| Tidal salt marsh is one of the important ecosystems of wetlands.It provides important ecosystem functions such as nursing habitats for fish and crustaceans,resting and feeding areas for migratory birds.It also supports biodiversity,filters contaminants,dissipates water energy,and offers intrinsic values such as aesthetics and education.Tidal salt marsh plays a significant role in resource and environment of coastal zone.With people's acquaintance on its importance,salt marsh ecosystem has become a hot topic of multi-disciplinary research.Tidal salt marsh vegetation affects the hydrodynamics and sedimentation process greatly.It can attenuate hydrodynamic and accelerate accretion as well as resist erosion of bed.In the context of the growing speed of the global sea level rise,this accretion effect seems more important than ever for tidal marsh survival.Besides the above benefits,the vegetation can also protect the engineering of the coastal zone by attenuating the water energy.All of these physical processes and engineering effects make tidal salt marsh vegetation one important study project.This study was funded by the National Science Foundation(44011770) and the Ministry of Science and Technology of China(2004CB720505).In this study,two exposed tidal wetlands,the Eastern Chongrning and the Jiuduansha,at the seaward side of the Yangtze River delta were selected as the study regions.Using advanced equipments,we observed the data of wave,flow speed and SSC(Suspended Sediment Concentration).Then,in the laboratory,sampled sediments were used to make grain-size analyzing,and sampled vegetations were used to measure the amount of sediments adhered by plants.Based on the subsequential results,we laid special stress on analyzing the attenuation of hydrodynamics,the trapping effect,the sediment grain size and stability of bed due to vegetations.The following conclusions are obtained:1.Influence of Salt marsh vegetations on hydrodynamics.Attenuation of wave energy and flow speed in S.alterniflora-S,mariqueter salt marsh was one order higher than that in tidal flat in magnitude.The average decrease rate of flow speed was 0.97%/m,and the highest decrease rate was 0.89%/m in salt marsh.In tidal flat,the average decrease rate and the highest decrease rate were 0.070%/m and 0.078%/m respectively.Decrease rate of wave height per unit bed level was 0.53%/cm in tidal flat,while that in S.mariqueter salt marsh was 2.60%/cm,and that in S. alterniflora salt marsh was 7.40%/cm.After analyzing,we found that the attenuation effect of vegetations on hydrodynamics was related with ecological parameters of plants.The higher, denser,and wider the plant was,the greater the attenuation effect on hydrodynamics was.The attenuation effect when the plant was submerged is different from that when the plant was not submerged.When not submerged,the plant can effectively attenuate the wave and flow.Therefore, we can know that the attenuation effect of S.mariqueter salt marsh on hydrodynamics was smaller than that of S.alterniflora salt marsh because the height of the former is smaller than the latter.2.Influence of Salt marsh vegetations on SSC.The SSC at Site 30 m away from the outer marsh edge was typically higher than that at Site 50 m away from the outer marsh edge at the trance in the Eastern Chongming.The tide-averaged SSC at Site 30 m away from the outer marsh edge was 2.44 kg/m3,and that at Site 50 m away from the outer marsh edge was 1.82 kg/m3.The maximum SSCs at the two sites were 4.59 kg/m3 and 3.35 kg/m3 respectively.Thus,we can draw the conclusion that the SSC of water close to sediment source(tidal fiat and creek) was higher. With the increase of the distance away from the landward,the SSC of water decreased.As a result of our analyzing,the mechanism of plant's influence on SSC dues to the settlement of the suspended particulate,the decrease of the resuspend power,caused by the decrease of the wave energy,and the trapping effect of plants on the sediments.3.Trapping effect of salt tidal marsh vegetation on suspended sediment.We measured weight of sediment trapped by vegetations sample in the Eastern Chongming and the Jiuduansha salt marsh.The results indicate:The dry weight of sediment adhering to plants ranged from 18.0 to 559.0 g/m2.On average,the amount of sediment adhering to S.alterniflora,P.australis and S. mariqueter was 220.6±172.7,64.9±38.1,and 31.6±10.0 g/m2,respectively.After analyzing, we find that the amount of sediment adhering to