Distribution And Fluxes Of Methane And Nitrous Oxide In Different Coastal Water Systems Of Eastern Hainan

CH₄ and N₂O are important greenhouse gases, which play significant roles in the global warming and atmospheric chemistry. Global oceans, especially rivers, estuaries and lagoons which are influenced by human activity severely, are net natural sources of atmospheric CH₄ and N₂O. Hence studies on the biogeochemistry of dissolved CH₄ and N₂O in these representative water systems will be helpful to understand the emission of CH₄ and N₂O from different water systems and their effect on environment.In this paper, distributions, water-air fluxes and influencing factors of CH₄ and N₂O in the representative water systems in eastern Hainan (i.e. rivers, estuaries lagoons and coastal waters) are studied detailedly. The production and consumption mechanisms, sediment-water (air) fluxes, sources and sinks of CH₄, N₂O in mangrove are studied as well. The main research results are as follows:1. Distributions and fluxes of CH₄ and N₂O are determined during the surveys on representative water systems in eastern Hainan from March 27th to April 15th in 2009. The results showed that concentrations of CH₄ and N₂O in rivers were generally high. CH₄ concentrations in Wenchang, Wenjiao and Wanquan rivers were (1492±541), (973±641) and (227±128) nmol·L^-1, respectively, while concentrations of N₂O were (45.70±0.60), (67.94±15.46) and (22.41±11.36) nmol·L^-1, respectively. CH₄ and N₂O concentrations in Wenchang and Wenjiao Rivers are much higher than those in Wanquan River due to the influence by serious human activities. The main source of CH₄ in Wanquan River was in-situ production as there was little exogenous input. On the contrary, besides waste water and mangrove pore water with rich CH₄ and N₂O being directly discharged into the Wenchang and Wenjiao Rivers, large amounts of organic matters were discharged into the rivers as well, and provide favorable conditions for the in-situ production of CH₄ and N₂O. CH₄ concentrations were (54.40±70.11), (151.69±96.51), (13.18±6.68) and (147.96±94.78) nmol·L^-1, respec- tively, for Bamen bay, Boao, Xiaohai and Laoyehai lagoons. The dissolved CH₄ in the lagoons mainly come from riverine inputs, input from groundwater and fringing mangrove forest and in-situ production and so on, the main sinks of methane in the lagoons were oxidation and outgassing. N₂O concentrations in the lagoons were (17.04±13.57), (30.20±16.58), (8.79±1.46) and (10.04±3.50) nmol·L^-1, respectively, for Bamen bay, Boao, Xiaohai and Laoyehai. N₂O concentrations in the lagoons with river-inputs were much higher than those without river-inputs. N₂O Concentrations in coastal waters were 7.14±0.48 nmol·L^-1, which is much lower than those in lagoons. Hence, lagoons act as net sources of dissolved CH₄ and N₂O in coastal water. The estimated air–water CH₄ ?uxes using the RC01 formula of Wenchang and Wanquan Rivers were 5967.0±5142.1 and 496.2±335.9μmol·m-2·d-1, respectively. Air–water N₂O ?uxes were (72.1±38.7) and (49.9±34.9)μmol·m-2·d-1, respectively, for the Wenchang and Wanquan Rivers. Air–water CH₄ ?uxes were 528.7±625.0, 441.7±473.3, 26.6±21.6 and 1287.8±1453.3μmol·m-2·d-1, respectively, for Bamen bay, Boao, Xiaohai and Laoyehai lagoons. Air–water N₂O ?uxes were (244.9±315.2), (85.3±59.3), (52.5±29.5) and (268.0±204.5)μmol·m-2·d-1, respectively. Rivers, estua- ries, lagoons and coastal waters in eastern Hainan are persistently supersaturated, which indicate that they are net sources of atmospheric CH₄ and N₂O.2. The sediment-air (water) fluxes, nitrification rates and denitrification rates were determined during surveys at mangrove in eastern Hainan in April and August 2009. The results showed that the sediment-air fluxes of CH₄ and CO₂ in spring were (12.5²32.5) and (34.5²56.3)μmol·m-2·d-1, respectively, whereas the sediment-air fluxes of CH₄ and CO₂ in summer were (11.4²230.22) and (39.8³53.7)μmol·m-2·d-1, respectively. The sediment-air fluxes of CH₄ and CO₂ in the mangrove forest showed great temporal and spatial variation. Besides the temperature, SO42-, salinity and tidal level, pneumatophores and crab burrows were important factors which could enhance CH₄ emission to the atmosphere. Both nitrification and denitrification can produce N₂O in sediments of mangrove, and denitrification is the dominant process. Mean while, the process of dissimilitary nitrate reduction to ammonium is found to be active in the mangrove sediment, but it is difficult to quantify it by now.