Assessing the origin of a massive cyanobacterial bloom in the Río de la Plata (2019): Towards an early warning system. (15th August 2020)

• Record Type: Journal Article
• Authors: Aubriot, Luis; Zabaleta, Bernardo; Bordet, Facundo; Sienra, Daniel; Risso, Jimena; Achkar, Marcel; Somma, Andrea
• Is Part Of: Water research. Volume 181(2020)
• Publication Date: 2020-08-15
• Subjects: Cyanobacterial bloom -- Remote sensing -- Climate -- Reservoir -- Estuary -- Early warning; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2020.115944
• Languages: English
• ISSNs: 0043-1354

Abstract

Abstract: The Río de la Plata estuary drains the second largest river basin of South America. The occurrence of frequent cyanobacterial blooms of the Microcystis and Dolichospermum complex in the Uruguayan coast are associated with high flows of Uruguay River due to rainy years. In summer 2019, a massive cyanobacterial bloom reached up to the Uruguayan Atlantic coast. This study seeks to unveil the origin and the environmental conditions that favored the occurrence of the last cyanobacterial bloom in the Río de la Plata, and to contribute with the development of an early warning system of cyanobacterial scum on Montevideo beaches. A complementary approach was applied with Sentinel-2 imagery, environmental data of monitoring programs of Salto Grande Reservoir and Montevideo beaches, hydro-meteorological information, and hydroelectric dam operation. Images were analyzed with the Normalized Difference Chlorophyll Index (NDCI), which allowed evaluating several water bodies within the same ranges. Positive anomalous rainfall increased river flows, particularly that of Uruguay and Negro rivers, which caused the opening of the dam spillways. NDCI maps showed that areas with high values (NDCI>0.06 ) in Salto Grande reservoir kept a similar surface area before and after the prolonged overflow period (8.7–7.8 km 2, before and after). In the Río Negro reservoirs, however, NDCI>0.06 coverage remarkably changed (62.5 km 2, Palmar reservoir), with a subsequent 56-fold reduction in the post-discharge of surface water. Twenty days after opening the spillways, Montevideo beaches were closed to swimming and the NDCI>0.06 surface reached 51.7 km 2 in the Río de la Plata coast. The dynamics of NDCI areas, the downstream bloom discharge, and the predicted Río de la Plata residual currents, suggest that the cyanobacterial bloom originated in the Negro River (Palmar reservoir). This bloom event was one of the worst that occurred in the Río de la Plata in last 20 years, circulated along the Uruguayan sub-corridor to the Atlantic coast along 690 km from its origin, and lasted three months on Montevideo coast. This is the first study that addresses the impact of cyanobacterial blooms from the Negro River reservoirs on the Río de la Plata estuary. Therefore, the Negro River basin is where the main efforts should be directed to mitigate massive cyanobacterial blooms. Graphical abstract: Image 1 Highlights: The greatest cyanobacterial bloom of the decade was evaluated in the Río de la Plata. Sentinel-2 NDCI-chlorophyll and hydrology revealed the bloom origin and displacement. The Palmar reservoir of Negro River originated and discharged the bloom downstream. High Uruguay River flow displaced the bloom along Río de la Plata coast to the Ocean. A framework of a four level early warning system based in on-line data is proposed.