**5. Conclusions**

#### **5.1 Large pond (7 ha and larger) simulations**

Of the 40 herbicide applications tested, significant effects from drift levels of 1 and 10% (the range possible in ponds equal to and larger than 7 ha) were noted for diuron (used for burndown) and atrazine. Diuron presents the greater risk for reduced water quality and for a longer time period (in excess of 4 wks). Atrazine effects are short-lived . Carfentrazone resulted in brief zooplankton reductions.

### **5.2 Surface scum algal populations simulations**

200 Herbicides – Properties, Synthesis and Control of Weeds

Aquaculture ponds often have surface floating scums predominately composed of cyanobacteria. These scum-forming algae are common in eutrophic ponds, including aquaculture ponds, especially during the growing season with warm temperatures and high nutrient loadings. Cyanobacteria in a suface scum state are unstable and prone to sudden die-offs (Boyd et al., 1975). The objective of this study was to test the effect of propanil on a

The experiment was conducted at the University of Arkansas at Pine Bluff mesocosm facility. A completely randomized design was used, with three replicates for each treatment in 12 mesocosms and with water of approximately 400 ug/l chlorophyll *a* from a goldfish pond. The treatments used were: a control with no propanil, 1%, 10% and 100% of the recommended field rates (0.45 kg/ha). Variables measured included: morning dissolved oxygen, pH, nitrite-nitrogen, total ammonia nitrogen, unionized ammonia , net primary productivity, chlorophyll *a* and phytoplankton composition. Methods followed Standard Methods (APHA 2005). Samples were taken before and after treatments were

*Microsystis* and *Anabaena* dominated the phytoplankton and formed the surface scum. Significantly lower DO and net primary productivity resulted after application in the 10% and full treatment. However, recovery was noted after 48 h. Also lower was pH following

In the earlier trials (Perschbacher et al., 1997, 2002) without surface scum algae, propanil at 10% drift resulted only in elevated chlorophyll *a*, but no significant differences were noted in chlorophyll *a* and phytoplankton composition in the present study. Thus, the significant negative impacts found in the present study were not expressed in previous studies, and the

Effects of propanil drift depended on the level of chlorophyll *a* found in the systems, in the study by Edziyie (2005). The greatest impact was on water quality in ponds with chlorophyll *a* levels 50-200 ug/l and lesser impacts below 20 and above 300 ug/l. Phytoplankton at high levels have been proposed to modify pesticide effects by sorption to the algae (Day and

Of the 40 herbicide applications tested, significant effects from drift levels of 1 and 10% (the range possible in ponds equal to and larger than 7 ha) were noted for diuron (used for burndown) and atrazine. Diuron presents the greater risk for reduced water quality and for

**4. Modifiying factors due to algal state from** *in situ* **mesocosm testing** 

pond with algal scums.

added.

**4.1 Methods and materials** 

**4.2 Results and discussion** 

application. Tan and UIA were higher on d 2.

difference is attributed to the algal state.

**5.1 Large pond (7 ha and larger) simulations** 

**5. Conclusions** 

Kaushik, 1987; Waiser and Robarts, 1997; Stampfli et al., 2011).

Algal populations forming scums appear more susceptible to these drift levels. Propanil levels which did not result in reductions in water quality in mixed water column populations, resulted in adverse reactions equal to the direct overspray. The concentration of algae at the surface and the propensity for algae in this stage to be unstable (crash) are judged responsible.

#### **5.3 Differing chlorophyll** *a* **level simulations**

Effects of propanil and atrazine drift, and perhaps of other herbicides, depend on the level of chlorophyll *a* found in the systems. The greatest impact of propanil was on water quality in ponds with chlorophyll *a* levels 50-200 ug/l and lesser impacts below 20 and above 300 ug/l. Absorption by algae, and other factors, may be responsible.

#### **5.4 Small pond (1.2 ha and smaller) simulations**

Simulations in small ponds, equal to or less than 1.2 ha, used drift rates up to 30%. Although atrazine and propanil did not cause concern, diuron caused DO drops that were below 3 mg/l for several days and recovery was not noted until 14 days.

#### **5.5 Beneficial aspects of herbicide drift**

Beneficial effects of atrazine, propanil and diuron included reduction or elimination of cyanobacteria, and reduced pH (Ludwig et al., 2007) and thus reduced UIA. Clorophyll *a* levels were stimulated by propanil and atrazine.

## **6. Acknowledgements**

Students Baendo Lihono, and Malisa Hodges, and Jason Brown from USDA/ARS SNARC ably assisted the study. Dr. J. Dulka and Dupont kindly supplied the Basis Gold and component herbicides. C. Guy Jr., N. Slaton, H. Thomforde, W. Johnson, J. Ross, R Scott, and J. Welch of the Cooperative Extension Service are thanked for providing information on herbicides and herbicide samples. Virginia Perschbacher kindly assisted in manuscript preparation. Funding was from State of Arkansas, USDA/CSREES Project ARX 05013 and Grant No. 2001-52101-11300 Initiative for Future Agriculture and Food Systems and Catfish Farmers of Arkansas Promotion Board Grant #6.
