• Effects of aerially-applied cotton herbicides, including diuron, on fish pond plankton
  Peter Perschbacher, Gerald Ludwig, and Hugh Thomforde
  
  USDA/ARS H.K. Dupree Stuttgart National Aquaculture Research Center

Three, aerially-applied cotton herbicides were tested for possible adverse impacts from drift on fish pond plankton. Phytoplankton biomass and productivity and zooplankton density were measured at four herbicide levels in triplicate 145-gallon outdoor pool mesocosms. Treatments were designed to evaluate simulated overspraying, and drift at low (1% of full field rates) and high levels (10% of the full rate), and no spraying (control). A 0.25-a earthen pond stocked with goldfish at the UAPB Aquaculture Research Station was the water source. Herbicides were evaluated at the approximate times of field application. Water was pumped into pools the day of the test and an intial preapplication water sample taken. Herbicide (without additives) was sprayed on the pool surface and evaluations repeated at 24 and 48 h post application. If effects were noted, selective evaluations were continued.
 
Commercial compounds and application rates (lb active ingredient per acre) in the order tested were: diuron (1.6), paraquat (0.94) and quizalofop (0.06). Fluazifop (0.188) had been tested as a soybean herbicide in previous trials and was not repeated. No measurable impact was noted with paraquat and quizalofop, as noted earlier with fluazifop. Diuron however impacted all phytoplankton parameters. Zooplankton samples are being analyzed at present and will be presented later.
 
Negative and positive impacts from diuron were noted after addition on March 5 and sampling was continued for an additional 4 weeks. Oxygen production was reduced after 24 h in all treatments in relation to dose. However, only at the full and 10% rates did critical conditions result. Chlorophyll levels dropped to 5% and 22% of control after 2 weeks at the full and 10% rates respectively and then slowly increased to 1/6 the control levels by the end of sampling at 4 weeks. As a result of the inhibition of oxygen production and reduced plankton levels, morning oxygen levels fell to ½ the control after 72 h in the full treatment. Critical oxygen levels of 3 ppm and below occurred after 1 week in the full and 10% treatments. Positive impacts noted in the full and 10% treatments included reduced levels of geosmin-producing straight chain Anabaena beginning at one week, with essentially complete elimination after 3 weeks. Green algae were stimulated. Zooplankton levels (adult and nauplii copepods) were reduced after week 2 and extending into week 3; and is judged to reflect lowered phytoplankton levels.
 
Fall application of diuron as a defoliant on September 23 resulted in similar effects to the spring application, but only at full, field rates.