marsh plants per unit of land area depends onthe plant properties,the SSC and the bed level.The general positive relationship between the amount of sediment and plant biomass:y=30.544e0.0004x(r=0.72,n=16,p＜0.001).The amount of sediment adhering to the stems and leaves per unit land area was less than that of adhering to the branching and flower/fruit structures.The SSC of water is higher,the amount of sediment adhering to marsh plants is bigger.The lower the bed level,the higher the frequency and duration of tidal submergence,and the more the sediment adhere to plant.The effect of each type plant is different.At the same position,the amount of sediment adhering to S.alterniflora(77.3 g/m2) was typically more than to P.australis(38.7 g/m2),and at the same position,the amount of sediment adhering to S.alterniflora(369 g/m2) was typically more than to S.mariqueter(44.8 g/m2). However,also on average,the amount of sediment adhering per plant biomass was highest for S. mariqueter(92.1±35.3 g/kg),lowest for P.australis(28.8±22.8 g/kg),and in between for S. alterniflora(57.5±32.9 g/kg).For the same species,plants closer to a creek of the lower marsh edge tend to trap more sediment.The amount of sediment trapped by vegetation per unit land area in the low marsh margin decreased at a rate of 1-3%/m with distance from the outer marsh edge bordering the mudflat or from the tidal creak where the SSC was higher.The difference of amount trapping by plant in different position attribute to different SSC and bed level.The dry weight of sediment adhering to plants drastically decreased from 10-15 to＜2 g/m2 upward from their base to their tip.The amount of particulate trapped by near-bed 51-0 cm section of the plants is over one third in total.The downward increasing trend in sediment adhering to the plants reflects the combined effect of plant biomass,SSC and inundation condition.The amount of particulate adhering to S.mariqueter in the initial stage of autumn(September) was six than that in the end stage of spring(May).The plants were wither away in winter,and the effect was disappearing.In the Eastern Chongming,we estimate that the contribution of S.mariqueter to the total deposition rate is＜10%and the contribution of P.australis to the total deposition rate may be either＞10% or＜10%.4.Influence of Salt marsh vegetations on A&E(accretion and erosion) process of bed.The daily average A&E rate of salt marsh was obviously less than that in tidal flat.For example,from 15 to 23 September 2006,the observation on a trance across salt marsh-tidal flat in the Eastern Chongming indicated that:The daily A&E rate in salt marsh varied from-1 to 3 mm/d,and the daily average A&E rate in salt marsh varied from-0.2 to 1.2 mm/d.However,the two A&E rates in tidal flat are-2～5 mm/d and-1～2.3 mm/d respectively.S.mariqueter can obviously affect the seasonal change of A&E of bed.For example,through our observations in Eastern Chongming, we found that the bed continuously accreted in season when S.mariqueter plants and the bed level can promote 20 cm in a growing season.In winter when the plants were withered away,the salt marsh becomes tidal flat,and the bed did not promote or be eroded.It can be concluded that the mechanism of plant's influence on A&E of bed attributes to attenuation of hydrodynamics caused by plants,trapping effect of plant and concretion of root.5.Influence of salt marsh vegetation on grain size of bed sediment.There was obviously negative relation between vegetation index(The product of mean height and coverage of plants) and mean size of surface sediment of north,middle and south salt marsh trances in the Eastern Chongming.The regression relationship was y=30.951e -5E-05x(r=0.9999,n=3,p＜0.01),which indicated that the higher and denser the plant was,the freer the sediment was.Taking S. mariqueter for example,the grain size(average grain size was 30.1±7.8μm) in winter when the salt marsh vegetations die away was two times larger than that in summer(average grain size was 16.4±3.8μm) when the bed was covered by plants.After analyzing,we found that the fining effect of the plants was also attributes to the attenuation of hydrodynamics and their trapping effect.The friction of vegetations induced attenuation of hydrodynamics,and enhanced the settling of fine grain.Meanwhile,the cover of plant prevented the erosion.The trapping effect of plant enhanced the deposition of fine grain to bed finally.
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