Species-specific effects of macrophytes on the anti-grazer morphological defense in Scenedesmus obliquus

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 multi-celled 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.

Automated summary

Predation and competition are the primary interspecies interactions in aquatic ecosystem, and understanding how these two stressors interactively affect organisms is critical to predict food web dynamics. This study revealed that both predation risk from zooplankton and allelopathy from different macrophytes induced multi-celled colony formation in the green alga Scenedesmus obliquus, with Daphnia cues having a stronger effect. The four macrophytes did not impact algal growth rate, but they did alter the morphological defense in algae in a species-specific manner.