High-frequency zones of phytoplankton blooms in the Río de la Plata Estuary associated with El Niño-Southern Oscillation

Abstract

In recent decades, the Río de la Plata Estuary has shown an increase in the frequency and intensity of phytoplankton blooms with negative impacts on production activities, human health, and biodiversity. Water quality monitoring programs provide samples from the coastal zone alone, which limits the collection of data inside the Estuary and the analysis of the spatio-temporal dynamics of phytoplankton blooms, as well as their relationship with flow rate. In this work, a systematic satellite monitoring of the Estuary was carried out for the first time. Sentinel-2 images captured during 2016–2021 were used along with the Normalized Difference Chlorophyll Index. It included one year of El Niño, one neutral year and two consecutive years of La Niña. Four zones with the highest frequency of bloom occurrence were delimited. Data on the extent and intensity in which blooms occurred were extracted and related to the flow rates of the main tributaries using Bayesian models. The most intense and frequent blooms were detected on the southern and northern coasts, respectively (maximum values of 515 km2 in January 2021, NDCI>0.06), followed by a wide area of intense phytoplankton development inside the Estuary. Blooms were more frequent in warmer months, with elevated Chl-a concentrations in 75% of the months of the study period on the Argentine coast, and 50% on the Uruguayan coast. Blooms were positively correlated with low flows. Therefore, the most extensive and intense bloom episodes occur during La Niña events. During El Niño, the high flows transport the biomass originating in the Estuary and in the hydroelectric reservoirs located upstream, which can even be transported along the northern coast. This work identified a recurrent pattern of phytoplankton blooms and the hydro-meteorological conditions that favor their magnification in a context of strong climate variability in the region and estuarine eutrophication.