Effects Of Nutrient Addition And Snail Herbivory On Vallisneria Natans,Epiphytic Algae And Water Nutritional Status

Due to the continuous input of nutrients such as nitrogen and phosphorus,the nutritional structure of freshwater ecosystems has changed,and its ecological functions have been seriously degraded,resulting in a series of problems such as deterioration of water quality.Based on the increasingly prominent problem of water eutrophication,the comprehensive restoration technology of aquatic organisms is widely used in the treatment of water eutrophication.Submerged macrophytes have important ecological functions in restoring the ecological function of water ecosystem and maintaining the stability of clear water.However,they are also affected by other aquatic organisms such as epiphytic algae,phytoplankton,snails,fish and so on.In this study,we took common fresh water omnivorous snails including Radix auricularia,Hippeutis umbilicalis and Pomacea canaliculata and the submerged plant Vallisneria natans as the research object.A mesocosm aquatic ecosystem was constructed.The control group and three treatment groups（V.natans group,snail group,snail grass group）were set up to study the effects of snails,submerged macrophytes and their coexistence on the nutrient situation of water and epiphytic algae under the condition of nutrient addition.The main results are as follows:（1）The grazing effects of R.auricularia,H.umbilicalis and P.canaliculata changed the community structure of epiphytic algae.Compared with the control group,the species of epiphytic algae in the V.natans group,snail group and snail grass group decreased significantly,the density and Chlorophyll a decreased significantly,and the evenness increased significantly（P<0.05）.However,compared with the V.natans group,there was no significant change in the density of epiphytic algae,Chlorophyll a and community diversity in the snail group and the snail grass group（P>0.05）.There was no significant difference in the density,Chlorophyll a and community diversity of epiphytic algae between the snail group and the snail grass group（P>0.05）.（2）R.auricularia,H.umbilicalis and P.canaliculata have different effects on the nutrients in water.Compared with the control group,the contents of NH₄⁺-N,NO₂^--N and TP in V.natans group,R.auricularia group and R.auricularia+V.natans group were significantly decreased.The contents of NH₄⁺-N,NO₂^--N and SRP were significantly decreased in H.umbilicalis group,and NO₂^--N and TP were significantly decreased in H.umbilicalis+V.natans group.The content of NO₂^--N in P. canaliculata group and P.canaliculata+V.natans group decreased significantly（P<0.05）.There was no significant difference in nutrient content of the water between the snail group and the snail grass group and the V.natans group,and there was no significant difference in the nutrient content of the water between the snail group and the snail grass group（P>0.05）.（3）The Chlorophyll a content of benthic algae in V.natans group was significantly lower than that in other treatment groups（P<0.05）.There was no significant effect on Chlorophyll a content of phytoplankton and benthic algae in water by R.auricularia and R.auricularia+V.natans,and there was no significant difference in the content of Chlorophyll a between the snail group and the snail grass group（P>0.05）.H. umbilicalis,H.umbilicalis+V.natans have no significant effect on the Chlorophyll a content of planktonic algae,but can significantly reduce the Chlorophyll a content of benthic algae.Compared with the V.natans group,the Chlorophyll a content of benthic algae in the H.umbilicalis+V.natans group increased significantly（P<0.05）.P. canaliculata and P.canaliculata+V.natans had little effect on phytoplankton.However,compared with the control group,V.natans group and P.canaliculata+V.natans group,the Chlorophyll a of benthic algae in P.canaliculata group increased significantly（P<0.05）.（4）With the continuous addition of nutrients,the number of leaves,total biomass and plant height of V.natans were not significantly affected by the grazing of R. auricularia,H.umbilicalis and P.canaliculata（P>0.05）.This study mainly explored the effects of freshwater omnivorous snails on the submerged plant V.natans,epiphytic algae and water nutritional status.It was found that the growth of V.natans was not significantly affected by R.auriculata,H. umbilicalis and P.canaliculata at a certain density.The community composition and quantity of epiphytic algae were significantly reduced,and the nutrient content of related water was also significantly reduced to varying degrees.This shows that the comprehensive application of freshwater omnivorous snails and submerged plant V.natans has certain positive significance for water ecological restoration and environmental governance.