The impact of a declining water table on observed carbon fluxes at a northern temperate wetland

Wetland biogeochemistry is strongly influenced by water and temperature dynamics, and these interactions are currently poorly represented in ecosystem and climate models. A decline in water table of approximately 6 cm/year was observed at a wetland in northern Wisconsin, USA over a period from 2001–2007. Eddy covariance measurements of carbon dioxide exchange in conjunction with the declining water table revealed an increase in ecosystem respiration of over 20% as water table depth fell through a range between 5 and 35 cm below the surface. Ecosystem respiration was not correlated with water table outside of this range. The limits of the range were dependent on temperature, with the effect of water table penetrating deeper at higher temperatures. Yearly average ecosystem production was approximately 20% higher in years with low water table than in years with high water table. As the water table declined, evapotranspiration decreased and ecosystem water use efficiency increased. Wetland net ecosystem exchange was not correlated with water table, but in 2007, a year with an exceptionally dry growing season, the wetland site was a net carbon source. These results suggest that changes in hydrology may not have a large impact on wetland carbon flux over inter-annual time scales due to opposing responses in both ecosystem respiration and productivity. However, this balance appears to be sensitive to changes in the seasonal distribution of precipitation.