Research Product

The use of aquatic plants in effluent toxicity evaluations is uncommon despite the availability of test methods and numerous recommendations for their use. It has been assumed that aquatic plants are less sensitive than animal test species and consequently, results from toxicity tests with invertebrates and fish have been used as a surrogate data base. This study evaluated the ability of such animal toxicity tests to provide 'safe' concentrations for in-stream periphyton. The toxicity of several samples of a treated municipal effluent were determined during 5-month tests to monitor short-term changes in periphyton photosynthesis (carbon-14 uptake) and observe effects on young production and survival of cultured daphnids and the fathead minnow. The effect levels from the various tests were compared. The effluent was seldom acutely toxic to Daphnia magna and the fathead minnow but it was consisently acutely and chronically toxic to Ceriodaphnia dubia. Significant inhibition and stimulation of periphyton photosynthetic activity occurred at concentrations of 6 to 39% effluent. Periphyton photosynthesis was a more sensitive effect parameter than animal survival and in some cases than Ceriodaphnia reproductive performance. Results indicate that effluent toxicity tests conducted routinely with daphnids and fish may not be sufficient to protect the aquatic flora in receiving waters. Therefore, estimates of phytotoxicity are needed for effective risk assessments for effluents.