Organic carbon burial efficiency in a subtropical hydroelectric reservoir

Raquel Mendonça,Simone Jaqueline Cardoso,Carlos Henrique Duque Estrada,Sarian Kosten,Fábio Roland,Marcos Paulo Figueiredo-Barros,Sebastian Sobek

doi:10.5194/bg-13-3331-2016

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 buried and deposited OC) in reservoirs are 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 subtropical 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 (9–17 %) 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 spatially resolved mean OC burial efficiency of 57 %. Being the first assessment of OCBE with such a high spatial resolution in a reservoir, these results suggest that reservoirs may bury OC more efficiently than natural lakes.

Highlights

The sediments of freshwater systems are important sinks of atmospheric CO2 assimilated through photosynthesis both in the aquatic and in the terrestrial environments (Stallard, 1998)
This study was conducted in Mascarenhas de Moraes, a large (272 km2) and deep hydroelectric reservoir located in southeastern Brazil (20◦17 1.52 S, 47◦3 46.53 W; Fig. 1), in the Cerrado biome, in a region characterized by subtropical climate
Sediment organic carbon (OC) burial was higher than or similar to OC mineralization in all pelagic sites in the Mascarenhas de Moraes reservoir, leading to site-specific OC burial efficiencies varying from 48 to 86 % in pelagic sites. These values are close to the upper end of OC burial efficiency (OCBE) of natural lakes

Summary

Introduction

The sediments of freshwater systems are important sinks of atmospheric CO2 assimilated through photosynthesis both in the aquatic and in the terrestrial environments (Stallard, 1998) This carbon sink corresponds to the fraction of the organic carbon (OC) that reaches the sediments and escapes mineralization by heterotrophic organisms and (photo)chemical reactions, remaining buried (accumulated). What is known about freshwater sediments is that OCBE tends to be strongly positively correlated with sedimentation rates, especially of terrestrially derived OC (Sobek et al, 2009; Gudasz et al, 2012) This explains, for instance, why hydroelectric reservoirs tend to be more important sites for OC accumulation, compared to lakes, as reservoirs receive relatively high inputs of fluvial sediments

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