Effects of Tebuthiuron on Characteristic Green Algae Found in Playa Lakes

There are many studies on the effects of tebuthiuron ( N ‐(5‐(1,1‐dimethylethyl)‐1,3,4‐thiadiazol‐2‐yl)‐ N,N′ ‐dimethylurea) on organisms, but only limited information regarding the impact that the herbicide has on green algae. Eleven species of green algae common to playa lakes were exposed tebuthiuron in 400‐mL microcosm cultures. The cultures were treated with 0.18 µ g/mL tebuthiuron, simulating the highest concentration of the herbicide found in natural runoff from agricultural areas treated with > 2.24 kg/ha tebuthiuron. Sixty‐six cultures were treated before active growth (first treatment) and 45 separate cultures were treated after maximum growth (second treatment). The first treatment was used to simulate the exposure of algal communities to tebuthiuron from runoff during the wet months of May and June. The second treatment simulated exposure to the herbicide once the community was established. Production and nutrient uptake by treated algal cultures was decreased by the first treatment with tebuthiuron. Of the eleven algal species selected, only Bracteacoccus minor was significantly inhibited by tebuthiuron in the first treatment. Photosynthetic pigments, total cell counts, and packed cell volumes for cultures exposed to tebuthiuron in the first treatment were lower than controls ( P < 0.05). Effects caused by the herbicide on the cultures were also evidenced by decreased water quality variables in cultures treated before active growth, with the exceptions of orthophosphates and chloride. No differences were detected between controls and treated samples 210 d after initial treatment with tebuthiuron. Once the algae reached stable cell numbers (second treatment), tebuthiuron demonstrated no significant ( P < 0.05) effects on the cultures. This study indicated that runoff from agricultural fields treated with concentrations > 2.24 kg/ha tebuthiuron may adversely affect the green algal communities of playa lakes.