Avaliação plurianual da carga de nutrientes e do índice do estado trófico em uma seção do rio Paraíba do Meio – Alagoas

Abstract

Due to population growth in the world, natural resources, especially aquatic ecosystems, are affected by an intense transformation. Characterized as a small river, the Paraíba do Meio river at the present time is suffering with a high degradation process due to the way in which its use for social and/or economic purposes is done by the local population. Therefore, the objective of this research was to determine the Trophic State Index (TSI) and the behavior of the nutrients load in a section of the Paraíba do Meio river (Alagoas - AL) during the period of two hydrological years (2016-2017) and its social and environmental implications. The flow rate and rainfall data were obtained through the HidroWeb portal of the National Agency for Water and Basic Sanitation – ANA. Flow rate and rainfall data were correlated in the section of the river for the period under study. The data referring to the physical-chemical parameters of the water were obtained from the Database of the Environmental Geochemistry Laboratory (IGDEMA-UFAL), which are: Ammonium (NH4), Nitrite (NO2-), Nitrate (NO3-), Dissolved Phosphorus, Dissolved Oxygen (OD), Turbidity (UNT), Water Temperature (°C), Chlorophyll a, Salinity, Potential of Hydrogen (pH), Electrical Conductivity and Total Suspended Solids (TSS), which were compared with the standards established by CONAMA Resolution Nº 357/2005. For the determination and analysis of the Trophic State in the water, the Trophic State Index (TSI) obtained through the Carlson method (1977) modified by Lamparelli (2004) was used. The total average rainfall for the period 1988-2017 was 1116.13 mm, whereas in 2016 the total annual rainfall was 921.44 mm and in 2017 it was 1866.9 mm. About flow rate, the average for the period 1988-2017 was 22.51 m3 s-1, with the annual average in 2016 being 7.97 m3 s-1 and 58.40 m3 s-1 in 2017. The correlation between the flow rate and rainfall of the period 1988-2017 showed Pearson and determination coefficients of 0.90 and 0.79, respectively. For the years 2016 and 2017, the coefficients were 0.85 and 0.73 and 0.95 and 0.88, respectively. The annual averages of ammonium were 76.57 μg.L-1 in 2016 and 49.5 μg.L-1 in 2017. About nitrite, the annual averages were 19.46 μg.L-1 in 2016 and 7.90 μg.L-1 in 2017. About nitrate, its annual averages were 126.48 μg.L-1 in 2016 and 227.67 μg.L-1 in 2017. In 2016 the annual average of phosphorus total was 111.34 μg.L-1 and 4.41 μg.L-1 for chlorophyll a, in 2017 the annual average of phosphorus total was 98.64 μg.L-1 and 13.15 μg.L-1 for chlorophyll a. The TSI ranged from 59 (eutrophic) to 66 (supereutrophic) in 2016 and from 58 (mesotrophic) to 72 (hypereutrophic) in 2017. It is concluded that rainfall and flow rate can modified the nutrient concentrations and their behavior in river water, and that the concentration of phosphate and dissolved inorganic nitrogen forms (nitrite, nitrate and ammonium) during the years 2016 and 2017 were, partly, changed by the flow rate. By analysing the TSI obtained, the river can be classified into two main zones of trophic state: eutrophic and supereutrophic, furthermore there are few months with mesotrophic and hypereutrophic conditions.