Toxicity and bioavailability of copper herbicides (clearigate, cutrine-plus, and copper sulfate) to freshwater animals

In designing aquatic herbicides containing copper, an important goal is to maximize efficacy for target species while minimizing risks for nontarget s pecies. To have a margin of safety for nontarget species, the concentration , duration of exposure (i.e., uptake), and form (i.e., species) of copper u sed for herbicidal properties should not elicit adverse effects on populati ons of nontarget species. To determine the potential for risk or adverse ef fects (conversely the margin of safety), data regarding the comparative tox icity of copper-containing herbicides are crucial. A series of comparative toxicity experiments was conducted, including baseline estimates of toxicit y (LC50s, LOECs), sensitive species relationships (thresholds and exposure- response slopes), and bioavailability of toxic concentrations and forms of copper 7 days after initial herbicide application. Aqueous 48-h toxicity ex periments were performed to contrast responses of Daphnia magna Strauss, Hy alella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promel as Rafinesque to copper herbicides: Clearigate(R), Cutrine(R)-Plus, and cop per sulfate. D. magna was the most sensitive aquatic animal tested for all three herbicides; 48-h LC50s for organisms exposed to Clearigate, Cutrine-P lus, and copper sulfate were 29.4, 11.3, and 18.9 mug Cu/L, respectively. I n terms of potency (calculated from the linearized portion of the exposure- response curves, which included 50% mortality), D. magna was the most sensi tive animal tested. Organisms exposed to Clearigate, Cutrine-Plus, and copp er sulfate had exposure-response slopes of 2.55, 8.61, and 5.07% mortality/ mug Cu/L, respectively. Bioavailability of Clearigate and Cutrine-Plus was determined by comparing survival data (LC50s) of test organisms exposed to herbicide concentrations during the first and last 48-h of a 7-day exposure period. Even in these relatively simplified water-only exposures, a transf ormation of copper to less bioavailable species over time was observed with a 100-200% decrease in toxicity (i.e., an increase in 48-h LC50s) for all four test animals. This series of laboratory experiments provides a worst-c ase scenario for determining the risk associated with the manufacturer's re commended application rates of Clearigate (100-1,000 mug Cu/L), Cutrine-Plu s (200-1,000 mug Cu/L), and copper sulfate (100-500 mug Cu/L) in natural wa ters for four nontarget freshwater animals.