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ECO-HEALTHY ESTUARINE WETLANDS Wave Attenuation by Two Contrasting Ecosystem Engineering Salt Marsh Macrophytes in the Intertidal Pioneer Zone Tom Ysebaert & Shi-Lun Yang & Liquan Zhang & Qing He & Tjeerd J. Bouma & Peter M. J. Herman Received: 11 April 2011 /Accepted: 20 September 2011 # Society of Wetland Scientists 2011 Abstract Tidal wetlands play an important role in dissi- pating hydrodynamic energy. Wave attenuation in vegeta- tion depends on plant characteristics, as well as on hydrodynamic conditions. In the pioneer zone of salt marshes, species co-occur that differ widely in their growth strategies, and it is anticipated that these species act differently on incoming waves. In this field study we investigated, under different hydrodynamic forcing and tidal inundation levels, the wave attenuating capacity of two contrasting pioneer salt marsh species that co-occur in the Yangtze estuary, China. Our study shows that vegeta- tion can reduce wave heights up to 80% over a relatively short distance (<50 m). Our results further indicate that Spartina alterniflora is able to reduce hydrodynamic energy from waves to a larger extent than Scirpus mariqueter, and therefore has a larger ecosystem engineer- ing capacity (2.5× higher on average). A higher standing biomass of S. alterniflora explained its higher wave attenuation at low water depths. Being much taller compared to S. mariqueter, S. alterniflora also attenuated waves more with increasing water depth. We conclude that knowledge about the engineering properties of salt marsh species is important to better understand wave attenuation by tidal wetlands and their possible role in coastal protection. Keywords Coastal protection . Scirpus mariqueter . Spartina alterniflora . Tidal wetlands . Yangtze estuary Introduction It is well recognized that certain organisms fundamentally modify, create or define their own habitat and have important impacts on the physical and chemical processes occurring in their environment. Where organisms cause a large and/or distinct modification of the abiotic environ- ment, such biologically mediated habitat modification is often referred to as ecosystem engineering (Jones et al. 1994, 1997). Intertidal environments are often inhabited by distinct structural ecosystem engineers such as salt marsh vegetation and epibenthic oyster reefs and mussel beds. These organisms form three-dimensional structures on an otherwise bare soft-sediment environment, which interact with the hydrodynamic forces caused by tidal currents and waves. Ecosystem engineering ability of organisms in these environments is often related to their capacity of reducing hydrodynamic energy (Leonard and Reed 2002; Möller 2006; Neumeier and Amos 2006; Bouma et al. 2005, 2010). In intertidal vegetation, wave attenuation is caused by the energy dissipation generated by the physical structure of the plants (Fonseca and Cahalan 1992; Möller et al. 1999; Mendez and Losada 2004; Koch et al. 2009). This attenuation of wave energy by vegetation generally causes an exponential decline of wave energy with distance across the salt marshes (Möller et al. 1999; Bouma et al. T. Ysebaert (*) : T. J. Bouma : P. M. J. Herman Center for Estuarine and Marine Ecology, Netherlands Institute of Ecology (NIOO-KNAW), P.O. Box 140, 4400 AC Yerseke, The Netherlands e-mail: [email protected] T. Ysebaert IMARES, P.O. Box 77, 4400 AB Yerseke, The Netherlands S.-L. Yang : L. Zhang : Q. He State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, Peoples Republic of China Wetlands DOI 10.1007/s13157-011-0240-1

Transcript of Wave Attenuation by Two Contrasting Ecosystem Engineering ... · ECO-HEALTHY ESTUARINE WETLANDS...

