TOXICITY OF 2,4-D ACID TO PHYTOPLANKTON

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.