Many North American blackwater rivers exhibit low dissolved O₂ (DO) that may be the result of benthic respiration. We examined how tree species affected O₂ demand via the quantity and quality of litter produced. In addition, we compared areal estimates of surface leaf-litter microbial respiration to sediment O₂ demand (SOD) and ecosystem respiration (ER) in stream and swamp reaches of a blackwater river to quantify contributions of surface litter decomposition to O₂ demand. Litter inputs averaged 917 and 678 g m^-2 y^-1 in the swamp and stream, respectively. Tree species differentially affected O₂ demand via the quantity and quality of litter produced. Bald cypress (Taxodium distichum) contributed most litter inputs because of its dominance and because it produced more litter per tree, thereby making greater relative contributions to O₂ demand in the swamp. In the stream, water oak (Quercus nigra) produced litter supporting lower fungal biomass and O₂ uptake rates, but produced more litter than red maple (Acer rubrum). Breakdown rates in the swamp were faster, whereas standing stock decreases were lower than in the stream, indicating greater organic matter retention. Surface litter microbial respiration accounted for 89% of SOD (6.37 g O₂ m^-2 d^-1), and 57 to 89% of ER in the swamp. Our findings suggest that surface litter drives the majority of O₂ demand in some blackwater swamps, and tree species with higher rates of litterfall may make larger contributions to ER. Forested swamps may be hotspots of O₂ demand in blackwater rivers because low water velocities enhance retention.