Pharmaceuticals are routinely detected at low concentrations in surface waters, but
effects on non-target organisms are not well understood. Microcosms were used to assess
ecological responses in freshwater ecosystems to a mixture offourtetracyclines commonly
used in veterinary and human medicine. Triplicate microcosms were treated with tetracycline,
oxytetracycline, doxycycline, and chlortetracycline, resulting in measured time-weighted
average total mixture concentrations of 0, 0.080, 0.218, 0.662, and 2.29 microM, respectively.
Responses were assessed in terms of structure and function based on measurements of
zooplankton and phytoplankton communities, ecosystem productivity, and water quality.
Effects were observed for some endpoints > or = the 0.218 microM treatment. The largest
responses were concentration-dependent reductions in total phytoplankton abundance
and species richness. Phytoplankton abundance recovered to control levels in all microcosms
after treatment was terminated, and resilience (time to return to normal operating
range during stress) was observed with respectto phytoplankton species richness. Zooplankton
were generally unaffected by the tetracyclines. Responses also included decreased
water clarity, lower oxygen concentration, and water temperature. Functional endpoints
showed varying sensitivity. On the basis of dissolved oxygen concentrations, community
respiration (R) increased while primary productivity (P) was unchanged with increased
treatment concentration. The effects observed occurred at considerably greater concentrations
than are currently measured in the environment, indicating minimal risk to aquatic
organisms.