Characteristics Of Micro-boundary Layer Around Submerged Plant Leaves And Stems And Its Ecological Effect

Submerged plants constitute an important natural component in shallow aquatic ecosystems.The micro-boundary layer(MBL)around submerged plants surfaces plays an important ecological role in macrophyte growth and nutrient transformation in the aquatic environment.The structure,composition,microenvironment,environmental factors and ecological effect of the micro-boundary layer around leaves and stems of submerged plants were investigated using the scanning electron microscope(SEM),high spatial resolution of microsensors(for OZ,pH and ORP),microchemical analysis and isotope tracer technique.Results showed that significant spatio-temporal variations and oxidation reduction heterogeneous environment exist in the micro-boundary layer around different host submerged plants during different growth stages.And the ecological effect of the micro-boundary layer and the control on nitrogen transformation in eutrophic water bodies were verified.The present study gives implications for better verifying the mechanisms of micro-boundary layer in regulating nutritional cycling in eutrophic waters.Conclusions are as follows:(1)The spatial structure,complex constituents,oxidation reduction heterogeneous microenvironment were significant in the MBL around submerged plants.Periphyton is an assemblage of algae,bacteria,fungi,protozoan,inorganic matter,and organic detritus that remains attached to submerged macrophyte surfaces and forms a special bio-water boundary layer.The thickness of diffusive boundary layer was 1062-1202 ?m,affected mainly by factors such as periphyton mass and macrophyte growing stages.The periphyton consisted of an important part of inorganic matter(CaCO3)with considerable amounts of silt and clay particles(76.59%-83.38%)from the water column observed in the periphytic layer.The nutrients content of periphyton were relative high with TOC and TN content 75.29-106.99 g�kg-1,2.43-7.01 g�kg-1,respectively,and could provide large amount substrate for organism.(2)The significant spatio-temporal variations existed in the MBL around leaves and stems of submerged plants.At the spatial scale,in the vertical direction of stems/leaves surface,the O2 concentration and pH in the MBL increased markedly with decreasing distance to the surface of leaves/stems and peaked at the leaves/stems surface of Potamogeton crispus.On contrary to micriprofiles of the O2 and pH,the ORP markedly decreased with reducing distance from the leaf/stem surface and reached the lowest at the leaf/stem surface.The O2,pH,ORP in the MBL around the mature leaf at the middle shoot was steep with greatest amplitude of fluctuation and that of senescent leaves at the basal shoot was relatively plain.The difference was mainly affected by plant physiological characteristics and the periphyton synergistically.At the temporal scale,O2,pH and ORP in the MBL varied significantly among different growing stages along the entire life cycle.The fluctuation of O2,pH and ORP in the micro-boundary layer was the lowest during seedling stages while its amplitude peaked(401.3 ?mol�L-1,8.37 and 428.51 mv)at stably growing stages.The O2 concentration at the surface of leaf displayed diurnal variations with a significant unimodal pattern.The periphyton increased continuously and reached maximum at decline stage.(3)The nutrients load,sediments type,light intensity and habitat were major environmental factors affected the MBL.The periphyton in the MBL of Vallisneria natans increased significantly with the proportion of lake sediment increase,and the periphyton mass was the highest of S5(lake sediments)and that of S1(clay)was lowest.The O2 concentration and amplitude in MBL,photosynthesis rate of Vallisneria natans,O2 concentration at leaf surface and DBL increased markedly with the irradiation increase.The nutrients load,sediments type and habitat affected the MBL by periphyton composition,periphyton mass and macrophytes growth.And the irradiation affected the thickness of MBL by photosynthesis of macrophytes releasing oxygen.(4)There were differences in micro-boundary layer between that of different host submerged marcophytes.The values and distributions of O2,pH and ORP in micro-boundary layer were different between different specie of submerged marcophytes.The values and amplitudes of fluctation of O2 and pH were greatest in MBL of Potamogeton malaianus,and those of Vallisneria natans were least,which related to the photosynthetic capacity and periphyton mass.There were differences in periphyton between different specie of submerged marcophytes.The masses of periphytic algae and protozoan were greatest for Potamogeton crispus,and those of Potamogeton malaianus were least.The thickness of periphyton,dry weight(DW),ash free weight(FADW),ash weight(AW),chlorophyll a content,TOC content and DBL were greatest for P.crispus,and those of Vallisneria natans were least.The contents of TN and TP in periphyton were greatest for P.malaianus with 7.01 g kg-1,0.39 g�kg-1,respectively,and those of P.crispus were least(2.43-10.15 g�kg-1).(5)The growth stages of macrophytes created large fluctuations and dynamics in O2,pH,and ORP at the stems and leaves surface by changing the thickness of the diffusive boundary layer around the macrophyte.O2 and pH decreased gradually with the distance increased from the stems and leaves surfaces in vertical direction during growth period.O2 concentration and pH in the MBL around the leaves and stems of P.malaianus and V.natans increased with macrophytes entering rapid growth stage,which accompanied by spatial differences gradually increasing.O2 concentration and pH in the MBL around the leaves and stems of the two macrophytes achieved peak at stable growth stage.The O2 concentration and pH in the MBL increased slightly or declined when entering the periphyton layer at decline stage.O2 concentration and the amplitude of fluctuation in MBL reduced to the minimum in December,and that of pH was comparable with June.However,the ORP had an opposite trend to that of O2 and pH.The magnitude and spatial distributions of 02,pH,and ORP in MBL around the leaves and stems of P.malaianus and V.natans were affected mainly synergistically by plant physiological character and periphyton in life cycle of macrophytes.(6)There was important ecological effect in the micro-boundary layer around submerged plants.The periphyton in micro-boundary layer negtively affected photosynthesis of submerged macrophyte P.Malaianus.The fluctuation and diffusion resistance of O2 in MBL increased by periphyton,and O2 fluxes decreased significantly in MBL.The maximum electron transport rate(ETRmax)of P.Malaianus leaves with periphyton(23.70 ?mol m-2�s-1,32.50 ?mol m-2�s-1 and 9.50?mol m-2�s-1)was lower than that of perphyton removed(23.93 ?mol m-2�s-1,66.5?mol m-2 �s-1 and 15.9 ?mol m-2�s-1)during seedling stage,stable growth stage and decline stage.The photosynthetic capacity of P.Malaianus with dense periphyton was lower than that of perphyton removed.The results suggest that a thick periphyton exerted negative effects on the growth of macrophytes by providing extensive shading and creating a barrier that hindered the transport of dissolved substances such as O2,and led to premature decline in macrophytes in the eutrophic Taihu Lake.There was denitrification activity in periphyton attached on submerged macrophyte P.Malaianus shoots,and that can be an important process of N removal in eutrophic Taihu Lake.Potential denitrification rates in periphyton attached on the surface of submerged macrophyte P.Malaianus ranged from 33.35 to 40.12?mol�-2�h-1(per lake area).These values were comparable with that of sediments.Potential denitrification rates varied spatially and temporally in Taihu Lake,with the highest and lowest denitrification rates appearing during autumn(Oct.)and during winter(Dec.),respectively.And the highest and lowest denitrification rates occurred in one of the most eutrophic algae-dominated state Meiliang Bay and in macrophyte-dominated state Xukou Bay with good water quality.Inorganic nitrogen content in water and periphyton attached on P.Malaianus shoots were important controlling factors on potential rates of denitrification.The denitrification in periphyton attached on submerged macrophyte P.Malaianus shoots can be of significant importance for the N turnover and cycle in eutrophic freshwater ecosystems.