Microbial race to colonise leaf litter in a littoral‐lake environment and its relation to nutrient dynamics

Abstract

Litter colonisation by fungi and bacteria is essential for decomposition. Although algae are not directly related to the decomposition process, in lentic environment, they usually colonise leaf litter rapidly and stimulate the activity of heterotrophs. We hypothesised that the timing of litter entering the water determines a rapid colonisation by algae, which affects fungal and bacterial colonisation, nutrient dynamics of leaf litter, and decomposition rates. We incubated two leaf litter species, Nothofagus antarctica and Potentilla anserina, in the littoral zone of a lake under two conditions: submerged from the beginning (aquatic: AQ) and near the shoreline (initially terrestrial: T‐A). The T‐A treatment was flooded on day 44, so the decomposition process continued in aquatic conditions. We estimated two decomposition rates, one before day 42 (k42) and one for the whole incubation period (ka), algal and fungal biomass, bacterial abundance, and nutrient (P and N) content in leaf litter throughout the experiment. The timing of litter entering the water affected decomposition rates, k42 and ka were faster in AQ than in T‐A conditions and only the ka was faster in P. anserina than in N. antarctica. Microbial colonisation was also affected. Algal biomass was always higher in the AQ treatment, but bacterial abundance differed between treatments and changed through the decomposition process. Fungal biomass depended on both the treatment and the litter species. Bacterial–algal and fungal–algal relationships in AQ treatment were synergistic and independent of time, while, with terrestrial exposure, microbial relationships depended on the stage of the decomposition process. Nutrient dynamics depended on the treatment and the species, but both species tended to gain or maintain nutrient content throughout the experiment. Fungi were related to N:P changes through the decomposition process. Our results suggest that the conspicuous growth of an autotrophic biofilm on some litter (in P. anserina) might retard decomposition but at the same time stimulate heterotrophic colonisation. Therefore, the study of decomposition in the littoral zone of lakes must include the autotrophic component of biofilms that grow on detritus, since it alters organic matter consumption by decomposers and nutrient dynamics.