The Response Processes And Mechanism Of V.Natans And H.Verticillata Co-existed To Change In Nutrient Concentration

Submerged macrophytes play an important role in the trophic status and ecosystem functions of shallow lakes.The addition of external nutrient affects the light and nutrient concentrations in the water,and has effect on the growth and morphological differentiation of submersed macrophytes.At the same time,the morphotypes of submersed macrophytes will also affect their absorption and accumulation of nutrients,which will further influence the growth of submersed macrophytes.Vallisneria natans and Hydrilla verticillata are the common submersed macrophytes in China.They belong to canopy and upright types,and mostly grow in lakes in coexistence.To understand the impact of external nutrient on V.natans and H.verticillata is helpful,which can provide a theoretical basis for the management of submersed macrophytes in Chinese lakes.Two treatments(low loading and high loading)and controls in twelve mesocosms were set up to investigate the responses of co-cultured V.natans and H.verticillata to nutrient addition.Water samples were collected every two weeks for measuring nutrients(TN,TP,et al),chlorophyll a(Chl a)of planktonic,total suspended solids(TSS)concentrations,particulate organic matters(POM)and light intensity at the sediment surface.Dry biomasses and nutrients(TN,TP)of roots and shoots of both species were measured at the end of the experiment.Results showed that under nutrient enriched conditions,both species reduced root: shoot ratio and increased nutrient storage in their biomasses.The root: shoot ratio of H.verticillata was significantly lower than V.natans.The biomass and nutrient storage in the H.verticillata increased with the rising of nutrients,but V.natans only increased under low nutrient loadings.The results indicated that the external nutrient can promote the growth and nutrient uptake of the submersed macrophytes.Under high nutrient loadings,H.verticillata.gained the advantages of the nutrient absorption and growth.This result may be related to the difference in morphotypes of V.natans and H.verticillata.