Evaluation of copper toxicity using site specific algae and water chemistry: Field validation of laboratory bioassays

Studies of metal toxicity to microalgae have predominantly been conducted using single non-target algae species and without due regard for the chemistry of the treated waters, leading to ineffective or excessive algaecide treatments. In this study, indigenous multi-algal species (Scenedesmus quadricauda, and Scenedesmus subspicatus and Oscillatoria agardhii) were used in laboratory toxicity bioassays under simulated field water chemistry (pH = 7.2, hardness = 196 mg L^-1 as CaCO₃, and alkalinity = 222 mg L^-1 as CaCO₃) to determine the optimum copper sulfate treatment dose to control algae growth in an irrigation canal. Toxicity bioassays were conducted using copper sulfate in chelated (with EDTA) and non-chelated (without EDTA) forms to assess the influence of the use of synthetic chelators in toxicity studies. Also, copper toxicity to the indigenous algae species was measured in the non-modified EPA test medium (pH = 7.5, hardness = 92 mg L^-1 as CaCO₃, alkalinity = 10 mg L^-1 as CaCO₃ and EDTA= 300 µg L^-1) to assess the impact of the water chemistry on algae inhibitory algal dosages. Under simulated water chemistry conditions, lower toxicity was measured in the test flasks with the chelated form of copper (96 h- EC₅₀= 386.67 µg L^-1 as Cu) as compared to those with the non-chelated metal (96 h-EC₅₀= 217.17 µg L^-1 as Cu). In addition, higher copper toxicity was measured in the test flasks prepared with the non-modified EPA medium using chelated copper (96 h-EC₅₀ = 65.93 µg L^-1 as Cu) as compared to their analogous microcosms with modified water chemistry (96 h-EC₅₀= 386.67 µg L^-1 as Cu), the increased water hardness and alkalinity in the latter case contributing to the decrease of the metal bioavailability. Results from laboratory experiments showed good correlation with copper dosages used in a small scale field testing to control algae growth, increasing confidence in laboratory bioassays.