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ABSTRACT
Mesocosm-testing of aerially applied herbicides propanil and Basis Gold (86.8% atrazine) and indicated high drift rates (10–20% of field rates) resulted in adverse impacts on aquaculture production-pond phytoplankton, zooplankton, and critical water-quality variables. Effects on fry pond phytoplankton, zooplankton, and water quality from these herbicides were evaluated at 30% of field application rates, equal to the worst-case scenario from drift to these smaller ponds with consequent higher concentrations. The study was conducted in 12 outdoor pool mesocosms of 500 L volume, filled with pond water from an adjacent 0.1 ha pond fertilized and prepared as recommended for sunshine bass, Morone chrysops × M. saxatilis, fry stocking. Plankton and water quality samples were taken just prior to herbicide application (day 0) and on days 1,2,3,4-zooplankton only, and on day 7 after application. Field application rates for Basis Gold and propanil were 0.8 kg active ingredient and 1.80 kg/ha, respectively. Significant changes from control treatment values were found for several parameters in both herbicide treatments. Following application on June 20, net primary productivity was significantly depressed on day 1 in the propanil treatments, but increased on days 2 and 3. Morning dissolved oxygen (DO) was lower on days 1–3, but not to critical levels. Also, in the presence of propanil, pH and consequently UIA were lower from day 1 to 3. Basis Gold reduced morning DO on days 2 and 3, but not net primary productivity. Nitrite-N, however, was significantly higher on day 1. Phytoplankton total numbers, and the cyanobacterium Chroococcus sp. which dominated, were reduced by propanil on days 1 -3; and similarly affected by Basis Gold on days 2 and 3. Numbers of green algae, Scenedesmus sp. and Coelastrum sp., and diatoms were, however, stimulated by propanil and diatoms by Basis Gold. Zooplankton were little affected by either herbicide. Thus, high levels of drift from propanil and Basis Gold should not result in negative impacts on fry production.