Spatiotemporal Variation on Water Quality and Trophic State of a Tropical Urban Reservoir: A Case Study of the Lake Paranoá-DF, Brazil

Damiana B Da Silva,Thiago B Lima,Jackeline Do S B Barbosa,Valéria R Bellotto

doi:10.3390/w13223314

Abstract

Nutrient enrichment and eutrophication are among the main problems that lead to the deterioration of water quality in lakes and reservoirs. In this study, spatial and temporal variations in the concentrations of organic and inorganic species of nitrogen and phosphorus in the water column of Lake Paranoá-DF (Brazil) were evaluated between 2016 and 2017. Seasonality was the main factor in the variations in concentrations of the investigated parameters. Additionally, we found differences in behavior for different nutrients and other variables that indicate different main sources of each nutrient as well as different biogeochemical processes predominating in each season. For example, the electrical conductivity (EC), dissolved silicon, PO43−, and NO3− showed mean concentrations significantly higher in the rainy season, indicating greater inputs in these periods (which is in part related to increasing soil leaching and runoff). Agricultural activities were the main source of NO3− and wastewater treatment plants (WWTP) proved to be the main source of nutrients, mainly NH4+ and all forms of phosphorus. These two allochthonous sources are also the determining factors of the trophic state and the degradation of the water quality of Lake Paranoá. The lake is in the transition process from a mesotrophic to a eutrophic condition.

Highlights

The goal of this study was to assess the water quality and dynamics of organic and inorganic species of nitrogen and phosphorus in Lake Paranoá, identifying possible seasonal and spatial differences in concentrations, and identifying the main sources of the nitrogen and phosphorus, as well as the predominant biogeochemical processes operating in the system and the current trophic state and trend
The data showed that there is no thermal stratification in Lake Paranoá (Figure 3), unlike those registered in other reservoirs located in the Northern region of Brazil such as Balbina, Tucuruí, Samuel, and Curuá-Una
In the absence of thermocline formation in Lake Paranoá, there are no vertical variations in the distribution of physicochemical and biological variables resulting from the restriction of homogenization of the water column due to the density barrier caused by a pronounced vertical difference in temperature

Summary

Introduction

Water quality and productivity of lakes and reservoirs are largely controlled by the quantity and quality of external nutrient loads [1]. In turn, can greatly intensify the natural eutrophication process, currently considered as one of the main problems affecting water quality [2,3]. Climate warming will tilt the CH4 balance towards higher lake emission with this impact exacerbated by the eutrophication of the lakes [4]. As has been widely reported in the literature, lakes and reservoirs are important sources of atmospheric methane. A recent estimate of global CH4 emission suggests that freshwater systems emit 103 Tg year− 1 of CH4 , with lakes contributing 62%

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