Abstract
Substantial amounts of organic matter (OM) from terrestrial ecosystems are buried as sediments in inland waters. It is still unclear to what extent this OM constitutes a sink of carbon, and how much of it is returned to the atmosphere upon mineralization to carbon dioxide (CO2). The construction of reservoirs affects the carbon cycle by increasing OM sedimentation at the regional scale. In this study we determine the OM mineralization in the sediment of three zones (river, transition, and dam) of a tropical hydroelectric reservoir in Brazil as well as identify the composition of the carbon pool available for mineralization. We measured sediment organic carbon mineralization rates and related them to the composition of the OM, bacterial abundance and pCO2 of the surface water of the reservoir. Terrestrial OM was an important substrate for the mineralization. In the river and transition zones most of the OM was allochthonous (56 and 48%, respectively) while the dam zone had the lowest allochthonous contribution (7%). The highest mineralization rates were found in the transition zone (154.80 ± 33.50 mg C m-2 d-1) and the lowest in the dam (51.60 ± 26.80 mg C m-2 d-1). Moreover, mineralization rates were significantly related to bacterial abundance (r2 = 0.50, p < 0.001) and pCO2 in the surface water of the reservoir (r2 = 0.73, p < 0.001). The results indicate that allochthonous OM has different contributions to sediment mineralization in the three zones of the reservoir. Further, the sediment mineralization, mediated by heterotrophic bacteria metabolism, significantly contributes to CO2 supersaturation in the water column, resulting in higher pCO2 in the river and transition zones in comparison with the dam zone, affecting greenhouse gas emission estimations from hydroelectric reservoirs.
Highlights
Significant efforts have been made to understand the carbon fluxes into and out of inland waters
The reservoir is mesotrophic with total phosphorus (TP) concentrations ranges of 25–37 μgL−1, total nitrogen (TN) concentrations ranges of 530–800 μgL−1, and chlorophyll-a concentrations ranges of 2–6 μgL−1 (Rangel et al, 2012)
The organic carbon (OC) mineralization rates in the sediment indicated that the allochthonous organic matter (OM) supply was an important fuel to the metabolism in the sediment of different zones of the Manso Reservoir
Summary
Significant efforts have been made to understand the carbon fluxes into and out of inland waters. Discerning the pathways of carbon flow, especially related to aquatic sediments, remains uncertain (Cole et al, 2007). The relationship between the allochthonous carbon loaded to aquatic ecosystems and the portion either mineralized or buried in sediments has yet to be better quantified (Battin et al, 2009; Sobek et al, 2011). Inland waters (e.g., lakes and reservoirs) receive large amounts of allochthonous organic matter (OM) transported from the surrounding watershed, part of which is sedimented. The limited knowledge of the lateral transport of carbon from soils, and its fates in lakes and reservoirs results in incomplete understanding of the terrestrial carbon balance (Tranvik et al, 2009)
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