ECO-HEALTHY ESTUARINE WETLANDS

Wave Attenuation by Two Contrasting EcosystemEngineering Salt Marsh Macrophytes in the IntertidalPioneer Zone

Tom Ysebaert & Shi-Lun Yang & Liquan Zhang &

Qing He & Tjeerd J. Bouma & Peter M. J. Herman

Received: 11 April 2011 /Accepted: 20 September 2011# Society of Wetland Scientists 2011

Abstract Tidal wetlands play an important role in dissi-pating hydrodynamic energy. Wave attenuation in vegeta-tion depends on plant characteristics, as well as onhydrodynamic conditions. In the pioneer zone of saltmarshes, species co-occur that differ widely in their growthstrategies, and it is anticipated that these species actdifferently on incoming waves. In this field study weinvestigated, under different hydrodynamic forcing andtidal inundation levels, the wave attenuating capacity oftwo contrasting pioneer salt marsh species that co-occur inthe Yangtze estuary, China. Our study shows that vegeta-tion can reduce wave heights up to 80% over a relativelyshort distance (<50 m). Our results further indicate thatSpartina alterniflora is able to reduce hydrodynamicenergy from waves to a larger extent than Scirpusmariqueter, and therefore has a larger ecosystem engineer-ing capacity (2.5× higher on average). A higher standingbiomass of S. alterniflora explained its higher waveattenuation at low water depths. Being much tallercompared to S. mariqueter, S. alterniflora also attenuatedwaves more with increasing water depth. We conclude that

knowledge about the engineering properties of salt marshspecies is important to better understand wave attenuationby tidal wetlands and their possible role in coastalprotection.

Keywords Coastal protection . Scirpus mariqueter .

Spartina alterniflora . Tidal wetlands . Yangtze estuary

Introduction

It is well recognized that certain organisms fundamentallymodify, create or define their own habitat and haveimportant impacts on the physical and chemical processesoccurring in their environment. Where organisms cause alarge and/or distinct modification of the abiotic environ-ment, such biologically mediated habitat modification isoften referred to as ecosystem engineering (Jones et al.1994, 1997). Intertidal environments are often inhabited bydistinct structural ecosystem engineers such as salt marshvegetation and epibenthic oyster reefs and mussel beds.These organisms form three-dimensional structures on anotherwise bare soft-sediment environment, which interactwith the hydrodynamic forces caused by tidal currents andwaves. Ecosystem engineering ability of organisms in theseenvironments is often related to their capacity of reducinghydrodynamic energy (Leonard and Reed 2002; Möller2006; Neumeier and Amos 2006; Bouma et al. 2005,2010). In intertidal vegetation, wave attenuation is causedby the energy dissipation generated by the physicalstructure of the plants (Fonseca and Cahalan 1992; Mölleret al. 1999; Mendez and Losada 2004; Koch et al. 2009).This attenuation of wave energy by vegetation generallycauses an exponential decline of wave energy with distanceacross the salt marshes (Möller et al. 1999; Bouma et al.

T. Ysebaert (*) : T. J. Bouma : P. M. J. HermanCenter for Estuarine and Marine Ecology,Netherlands Institute of Ecology (NIOO-KNAW),P.O. Box 140, 4400 AC Yerseke, The Netherlandse-mail: [email protected]

T. YsebaertIMARES,P.O. Box 77, 4400 AB Yerseke, The Netherlands

S.-L. Yang : L. Zhang :Q. HeState Key Laboratory of Estuarine and Coastal Research,East China Normal University,3663 North Zhongshan Road,Shanghai 200062, People’s Republic of China

WetlandsDOI 10.1007/s13157-011-0240-1

2005, 2010). Plants at the transition between the salt marshand the bare mudflat (i.e., pioneer zone) thus experience thelargest hydrodynamic forces.

