Atrazine selects for ichthyotoxic Prymnesium parvum, a possible explanation for golden algae blooms in lakes of Texas, USA.

Prymnesium parvum Carter is a mixotrophic haptophyte which, under certain environmental conditions, produces potent toxins responsible for fish kills around the world since the 1930s. Many P. parvum blooms have occurred in catchments where crop agriculture is a dominant land use; however, the effects of herbicides on bloom dynamics have not yet been investigated. Aquatic microbial communities containing P. parvum were subjected to two separate experiments involving the addition of either atrazine or glyphosate at varying concentrations. After 14, 21, and 28 days at 10 μg/l atrazine we observed that the relative abundance of P. parvum was significantly higher compared to the control. After 28 days, the relative abundance of P. parvum was approximately 53% higher in 10 μg/l atrazine compared to the control. Glyphosate exhibited no statistically-significant effect on the relative abundance of P. parvum. Inadequate characterization of the microbial community and uncertainty due to ecological and allelopathic effects of P. parvum made it difficult to establish strong relationships between herbicide sensitivity and nutritional mode. Large volumes of mobile and persistent herbicides with high toxicity to phytoplankton are used in cotton defoliation in Texas prior to the typical P. parvum pre-bloom period. These results have important implications for management, such as whether reduction in herbicide runoff could decrease the frequency and duration of P. parvum blooms in the future.