Chronic effects of atrazine on tolerance of a benthic diatom

The effects of prior chronic exposure to atrazine on responses to subsequen t acute exposures were investigated using a common benthic diatom. Clonal, axenic cultures of Craticula cuspidata were established from the Platte Riv er (Nebraska) and obtained from a culture collection (unlikely prior exposu re to atrazine). All cultures received a chronic 67-d treatment of I mu g/L atrazine. and growth was monitored using fluorometric detection of chlorop hyll a. Chronic atrazine exposure significantly reduced growth rate only du ring the first day of treatment (p = 0.0001): no significant effect was det ected throughout the remainder of the 67-d period. Following the chronic tr eatment, clones were exposed to six atrazine concentrations (83, 188, 402, 860, 1,782, and 3.250 mu g/L) to ascertain whether prior exposure influence d the tolerance of this diatom. Prior chronic exposure had a negative effec t on growth following subsequent exposures to higher concentrations. A sign ificant decline in growth was detected on days 7, 9, and 12 between previou sly exposed and control clones at 83 mu g/L of atrazine, The lack of increa sed tolerance in C. cuspidata after a realistic chronic exposure indicates that the levels of atrazine presently found in many lotic systems may inhib it the growth of periphyton during periods of higher pulses of atrazine cha racteristic of spring runoff events.