Both microbes and macroinvertebrates mediate the contribution of different litter species to benthic oxygen demand

Many blackwater rivers exhibit low dissolved oxygen due, in part, to microbial oxygen uptake as leaves decompose.  Riparian forest composition can potentially affect benthic oxygen demand by contributing leaf litter of differing quality which determines microbial biomass.  In a hypothetical system receiving equal-mass inputs of leaves from several tree species, contributions to oxygen demand should vary temporally among species for two reasons:  because of leaf quality effects on microbial biomass, and because masses of leaves remaining in the system decline at different rates among species.  Although microbial biomass determines respiration rates per gram of leaf, macroinvertebrates may be key drivers of breakdown.  We tested these hypotheses by incubating leaves of five tree species in two hydrologically different reaches (third-order stream and sixth-order swamp) of a blackwater river draining Georgia’s coastal plain. We measured respiration, decomposer biomass, and leaf mass loss through time.  Microbial respiration differed significantly among leaf litter species in both reaches, but was correlated with fungal biomass only in the third-order stream, possibly due to oxygen limitation within leaves in the swamp.  Although macroinvertebrate diversity was lower in the swamp, shredder biomass was significantly higher, and rapid consumption reduced contributions of labile leaf litter to benthic oxygen demand.