli ofthe Yellow River showed seasonal variations with the highest values in winter and thelowest in spring. There is little change of dissolved CH₄in other seasons. Unlike CH₄,dissolved N₂O showed a relatively high concentration in winter and summer whilelow concentration occurred in spring and autumn. CH₄concentrations in the YellowRiver correlated positively with the monthly flow rates, while N₂O showed a negativecorrelation with water temperature. The input of CH₄and N₂O from the Yellow Riverto the Bohai Sea in2010-2011was estimated to be4.4×10⁵mol/yr and5.1×10⁵mol/yr,respectively. The incubation experiments at Station Kenli showed that the potentialproduction and oxidation rate ranges for CH₄in the water of the Yellow River waterwere （0.4⁰.9） nmol CH₄·L^-1·d^-1nmol CH₄·L^-1·d^-1and （0.3³.5） nmol CH₄·L^-1·d^-1, andthe sedimentary potential CH₄production and oxidation rate ranges are （1.3⁷.4）nmol CH₄·Kg^-1·d^-1and （0⁵.0） nmol CH₄·Kg^-1·d^-1. The seasonal variations of potential production and oxidation rate showed a higher value in summer and autumn and alower value in winter. The highest sedimentary potential production rate occurred insummer and the lowest in winter, while for sedimentary oxidation production rate, themaximum appeared in autumn and no obviously oxidation was detected in winter. Asfor N2O, the ranges of potential total production rate, potential nitrification anddenitrification rate in the Yellow River water were （-0.80¹.05） nmol N·L^-1·h^-1、0.42⁰.46nmol N·L^-1·h^-1and29.46⁶1.09nmol N·L^-1·h^-1, while in the sediment therate ranges were （-0.64¹.12） nmol N·Kg^-1·h^-1，（0.21⁰.52） nmol N·Kg^-1·h^-1and（4.17³14.4） nmol N·Kg^-1·h^-1, respectively. The potential total production rates in theYellow River correlated positively with incubation temperature, and these potentialrates in water and sediment have seasonal variation with a higher value in summerand lower one in autumn, and in winter the water and sediment are sinks of N2O.（2） The seasonal variation of dissolved CH₄and N₂O was studied from January2010to December2011in Yangtze River. The average concentrations of CH₄and N2Owere （143.08±49.73） nmol/L and （18.05+8.20） nmol/L. CH₄and N₂O had seasonalvariations with the highest values occurring in autumn and the lowest in spring. Thedissolved CH₄and N₂O inputs to the East China Sea and Yellow Sea via the YangtzeRiver were estimated to be3.5Gg·a^-1and0.4Gg·a^-1respectively, which plays animportant role for the distribution of CH₄and N₂O and the biogeochemical cycles of Cand N in the Yangtze River estuary. A cruise was conducted on the Yangtze estuary inJuly2011. Horizontal and vertical distribution of CH₄and N₂O showed that theaverage dissolved CH₄in the surface and bottom waters were （64.29±29.82） nmol/Land （87.94±43.80） nmol/L, respectively, and N₂O were （30.16±12.68） nmol/L and（33.37±15.91） nmol/L. CH₄and N₂O concentrations in bottom waters are higher thanthose in surface, which illustrated the sediment act as sources of CH₄and N2O. Thehorizontal distributions of CH₄and N₂O in Yangtze estuary decreased gradually fromthe inner to the outer estuary. Dissolved CH₄and N₂O concentrations initiallydecreased with increasing salinities at0–10psu. The CH₄and N₂O in surface water ofYangtze River estuary are supersaturated, which indicates that the Yangtze Riverestuary was net sources of atmospheric CH₄and N₂O in the summer of2011.