Abstract
Microcystins, potent toxins produced by cyanobacteria, occur widely in aquatic systems across the world, and are a known environmental and public health hazard. Therefore, it is important to understand their fate in aquatic systems. As an important component of aquatic systems, sediments play multiple roles in the distribution and natural attenuation of microcystins. However, systematic studies identifying the effects of environmental variables on the variability of microcystins in lake sediments are lacking, and the contribution of sediments to the removal of these toxins in the water body are not yet clear. Hence, this research aimed to: (a) investigate the variability of microcystins in lake sediments; and (b) identify the relative contribution of the biodegradation and adsorption ability of sediments to the removal of microcystins from the water. As research into microcystins in lake sediments has been hindered by the lack of an effective analysis method, a further aim of this study was to develop a method to quantify these toxins in sediment samples using supercritical carbon dioxide.
The first part of this research involved a field study, analysing microcystin concentrations in lake sediments and their correlation with environmental variables. Microcystins were detected in all sediment samples, even at one of the sampling sites with negligible cyanobacterial biomass present in the water. The concentration of these toxins in lake sediments had a weak, but significant correlation with intracellular microcystins, total microcystins and cyanobacterial biomass in the water. Furthermore, their variability of in lake sediments could be explained by a combination of total microcystins in the water, cyanobacterial biomass in the water, pH and temperature.
In the second part of this research, changes in the concentration of microcystin-LR (MCLR) in the water in the presence of sediments were quantified in a laboratory experiment. The results of this experiment showed that each time MCLR was added to sterile lake water in the presence of sediments, MCLR concentration decreased significantly following an exponential decay curve, with no observed lag phase. Comparison between different treatment conditions implied that the adsorption and biodegradation ability of sediments caused the MCLR removal and that biodegradation was the dominant mechanism.
The final part of this research investigated the use of supercritical carbon dioxide in quantifying microcystins in sediment samples. A protocol was developed which included the optimisation of extraction conditions using supercritical carbon dioxide. This protocol was use to quantify microcystin concentrations in natural field samples. The results showed that for sediment samples with added MCLR, the conventional method recovered more spiked MCLR but fewer microcystin variants. In contrast, supercritical carbon dioxide with water as modifier extracted a higher amount of total microcystins.
Overall, this research highlights the wide occurrence of microcystins in the sediments of the studied lake, and the biodegradation ability of sediments to remove microcystins quantitatively from the water. This study suggests that researchers and water management authorities should include sediments when assessing the potential hazards and fate of microcystins in aquatic systems.
The first part of this research involved a field study, analysing microcystin concentrations in lake sediments and their correlation with environmental variables. Microcystins were detected in all sediment samples, even at one of the sampling sites with negligible cyanobacterial biomass present in the water. The concentration of these toxins in lake sediments had a weak, but significant correlation with intracellular microcystins, total microcystins and cyanobacterial biomass in the water. Furthermore, their variability of in lake sediments could be explained by a combination of total microcystins in the water, cyanobacterial biomass in the water, pH and temperature.
In the second part of this research, changes in the concentration of microcystin-LR (MCLR) in the water in the presence of sediments were quantified in a laboratory experiment. The results of this experiment showed that each time MCLR was added to sterile lake water in the presence of sediments, MCLR concentration decreased significantly following an exponential decay curve, with no observed lag phase. Comparison between different treatment conditions implied that the adsorption and biodegradation ability of sediments caused the MCLR removal and that biodegradation was the dominant mechanism.
The final part of this research investigated the use of supercritical carbon dioxide in quantifying microcystins in sediment samples. A protocol was developed which included the optimisation of extraction conditions using supercritical carbon dioxide. This protocol was use to quantify microcystin concentrations in natural field samples. The results showed that for sediment samples with added MCLR, the conventional method recovered more spiked MCLR but fewer microcystin variants. In contrast, supercritical carbon dioxide with water as modifier extracted a higher amount of total microcystins.
Overall, this research highlights the wide occurrence of microcystins in the sediments of the studied lake, and the biodegradation ability of sediments to remove microcystins quantitatively from the water. This study suggests that researchers and water management authorities should include sediments when assessing the potential hazards and fate of microcystins in aquatic systems.
| Original language | English |
|---|---|
| Qualification | Doctor of Philosophy |
| Publication status | Unpublished - 2015 |
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