Species‐specific effects of leaf litter leachate on the aquatic microbial community and the ratio of heterotrophic to autotrophic biomass

Abstract Dissolved organic carbon (DOC) released from submerged leaf litter is an important allochthonous carbon source for heterotrophic bacteria that enhances the detrital food chain in freshwater ecosystems. In contrast, nitrogen (N), phosphorus (P) and micronutrients released during the leaching process support primary production and enhance the grazing food chain in these ecosystems. This study investigated how DOC and dissolved N and P from the leaf litter of two temperate broadleaf and two coniferous tree species affect heterotrophic and autotrophic biomass by incubating the freshwater microbial community in the culture media prepared from the leaf litter leachate. The results showed that heterotrophic biomass, including bacteria, heterotrophic‐flagellated protists and fungal zoospores, was the highest in the leaf litter leachate, which contained the highest DOC concentration relative to dissolved N and P (Japanese hemlock). In contrast, autotrophic biomass, including small algae, autotrophic flagellated protists and cyanobacteria, was associated with concentrations of dissolved N and P in the leaf litter leachates. Amendment of basal nutrients other than N and P, including micronutrients, trace elements and vitamins, increased autotrophic biomass in two leaf litter leachates (oak, Siebold's beech), suggesting that some elements other than N and P were deficient relative to the requirements of autotrophic organisms. These results indicated that the elemental composition of leaf litter leachates influences microbial community structure and heterotrophic versus autotrophic biomass in freshwater ecosystems. More importantly, the present results suggest that changes in forest vegetation surrounding freshwater ecosystems alter detritus‐based heterotrophic production relative to autotrophic production through the changes in the elemental composition (i.e. stoichiometry) of leaf litter inputs.