The effects of water depth on the growth and reproduction of Potamogeton crispus in an in situ experiment

Aims Water level is one of the most important determinants of the distri-bution and composition of submersed aquatic vegetation in shal-low lakes.Without changes in nutrient loading,large-scale declines in submerged macrophytes in shallow lakes are largely attributed to strong external or internal forces,including changes in water level.As a winter-active submerged macrophyte,Potamogeton crispus has important functions in aquatic ecosystem.The objec-tives of our study were to reveal the effects of water depths on the germination,growth,reproduction and morphological changes of P.crispus under natural environmental conditions;identify the optimum water depth range for colonization of P.crispus;and pre-dict the succession course for P.crispus in Lake Taihu.Methods A long-term in situ experiment was performed to study the effects of water depth on the growth and morphology of P.crispus across the entire life cycle.Plants were grown in the improved cross type of rhizotrons for 30 weeks at three different water depths(60,120 and 200 cm)in Dongtaihu Bay of Lake Taihu.We measured the plant height,root length,the length and width of leaves and counted the numbers of leaves,ramets,turion spikes and roots of each plant on each observation day during the experiment.Important Findings Water depths ranging from 60 to 120 cm were favourable for above-ground vegetation growth,root tissue growth and the reproductive ability of P.crispus.At water depth of 200 cm or beyond,the distri-bution of P.crispus will be limited in the following year because of the lack of turion formation and the severe inhibition of ramet pro-duction.The relationship between above-ground biomass and growth time at three different water depths fits a logistic growth curve well(P<0.001).The rapid growth and distribution with high density of P.crispus at water depth near 60 cm during the reproductive stage is not favourable for growth of other submerged species.So a continuous water depth of~60-120 cm is the optimum growth depth range for the conservation and restoration of P.crispus in Lake Taihu.In addi-tion,the morphological parameters values acquired can replace or provide the ranges for the value of GmaxSB(the maximum growth rate of submerged plants,in the unit of d^(−1))obtained via calibration in the control equations of submerged plant growth in the Eco-Taihu model,which is a three-dimensional ecological model of Lake Taihu.

How do water depth and harvest intensity affect the growth and reproduction of Elodea nuttallii(Planch.)st.John?

Aims both high and low densities of macrophyte vegetation can impair its ecosystem service function.Harvesting is often applied to macrophyte vegetation to maintain an appropriate density.Vegetation harvesting has occasionally gone awry and caused catastrophes,such as vegetation disappearance and cyanobacterial dominance in waterways and lakes.because water depth influences macrophyte density at all life stages,the simultaneous influences of harvesting and water depth should be carefully examined.Thus,this study aims to quantify the effects of differently harvesting Elodea nuttal-lii on its growth and reproduction at different water depths in field experiments.Methods Four harvest intensities(harvesting E.nuttallii plant heights equal to 25%,50%,75%and 100%of the water depth)were applied to E.nuttallii growing at four different water depths(60,90,120 and 150 cm).Plant length and root length were measured.The node number,root number of each plant and number of floating plants were counted before harvesting.The harvested plant were dried to a constant weight for dry weight determination.Important Findings The rate of increase in the length and shoot number of E.nuttallii varied from−0.012 to 0.440 day^(−1) and from−0.020 to 0.639 day^(−1),respec-tively.Water depth>150 cm would limit E.nuttallii growth.Elodea nuttallii responded to increasing water depths and low-intensity har-vesting by increasing internodal length and decreasing shoot number.The larger internodal length of E.nuttallii observed in relatively deeper water was also induced by the physical strain generated by its buoy-ancy as its specific gravity was less than water’s.The physical mecha-nism of removing the plant canopy by harvesting decreased E.nuttallii buoyancy and prevented floating.Harvesting increased plant produc-tion in shallow waters<90 cm deep.moreover,it is also necessary to perform three medium-intensity harvests at a water depth of 120 cm and one low-intensity harvest or no harvesting at a water depth of 150 cm to achieve longer lifetimes and less biomass near the water surface when the plants reach or approach the water surface.