Variação espaço-temporal de cianobactérias no Baixo Rio Tapajós, Pará, Brasil

Abstract

Cyanobacterial blooms have occurred in the Tapajós River, Western Pará, Brazil. This overgrowth of potentially toxic cyanobacteria has been recorded worldwide, often associated with anthropized lentic environments. The phenomenon is less frequent in lotic environments, and there is a shortage of investigations on the subject in the Amazon. The present study aimed to analyze the spatial and temporal variation of cyanobacteria in water, collected on a monthly scale for 10 months on five beaches for recreational use on the Tapajós River, and to verify the condition of bathing and potability regarding the presence of cyanobacteria. The richness in species of cyanobacteria was evaluated by the number of taxa per sample and quantitative analysis of cyanobacteria was performed by the method of Utermöhl (1958). There were 22 taxa of cyanobacteria distributed in 11 genera, of these Microcystis, Dolichospermum, Aphanocapsa, Lyngbya, and Planktothrix present potentially cyanotoxin-producing species. Throughout the sampled period cyanobacteria were produced in the water and no spatial variation in the richness and population density of cyanobacteria was found. On the other hand, the temporal difference of cyanobacteria was found in which the highest values of richness and density were recorded during the period of least precipitation and during the low water and low water phases. It is suggested that, during the drought and low water and low water period, the effect of upsetting and resuspension of deep sediment and nutrients, coupled with hydrodynamic conditions of the Tapajós River, favorable limnological conditions the growth of cyanobacteria, which can be intensified by anthropic action. The peak tourist season coincided with the time when there were the largest blooms of cyanobacteria. During the months of June and December the water of the Tapajós river was inappropriate for recreational contact. In addition, in 34% of the investigated samples, there was the possibility of human contamination via water consumption during bloom moments, as it was in disagreement with the regulations. Although the notification of spot blooms of potentially toxic cyanobacteria in the Tapajós River is a sufficient warning about the possibility of cyanotoxins being released into the water, the monitoring of these toxins cannot be overlooked by the authorities.