Interactive multiple‐stressor effects of the antibiotic monensin, cattle effluent and light on stream periphyton

Summary Streams draining agricultural land are simultaneously exposed to multiple stressors including agricultural antibiotics and effluent from surface water run‐off. The potential interactive effects of these contaminants on stream benthic communities are poorly understood. Using a streamside channel experiment with a full factorial, repeated‐measures design, we assessed the individual and combined effects of the veterinary antibiotic monensin (four concentrations versus control), dairy cattle effluent (added versus control) and differing light levels (reduced versus ambient) on benthic stream periphyton (biomass accrual, community composition and ecological algal guilds). Antibiotic and effluent treatments were applied during a 24‐hour pulse to simulate run‐off from farmland, while light was manipulated throughout the experiment, to mimic the presence or absence of riparian shading. Periphyton taxon richness was decreased by the two highest antibiotic levels 8 days after the pulse. Multivariate community composition (relative abundances of the 15 most common periphyton taxa) was also affected by all three stressors 8 days post‐pulse. However, the periphyton community recovered within 16 days post‐pulse. Interactions among stressors were fairly common, occurring for two of three ecological algal guilds and six of the 15 common taxa. Nevertheless, interactions were generally subtle, without any strong synergistic or antagonistic patterns. Our study shows that the antibiotic monensin can negatively affect stream periphyton communities, but also implies short pulses may not have lasting impacts. The lack of strong synergistic or antagonistic interactions between monensin and light or effluent indicates monensin effects on stream periphyton may be predicted reasonably well based on single‐stressor studies.