Many blackwater streams and rivers of the Coastal Plain exhibit seasonally low levels of dissolved oxygen, due in part to microbial oxygen uptake during aerobic decomposition of organic matter. Much of this matter, whether in dissolved or particulate form, is produced by the floodplain forest. Forest composition in blackwater ecosystems changes through natural succession and anthropogenic disturbance, but we have a limited understanding of effects of these changes on oxygen dynamics. In a hypothetical system that receives equal-mass inputs of leaves from several common tree species, contributions to oxygen demand should vary temporally among species for two reasons: because of immediate effects of specific leaf quality on microbial oxygen uptake, and also because the biomass of each species remaining in the system will decline at different rates among species. We tested these hypotheses using the leaves of five common tree species in manipulative experiments. Total oxygen demand generated over the decomposition trajectory of each leaf litter species is extrapolated from repeated measurements of microbial oxygen uptake and leaf mass loss throughout the breakdown process. Leaf litter species that exhibited highest microbial oxygen uptake rates had short-lived effects due to rapid rates of breakdown and CO₂ release, and slowly-decomposing species had greater relative oxygen demand contributions as time progressed.