Seasonal variation of cyanobacteria blooms and microcystin concentrations in an Amazonian river system.

Microcystins (MCs) are hepatotoxic cyanobacterial metabolites that pose risks to aquatic ecosystems and human health. Cyanobacterial blooms are promoted by high nutrient availability and elevated temperatures, negatively affecting water quality, supply, and recreational use. Despite these concerns, information on MCs occurrence in Amazonian waters remains limited. This study evaluated the seasonal variation of MCs concentrations in the Tapajós River and examined their relationship with environmental parameters throughout the hydrological cycle. We hypothesized that microcystin occurrence varies across the hydrological cycle of the Tapajós River, particularly during periods when hydrological conditions favor longer water residence times and the development of cyanobacterial biomass. Water samples were collected during 2020-2022 during different hydrological phases. A total of 37 samples were separated into filtered and biomass fractions and analyzed using high-performance liquid chromatography. MC-LR concentrations ranged from 0.056 to 1.353 μg/L. The highest cyanobacterial densities were recorded in Suruacá and Parauá, with dominance of the genus Dolichospermum. Generalized Linear Mixed Models indicated that MCs occurrence was negatively associated with water temperature and positively associated with dissolved oxygen. The hydrological cycle explained 43% of the variance, with higher MCs occurrence during dry and receding water periods, highlighting the need for monitoring in the Tapajós River.