In the pioneer zone of salt marshes, species co-occurthat differ markedly in growth strategy. For example theemergent macrophyte Spartina anglica and the sea grassZostera noltii experience similar waves and currents, butnevertheless show remarkably contrasting shoot stiffness.Their growth form reflect differences in physiology, withthe flexible Z. noltii being an aquatic species that has totolerate ebb periods, vs. the stiff S. anglica being a‘terrestrial’ species that has to withstand regular flooding.Their difference in shoots stiffness also represents a trade-off related to ecosystem engineering. An increased shootstiffness enhances a species’ capacity to reduce hydrody-namic wave energy (i.e., proxy for ecosystem engineeringability) but also causes an increase in the drag forces thatneed to be resisted (i.e., proxy for ecosystem engineeringcosts); decreasing shoot stiffness restricts both aspects(Bouma et al. 2005). The ‘terrestrial’ species that inhabitthe pioneer zone of the marsh can also have distinctlydifferent growth forms (e.g., see Bouma et al. 2010). Butfew field measurements have been undertaken to directlyquantify the effect of these different growth strategies onwave damping. Here we compare two contrasting pioneerspecies that co-occur in the Yangtze estuary (China),where the invasive stiff Spartina alterniflora now co-occurs with the indigenous and much more flexibleScirpus mariqueter. The difference in shoot stiffness andheight between both species suggests that the invasion ofS. alterniflora has enhanced the ecosystem engineeringability of marsh vegetation by enhanced wave attenuation.However, S. mariqueter tends to grow much denser than S.alterniflora, and density is known to compensate for stiffness(Bouma et al. 2010). So this raises the question to whatextent the replacement of the native S. mariqueter by theinvasive S. alterniflora may have changed the habitatmodifying characteristics of the ecosystem in terms of waveattenuation.

Knowledge about the ecosystem engineering effects ofsalt marsh plants is very relevant from a management pointof view (Borsje et al. 2011). Recent studies have confirmedthe widely-held view that intertidal salt marshes and othercoastal biotic structures are highly efficient in dissipatingtidal currents and incident wave energy (Koch et al. 2009).The latter is considered an important ecosystem service,since it forms a physical protection of the coasts andtherefore permits the relaxation of design criteria for flooddefence (Barbier et al. 2008; Koch et al. 2009). Actualcoastal protection by salt marshes will depend on manyfactors, including tidal inundation level, seasonal plantcharacteristics (height, biomass, stem density), and timingof natural processes such as storms, which results in a

spatial and temporal heterogeneity in the ecosystemfunction of wave attenuation (Koch et al. 2009). Althoughflume experiments have captured some of these aspects, byshowing that wave damping by salt marsh pioneer speciesis strongly affected by vegetation characteristics likevegetation density, rigidity, and standing biomass (e.g.Bouma et al. 2005, 2010), the flume dimensions make itimpossible to study the interaction with flooding height.

The aim of this study is to investigate in the field,under different hydrodynamic forcing and different tidalinundation levels, the wave attenuating capacity of twocontrasting salt marsh species that co-occur in the lowerpioneer zone of the Yangtze estuary: the native, shortand flexible Scirpus mariqueter and the invasive, tall andstiff Spartina alterniflora. The following hypotheses weretested: (1) wave attenuation by S. alterniflora will onaverage be larger as compared to wave attenuation by S.mariqueter; (2) due to its taller height, S. alterniflora willattenuate waves at higher water depths compared to theshorter S. mariqueter. We estimated in detail in the fieldthe vegetation density, height, and structure of the twoplants and measured wave attenuation along a transectrunning through both types of vegetation during severaltidal cycles.

Methods

Study Area

Field measurements were done on an exposed tidal wetlandon Chongming Island, at the seaward side of the Yangtzeestuary (Fig. 1). The Yangtze (or Changjiang) River is thelongest river in China, ranking third in the world. Itsdischarge into the sea is about 26,500 m3 s−1 on average,and can be as high as 100,000 m3 s−1 at peak flood. Thesediment discharge from the Yangtze into the estuary is ashigh as 490 Mt Y−1 (1 Mt=106 t) in the 1950–1960s, theworld’s fourth largest, but has been decreasing since the1970s, mainly because of dam construction in the riverbasin (Yang et al. 2002, 2005). Since the closure of theThree Gorges Dam, the largest river damming project in theworld, the Yangtze sediment discharge to the sea hasdecreased to less than 200 Mt Y−1, which has resulted indownstream channel erosion and conversion of the deltaicfront from progradation to recession (Yang et al. 2011). Thetides in the estuary are irregularly semidiurnal; at EasternChongming the tidal range is 2.5 m on average and 3.5 mduring spring tides (max. 5.0 to 6.0 m). The wind speed inthis area is highly variable, with multi-year averages of 3.5to 4.5 m s−1 and a maximum of 36 m s−1 recorded at thegauging stations (Yang et al. 2008). Controlled by themonsoon, southeastern winds prevail in summer and

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northwestern winds prevail in winter. The mean andmaximum wave heights are 1.0 and 6.2 m, respectively, atthe delta front (Yang et al. 2008).

