The origin and fate of sediment organic carbon in tropical reservoirs

Abstract: Recently, the construction of reservoirs has boomed, particularly in the tropics, but the impact of reservoirs on the global carbon cycle is not evident. Reservoirs accumulate sediments that simultaneously bury organic carbon (OC) and thus act as a C sink, and also produce methane (CH4) and thus emit a strong greenhouse gas. High temperature, internal production and sedimentation rates in tropical reservoirs may enhance both OC burial and CH4 production, however to a currently unknown extent. This thesis investigates the efficiency of the OC sink as well as the OC sources that feed into OC burial and CH4 production in four contrasting tropical reservoirs in Brazil. The results demonstrate that reservoir sediments receive both terrestrial and aquatic OC, and that terrestrial OC is more prevalent in reservoirs with low internal production, and in river inflow bays. Aquatic OC is present in the sediments of all studied reservoirs, particularly in the reservoirs with high internal production and at sites that are closer to the dam. Reservoirs that experience anoxic conditions or high sediment deposition rates are likely to bury terrestrial OC at higher efficiency than oxic environments, such as oxygenated reservoirs, rivers, floodplains and sea, while aquatic OC degrades as similar rates in both oxic and anoxic environments. Deposition of OC in anoxic sediment, however, results in high CH4formation rates that strongly depend on sediment age and nitrogen content. The CH4 formation decreases exponentially with sediment age, but never ceases completely in the studied reservoir sediment. CH4 formation is highest but decreases more rapidly over time in sediment with a high share of nitrogen-rich aquatic OC, indicating that management of nutrient input into the reservoir may decrease sediment CH4 formation.The thesis illustrates that reservoir sediments bury aquatic OC and also bury terrestrial OC with high efficiency, which represents an anthropogenic carbon sink that decreases the carbon footprint of hydropower. Simultaneously, the reservoir sediment produces CH4 that may be emitted into the atmosphere and consequently elevates the carbon footprint of hydropower. However, reservoir CH4 emission may be mitigated by reducing nutrient input into rivers and reservoirs.