Salt pulses effects on in-stream litter processing and recovery capacity depend on substrata quality.

Abstract

Human activities have greatly extended and intensified freshwater salinization, which threatens the structure and functioning of streams and rivers. Research on salt effects on in-stream processes has been strongly biased towards chronic salinization at constant levels. The aim of this study was to assess microbial mediated decomposition of two leaf species contrasting in quality (alder and oak) and associated descriptors, during salt-pulsed contamination (salinization period) and after cessation of salt additions (recovery period). Leaves were incubated in a mountain stream (central Portugal) longitudinally divided over 22 m. Half of the stream (salinized half) was subjected to daily short-term sharp salinity increases (conductivity up to ~48 mS cm-1) during 7 days while the other half (control half) was used as control. During the salinization period, salt exposure negatively affected mass loss and microbial respiration rate of alder (high-quality resource) while effects on fungal sporulation rate were independent of leaf quality. Fungal biomass was not impacted. After the recovery period, mass loss and respiration rate in both leaf species were similar between experimental stream halves. Fungal biomass associated with oak was enhanced and sporulation rate of alder, maintained in the previously salinized half, remained depressed. These results point out that the effects of salt pulses may be more deleterious in streams exclusively lined by high (vs. low) quality riparian trees as a result of a less efficient microbial-mediated leaf processing, and a reduced contribution to the conidial pool, even beyond the salinization period.