Recovery of duckweed following chronic herbicide exposure: implications for risk assessment

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Lau, Carlie
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Herbicide exposure in surface water usually occur as pulses, which may allow aquatic primary producers to recover from the exposure. In this thesis, the quality of published studies on the recovery of aquatic primary producers (macrophyte, periphyton and phytoplankton) after herbicide exposure, as well as the recovery potential of two duckweed species after 2,4-D and atrazine exposure were examined. Forty peer-reviewed recovery studies on aquatic primary producers were reviewed and the reliability and ecological relevance of these studies were evaluated using transparent criteria. The literature review revealed a low percentage of recovery studies from both exposure (i.e., 30%) and recovery (i.e., 35%) periods with >50% of total strength for the method score and >50% for the total endpoint ecological relevance score for risk assessment. We recommend the development of standard protocols for recovery and guidelines to interpret recovery results in ecological risk assessment to improve the quality of future studies. In addition, the recovery potential of Lemna gibba and Lemna minor after 7, 14, 21 and 28 days of 2,4-D and atrazine exposure, followed by a transfer to clean media for 7 days were investigated. The EC50s and NOECs from the recovery period on the relative growth rate of Lemna sp. after the exposure were ≥ 558 μg/L and ≥ 117 μg/L for 2,4-D, and ≥ 271 μg/L and ≥ 89 μg/L for atrazine. The toxicity values suggest that the duckweed had no adverse effect after 2,4-D exposure, and were able to recover after atrazine exposure. We also evaluated the toxicity effect of 2,4-D and atrazine on duckweed after a 60-day continuous exposure at regulation concentrations with test concentrations up to 8 μg/L for 2,4-D and 20 μg/L for atrazine. We conclude that the current regulations are likely able to protect freshwater macrophytes from chronic exposure. Overall, we recommend that the recovery of macrophytes be considered in ecological risk assessments to ensure that future decisions are made with realistic and relevant data.
Recovery, Herbicide, Risk assessment, Primary producer