A contemporary challenge for ecologists is to understand how human activities influence species abundance and diversity. Aquatic communities are exposed to a diversity of anthropogenic chemicals including antimicrobials, a contaminant of emerging concern. Toxicological studies demonstrate that antimicrobials can have direct toxic effects on aquatic organisms but at concentrations that are many times higher than are observed in nature. Aquatic systems are more often exposed to concentrations of antimicrobials several orders of magnitude lower than what is considered directly toxic. To date, few studies have investigated the direct and indirect effects of environmentally-relevant concentrations of antimicrobials at the community level. Towards this goal, we investigated the effects of a common antimicrobial sulfadimethoxine (SDM) on wetland communities using aquatic mesocosms. We constructed 20 wetland communities with 19 leopard frog tadpoles and a uniform assemblage of cladocerans, copepods, phytoplankton, and periphyton. We exposed mesocosms to either 0 ug/L SDM or 1 ug/L SDM every four days for 32 days. We measured total algal abundance (chlorophyll-a), blue-green algal abundance (phycocyanin), zooplankton abundance (cladoceran and copepod counts), and periphyton abundance (biomass) at multiple time points. At the end of the experiment, we quantified tadpole survival stage, snout-vent length (SVL) and mass.
Results/Conclusions
We found that SDM treated mesocosms had 36% more blue-green algae compared to control mesocosms by day 24. The ratio of blue-green algae to total algae also increased by 55% in SDM mesocosms compared to the control mesocosms by day 24. By day 28, this shift in the phytoplankton community composition to more unpalatable blue-green algae resulted in cladoceran abundances that were on average 6.5 times lower than the control. Copepod abundance was unaffected by the shift in the phytoplankton community. SDM also caused a significant increase in periphyton biomass on days 4 and 28 (50% and 70% respectfully), however this did not result in a significant difference in tadpole survival, stage, SVL, or mass. Antimicrobials are categorized as practically non-toxic by the EPA when considered in single species tests. However, when layers of biological complexity are added, concentrations of SDM several orders of magnitude below directly toxic levels still caused a reduction in zooplankton. We conclude that traditional assessments of chemical toxicity may not always be sufficient for predicting the ecological impacts of antimicrobials in aquatic environments.