Honors College Theses

Publication Date



Biology (B.S.B.)

Document Type and Release Option

Thesis (open access)

Faculty Mentor

Dr. Risa Cohen


Agricultural herbicides enter aquatic environments after rain events where they affect microscopic aquatic plants (phytoplankton) and animals (zooplankton) that form the base of aquatic food webs. Atrazine, an herbicide with low solubility in water, is often mixed with surfactants such as alkyl polyglucoside (APG) to improve effectiveness. Although APG has low-toxicity, a potential drawback to increased atrazine solubility is greater adverse effects on aquatic organisms. I hypothesized that atrazine and APG decrease phytoplankton abundance more than atrazine alone. Specifically, I predicted phytoplankton abundance should 1) decrease with increasing concentrations of the mixture compared to the same atrazine concentrations individually, and 2) decrease more with zooplankton grazers that consume phytoplankton. The responses of phytoplankton (Chlorella sp.) to atrazine with or without APG and with or without grazers (Daphnia magna) were examined over 2 weeks in freshwater microcosms. First, Chlorella sp. received atrazine (0, 1, 5, or 25 µgL-1) with or without APG (n=5). In a second experiment, 10 D. magna were also added to each microcosm. Only the highest concentration of atrazine decreased phytoplankton abundance, independent of APG addition. Surprisingly, mixing APG with low and medium concentrations of atrazine either had no effect or appeared to reduce atrazine toxicity to Chlorella sp. As expected, grazers decreased Chlorella sp. abundance. However, increasing atrazine concentration adversely affected D. magna abundance both alone and in mixture with APG. These results suggest that high concentrations of atrazine decrease the abundance of non-target planktonic organisms and mixing atrazine with APG does not appear to increase atrazine toxicity.