Effects of tebuthiuron on characteristic playa lake algae of the Southern High Plains: A microcosm study
Price, David John
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With use of tebuthiuron on pastures and rangelands, the possibility of adverse environmental effects on playa ecosystems has increased. Runoff from tebuthiuron-treated areas could provide an inflow of the herbicide into playas, where it accumulates in sediments. Tebuthiuron may affect algal populations in playas, thus reducing the amount of food available to invertebrates and various wildlife species. The objective of this research was to determine the impacts that tebuthiuron had on 11 algal species, which were treated at the beginning and end of the increase of algal growth. The two treatments represented inflow of water from rainfall into playas. The objective was accomplished by interpreting the dynamics of algal communities in one-phase microcosms treated with 0.18 ppm tebuthiuron. Variables monitored included cell numbers, cell volume, concentrations of photosynthetic pigments, composition of algal communities, and changes in available nutrients. Differences between control and treatment means of these variables were determined with least significant difference (LSD) tests after a significant (P <0.05) analysis of variance. Total alkalinity and pH increased during the study. Differences between control and first treatment means were observed for pH and total alkalinity. Dissolved oxygen increased and carbon dioxide decreased indicating that photosynthesis occurred in the microcosms. Inhibition of carbon dioxide levels were related to the herbicidal inhibition of photosynthetic pigments. An increase in total alkalinity resulted in decreased hardness, due to precipitation of metal salts. The decrease in hardness affected buffering capacities. Nitrification caused decreased nitrate levels, due to conversion of ammonia to nitrite. Nitrate and nitrite exhibited significant differences between control and treatment means for the the first treatment. Chloride levels increased while orthophosphate decreased indicating use by algal cells. Chloride and orthophosphate means were not different between control and first treatment. However, chloride means were different between control and second treatment. The only species affected by tebuthiuron was Bracteacoccus minor; however, as a community, the algae were inhibited by tebuthiuron. Packed-cell volume and photosynthetic pigment levels increased during the study; however, inhibition by tebuthiuron was detected in treated samples. No significant conclusions were drawn from the treatment at the end of the increase of growth, due to variability in the data. It was concluded that 0.18 ppm tebuthiuron, applied before growth increase, inhibited the algal community tested.