Effects of common aerially applied rice herbicides on the plankton communities of aquaculture ponds

Ten common aerially applied herbicides for rice were tested for possible adverse impacts on pond phytoplankton and zooplankton and critical water quality variables. Treatments simulated direct spraying of ponds and amounts of drift judged able to reach the pond at 1/10 and 1/100 direct rates. The study was conducted in 12, 500-l outdoor pool mesocosms. Pond water from an adjacent baitfish rearing pond was used. Water quality measurements were made prior to application and at 24 and 48 h after application. Commercial compounds tested and direct rates in kg active ingredient ha −1 were clomazone (0.6), thiobencarb (3.4), pendamethalin (1.1), propanil-dry, flowable (4.5), quinclorac (0.6), halosulfuron (0.07), bensulfuron methyl (0.07), triclopyr (0.4), 2,4- d-amine (1.7), and molinate (5.6). In addition, propanil was evaluated in a second application scenario at the same rates and to the same pond water sample as the initial application. Except for propanil, the rice herbicides produced no measurable effect on pond plankton or associated water quality. Following the first application at full rate for propanil, morning oxygen levels were reduced to critical levels (below 3 mg/l) for 2 days. After a second application of propanil at the full and 1/10 rates, oxygen levels were not decreased, although there were significant increases in chlorophyll a and zooplankton concentrations.