Influence of nitrogen and phosphorus concentrations and ratios on Lemna gibba growth responses to triclosan in laboratory and stream mesocosm experiments.

ABSTRACT

The effects of co-occurring nutrient and contaminant stressors are very likely to interact in aquatic systems, particularly at the level of primary producers. Site-specific nitrogen (N) and phosphorus (P) concentrations are often much lower and differ in relative availability than those used in nutrient-saturated laboratory assays for aquatic plants, which can introduce uncertainty in prospective ecological hazard and risk assessments. Because triclosan, an antimicrobial agent included in personal care products, potentially presents high relative risk among antimicrobial agents to aquatic plants and algae, we performed laboratory experiments with the model aquatic macrophyte Lemna gibba across a gradient of environmentally relevant NP levels with and without triclosan co-exposure. Frond numbers (7 d) were significantly higher in NP treatments of 16 and 23 but were lower in NP of 937 and 2,500 treatments relative to standardized control media (NP=3). When triclosan co-exposure occurred at high nutrient concentrations, frond number median effective concentration values at NP 0.75, 3, and 16 were more than twofold lower than triclosan median effective concentration values in low nutrient media NP ratios. However, a triclosan median effective concentration for frond number was twofold lower at NP of 2,500 than at other NP ratios in low concentration media. Influences of P enrichment on triclosan toxicity to L. gibba were further explored during a 14-d outdoor experimental stream mesocosm study. Effects of 2.6 and 20.8 microg L(-1) triclosan on L. gibba growth rates were more pronounced with increasing P treatment levels, which was generally consistent with our laboratory observations. Findings from these laboratory and field studies indicate that site-specific nutrient concentrations and ratios should be considered during assessments of primary producer responses to chemical stressors.