Assessing triclosan-induced direct and multi-trophic cascading effects in a sub-tropical freshwater ecosystem

Triclosan, a synthetic broad-spectrum antimicrobial agent, is commonly used in personal care products, household goods, and industrial items. Its extensive use and subsequent release have led to frequent detections in aquatic ecosystems worldwide, potentially harming ecosystem health. Therefore, an outdoor mesocosm experiment was conducted to assess the effects of triclosan on community structure (microbes, phytoplankton, zooplankton, and macroinvertebrates) and ecosystem functioning in freshwater ecosystems under sub-tropical conditions. Triclosan was applied daily in pulses at nominal concentrations of 0.05, 0.5, 5, and 50 µg/L over a three-week treatment period, followed by a five-week recovery period. Triclosan significantly affected the structure of microbial (NOEC = 0.5 µg/L), phytoplankton (NOEC = 5 µg/L), and zooplankton (NOEC = 5 µg/L) communities. In contrast, no significant effects were observed on the macroinvertebrate community in response to triclosan exposure. Within the microbial community, the phyla Cyanobacteria and Bacteroidetes showed consistent responses to triclosan exposure, with taxa in the Cyanobiaceae family being the most adversely affected. Among phytoplankton taxa, the cyanobacteria Cylindrospermopsis sp. had the lowest consistent NOEC (<0.05 µg/L), while the lowest NOEC (0.5 µg/L) among zooplankton taxa was observed for the copepods nauplii and Mesocyclops leuckarti. Piecewise structural equation modelling revealed that triclosan exposure resulted in significant direct and indirect effects across multiple trophic levels, ultimately disrupting ecosystem functioning related to carbon and nitrogen cycling. Hence, our study highlights the threat posed by environmentally relevant concentrations of triclosan to both community structure and ecosystem functioning in sub-tropical freshwater ecosystems.