Phytoplankton community responses to sub-inhibitory levels of four contaminants of emerging concern

Contaminants of emerging concern (CECs) are increasingly prevalent in freshwater systems, yet their effects on phytoplankton communities remain poorly understood. We examine how a representative freshwater assemblage responds to four ubiquitous CECs [carbamazepine, diclofenac, 1H,1H,2H,2H-perfluorooctanesulfonic acid (6:2 FTS) and 6ppd-quinone (6PPD-Q)] at environmentally relevant concentrations, monitoring changes in chlorophyll a (chl a), taxonomic composition and photosynthetic efficiency. Bayesian segmented regression (BSR) identified pollutant-specific ecological thresholds (breakpoints). Phytoplankton responses varied markedly among contaminants. 6:2 FTS showed the most consistent above- and below-breakpoint directional slope patterns; most algal groups were modestly stimulated at low concentrations, declining above the threshold. Diclofenac showed tight clustering of algal group breakpoints. Carbamazepine showed dispersed breakpoints, with several sensitive taxa responding above the community-level threshold. 6PPD-Q produced the highest and most variable breakpoints, with significant early declines in chl a. These results suggest that phytoplankton sensitivity to CECs is strongly influenced by contaminant-specific mechanisms and taxa-specific physiology. Chemicals with consistent effects (e.g. 6:2 FTS) can be monitored using community-level metrics, whereas compounds with heterogeneous thresholds (e.g. carbamazepine and 6PPD-Q) require multi-taxa monitoring to detect early ecological changes. Applying BSR to natural communities provides a robust framework for assessing sub-inhibitory impacts of contaminants, thereby enhancing ecological risk assessments.