Abstract
Although multiple herbicide exposures are more prospective to occur in water, many previous studies were carried out as single herbicide exposure. To investigate the toxic effect of prometryn on cyanobacteria and water qualities, single and double prometryn exposures (at different growth phases) on Microcystis aeruginosa growth and concentrations of nutrients were compared after a 44-day experiment. Results indicated that under single exposure, maximum inhibition rates were 4.7–12.0% higher than those under double exposures. Correspondingly, the maximum Microcystis aeruginosa densities and growth rates under single exposure were 10.3–21.1% and 19.5–37.7% lower than those under double exposures (p < 0.05), respectively. These findings revealed that repeated prometryn exposures resulted in a reduction in biological effects, because the time of application and the concentration injected during the first application were both significant factors in the biological effects of prometryn. Prometryn exposure scenarios did not have a significant effect on nutrient or nutrient consumption concentrations (p > 0.05). In general, the pattern of nutrient limitation showed a shift from phosphorus to nitrogen limitation. The quantified relationships between Microcystis aeruginosa growth rates and consumed nutrients were studied. Based on the above findings, we believe that a high-dose and single prometryn exposure is a more effective exposure pattern for limiting cyanobacteria growth.
Highlights
As reported by Kniss [1], with a steady, linear trend, the intensity of herbicide use has increased over the last 25 years
The kinetics of M. aeruginosa growth process, concentrations of nutrients, and concentrations of consumed nutrients are computed from the 10th day onwards, because relative algae density counting errors are relatively large when algae densities are low at the start of the experiment [20]
Equation (1) can describe the variation of M. aeruginosa growth with time with good accuracy (R2 = 0.966–0.998), and this is in agreement with the reported results [19]
Summary
As reported by Kniss [1], with a steady, linear trend, the intensity of herbicide use has increased over the last 25 years. Herbicide toxicity has been reported in aquatic organisms, such as microalgae [2]. Microalgae are organisms of ecological importance in the aquatic food chain, so the structure and function of the entire aquatic ecosystem can be affected by ecosystem toxic stress caused by herbicides [3,4]. Prometryn in water is quite stable with a long half-life time (above 390 d) and difficult to degrade [9,10]. Prometryn can bioaccumulate in the food web and be transported to long distances, making it a persistent pollutant in the environment [8]. Prometryn has been banned in several countries and regions. It is still being widely used in China, South Africa, and many other countries [11]
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