Satellite monitoring of chlorophyll-a threshold levels during an exceptional cyanobacterial bloom (2018-2019) in the Río de la Plata

ABSTRACT

The Río de la Plata estuary, located in southeast South America, is of great social and environmental relevance for the region. In the summer of 2018–2019, an exceptional cyanobacterial bloom was detected along the northern coast of the estuary, which greatly affected tourism and recreation, as reported in previous studies and the public press. This is the first work that uses satellite images to quantitatively monitor the evolution of this bloom in an intermediate zone of the estuary, including its initial and ending stages. Chlorophyll-a threshold maps of 10 and 24 µg/L were detected from Sentinel-2 imagery between November 2018-May 2019, while the presence of cyanobacteria was confirmed from phytoplankton field samples and a continuous (point) record of phycocyanin fluorescence. The combination of both satellite and in-situ information was crucial to analyze the spatiotemporal variability of the bloom, and to study its qualitative relationship to environmental drivers: local winds, discharge of the Río de la Plata main tributaries, discharge of a local tributary, water temperature and salinity. Our study shows the complexity of short-term bloom dynamics in a coastal region of the Río de la Plata, particularly highlighting the role of local winds on surface transport and mixing conditions.

RESUMEN

El estuario del Río de la Plata, ubicado al sureste de América del Sur, posee una gran relevancia social y ambiental para la región. En el verano 2018-2019 ocurrió una floración excepcional de cianobacterias a lo largo de la costa norte del estuario, afectando el turismo y la recreación, según se informó en trabajos anteriores y en la prensa. Este es el primer trabajo que utiliza imágenes satelitales para monitorear cuantitativamente la evolución de esta floración en una zona intermedia del estuario, incluyendo su etapa inicial y final. A partir de imágenes de Sentinel-2, se obtuvieron mapas de umbrales de clorofila-a de 10 y 24 µg/L entre noviembre de 2018 y mayo de 2019. La presencia de cianobacterias se confirmó a partir de muestreos de fitoplancton y de un registro continuo (puntual) de fluorescencia de ficocianina. La combinación de información satelital y de campo fue crucial para analizar la variabilidad espaciotemporal de la floración, y para estudiar su relación cualitativa con distintos forzantes ambientales: vientos locales, caudales, temperatura y salinidad. Este trabajo evidencia la complejidad de la dinámica de una floración en la costa del Río de la Plata, destacándose el rol de los vientos locales en el transporte superficial y las condiciones de mezcla.

KEYWORDS:

• Estuary
• sentinel 2
• coastal waters
• optically complex waters
• water quality
• remote sensing
• algal bloom

Acknowledgments

This work was supported by the Agencia Nacional de Investigación e Innovación (ANII, National Research and Innovation Agency, Uruguay) under Grant FMV-1-2017-1-136098; Comisión Sectorial de Investigación Científica (CSIC, Scientific Research Committee) of the Universidad de la República, Uruguay, under the 2017 fund for scientific equipment; CSIC and UTE (National Electric Administration of Uruguay) under Grant ID24 (2017); and the Comisión Académica de Posgrado (Academic Graduate Committee) of the Universidad de la República, Uruguay, under the fellowship for graduate studies granted to F. Maciel. The authors want to thank Matías González, Rodrigo Mosquera, and the staff of Movil 1 S.A. and Techno Dive S.A. for their assistance during field measurements; and the assistance of Katherine Bombi in laboratory analyses. Part of the data used in this work was collected during field campaigns in the frame of an outreach project for UTE, and the authors also want to thank the co-responsible scientist in the project, Dr. Rafael Arocena.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

The data registered at the measurement station is available for academic studies by request to the corresponding author of this work, and after getting authorization from UTE.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/23863781.2023.2264511

Additional information

Funding

This work was supported by the Agencia Nacional de Investigación e Innovación [FMV-1-2017-1-136098]; Comisión Sectorial de Investigación Científica [CSIC-UTE ID24 (2017)].