Field level evaluation and risk assessment of the toxicity of dichloroacetic acid to the aquatic macrophytes Lemna gibba, Myriophyllum spicatum, and Myriophyllum sibiricum

Dichloroacetic acid (DCA), a haloacetic acid, is a common contaminant of aquatic ecosystems. A study to investigate potential phytotoxic effects on rooted and floating macrophytes (Myriophyllum spicatum, M. sibiricum, and Lemna gibba) was conducted. Replicate 12,000 L outdoor microcosms (n=3) were treated with 3, 10, 30, and 100 mg/L of DCA that had been neutralized to the sodium salt, plus controls. Plants were sampled regularly over 21 days and assessed for a variety of endpoints including plant growth, root growth, number of nodes, wet and dry mass, chlorophyll-a, chlorophyll-b, carotenoids, and citrate levels. EC10, EC25, and EC50 values were calculated for each endpoint that exhibited a concentration–response. Overall, M. sibiricum was slightly more sensitive than M. spicatum to DCA exposure. The most sensitive plant endpoints were wet mass and plant length. Pigments showed no response with exposure to DCA. The probability of current concentrations of DCA in Canadian lake water and Swiss river waters exceeding thresholds of toxicity derived from single species effect measure distributions (EC10s) is 0.01%. The use of effect measure distributions holds promise as a new risk assessment technique for aquatic plants. Currently, environmental levels of DCA do not pose a risk to these plants.