Freshwater sediment pesticide biodegradation potential as an ecological indicator of microbial recovery following a decrease in chronic pesticide exposure: A case study with the herbicide diuron

The aim of this study was to evaluate the use of freshwater sediment biodegradation potential as an ecological indicator for monitoring microbial recovery following a decrease in chronic pesticide exposure. For this purpose, a four-year case study (2008–2011) was conducted in a small stream (Morcille river) long exposed to high diuron concentrations, increasing from upstream to downstream. Our results show that the ban on diuron in December 2008 resulted in a progressive decrease in its concentrations in the Morcille river over the survey period. However, diuron remained present in the water three years after the ban. The spatio-temporal variations in the sediment biodegradation potential were assessed by radiorespirometry using [ring-U-14C] diuron to estimate diuron mineralization potentials. Between autumn 2008 and autumn 2011, mean diuron mineralization percentage after 15 weeks of incubation decreased by 65% downstream and by 82% in the intermediate sector, and mean 10% diuron dissipation time values increased between 143% (downstream) and 210% (intermediate). Thus the decrease in the level of chronic diuron exposure in the river also caused a fall in sediment diuron-mineralizing capacities, revealing a corresponding recovery of microbial communities. Our results show that the use of freshwater sediment biodegradation potential may be useful for assessing microbial recovery after a decrease in chronic exposure to pollutants, opening prospects for developing a new class of ecological indicator to monitor the recovery of biological quality of water resources. In this way, the use of molecular approaches based on direct extraction of nucleic acids from environmental matrices and their subsequent analysis by PCR-based approaches to quantify the abundance of pesticide-degrading communities could represent a promising alternative.