D. magna were exposed to rapid, extreme shifts in water hardness and/or alkalinity.
Without a precipitation event, the tested shifts were not lethal to D. magna.
If CaCO3 precipitated in test water, 98–100% of D. magna died within 96 h.
CaCO3 precipitate from a mining impacted stream contained cemented organisms.
Rapid CaCO3 precipitation may cause species loss and wide-scale ecosystem effects.