Abstract
This study investigated the dynamics of cyanobacteria in two deep, eutrophic reservoirs in a semi-arid region of Brazil during periods of stratification and destratification. Four collections were carried out at each reservoir at two depths at three-month intervals. The following abiotic variables were analyzed: water temperature, dissolved oxygen, pH, turbidity, water transparency, total phosphorus, total dissolved phosphorus, orthophosphate and total nitrogen. Phytoplankton density was quantified for the determination of the biomass of cyanobacteria. The data were analyzed using CCA. Higher mean phytoplankton biomass values (29.8 mm(3).L(-1)) occurred in the period of thermal stratification. A greater similarity in the phytoplankton communities also occurred in this period and was related to the development of cyanobacteria, mainly Cylindrospermopsis raciborskii (>3.9 mm(3).L(-1)). During the period of thermal destratification, this species co-dominated the environment with Planktothrix agardhii, Geitlerinema amphibium, Microcystis aeruginosa and Merismopedia tenuissima, as well as with diatoms and phytoflagellates. Environmental instability and competition among algae hindered the establishment of blooms more during the mixture period than during the stratification period. Thermal changes in the water column caused by climatologic events altered other physiochemical conditions of the water, leading to changes in the composition and biomass of the cyanobacterial community in tropical reservoirs.
Highlights
The dominance of cyanobacteria in aquatic ecosystems is often associated with CO2 availability (Shapiro 1990, Caraco and Miller 1998), high temperatures (Bouvy et al 2000, Huszar et al 2000, Figueredo and Giani 2009, Soares et al 2009a), low luminosity (Niklisch and Kohl 1989), high pH (Reynolds and Walsby 1975, Briand et al 2002), high concentrations of nutrients (Watson et al 1997), a low N:PÊNIO W
Eighteen species of cyanobacteria belonging to Chroococcales, Oscillatoriales and Nostocales were identified
The phytoplankton biomass was predominantly composed of cyanobacteria, especially in the period of thermal stratification, which accounted for 92% to 97% and 63% to 87% of the total biomass in the Arcoverde and Pedra reservoirs, respectively
Summary
The dominance of cyanobacteria in aquatic ecosystems is often associated with CO2 availability (Shapiro 1990, Caraco and Miller 1998), high temperatures (Bouvy et al 2000, Huszar et al 2000, Figueredo and Giani 2009, Soares et al 2009a), low luminosity (Niklisch and Kohl 1989), high pH (Reynolds and Walsby 1975, Briand et al 2002), high concentrations of nutrients (especially phosphorus) (Watson et al 1997), a low N:PÊNIO W. The dominance of cyanobacteria in aquatic ecosystems is often associated with CO2 availability (Shapiro 1990, Caraco and Miller 1998), high temperatures (Bouvy et al 2000, Huszar et al 2000, Figueredo and Giani 2009, Soares et al 2009a), low luminosity (Niklisch and Kohl 1989), high pH (Reynolds and Walsby 1975, Briand et al 2002), high concentrations of nutrients (especially phosphorus) (Watson et al 1997), a low N:P. There is consensus among researchers regarding the factors that best explain the dominance and blooms of cyanobacteria, such as ample nutrient availability, an increase in temperature, stratification, an increase in residence time and salinization (Paerl and Huisman 2008). Nutrients are an important factor, physical variables better explain changes in the composition of species in systems dominated throughout the year by cyanobacteria (Soares et al 2009a)
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