Abstract
The world’s largest mangroves ecosystem, the Sundarbans, being highly productive and a place for extensive organic matter cycling, is considered to be the hotspot for biogeochemical studies in the tropical estuarine environment. Hence, the spatial and temporal dynamics of the biogenic gases (CO2, CH4, and N2O), also known as radiatively active gases, were measured in mangrove-dominated estuaries of the system. In addition to spatial and seasonal observation, three full tidal cycles were observed at one site. Results showed that the air/water gas saturations were widely distributed and highly variable along the stretch. The gas saturations showed varying responses to salinity and tidal fluctuations. This indicated that localized biogeochemical processes may be more influential than simple mixing and dilution processes in controlling the variability of these gases. The surface waters were always supersaturated with CH4 (Up to 13,133%) relative to the atmosphere. However, N2O ranged from 8 to 1,286% and CO2 from 30 to 2075%. N2O fluxes were ∼4.8 times higher in the pre-monsoon than the post-monsoon. CH4 fluxes were ∼3.6 times higher in the pre-monsoon than both the monsoon and the post-monsoon. CO2 fluxes were ∼10 times higher in the monsoon than both the pre-monsoon and the post-monsoon. The seasonality in the gas saturation could be linked more to the availability of substrates than physicochemical parameters. Overall, air/water CH4 fluxes varied maximally (0.4–18.4 μmol m−2 d−1), followed by CO2 fluxes (−0.6–10.9 mmol m−2 d−1), and N2O fluxes varied the least of all (−0.6–5.4 μmol m−2 d−1). Interestingly, CH4 and N2O fluxes were positively correlated to each other (p < 0.05), suggesting organic matter decomposition as the key factor in the production of these two gases. Finally, these water–air CO2, CH4, and N2O flux estimates show that the estuaries are a modest source of CH4 but fluctuate between sources and sinks for CO2 and N2O gases.
Highlights
Rapid changes in the global climate being of primary concern have recently led to investigations regarding the recent developments in the phenomenon of radiative forcing
The distribution was random throughout the extent of the study area, corroborating the studies on N2O that identify the gas as spatially heterogeneous (Murray et al, 2015)
Our study suggests that heterotrophy is suppressed even in the premonsoon despite the advective input of organic matter from the intertidal areas
Summary
Rapid changes in the global climate being of primary concern have recently led to investigations regarding the recent developments in the phenomenon of radiative forcing. According to IPCC 2014, the concentrations of the major greenhouse gases like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have been major contributors in radiative forcing, more than solar radiation. The radiative equilibrium of the Earth can be influenced by CO2, CH4, and N2O, causing a net gain of solar energy by the atmosphere, infrared radiation (IR), which eventually causes warming. These greenhouse gases can, be termed alternatively as radiatively active gases or RAGs. Detailed accounts of each of these gases have since been obtained from studies first carried out in the oceans and seas (Bange, 2008; Forster et al, 2009). It is imperative to quantify CO2, CH4, and N2O because the global greenhouse gas flux budget needs accurate information on the contribution of these gases (Khalil et al, 2002; Wuebbles and Hayhoe, 2002)
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