Abstract
Harmful cyanobacterial blooms (cHABs) are increasing in their frequency, duration and intensity on a global scale, posing a serious threat to human health and the environment. The current trend in cyanobacterial prevalence is influenced by nutrient enrichment, watershed modification and global climate change, however these factors alone do not adequately predict bloom formation. The role that common agrochemicals play in modifying phytoplankton communities remains poorly understood but the literature suggests that these compounds may play an important role in cHABs. To understand if there is a differential effect of the herbicide glyphosate on the members of a freshwater phytoplankton community, a dose-dependent response was explored on a simulated phytoplankton community using single species cultures of the cyanobacterium Microcystis aeruginosa and the green alga Haematococcus pluvialis. M. aeruginosa was outcompeted at medium (10 mg L-1) and high (100 mg L-1) concentrations of glyphosate. However, this relationship was reversed as M. aeruginosa outcompeted a robust competing alga, H. pluvialis, at 1.0 mg L-1 glyphosate concentrations. Cyanobacterial dominance at low concentrations of glyphosate is concerning because these concentrations (~1.0 mg L-1) are the most commonly observed in nature. Furthermore, this study supports the notion that glyphosate-based herbicides proliferate cHABs through eutrophication, not through inhibition of competing phytoplankton groups.
