Low pH preempts bloom development of a toxic haptophyte

► A number of factors influence the development of Prymnesium parvum blooms; pH influences the magnitude of ambient toxicity caused by these blooms. ► In two 21 d studies examining pre-bloom and bloom development conditions, neutral pH levels preempted harmful P. parvum bloom development. ► Population densities of P. parvum never reached bloom proportions at pH 7 or 7.5; however, harmful blooms occurred at higher pH (8.5). ► Here we report for the first time that pH appears to govern P. parvum bloom formation and associated toxicity to aquatic life in inland waters.