, Volume 37, Issue 2, pp 381-398

Carbon Dioxide and Methane Emissions from Mangrove-Associated Waters of the Andaman Islands, Bay of Bengal

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Abstract

We estimated CO2 and CH4 emissions from mangrove-associated waters of the Andaman Islands by sampling hourly over 24 h in two tidal mangrove creeks (Wright Myo; Kalighat) and during transects in contiguous shallow inshore waters, immediately following the northeast monsoons (dry season) and during the peak of the southwest monsoons (wet season) of 2005 and 2006. Tidal height correlated positively with dissolved O2 and negatively with pCO2, CH4, total alkalinity (TAlk) and dissolved inorganic carbon (DIC), and pCO2 and CH4 were always highly supersaturated (330–1,627 % CO2; 339–26,930 % CH4). These data are consistent with a tidal pumping response to hydrostatic pressure change. There were no seasonal trends in dissolved CH4 but pCO2 was around twice as high during the 2005 wet season than at other times, in both the tidal surveys and the inshore transects. Fourfold higher turbidity during the wet season is consistent with elevated net benthic and/or water column heterotrophy via enhanced organic matter inputs from adjacent mangrove forest and/or the flushing of CO2-enriched soil waters, which may explain these CO2 data. TAlk/DIC relationships in the tidally pumped waters were most consistent with a diagenetic origin of CO2 primarily via sulphate reduction, with additional inputs via aerobic respiration. A decrease with salinity for pCO2, CH4, TAlk and DIC during the inshore transects reflected offshore transport of tidally pumped waters. Estimated mean tidal creek emissions were ∼23–173 mmol m−2 day−1 CO2 and ∼0.11–0.47 mmol m−2 day−1 CH4. The CO2 emissions are typical of mangrove-associated waters globally, while the CH4 emissions fall at the low end of the published range. Scaling to the creek open water area (2,700 km2) gave total annual creek water emissions ∼3.6–9.2 × 1010 mol CO2 and 3.7–34 × 107 mol CH4. We estimated emissions from contiguous inshore waters at ∼1.5 × 1011 mol CO2 year−1 and 2.6 × 108 mol CH4 year−1, giving total emissions of ∼1.9 × 1011 mol CO2 year−1 and ∼3.0 × 108 mol CH4 year−1 from a total area of mangrove-influenced water of ∼3 × 104 km2. Evaluating such emissions in a range of mangrove environments is important to resolving the greenhouse gas balance of mangrove ecosystems globally. Future such studies should be integral to wider quantitative process studies of the mangrove carbon balance.

Communicated by Alberto Vieira Borges