OBJECTIVE:

To assess the spatiotemporal dynamics of cyanobacteria on beaches in Barcarena, Pará State, Brazil, relating them to physicochemical, nutrient, and climatic variables, while evaluating water quality based on Brazilian legislation.

MATERIALS AND METHODS:

Samples were collected at three equidistant points along Conde, Itupanema, and Caripi beaches in September 2021 and March 2022 for quantitative analyses of cyanobacteria, photosynthetic pigments, and physicochemical factors. Spatial and temporal variability was observed, with precipitation, wind, temperature, electrical conductivity, dissolved oxygen, nitrate, and alkalinity being the most relevant variables.

RESULTS:

A total of 41 cyanobacterial species were identified, with Planktolyngbya limnetica, Aphanocapsa delicatissima, and Aphanizomenon sp. as the most abundant. The driest month showed the highest density (490.0 ± 304.0 cells·mL⁻¹) and lowest species richness (23 species). Conde Beach presented the lowest overall density.

CONCLUSION:

Despite these variations, all beaches were classified as suitable for bathing. The findings indicate that hydrological factors and rainfall regimes strongly influence cyanobacterial community structure. In the context of climate change, this study provides a reference for future research and environmental monitoring, emphasizing the presence of potentially toxic species and associated health risks in Amazonian fluvio-estuarine environments.