Water quality and cyanobacterial blooms in the Piancó-Piranhas-Açu Watershed, Semi-arid Region of Northeastern Brazil: relationships with education toward sustainability

Abstract

Greater emphasis has been attached to aquatic ecosystems in recent decades, as these are associated with indicators determinant of sustainability indices. The environmental impacts of these ecosystems with potential risk to human health in the semi-arid region of the state of Rio Grande do Norte have been frequently observed. This study aimed at evaluating the cyanobacteria density, and the indices that indicate the Piancó-Piranhas-Açu Watershed quality. It also targeted to identify the factors that impact most the blooming of these aquatic organisms. The sampling design consisted of 12 collections in different seasonal periods from 2009 to 2016, carried out by the Água Azul Program on 6 points along the basin. Environmental variables were used to analyze the Water Quality Index (WQI), Trophic State Index (TSI), and Cyanobacteria Density (CD). The mean WQI indicated ‘Regular’ water status for points PPA3 and PPA4, while PPA1, PPA2, PPA5 and PPA6 resulted in ‘Good’. As for the TSI, the levels indicate average degree of eutrophication (mesotrophy) on three sampling points, high degree of eutrophication (eutrophy) on two points, and low degree of eutrophication (oligotrophy) on one point. In this sense, human activities - especially diffuse polluting sources - around the spring seemed to worsen these scenarios. The density of potentially toxic cyanobacteria was high, exceeding CONAMA Resolution 357/2005 that recommends a maximum value of 50,000 cells/mL, and the Ministry of Health Ordinance 2914/2011 that recommends a threshold of 20,000 cells/mL. Regarding the general correlation between CD and seasonality we found significant influence (p < 0.05), with greater proliferation of these organisms directly related to periods of drought. We concluded that although the WQI indicated that dams’ waters are suitable for multiple uses, the TSI had limitations, considering the excessive primary productions associated with the presence of cyanobacteria and, thus, the potential occurrence of cyanotoxins ensuing from pollutant loads. The study led us to conclude that the adoption of education practices toward sustainability would help environmental preservation and local social development. It would also enable the establishment of effective priority public policies on mitigation of water pollution.