Organic carbon burial efficiency in a large tropical hydroelectric reservoir

Abstract

Abstract. Hydroelectric reservoirs bury significant amounts of organic carbon (OC) in their sediments. Many reservoirs are characterized by high sedimentation rates, low oxygen concentrations in bottom water, and a high share of terrestrially derived OC, and all of these factors have been linked to a high efficiency of OC burial. However, investigations of OC burial efficiency (OCBE, i.e. the ratio between OC buried and deposited) in reservoirs is limited to a few studies, none of which include spatially resolved analyses. In this study we determined the spatial variation in OCBE in a large tropical reservoir and related it to sediment characteristics. Our results show that the sediment accumulation rate explains up to 92 % of the spatial variability in OCBE, outweighing the effect of other variables, such as OC source and oxygen exposure time. OCBE at the pelagic sites varied from 48 to 86 % (mean 67 %) and decreased towards the dam. At the margins, OCBE was lower (9 to 17 %) due to the low sediment accumulation in shallow areas. Our data show that the variability in OCBE both along the rivers-dam and the margin-pelagic axes must be considered in whole-reservoir assessments. Combining these results with a spatially resolved assessment of sediment accumulation and OC burial in the studied reservoir, we estimated a whole-basin OC burial efficiency of 57 %. Being the first whole-basin assessment of OCBE in a reservoir, these results suggest that reservoirs may bury OC more efficiently than natural lakes.