Abstract
AimDrought periods often occur in Brazilian semiarid region and are supposed to induce water quality degradation by changes in physical, chemical and biological properties of freshwater ecosystems. Reservoirs in this region are used as drinking-water supplies and are exposed to wide volume fluctuations during drought periods due to lack of precipitation and high evaporation rates. This study aimed to identify patterns on water quality of two reservoirs during a long drought period. It was expected that more arid and shallower conditions would favor algal growth by enhancing nutrient availability, causing a decrease on water quality.MethodsThe study was based on monthly sampling over 20 months (May 2011 to December 2012) at two tropical reservoirs on Brazilian semiarid region. Precipitation and volume data were obtained from environmental agencies. Transparency was measured on field using a Secchi disk and conductivity, nutrients, suspended solids and chlorophyll-a were analyzed on laboratory. Temporal changes in all environmental variables were analyzed in each reservoir using two-way cluster analysis and also principal component analysis (PCA).ResultsThe volume of both reservoirs decreased considerably over the study because of low or shortage of precipitation. It was possible to detect two opposite patterns of chlorophyll-a in each reservoir throughout the drought season: in the first one phytoplankton growth was favored, while in the second one chlorophyll-a decreased by high inorganic turbidity. Both reservoirs tended to increase their turbidity and conductivity during the drought period due to shallow conditions, which probably contributed to sediment resuspension.ConclusionsWater level reduction during the extended drought period, contributed for water quality degradation due to high algal biomass and also high turbidity found during drought period. Local factors, as the nature of suspended solids, play an important role on predicting water quality.
Highlights
The frequency and intensity of precipitation, as well as evaporation rates can cause wide ranges in the water level of freshwater ecosystems, especially in reservoirs
The data accounts for more eutrophic conditions for Dourado reservoir (Table 1)
Both reservoirs showed a considerable reduction in their water levels, which was followed by reduced water quality in terms of high Chl-a concentrations or suspended solids
Summary
The frequency and intensity of precipitation, as well as evaporation rates can cause wide ranges in the water level of freshwater ecosystems, especially in reservoirs. Previous studies have found that shallower conditions in semiarid reservoirs are associated with higher nutrient concentrations (TP and NO3), conductivity, turbidity and algal biomass, leading to water quality degradation and more eutrophic conditions (Naselli-Flores, 2003; Geraldes & Boavida, 2005, 2007, Mac Donagh et al, 2009). Eutrophic conditions can favor algal growth because of greater nutrients availability and volatile solids concentration, which by consequence, increases organic turbidity. Algal biomass, even in rich nutrient conditions, could be decreased by high fixed suspended solids (inorganic turbidity) leading to low light availability (Kang et al, 2013). Light and nutrients could act as limiting factors on algal biomass (Reynolds, 2006)
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