New lake in a changing world: the construction and filling of a small hydropower reservoir in the tropics (Rio de Janeiro, Brazil).

Climate change has affected rainfall patterns in tropical regions, where simultaneous demands for water and energy, habitat loss, declining biodiversity, and spread of invasive species have reflected a rapidly changing world underway. In Brazil, hydropower generation accounts for 64% of the electricity matrix, which presently includes 1007 small hydropower plants (SHPs) having many others under construction or planned. This paper aimed to evaluate changes in water quality, plankton communities, and benthic macroinvertebrates during dam construction, filling, and the first year of operation of a SHP. Suspended solids, turbidity, and silica were variables that highlighted the impact of this construction on the river. Fast changes in water quality (increases in calcium, chlorides, and nitrate) and on aquatic communities (i.e. euglenophyceans and testate amoebae increased in numbers) were detected during the filling phase. Following SHP construction, the concentrations of metals and total phosphorus tended to decrease. Two striking findings observed in the aquatic communities from the riverine conditions to the new lake were the increase in picocyanobacteria abundance, expanding population stocks throughout the river basin, and the constant presence of the invasive mollusc Corbicula fluminea in the macroinvertebrate assemblage, revealing once again its resistance to environmental variability. The lake soon became a natural trap for ions from the drainage basin, as revealed by the increase in electrical conductivity, ammonium, potassium, and magnesium concentrations and the abundance of cyanobacteria, highlighting the need for watershed management to improve ecological conditions in the lake.