An unpolluted regulated stream and its recovery gradient dependency from environmental variables

Unpolluted regulated stream and its recovery gradient dependency

Résumé

We studied the recovery gradient of a thermally altered regulated unpolluted piedmont stream and its dependency from the main environmental variables. Complex interactions between nutrients and the main physical parameters were identified as responsible of the observed trends. Thus, seasonal fluctuations of parameters such as dissolved oxygen and temperature in the hypolimnion of the upstream dam influenced the concentration of nitrogen compounds entering the stream from the hypolimnion to downstream sections, thus affecting invertebrate communities. Macroinvertebrate assemblages were studied in a 1.25 km river reach downstream from the dam during a 4-year period (2019-2022). Environmental parameters and nutrients were measured and hydrological alteration indices were calculated for the longitudinal gradient survey. The goal was to unravel the main environmental variables linked to the dam-induced hydro-morphological pressure affecting macroinvertebrate communities. Our analysis revealed natural-born hypolimnetic ammonia as the principal contributing physicochemical or hydrological parameter to community structure or biomonitoring metrics. Ammonia enters the river system mainly during the dam thermal stratification periods (spring-summer) and macroinvertebrate communities showed the highest recovery rates in the autumn, when the stream can achieve a good ecological status even as near as 25 m below the dam wall when ammonia concentrations are low.