Zoning the risk of transporting phytoplankton blooms from multiple reservoirs to the primary drinking water intake in Uruguay

Abstract

Abstract Cyanobacterial blooms pose potential health and economic risks when they reach drinking water intakes. In basins with intensive agricultural production, there are numerous small and medium-sized (0.25-60 ha) productive freshwater reservoirs. These reservoirs create favorable conditions for the excessive growth of phytoplankton, which can be transported downstream after heavy rainfall events. These environments have been identified as potential sources of cyanobacterial inoculum in Uruguay's main drinking water intake, located in the Santa Lucía River Basin. To pinpoint areas with the highest risk of cyanobacterial biomass export, this study integrated chlorophyll-a concentration monitoring in 325 reservoirs using Sentinel-2 satellite images, along with meteorological and structural information from the reservoirs. An Export Index (EI) for biomass was developed, and spatial distribution patterns were explored through geostatistical and multivariate analyses. According to the EI, the western and southern zones, characterized by intensive agricultural land uses such as dairy farming and horticulture, were the most at risk for biomass export. Conversely, the northeastern zone exhibited the lowest risk, aligning with extensive cattle ranching practices associated with low nutrient export to aquatic systems. This study marks the initial effort to assess the risk of transporting cyanobacterial inoculum from numerous reservoirs to Uruguay's primary drinking water intake, providing valuable input for the development of early warning systems for the occurrences of cyanobacterial blooms.