Combined Effects Of Nutrient Enrichment And Herbivory On Growth Of Submerged Macrophytes

Submerged macrophytes,as an important primary producer,play a key role in maintaining the functions and biodiversity in shallow lakes.The nutrient enrichment in water can promote the growth of submerged macrophytes within a certain range.Above critical thresholds in water nutrient concentrations,however,submerged macrophytes have declined or collapsed in many lakes with negative consequences for important ecosystem functions.This can be accompanied by a shift from clear to turbid,phytoplankton-dominated conditions.The mechanisms behind this decline are complex and subject to debate.In recent years,more and more studies have shown that herbivory have strong effects on growth of submerged macrophytes.However,few study has examined the role of herbivory in the decline of submerged macrophytes in eutrophic lakes.China is facing the risks of severe lake eutrophication,the irrational stocking of herbivorous fish and the spread of invasive herbivores.Thus,revealing whether herbivory is a trigger for the rapid decline of submerged macrophytes and the mechanisms of the decline of submerged macrophytes in these eutrophic lakes can provide valuable knowledge and insights for lake restoration and management.In this study,we conducted mesocosm experiments to examine the single and combined effects of nutrient enrichment and herbivory on submerged macrophytes and the mechanisms of them with different animals like Pomacea canaliculata,Procambarus clarkii,and simulated grazing of fish and birds.The results were as follows:1.Nutrient enrichment alone enhanced the growth of Vallisneria natans,although it could also promote the growth of phytoplankton within a certain range of nutrient concentration(TP=0.025?0.13 mg/L).Above a threshold of nutrient concentration(TP=0.2 mg/L),however,the growth of submerged macrophytes did not increase,indicating the positive effect of the nutrient enrichment on macrophytes was suppressed by the shading of phytoplankton.2.P.canaliculata had a strong negative effect on V.natans,while.P.clarkii had a relatively small direct consumption but caused lots of V.natans floating.The results of simulated grazing showed that the greater herbivory,the greater the negative effects on Hydrilla verticillata.The effects of simulated grazing by clipping on V.natans were not significant,indicating V.natans was tolerant to the damage of leaves.3.Nutrient enrichment and herbivory of P.canaliculata and P.clarkii had antagonistic effects on growth of V.natans.That is,nutrient enrichment promoted the growth of V.natans,while herbivory inhibited it within the range of nutrient concentration in our experiment.While,in moderate nutrient treatment,the consumption of P.canaliculata on V.natans was faster than in low nutrient treatment,indicated that the negative top-down effects by P.canaliculata on V.natans were enhanced by nutrient enrichment.The combined effects of nutrient enrichment and simulated grazing were also antagonistic when TP concentration below xx.Nutrient enrichment promoted the growth of H.verticillata,while herbivory inhibited it.However,in high nutrient treatment(TP=0.2 mg/L),the combined effects of nutrient and herbivory on H.verticillata turned into synergistic effects.Both nutrient enrichment and herbivory had negative effects on growth of H.verticillata,resulting the greatest biomass loss in highest nutrient level and strongest herbivory treatment.4.Nutrient enrichment enhanced the effect of P.canaliculata on V.natans by increasing macrophyte nutritional quality and palatability.In addition,macrophyte consumption was faster and P.canaliculata grew better in moderate nutrient treatment than in low nutrient treatment,which leading to a negative feedback loop.However,nutrient enrichment had no significant effect on the palatability of V.natans for P.clarkii,and the main effect of P.clarkii on V.natans was damage.The effects of nutrient enrichment and herbivory of P.clarkii on V.natans were the additive of the two factors.5.Herbivory had negative effects on water quality.The concentration of ammonia nitrogen in water column was significantly increased while the concentration of dissolved oxygen and p H was significantly decreased by the presence of P.canaliculata or P.clarkii.Herbivory might increase the chance for the shift of lakes to turbid states.In this study,nutrient enrichment significantly increased the phytoplankton biomass.Herbivory of P.canaliculata,simulated grazing of fish and birds had very strong effects on macrophytes.When the density of P.canaliculata was above 4.5 individuals/m~2(144 g/m~2),the V.natans in morderate nutrient treatment collapsed.Simulated grazing of fish and birds seriously reduced the biomass of H.verticillata.Thus it might trigger the steady-state transition of the lake with the biomass of phytoplankton increased and submerged macrophytes decreased.This study revealed the possible patterns and mechanisms of nutrient enrichment and herbivory on submerged macrophyte.It suggested that herbivory may enchance the chance of the shift from clear to turbid water state in shallow lakes during the process of eutrophication.It provided a theoretical basis for further revealing the mechanisms of submerged macrophytes decline,and also provided the knowlege for the protection and restoration of submerged macrophytes and the management of lake ecosystems.