Meteorological and potential climatic influence on high cyanobacterial biomass within Patos Lagoon (southern Brazil): A case study of the summer of 2019–2020

Abstract

Cyanobacterial blooms are a potential threat to human communities and ecosystems. Since the late 1980s, researchers have reported harmful cyanobacterial colonies in Patos Lagoon (PL), the largest coastal lagoon in South America. Most studies concerning harmful blooms in PL have focused on its biology and on its southernmost estuarine region, with little information about its displacement inside the lagoon and the influence of physical forces on its dynamics. This study uses satellite-derived information (normalized difference chlorophyll-a index – NDCI), river discharge data, and meteorological data (wind speed and direction, rainfall, and air temperature) to analyze two bloom episodes in PL, during the austral summer of 2019/2020, specifically in its larger, limnic portion. A 30-year meteorological time series was used to contrast the same summer period. Two remote sensing images from Sentinel-2 were taken of PL margins, near their central portion. The summer of 2019/2020 was drier when compared with the historical data, characterizing low river discharge. This environmental condition was coupled with high temperature, which implies thermal stratification in summer even at 2-m depth sites, which might have promoted cyanobacterial growth and accumulation inside PL. Moreover, weak winds (<<6 m s−1) seemed to accumulate cyanobacterial patches on the water surface, including after vertical mixing caused by strong winds (>6 m s−1). The NDCI values represented the two days of blooms, with higher values occurring under higher water temperatures and low wind speeds.