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Anthropogenic emissions of polycyclic aromatic hydrocarbons (PAHs) cause severe ecological impacts by contaminating natural water bodies, affecting various biological groups, and altering interspecies relationships and ecological functions. This study examined the effects of two typical PAHs, phenanthrene (Phe) and naphthalene (Nap), on the anti-grazing defense mechanisms of Tetradesmus obliquus, a primary producer in freshwater food chains. Four non-lethal concentrations (0.01, 0.1, 1, and 10 mg L–1) of Phe and Nap were tested and the population growth, photosynthetic capacity, pigment content, and morphological defense of T. obliquus were analyzed. The results indicated that Phe and Nap inhibited both the growth rate and formation of defensive colonies of T. obliquus induced by Daphnia grazing cues, and the inhibition ratio increased with concentration. Phe and Nap significantly shortened the defense colony formation time of T. obliquus. Phe and Nap significantly suppressed photosynthesis in the early stages; however, the photosynthetic efficiency recovered over time. These findings highlight the high sensitivity of grazing-induced colony formation in T. obliquus to Phe and Nap at non-lethal concentrations, which could affect the interactions between phytoplankton and zooplankton in aquatic ecosystems. Our study underscores the influence of Phe and Nap on the defense mechanisms of phytoplankton and the consequential effects on ecological interactions within freshwater ecosystems, providing insight into the complex impacts of pollutants on phytoplankton-zooplankton relationships. Therefore, it is necessary to consider interspecific interactions when assessing the potential negative effects of environmental pollutants on aquatic ecosystems.

Predation and competition are the primary interspecies interactions in aquatic ecosystem. Understanding how these two stressors interactively affect the organisms is critical to predict the food web dynamics. Here, we investigated the responses of the green alga Scenedesmus obliquus to predation risk from zooplankton (Daphnia magna) and to allelopathy from four macrophytes (Cabomba furcate, Nymphoides hydrophylla, Lemna minor and Chara sp.) separately and jointly. Neither the Daphnia-derived cues nor the four macrophyte exudates affected the algal growth rate compared with those in control. Nonetheless, both the two stressors induced the multicelled colony formation in S. obliquus, which was, however, much more strongly induced by the Daphnia cues. In addition, the four macrophytes differentially altered the anti-grazer morphological defense in algae: exudates from Nymphoides and Chara markedly promoted the enhancing effect of Daphnia cues on the eight-celled colonies formation, resulting in the increased number of cells per particle of the algal population. Nonetheless, more four-celled colonies but fewer eight-celled colonies were formed under Lemna exudates exposure, in contrast to the unchanged multicelled colonies growing in Cabomba exudates under Daphnia cues exposure. The results revealed that macrophytes allelopathically disturbed the anti-grazer morphological defense in phytoplankton even though they had no direct impact on algal growth, and this interference was species-specific in macrophyte. As a result, the predator-prey interactions between herbivores and phytoplankton can be potentially changed differently in different macrophyte-dominated freshwaters, contributing to the new cognition of the important role of macrophytes in ecosystem functioning. It suggests that the algal morphology can function as a useful indicator of phytoplankton response to simultaneous stresses from predation and competition.