DYNAMICS OF HERBICIDE CONCENTRATIONS IN MISSISSIPPI DELTA OXBOW LAKES AND THE ROLE OF PLANKTONIC MICROORGANISMS IN HERBICIDE METABOLISM

Authors: Zablotowicz, Robert; Locke, Martin; Lerch, Robert; Knight, Scott

Submitted to: American Chemical Society Symposium Series
Publication Type: Book / Chapter
Publication Acceptance Date: 7/1/2002
Publication Date: 3/1/2004
Citation: Zablotowicz, R.M., Locke, M.A., Lerch, R.N., Knight, S.S. 2004. Dynamics of herbicide concentrations in mississippi delta oxbow lakes and the role of planktonic microorganisms in herbicide metabolism. American Chemical Society Symposium Series 877. In: Nett, Mary T., Locke, Martin, A., and Pennington, Dean A., editors. Water Quality Assessments in the Mississippi Delta. Oxford University Press, New York, NY. pp. 134-149.

Interpretive Summary: Understanding processes and management practices in watersheds as they relate to herbicide occurrence in surface water is a challenge to maintaining water quality. As a component of the Mississippi Delta Management Systems Evaluation Area Project (MSEA), herbicide concentrations and aquatic microbiology in three oxbow lakes from experimental watersheds were monitored. Herbicide accumulation and persistence in the lakes was related to watershed management practices, e.g., a combination of a cover crop and reduced tillage in one watershed lowered fluometuron herbicide concentrations. Herbicide incorporation, and factors such as hydrology and microbiological characteristics of the water influenced atrazine and metolachlor concentrations in the other lakes. Laboratory results showed that one green algae species isolated from the lakes had potential for degrading herbicides. Results from this research should help farmers and regulators address Total Maximum Daily Load (TMDL) issues by improving understanding of the relationship between land management practices and the fate of herbicides in surface water bodies.

Technical Abstract: The small oxbow lakes central to the Mississippi Delta MSEA project provided a model system for evaluating the effects of watershed management practices on dynamics of herbicide concentrations and planktonic populations. In 1996 and 1997, cotton was planted to about 50% of the area of three watersheds, and maximum fluometuron concentrations of 5.7, 5.0 and 12.4 ug L**-1 were observed in Beasley, Deep Hollow and Thighman lake water samples, respectively. The metabolite desmethyl fluometuron was present in all lakes (2.0 to 4.0 ug L**-1). In 1998, significant areas of Beasley and Thighman watershed were planted in corn. Maximum concentrations of atrazine and metolachlor observed in Thighman lake in 1998 were 15.0 and 7.2 ug L**-1, respectively, occurring in May. In Beasley lake, maximum concentrations of atrazine and metolachlor were 2.5 and 1.8 ug L**-1, respectively, occurring in late summer. Although differences in herbicide dissipation in the lakes may be partly explained by hydrology, cropping and management practices, the microbiological characteristics of the lake also need to be considered. Differences in planktonic populations and activity were observed among the lakes, e.g., Thighman lake had the highest enzymatic activity and bacterioplankton populations, and Beasley the lowest. The lowest suspended solids typically and the highest algal populations were found in Deep Hollow lake during the rainy season, when the other lakes were sediment stressed.