Variations of carbon dioxide fluxes over a southern inland water in the USA

Inland waters are important conduits of terrestrially fixed carbon to the atmosphere. Their role in the regional and even global biogeochemical cycles may be much more important than previously thought. Therefore, it is necessary to quantify their magnitudes and temporal variations as well as controlling factors through direct measurements of CO2 fluxes and other meteorological and limnological variables in the water-atmosphere system. Here we analyzed eddy covariance-based measurements of CO2 fluxes over the Ross Barnett Reservoir (32°26'N, 90°02'W; 117.5 m a.s.l.) (which is always ice-free), with a mean depth of 5 m and surface area of approximately 134 km2 in central Mississippi in 2008. Our results indicated that CO2 fluxes exhibited distinctive diurnal and monthly variations. Monthly average diurnal cycles of CO2 fluxes showed diurnal sine waves, with maximums occurring in the early morning (about 3:00 – 5:00 LT) and minimums in the late afternoon (about 17:00 – 18:00 LT). It was found that these variation patterns corresponded well to changes in atmospheric stability in the atmospheric surface layer over water. The daily maximums for CO2 fluxes ranged from about 0.2 µmol mol-1 to 4.2 µmol mol-1, whereas the daily minimums ranged from about 0.1 µmol mol-1 to 1.0 µmol mol-1. Monthly mean CO2 fluxes were high from March to May and from October to December and low for the rest months, ranging from 0.1 µmol mol-1 to 0.3 µmol mol-1. Variation patterns of monthly mean CO2 fluxes were best explained by those of monthly precipitation. This correspondence may suggest that higher CO2 effluxes were due to the increased terrestrial carbon inputs from the catchment by precipitation events. It was estimated that annual carbon efflux from this reservoir was about 0.8 Mg C ha-1 yr-1, which is slightly smaller than, but comparable to the estimate for tropical inland waters.