Zonal and tidal effects on trace gas flux in a mangrove ecosystem in B.C.S, Mexico

Background/Question/Methods

Mangrove ecosystems are important in terms of their contribution to trace gas budgets, where anoxic conditions within waterlogged sediments allow for the production of N₂O and CH₄ as microbial intermediate and end products. The decomposition or organic matter by bacteria and production of radiatively important gases is based on the availability of electron acceptors as well as the abundance of different assemblages of metabolic bacteria. In the presence of oxygen, aerobic respiration occurs, creating suboxic and anoxic zones where nitrification, denitrification, iron reduction, sulfate reduction, and methanogenesis occur. The aims of the present study are not only to quantify the flux of trace gases but also to elucidate how tidal and zonal shifts in electron acceptors and aboveground productivity affect the flux of CO₂, CH₄, and N₂O. The present study took place within the Bahia Magdalena Lagoon Complex (BMLC) located on the Pacific coast of Baja California Sur, Mexico. All sampling took place along a land to sea transect in order to capture differences in productivity, substrate availability, and trace gas flux along zones subject to varying periods of inundation during tidal regimes. Six collars for measurements of N₂O and CH₄ gas flux were placed along each of 5 land to sea transect and sampled using the static chamber technique and analyzed using gas chromatography. CO₂ flux data was collected using the LI-7000 gas analyzer using the dynamic closed chamber method. Sediment cores and pore water samples were collected within a 1m area of collars and used to determine concentration of dissolved gases, as well as nitrate, ammonium, phosphorus, and Fe²⁺concentrations.   Litter was captured within traps and used as a proxy for productivity within each tidal zone. 

Results/Conclusions

Preliminary data revealed the flux of CH₄ from mangroves to be on the low end of observed values (-15.78 to 13.31 mmolm²h¹), presumably due to methanotrophy and substrate limitation as noted in previous studies.  CO₂ flux, average of 6.24 mM C m^-2d^-1, showed zonal pattern, though small sample size precluded statistical analysis, with the largest fluxes observed in the high tide zone. Fluxes of N₂O ranged from -4.19 to 6.57 mmolm^-2h^-1 with a moderate zonal pattern in N2O emission, with the largest fluxes observed in mid and high tidal zones and significantly reduced fluxes in low tidal zones (p=0.10). The lack of a significant difference between mid and high tidal zones could be due to the zonal overlap of nitrous oxide production.