Year-round CO2 emissions from the drawdown area of a tropical reservoir: Strong seasonal and spatial variation

A growing body of literature points to drawdown areas as important sources of atmospheric CO2 2 within reservoirs. Yet seasonal and temporal patterns of CO2 2 flux from periodically exposed sediments in drawdown areas remain poorly understood. Here we evaluate the annual and diel (24-hour cycle) variations in CO2 2 emissions from sediments periodically exposed to the atmosphere. We sampled sediments in the drawdown area of a tropical reservoir, which encompassed two primary adjacent land covers-grassland and forestland-in the watershed of a reservoir located in southeastern Brazil. We also experimentally assessed the effect of rewetting on CO2 2 emissions from exposed sediments. We found large variations in emissions during all hydrological periods (from 10 to 10116 mg C/m-- 2 (- |-) d-1),-1 ), except for the late rainy period. Land use and how distant dry sediments were from the open water significantly affected drawdown CO2 2 emissions, with higher emissions occurring in areas surrounded by forest than those adjacent to grassland. Our diel-cycle analysis did not show significant variation of emissions over daily cycles. Furthermore, a rewetting experiment indicated a significant increase in emissions 30 min after the onset of the rewetting event. Although drawdown areas only cover 20 % of the reservoir's area, they account for 80 % of the reservoir's total CO2 2 emissions. Ultimately, single-time measurements can lead to considerable underestimation (up to 52 %) or overestimation (up to 190 %) of whole- reservoir CO2 2 emissions.