| Submerged macrophytes play an important role in stabilizing clear-water conditions and maintaining biodiversity in shallow lakes.With nutrient enrichment,submerged macrophyte communities have declined or even totally lost in many lakes and these lakes shift from clear-water state to turbid state,inducing strong negative effects on the structure and function of theses lake ecosystems.However,the mechanisms behind submerged macrophyte communities decline in the process of lake eutrophication remain unclear.In recent years,herbivory have been found playing a key role in regulating the abundance and community composition of submerged macrophyte,but few study has examined the role of herbivory in the decline of submerged macrophyte communities in eutrophic lakes.Submerged macrophyte may suffer both eutrophication and heavy herbivory in many lakes in China since a large number of lakes are under eutrophication conditions and the unreasonable herbivorous fish aquaculture and invasive herbivores are widespread in these lakes.Examining the effects of herbivory on submerged macrophyte communities under different nutrient conditions can reveal whether nutrient enrichment will strengthen the top-down control of herbivores on submerged macrophytes and test whether herbivory will induce rapid decline of submerged macrophyte in eutrophic lakes,thus it is of great significance to the protection and restoration of submerged macrophyte communities in lakes and understanding the mechanisms of submerged macrophytes decline under eutrophic lakes.In this study,we constructed a mesocosm experiment and built submerged macrophyte communities composed of submerged macrophytes(Vallisneria denseserrulata,Hydrilla verticillata,Potamogeton maackianus),which are common in Yangtze River.Subsequently,we set up different nutrient gradients and put herbivores(Pomacea canaliculata,Ctenopharyngodon idellus)into systems.Thro μgh measuring the changes in the community biomass,community structure,water chlorophyll content and herbivores biomass to assess the responses of submerged macrophyte communities and water quality to grazing pressure,eventually examining the effects of herbivory on submerged macrophyte communities under the various nutrient conditions.The results were as follows:1.Nutrient enrichment had a greater promoting effect on growth of submerged macrophytes than the effect shading by phytoplankton within the nutrient concentration range in our experiments,altho μg h it could also promote the growth of phytoplankton.Nutrient enrichment had positive effect on growth of the three submerged macrophytes,but the positive effect on H.verticillata was greater than that on V.denseserrulata and P.maackianus.2.The presence of herbivores significantly reduced the biomass of submerged macrophyte communities,and the dominant species shifted from H.verticillata to V.denseserrulata.3.Nutrient enrichment and herbivory had antagonistic effects on growth of submerged macrophyte.That is,nutrient enrichment promoted the growth of submerged macrophyte,while herbivory inhibited it at the range of nutrient concentration in our experiment.However,consumption of herbivores on submerged macrophyte was faster in moderate nutrient treatment than in low nutrient treatment,indicated that the negative top-down effects by herbivores on macrophytes was enhanced by nutrient enrichment.4.Herbivory had negative effects on water quality and the effects were not equal in different nutrient conditions.The concentration of total nitrogen,total phosphorus,ammonia nitrogen,turbidity and phytoplankton chlorophyll a in water column was significantly increased while the concentration of dissolved oxygen were significantly decreased by the presence of P.canaliculata or C.idellu.Compared with low nutrient treatment,herbivory had a greater influence on water quality in moderate nutrient treatment.In this study,the effects of herbivory on submerged macrophyte communities under different nutrient conditions were examined using an mesocosm experiment.Our results indicated that herbivory had stronger negative effects on submerged macrophyte communities and water quality in eutrophic lakes,thus herbivory may increase the risk of these lakes shifting from clear-water state to turbid-water state.This study improved our understanding of the mechanism of submerged macrophyte decline in shallow lakes and provided insights for the protection of submerged macrophytes communities and the management and restoration of shallow lakes. |
