Ecotoxicologia de cianobactérias no canal principal do Baixo Rio Tapajós, Santarém, Pará, Amazônia, Brasil

Abstract

Cyanobacteria are natural components of phytoplankton and its accelerated development (flowering) has been associated with environments under strong human pressure. The concern with the increase of these organisms is a function of its potential to produce cyanotoxins and the cause for this production are not well understood. In Rio Tapajós, frequent episodes of blooms generate concern because it is one of the major waterways of the West Region of Pará For this reason it was aimed to investigate the ecotoxicity and community structure of cyanobacteria in the Tapajós River and their relationship with limnological and climatic factors. Samples were collected from October 2010 to October 2011 in the main channel of the Lower Tapajós River. Were analyzed: richness, diversity, evenness, density, chlorophyll-a, microcystin, water temperature and atmospheric, transparency, turbidity, color, quota, electrical conductivity, total dissolved solids, oxygen, pH, phosphorus, nitrogen series, precipitation and speed the wind. Was found eleven species of cyanobacteria distributed in ten genres: Aphanothece (1 spp.), Chroococcus (1 spp.), Dolichospermum (antes Anabaena) (2 spp.), Lyngbya (1 spp.); Merismopedia (1 spp.), Microcystis (1 spp.), Oscillatoria (1 spp.), Pseudanabaena (1 spp.), Synechococcus (1 spp.) and an unidentified taxon, Cyanobacteria (1 spp.). These Synechococcus and Microcystis were the most frequent. Microcystin levels were detected throughout the period and its maximum concentration was associated with the proliferation of Microcystis. Among the parameters investigated, the more influence on the ecotoxicology of cyanobacteria were the availability of nitrogen and phosphorus, light winds and low level of the river. Although no significant difference was found regarding seasonality, it was found that the growth of cyanobacteria concentrated early in the rainy season when the river received a higher amount of nitrogen and phosphorus. The flood pulse exerted significant influence on the richness of cyanobacteria, occurring during the rising waters the greatest richness. In this sense, the sharp human settlement and land use, especially the right bank of the Rio Tapajós, with activities such as agriculture and livestock, have affected the nutrient cycle in the basin as they increase the input of nutrients in the watershed. These changes trigger limnological, considering that the 60s to the present electrical conductivity increased by a factor of 2 of the Tapajós and water 1.2 meters has become less transparent. This variation highlights the contribution of organic and inorganic compounds in the river, providing a disturbance in the system with excessive development of cyanobacteria. Although compliance throughout the period, with the standards established by the guidelines of CONAMA Resolution 357/2005 and Decree 2.914/2011 of the Ministry of Health, the frequent presence of the genera Microcystis, Dolichospermum and Oscillatoria (potentially toxic genera) plus the detection of microcystin throughout the period, set up an alert to public health.