Contribution of methane and lateral carbon fluxes in a temperate marsh carbon budget

Background/Question/Methods

Lateral transport of dissolved organic carbon (DOC) and particulate organic carbon (POC) can account for a significant portion of the carbon budget in coastal wetlands. The allochthonous carbon imported from adjacent terrestrial ecosystems might be released as CO₂ or CH₄ or deposited in sediments in these systems. We examined the relevance of CH₄ emission and lateral hydrological carbon fluxes in context of ecosystem C budget in a 129-ha managed marsh wetland located in the Winous Point along the Lake Erie shoreline from March 2011 to March 2013. CO₂ and CH₄ fluxes were measured using an eddy covariance tower. The DOC and POC fluxes were measured at the three inlets from adjacent croplands/woodlands and one outlet to the lake. Carbon sedimentation was obtained by the feldspar marker method.        

Results/Conclusions

The climate was warmer and dryer in 2012 than 2011. The water level in the marsh was maintained 0.1~0.7 m above ground by controlling the outflow gate. Gross ecosystem productions (GEP) were estimated 553 and 491gC m^-2 yr^-1 in 2011 and 2012, respectively. Ecosystem respirations (ER) were 527 and 490 gC m^-2 yr^-1 in 2011 and 2012, corresponding to 95% and 100% of the GEP. CH₄ emission showed a clear seasonality with high value of ~600 mg m^-2 day^-1 on hot summer days. Overall, the CH₄ fluxes can be reasonably modeled by the surface water temperature (R²= 0.86-0.88). The annual CH₄ emissions were 47 and 63gC m^-2 yr^-1 in 2011 and 2012 (8% and 13% of the GEP), respectively, which is higher than the average (24±6 gC m^-2 yr^-1) for North American wetlands. The lateral DOC/POC transports showed a typical flow-in/flow-out regime in 2011. The three inlets imported 80/8 gC m^-2 yr^-1 DOC/POC, while the outlet exported 36/4 gC m^-2 yr^-1. In contrast, the outlet was less frequently opened in 2012 in order to maintain inundated conditions in the marsh. During the dry late summer and fall, the water flow reversed in the inlets and exported significant amount of DOC/POC back to the upstream farm ditches. The annual DOC/POC imports from the inlets reduced to 11/ 3 gC m^-2 yr^-1. The DOC/POC outflows reduced to 7/2 gC m^-2 yr^-1. Clearly, the lateral transports and CH₄ emissions account for significant portion (2-16% and 8-13% of annual GEP) in the marsh carbon budget. Hydrological management, by maintaining flooding and controlling the inflow/outflow regime, has considerable influence on wetland carbon cycles.