Elevated atmospheric CO2 lowers leaf litter nutritional quality for stream ecosystem food webs

Up to 99% of the carbon fuelling the food webs of temperate woodland streams is derived from inputs of terrestrial leaf litter. Aquatic bacteria, fungi, and detritivore invertebrates directly utilize these inputs, transferring this energy to other components of the food web. Increases in atmospheric CO2 could indirectly impact woodland stream food webs by chemically altering leaf litter. This study evaluated CO2-induced chemical changes in aspen (Populus tremuloides) leaf litter, and the corresponding effects on stream bacteria, fungi and leaf-shredding cranefly larvae (Tipula abdominalis: Diptera). Leaf litter from plants grown under elevated CO2 had decreased nutritional value to aquatic decomposers and detritivores because of higher levels of structural compounds and lower nitrogen content. Consequently, elevated CO2-grown leaf litter supported 59% lower bacterial production in a stream than litter grown at ambient CO2 levels, while not affecting fungal biomass. Larval craneflies fed elevated CO2-grown microbially colonized leaves consumed less, assimilated less, and grew 12 times slower than their ambient fed counterparts.