Effects Of Pomacea Canaliculata And Procambarus Clarkia On The Growth Of Submerged Macrophytes And The Related Physicochemical Factors

The golden apple snail(Pomacea canaliculata)and the red swamp crayfish(Procambarus clarkii)are two widespread invaders in(sub)tropical areas in China.The invasion of golden apple snail and red swamp crayfish destroy submerged macrophytes in shallow lakes,which may lead to the shift from a stable clear state to a turbid state.At present,most of the studies focused on the effects of the two invasive species on aquatic macrophytes,water quality and ecosystem services,respectively.However,few studies have examined the potential effect of co-occurrence.In the present study,we performed three experiments using control experiments:(1)Effect of golden apple snail grazing on three submerged macrophytes and the related physicochemical factors;(2)Effects of different densities of red swamp crayfish on Vallisneria denseserrulata and the related physicochemical factors;(3)Effects of co-occurrence of invading golden apple snail and red swamp crayfish on V.denseserrulata-dominated clear-water ecosystems.The main findings are bellow:(1)We assessed the preference of golden apple snail for three submerged plants,namely V.denseserrulata,Hydrilla verticillata,and Myriophyllum spicatum—which have been largely planted for recovering Chinese eutrophic shallow lakes.The results indicated that the attachment preference of golden apple snail for submerged macrophytes was in the following order:V.denseserrulata>H.verticillata>M.spicatum;moreover,golden apple snail grazing negatively affected plant survival in the following order of magnitude:H.verticillata>M.spicatum>V.denseserrulata.H.verticillata was the most vulnerable to snail grazing and died out rapidly.M.spicatum leaves had been completely consumed by the end of the experiment.V.denseserrulata was the least influenced by golden apple snail.Compared with the controls,the concentrations of total nitrogen(TN),ammonia nitrogen(NH₃-N),total phosphorus(TP),and soluble reactive phosphate(SRP)were higher with the H.verticillata+V.denseserrulata treatment;TP and SRP concentrations were higher with the V.denseserrulata+M.spicatum treatment;and TN concentrations were higher with the V.denseserrulata+M.spicatum treatment.These results suggested that the grazing preference of P.canaliculata alters the competitive advantage of submerged macrophyte.However,none of the submerged macrophytes prevented P.canaliculata grazing,resulting in decreases in the production of the submerged macrophytes and in the likelihood of clear-water state establishment.(2)We studied the effects of different densities of red swamp crayfish on V.denseserrulata with or without shelter.The concentrations of periphyton on the surface of V.denseserrulata in high-density treatment without shelter was higher than in other treatments and the controls,while the biomass of V.denseserrulata in high-density treatment without shelter was lower than in low-density treatments.The TSS concentrations in the high-density treatments without shelter was higher than in the low-density treatments without shelter,while there was no significant difference between the high-density treatments with shelter and the low-density treatments with shelter.The TSS concentrations was no significant difference between the low-density treatments with or without shelter,whereas was lower in the high-density group with shelter than without shelter.The results suggested that high density of red swamp crayfish would increase the water disturbance and damage submerged macrophyte.The shelter could reduce the intraspecific competition of red swamp crayfish and reduce disturbance.(3)We performed a 20-day experiment in 16 mesocosms-of which 4 contained only two snails,4 contained only two crayfish,4 contained two snails and two crayfish,and 4 controls-to test the effects of additive co-occurrence.Compared with the control,in the only snail treatment,TN concentrations in the water increased on the twentieth day,while the length,number,and biomass of submerged macrophytes decreased.In the only crayfish treatment,TN,TP,and TSS increased compared with the control,while the length,number,and biomass of submerged macrophytes decreased.In the snail+crayfish treatment,nutrients and turbidity concentrations did not differ between the snail+crayfish and the crayfish treatment,and the length and biomass of plants did not differ between the snail+crayfish and the snail treatment.Though additive co-occurrence,snail and crayfish together had no synergistic effects on V.denseserrulata and the physical and chemical variables studied.These findings suggest that golden apple snails mainly affected the lake ecosystem by plant grazing,while red swamp crayfish disturbed the sediment by increasing nutrients in the water and resuspension.