Honors College Theses

Publication Date



Biology (B.S.B.)

Document Type and Release Option

Thesis (open access)

Faculty Mentor

Dr. Risa A. Cohen


Widespread use of agrochemicals such as herbicides and antibiotics increases their likelihood of entering aquatic systems in mixture. Despite different modes of action, atrazine (herbicide) and tetracycline (antibiotic) adversely affect non-target photosynthetic organisms, such algae and macrophytes, with the potential to reduce food availability to higher trophic levels. However, the effects of simultaneous exposure to both contaminants have yet to be determined. I hypothesized that a mixture of atrazine and tetracycline affects freshwater communities differently than each compound alone. A microcosm experiment was conducted to test effects of environmentally relevant concentrations of atrazine (1 μg L-1) and tetracycline (1 μg L-1), alone and together, on the green microalga Chlorella sp., the duckweed Lemna minor, and the microscopic crustacean Daphnia magna in a greenhouse over a period of 10 days. The endpoints measured were Chlorella sp. cell density and chlorophyll a concentration, L. minor growth and tissue condition, and mortality and reproduction of D. magna. Atrazine appeared to decrease Chlorella sp. abundance but not enough to reduce food availability to D. magna, whose reproduction and mortality were unaffected. Tetracycline and atrazine decreased L. minor abundance individually and in combination. Furthermore, the reduction in the combination treatment appeared to be additive. Growth inhibition was highest in the combination treatment. The greater adverse effects associated with mixtures of atrazine and tetracycline on L. minor compared to the individual compounds suggests increased potential for population decline over the long term. Losses of aquatic plants in turn may alter aquatic community composition and species interactions.