The toxic effects of 2,4-D on Phaedoactylum tricornutum (Bohlin) and D
unaliella tertiolecta (Butcher), two species of phytoplankton well sui
ted to bioassay studies and responsive to pollutants, were studied by
monitoring changes in growth in terms of cell populations, chlorophyll
fluorescence and the rate of (CO2)-C-14 assimilation. Short term bioa
ssays, batch and continuous cultures were studied. Pure 2,4-D acid app
eared more toxic than the commercial amine form of the herbicide but t
his may have been due to small quantities of acetone present in the so
lvent. Concentrations of amine herbicide in excess of 100 mg l(-1) ext
ended the duration of the lag phase and inhibited growth but smaller c
oncentrations stimulated growth, the amine being consumed by phytoplan
kton in preference to nitrate. Continuous culture confirmed the abilit
y of phytoplankton to adapt slowly to herbicide concentrations even as
high as 500 mg l(-1). It is suggested that green algae adapt more rap
idly to environmental change than do diatoms.