Chemical repellency and adverse effects of free-floating macrophytes on the cladoceran Ceriodaphnia dubia under two temperature levels

Abstract

Macrophytes can structure biological assemblages in subtropical lakes. Zooplankton often uses them as a refuge, but in the absence of predators many species frequently avoid them. It is hypothesized that plant exudates can act as “repellent factors” because they can be detrimental to the animals’ life history. This work aimed to test both assumptions: whether exudates of two free-floating species, Eichhornia crassipes and Salvinia biloba, repel the cladoceran Ceriodaphnia dubia and whether they affect its life history. Furthermore, given the possibility that increasing temperatures can modify the observed responses, the experimental research was carried out at two temperature levels: 21 and 25 (±1) °C. First, an avoidance experiment was performed inside narrow aquaria containing the plants, so that repellence was analyzed according to the location of animals after 30, 60 and 90 min. Second, for the life cycle experiments, cladocerans were raised in macrophyte-conditioned water and their lifespan, growth and reproduction were daily monitored. Macrophytes’ exudates effectively repelled C. dubia and affected its life history by decreasing its life expectancy, increasing the age at first reproduction and lowering the number of eggs per female. The magnitude of such effects depended on the treatment, i.e. both plants together caused more negative effects than E. crassipes alone, and this latter was more negative than S. biloba. At higher temperature, the negative effects were intensified but C. dubia maintained the same order of sensitivity to each treatment. This suggests that temperature variations, within a tolerance range, do not alter the allelopathic interactions between the studied species.