The invasive aquatic fern Azolla filiculoides negatively impacts water quality, aquatic vegetation and amphibian larvae in Mediterranean environments

Among invasive species, floating plants have a great impact on aquatic ecosystems, forming dense mats that cause a great alteration of the physical and chemical water features. We experimentally assessed the effect of Azolla filiculoides mats on Mediterranean temporary waters using mesocosms with sediments from an invaded marsh and amphibian larvae from surrounding wetlands. Azolla filiculoides formed a dense mat over the water surface in the experimental tanks, which decreased pH and oxygen concentration, and increased nutrients, nitrogen and phosphorus compounds in the water compared to mesocosms lacking the exotic fern. Macrophyte abundance and richness were reduced under the Azolla mat. Invaded mesocosms also had higher phytoplankton abundance and different zooplankton composition, which was mainly characterized by higher abundance of juvenile copepods. Regarding amphibian development, tadpoles requiring a long developmental period, the western spadefoot toad, Pelobates cultripes, had significantly lower survival in the presence of A. filiculoides (4.8% compared to 60% survival in fern-free mesocosms). For tadpoles of the Mediterranean tree frog, Hyla meridionalis, which develop faster, we did not detect effects on survival, but on body size and physical condition of metamorphs. The formation of A. filiculoides mats produced a deterioration of the trophic web, as it caused most sources to be depleted in 13C and 15N. Primary consumers, tadpoles and zooplankton, were also 13C depleted, although the diet of tadpoles from Azolla mesocosms was mainly composed of the invasive fern. We therefore conclude that A. filiculoides has a negative impact on physical and chemical water features, reduces the richness and biomass of the submerged macrophyte guild and reduces survival and body condition of tadpoles. Overall, A. filiculoides causes substantial deterioration of the food web of the aquatic ecosystem.