Biosphere-Atmosphere Exchange of Greenhouse Gases in a Subtropical Mangrove Wetland in Hong Kong

Prof. LAI Yuk Fo, Derrick

This project quantified the magnitude, temporal variations, and environmental controls of the biosphere-atmosphere exchange of greenhouse gases, including CO2 and CH4, from a subtropical mangrove ecosystem in Hong Kong. We found that our subtropical mangrove site on one hand was a significant CO2 sink sequestering about 758-890 g CO2-C m-2 year-1 over the study period, while on the other hand was a net CH4 source emitting about 11 g CH4-C m-2 year-1. Hence, the overall climatic impacts of mangrove wetlands should take into account the exchange of both CO2 and CH4 with the atmosphere. CO2 flux showed a clear diurnal pattern, with the maximum gross primary production and net CO2 uptake being observed at noon. CH4 flux also a distinct diurnal trend with a higher emission from noon to late afternoon. Seasonally, we found that our mangrove had a greater mean net CO2 uptake during the dry season than wet season, in contrary to the conventional wisdom that ecosystems would sequester more atmospheric CO2 in the wet seasons owing to a greater availability of sunlight and moisture. This was partly because of a significant increase in ecosystem respiration during the warmer wet season that led to considerable CO2 loss, as well as an increase in salinity during the dry season that supported greater photosynthetic activity of mangrove plants. We also observed a significantly higher mean CH4 emissions during the wet seasons than the dry seasons in our mangrove site, possibly because of a higher temperature and lower salinity that supported CH4 production. We observed inter-annual variability of CO2 fluxes in our mangrove sites, with annual CO2 flux in the second year being about 15% lower than the first year, in response to changes in temperature and solar radiation. In contrast, inter-annual variations of CH4 fluxes were rather small. We found that light intensity, air temperature, salinity, tidal level and vapor pressure deficit played a role in governing temporal variations in CO2 flux, while soil temperature, salinity and tidal level would control the variations in CH4 flux.