Impact Of Herbivory By Invasive Apple Snail (Pomacea Canaliculata) On Wetland Aquatic Plants And Nutrients In Water Body

The wetland ecosystem is a complex, which has many characteristics and functions related to hydrology, ecosystem services, self-maintenance, autoregulation and so on. As one of the wetland ecosystem, freshwater ecosystem, especially lakes and ponds, is very vulnerable to invasive species because of the discontinuous habitats, thus, its functions are prone to be crippled.As to freshwater system, grazing by invasive species can affect many aspects; however, most studies have focused on the direct effects on macrophytes. Therefore, we conducted two mesocosm experiments and one laboratory experiment not only to further the determination of the herbivory of the invasive apple snail (Pomacea canaliculata) but also to investigate the impacts of apple snail on filamentous algae,phytoplankton and nutrients in water body.In a freshwater pond in Hong Kong Wetland Park, 500 g of Myriophyllum aquaticum or Eichhornia crassipes with 0, 2, 4 or 8 apple snails in 1 m 1 m 1 m enclosures were confined for approximately 1 month. All related parameters were determinated at regular intervals. According to the results of 2 mesocosm experiments, the laboratory experiment of feeding the apple snail with filamentous algae (mainly Spirogyra sp.) was designed to quantify the herbivory.The results showed that apple snails grazed heavily on both species of macrophytes, with higher overall weight losses at higher snail densities. The damage patterns differed between the two macrophytes. In M. aquaticum experiment, both leaves and stems suffered from substantial herbivory, whereas in E. crassipes experiment, only the roots suffered significant weight reduction.In addition to grazing on macrophytes, apple snails appeared to have controlled the growth of filamentous algae, as these did not develop in the snail treatments. Howerer, the growth of filamentous algae reached 80.3 g m^-2 in the control with no apple snail, forming a spongy pond scum.The ability of P. canaliculata to control filamentous algae was supported by the laboratory experiment where the consumption was as high as 0.25 g g^-1 snail DW d^-1.Nitrogen and phosphorous concentrations in water body remained low throughout both experiments and were not correlated with apple snail density. The treatment effects on chlorophyll a (Chl a) and phytoplankton composition varied in the two experiments. In the M. aquaticum experiment, with the increasing of snail density, Chl a increased, and the phytoplankton community became dominated by Cryptophyceae.In the E. crassipes experiment, Chl a level was independent of snail density, but with increasing snail density, the phytoplankton community became co-dominated by Cryptophyceae, Chlorophyceae and Bacillariophyceae.Based on this research, P. canaliculata could cause multiple effects on wetland biodiversity and functions, in this way, management strategies should be developed to prevent its further spread. In invaded wetlands, strategies should be developed to eradicate the apple snail and re-introduce native snails which can control the development of filamentous algae, and avoid the deterioration of water body due to pond scum.