Effects of increased nutrient loading and water level caused by extreme flooding on Hydrilla verticillata,periphyton and water properties
Extreme flooding significantly affects the habitat of aquatic plants.Its knock-on effects may be one of the reasons for a massive decline in submerged macrophytes.Increased nutrient loading and rising water level are major factors affecting the repro-duction and growth of submerged macrophytes.Therefore,we used H.verticillata,a typical submerged macrophyte in the lake,as the target species.A simulation study was conducted to explore the effects of water-level fluctuations(an increase in water level within a short period of time)and nutrient pulses(an increase in nutrient concentration within a short period of time)caused by extreme flooding events on the submerged macrophyte biomass,growth and reproductive traits,water physicochemical properties and periphyton.Four water level and nutrient loading scenarios were set up,i.e.,control(stable water level of 75 cm),gradual increase of water level from 75 cm to 150 cm+N(nitrogen)and P(phosphorus)inputs,sudden increase in water level from 75 cm to 150 cm+N and P inputs,and stable water level of 75 cm+N and P inputs.The latter three treatments had identical total N and P inputs.Phytoplankton,periphyton,and N and P concentrations in the water column were monitored in a 90-d period.The results showed that the sizes of females were greater than those of males in the control,while the relative growth rate(RGR),root biomass and the reproductive organ number in males were higher than those in females in the extreme precipitation treatment(sud-den change in water level).These findings demonstrated an evident sexual dimorphism in H.verticillata.Females responded more sensitively than males to water-level fluctuations(sudden and gradual water level change)and nutrient pulses.Both sudden and gradual increase in water level suppressed root biomass,RGR and the main branch number of plants,whereas plant height,aboveground and reproductive biomass remained unchanged.At the same time,a sudden rise in water level inhibited periphyton growth.Nutrient pulses increased the N concentration in the overlying water,promoted phytoplankton and periphyton growth,and significantly inhibited the biomass of shoots,roots and reproductive tissues.Within the designed range of water levels(75-150 cm),increased water level had no significant effect on the aboveground growth and reproduction of plants,while the combined effect of increased N and P inputs and water-level fluctuations had a negative effect on both aboveground biomass and reproductive output.Thus,the nutrient pulses aggravated the effect of water-level fluctuations on H.verticillata.In conclusion,the joint effect of water-level fluctuations superimposed on nutrient pulses caused by extreme flooding events will have a significant inhibitory effect on submerged macrophytes.With the increasing intensity and frequency of precipitation in the future,it would cause considerable impacts on aquatic ecosystems.
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