Benthic diatoms as indicators of herbicide toxicity in rivers
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Agricultural herbicides are common pollutants of freshwater environments and pose a potential threat to aquatic biota. Assessing the impacts of herbicide pollution on primary producers such as benthic diatoms is essential in protecting freshwater ecosystems from degradation. Benthic diatoms are highly responsive to changes in environmental conditions and changes in community composition can be used to assess the ecological health of rivers. This thesis aims to investigate the impact of herbicide toxicity on benthic diatoms and to determine whether benthic diatoms are suitable indicators of herbicide toxicity in rivers that flow into the Great Barrier Reef (GBR). This was achieved through a series of scientific studies, each addressing key questions regarding the effects of herbicides on benthic diatoms. Benthic diatoms exposed to herbicides in rapid toxicity tests showed varying sensitivity to herbicides, some taxa being highly sensitive whilst others were unaffected by herbicide exposure. The relative sensitivity of the diatom taxa was consistent between herbicides with differing modes of action and was not altered under reduced light intensities. Prior pollution of the collection site was influential in determining response of diatom communities to herbicide exposure; the diatom community from a highly polluted agricultural stream was less affected than the community collected from a reference site with no history of prior exposure. My thesis identifies individual diatom taxa that are most at risk of herbicide toxicity and also taxa that are tolerant and able to thrive under high herbicide concentrations. This study found that benthic diatom communities within the GBR catchment were affected by herbicide toxicity, showing a decline in sensitive taxa with increasing contamination of the site, after the wet season. Diatom communities were also influenced by other environmental variables such as nutrients and salinity and separating the individual effects of herbicides will require further research. My thesis demonstrates the effects of herbicide toxicity on benthic diatoms at both the species and community levels. Each study in this thesis provides new insights into the effects of herbicide exposure on natural benthic diatom communities and contributes to the field of aquatic ecotoxicology. As a whole, my thesis illustrates the great potential that benthic diatoms have to assess agricultural impacts, including herbicides in rivers of the GBR catchment area.
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