AQUATIC PHYTO-TOXICITY OF 23 PESTICIDES APPLIED AT EXPECTED ENVIRONMENTAL CONCENTRATIONS

Environment Canada uses an Expected Environmental Concentration (EEC) in evaluating the hazard of pesticides to nontarget aquatic organisms. This concentration is calculated by assuming an overspray of a 15 cm deep waterbody at the label application rate. The EEC of pesticides is then related to the EC50 (concentration causing a 50% reduction in a chosen toxicity endpoint) for a given aquatic test organism. At presen t, the use of an uncertainty factor is suggested in the literature if only a few species are tested because of important interspecific diffe rences in pesticide sensitivity. The phytotoxicity of the EEC of 23 di fferent pesticides to ten algae (24 h inhibition of C-14 uptake) and o ne vascular plant (7-day growth inhibition) was determined in an effor t to examine the question of interspecific sensitivity and its relatio n to the development of pesticide registration guidelines. Chemicals i ncluded five triazine herbicides (atrazine, cyanazine, hexazinone, met ribuzin, and simazine), four sulfonylurea herbicides (chlorsulfuron, m etsulfuron-methyl, ethametsulfuron-methyl, triasulfuron), two phenoxya lkane herbicides (2,4-D and MCPA), two pyridine herbicides (picloram a nd triclopyr), a substituted urea, an amine derivative, and an imidazo linone herbicide (tebuthiuron, glyphosate and imazethapyr, respectivel y), a bipyridylium (diquat), a hydroxybenxonitrile (bromoxynil), an al dehyde (acrolein) and an acetanilide (metolachlor) herbicide, as well as two carbamate insecticides (carbofuran and carbaryl) and a triazole derivative fungicide (propiconazole). Test organisms were selected ba sed on ecological relevance and present use in test protocols. Organis ms included green algae (Scenedesmus quadricauda and Selenastrum capri cornutum), diatoms (Nitzschia sp. and Cyclotella meneghiana), cyanobac teria (Microcystis aeruginosa, Oscillatoria sp., Pseudoanabaena sp., A nabaena inaequalis and Aphanizomenon flos-aquae) and a floating vascul ar plant, duckweed (Lemna minor). The five triazine herbicides, acrole in and diquat inhibited the carbon uptake of an algae, diatoms and cya nobacteria by more than 50%. Two other pesticides, carbaryl and tebuth iuron, caused more than 50% inhibition in 90% of the algae tested. Nin e of the 23 pesticides, five of which were triazine herbicides, were t herefore highly phytotoxic to algae. Twelve pesticides inhibited growt h of duckweed by more than 50%. Once again, all five of the triazine h erbicides were among this group, as well as three sulfonylurea herbici des and acrolein, diquat, metolachlor and tebuthiuron. Duckweed was th e most sensitive organism tested, being equally affected by all pestic ides causing algal phytotoxicity (with the exception of carbaryl), as well as being acutely affected by sulfonylurea herbicides. Green algae were least sensitive to diquat; diatoms and one cyanobacterium were t he only organisms that showed sensitivity to glyphosate. Through testi ng the phytotoxicity of a variety of agricultural pesticides to a wide range of algal taxa, it is evident that there are considerable differ ences in sensitivity among species and that the use of an uncertainty factor is necessary to provide an acceptable margin of safety in evalu ating the hazard presented by these chemicals to the aquatic environme nt.