Triazine herbicides alter growth competition to promote cyanobacteria blooms

Pesticides and herbicides applied during intensive agriculture can contaminate aquatic environments where they have selective effects on phytoplankton, alter community structure and have been hypothesized as a mechanism promoting cyanobacteria blooms. We used two-species competition cultures with different relative starting concentrations of the chlorophyte Oesmodesmus asymmetricus and the cyanobacterium Anabaena circinalis to determine whether outcomes of growth competition could be reversed by the triazine herbicide atrazine. In 10 day competition experiments, 0. asymmetricus out-competed A. circinalis in the absence of atrazine, except when starting species dominance was 4 to 1 in favour of A. circinalis. In contrast, environmentally relevant concentrations of atrazine resulted in A. circinalis out-competing 0. asymmetricus even when starting dominance favoured 0. asymmetricus by 4 to 1. Atrazine inhibited both species during exponential growth, and was more severe for 0. asymmetricus. However during the latter half of experiments A. circinalis suppressed 0 . asymmetricus by release of an allelopathic factor. Our findings show triazine herbicides can favour A. circinalis, reverse competition outcomes even from a position of sub-dominance, and shift temperate community structure toward cyanobacterial dominance.