Toxicity of atrazine to the estuarine phytoplankter Pavlova sp. (Prymnesiophyceae): increased sensitivity after long-term, low-level population exposure.

Phytoplankton are potentially more at risk to the adverse effects of herbicides than many other organisms in estuarine ecosystems. The focus of this study was to characterize the toxicity of a widely used herbicide, atrazine, to a single species of phytoplankton. The nanoplankter Pavlova sp. was grown under controlled laboratory conditions and was used to elucidate short-term, high-level population effects of atrazine. Secondly, a long-term, multigenerational population exposure was performed with a low level of atrazine followed by an additional short-term, high-level exposure on the same population with no recovery period. The 96-h growth rate of 50% effective concentration (EC50) for Pavlova sp. was 147 [microg/L (95% CI = 116.4-178.7 microg/L). Long-term exposure at 20 [microg/L for four (batch culture) growth cycles (approximately 20 generations) had no significant effect on the growth rate of Pavlova sp. except during the fourth growth cycle. However, a subsequent short-term, high-level 96-h exposure inoculated from the long-term, low-level exposed population showed increased sensitivity to atrazine (96-h growth rate EC50 = 96.0 microg/L, 95% CI = 90.2-103.7 microg/L). Multigenerational exposure to atrazine appeared to render phytoplankton significantly more sensitive (35%) to atrazine effects. Given the documented persistence of atrazine in surface waters, long-term exposure to low levels of atrazine without recovery may lead to shifts in species sensitivity and potential alterations in phytoplankton population dynamics.