Aquatic plants (macrophytes) are important components of freshwater ecosystems and serve numerous purposes that structure aquatic communities. Although macrophytes represent an essential component of stable aquatic communities, invasive macrophytes can negatively alter ecosystem properties. The success of invasive macrophytes may be related to allelopathic interactions that can have strong impacts on phytoplankton communities.

Ludwigia grandiflora is an invasive macrophyte in the north-west of France which competes with indigenous macrophytes (e.g. Mentha aquatica) and phytoplankton species. We tested over a yearly cycle of development, the allelopathic effects of Ludwigia on three target phytoplankton species as well as the allelopathic effects of both Ludwigia (i.e. the invasive species) and Mentha (i.e. the indigenous species) on the seed germination of Ludwigia. Scenedesmus communis and two Microcystis aeruginosa strains (one toxic producing microcystins and one non-toxic) were selected as phytoplankton species. Firstly, we examined the allelopathic effects of Ludwigia leaf extract on the photosynthetic activity of the phytoplankton species according to three distinct seasons (spring, summer, autumn). Secondly, we examined the allelopathic effect of both Ludwigia and Mentha leaf extracts on seed germination of Ludwigia according to the same seasons.

We observed a strong seasonal effect of allelopathic interactions and a differential effect on the three phytoplankton species. The photosynthetic activities of Scenedesmus and the toxic Microcystis were stimulated at low dose extract whereas the photosynthetic activity of the non-toxic Microcystis was inhibited according to the increase of dose extract. The photosynthetic activities of the three phytoplankton species decreased at high dose extract in summer. The germination rate of Ludwigia was stimulated in presence of both Ludwigia and Mentha leaf extracts. The stimulation of germination rate was higher in summer compared to the two other seasons, concomitant to the strong negative effects observed on the phytoplankton species.