Mangrove Methane Biogeochemistry in the Indian Sundarbans: A Proposed Budget

Manab K Dutta,Sandip K Mukhopadhyay,Thomas S Bianchi

doi:10.3389/fmars.2017.00187

Manab K Dutta, Sandip K Mukhopadhyay + Show 1 more

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https://doi.org/10.3389/fmars.2017.00187

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Abstract

Biogeochemical cycling of CH4 was investigated at Lothian Island, one of the relatively pristine islands of Indian Sundarbans and its adjacent Saptamukhi estuary, during June 2010 to December 2012. Intertidal mangrove sediments were highly anoxic and rich in organic carbon. Mean rates of methanogenesis were 3547 and 48.88 µmol m-3 wet sediment d-1, for intertidal (up to 25 cm depth) and sub-tidal sediments (first 5 cm depth), respectively. CH4 in pore-water was 53.4 times more supersaturated than in adjacent estuarine waters. This resulted in significant CH4 efflux from sediments to estuarine waters - via advective and diffusive transport. About 8.2% of the total CH4 produced in intertidal mangrove sediments was transported to the adjacent estuary through advective flux, which was 20 times higher than diffusive CH4 flux. Mean CH4 concentrations in estuarine surface and sub-surface waters were 69.9 and 56.1 nM, respectively, with a dissolved CH4 oxidation rate in estuarine surface waters of 20.5 nmol L-1 d-1. An estimated 0.09 Gg yr-1 of CH4 is released from estuaries of Sundarbans to the regional atmosphere. The mean CH4 mixing ratio over the forest atmosphere was 2 ppmv. On annual basis, only 2.75% of total supplied CH4 to the forest atmosphere was transported to the upper atmosphere via biosphere-atmosphere exchange. Mean CH4 photo-oxidation rate over the forest atmosphere was 3.25 x 10-9 mg cm-3 d-1. Using new and and previously published data we present for the first time, a CH4 budget for Sundarbans mangrove ecosystem which in part, revealed the existence of anaerobic CH4 oxidation in the mangrove sediment column.

Highlights

Methane (CH4) is the key gas produced in anaerobic environments and represents the second most abundant greenhouse gas associated with climate change (Forster et al, 2007)
Eh values of mangrove surface sediment varied between −119.8 and −103.2 mV having a mean of −111.4 ± 6.78 mV (Figure 2)
In contrast to the Sundarbans mangrove ecosystem, mangrove sediments of New Caledonia were found to have positive redox potential values, to a sediment depth of 30 cm, with negative Eh values only found in considerably deeper sediments (Deborde et al, 2015)

Summary

Introduction

Methane (CH4) is the key gas produced in anaerobic environments and represents the second most abundant greenhouse gas associated with climate change (Forster et al, 2007). The cause of this large increase in atmospheric CH4 is not fully understood, but is probably related to a surge in CH4 emissions from wetlands that contribute approximately 20–39% of the annual global atmospheric CH4 budget (Hoehler and Alperin, 2014). Sedimentary-derived CH4 can escape to the adjacent water/atmosphere via diffusive evasion, ebullition, and plantmediated transport (Chanton and Dacey, 1991). Past work has shown that diffusion is the least effective pathway (Chanton and Dacey, 1991; Laanbroek, 2010; Bridgham et al, 2013), with plant-mediation being the most globally-important source to the atmopshere from shallow-water ecosystems-which are largely freshwater and with emergent rooted plants (Bastviken et al, 2011). Tidal effects can introduce another pathway of transport whereby during low-tide conditions, CH4 -rich pore water is transported to adjacent creeks and estuaries through hypsometric gradients (Deborde et al, 2010; Santos et al, 2012; Stieglitz et al, 2013)

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