Wastewater discharge with phytoplankton may favor cyanobacterial development in the main drinking water supply river in Uruguay.

The main drinking water source supplying Uruguay (Santa Lucía River, SLR) was threatened in 2013 by a cyanobacterial bloom transported downstream to the water treatment plant that provides water to half of this country population. Several eutrophic reservoirs and stabilization ponds located in the river basin may have been the source of cyanobacterial populations. Such conditions may be common in productive basins; however, few studies have evaluated the impact of microalgae from wastewater stabilization ponds on rivers and its viability downstream. The effect of a dairy wastewater effluent on SLR was studied by means of nutrient and chlorophyll a loads, phytoplankton composition, and effluent incubation in river water in order to evaluate the potential development of cyanobacteria. Total phosphorus and nitrogen loads of the effluent reached up to 25% and 17% of SLR, respectively, while chlorophyll a was up to 37%. The upstream-downstream evaluation showed an increase in dissolved phosphorus and chlorophyll a. The effluent phytoplankton (14.16 mm3 L-1) was dominated by organisms < 10 μm and diatoms (91%), and 3% of cyanobacteria. Cyanobacteria were not detected in SLR, though it appeared downstream of the effluent discharge. At the end of the bioassay, cyanobacterial biomass became the dominant group (37%). This study shows the potential development of cyanobacteria present in industrial effluents when diluted in river water. The effect of phytoplankton discharge from stabilization ponds is not generally considered in monitoring assessments and environment management, despite representing a particular risk if the water body is used as a drinking water source.