Leaf-litter leachates from helophytes enhance the activity of freshwater biofilms receiving inputs from a wastewater treatment

Abstract

Bioengineering techniques use aquatics plants (i.e., helophytes) to restore degraded stream habitats. However, helophytes can alsocontribute to ameliorate stream water quality because they uptake dissolved nutrients and they provide habitat and labile carbon tomicrobial communities in the rhizosphere. We hypothesize that during plant senescence, leachates from helophyte leaf litter wouldserve as a labile carbon source to promote grow and microbial activity of biofilms on sediments. We tested the effect of leachatesfrom two common helophytes used in bioengineering techniques (Iris pseudacorus and Phragmites australis) and compared biofilmbiogeochemical responses with those from a brewery by-product rich in sugars. In the laboratory, we exposed biofilms developedon artificial flumes receiving water from a wastewater treatment plant (WWTP) effluent and planted with Iris and Phragmites to thethree different carbon sources (the two leachates and the brewery by-product) and to the water from the WWTP effluent which servedas control. The total microbial abundance increased during the incubation experiments with labile carbon compared to the control,regardless of the carbon source considered. The three labile carbon sources clearly enhanced denitrification and aerobic respirationof the biofilms compared to the control. However, the abundance of denitrifying bacteria was not influenced by the treatments considered.Overall, results suggest that leaf litter leachates from helophytes used in stream restoration can promote aerobic respirationas well as denitrification of stream biofilms, which can contribute to enhance the capacity of streams to transform and reduce N excessesfrom the WWTP effluent inputs.