With plenty of fine-grained sediment coming from theYangtze River and mesotidal to macrotidal conditions,intertidal wetlands are broadly developed in the estuary

Fig. 1 The Yangtze estuary (a, b) with the study site on easternChongming Island (c). The positions of the sensors on the fivedifferent days (5/9 till 9/9) are shown in (d), including the elevationgradient along the sensors. The photos (e, f) show the salt marshpioneer zone on eastern Chongming Island. (e) Overview photo of the

pioneer zone when the tide is coming in, in front the near-submergedvegetation of Scirpus mariqueter is visible, together with the muchtaller and emerged Spartina alterniflora in the right side of the photo.(f) Detail of the study area where pressure sensors were placed in theS. mariqueter and S. alterniflora vegetation at different positions

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(Fig. 1b). Salt marshes and mudflats have been expandingin a seaward direction over time in the Yangtze estuary(Yang et al. 2001), and were regularly reclaimed from thesea (on eastern Chongming Island the most recent recla-mations date from 1998 and 2001). Since then, the saltmarsh has again advanced for several hundred meters,although in recent years the rate of progradation seems tohave slowed. At present, the maximum width of theintertidal wetland from the low water mark to the seawallin Eastern Chongming is 8 km, with the upper portioncovered by marsh vegetation (maximum width of 2.5 km)and the rest being bare mudflat (Fig. 1c).

The native salt marsh vegetation on eastern Chongmingis dominated by S. mariqueter in the pioneer zone andlower marsh and Phragmites australis in the higher marsh.Scirpus mariqueter is a perennial rhizomatous, corm-forming sedge, mainly distributed in the salt marshesaround the Yangtze estuary. This species is endemic toChina. The above-ground shoot is usually composed of oneor two leaves and is about 10–80 cm in height. All theabove-ground shoots die off at the end of the growingseason, whereas below-ground parts (corm and rhizome)persist for several years. In 2001, S. alterniflora wasintentionally introduced into the northern marsh of EasternChongming Island (Li et al. 2009) for enhancing accretion.Spartina alterniflora is a perennial rhizomatous grassspecies native to the east coast of America. It grows 1.5–2.0 m in height, and has smooth, hollow stems that bearseveral leaves up to 20–60 cm long and 1.5 cm wide at theirbase, which are sharply tapered and bend down at their tips.The introduction of S. alterniflora in eastern Chongminghas led to a rapid range expansion of this species (Li et al.2009; Xiao et al. 2010), and S. alterniflora replaced S.mariqueter and P. australis in the northern part of EastChongming to a large extent. Certain areas are now 100%monocultures of S. alterniflora, but more to the south amixed vegetation occurs, often with S. mariqueter fringingthe bare mudflat and S. alterniflora somewhat higher in theintertidal zone (Fig. 1e–f). Measurements were done in thismixed vegetation type, where a small fringe of S.mariqueter abruptly shifted into a S. alterniflora commu-nity. Because of the limited pressure sensors available andthe need for a measuring platform, we were not able tomeasure complete monocultures simultaneously, but usedthis vegetation gradient instead. No tussocks or patches ofvegetation occurred on the bare mudflat in front of themeasurements.

Wave Attenuation Measurements

Measurements were done during five consecutive days inlate summer (5 until 9 September 2005, a period with fullygrown plants) under spring tide conditions in the northern

part of East Chongming Island at the mudflat–salt marshtransition zone. Complete tidal cycles were measuredduring 6, 7, and 9 September. On 5 September nomeasurements are available for the beginning of the flood,and on 8 September only part of the tidal cycle wasmeasured, due to technical problems.

Three bottom mounted Druck PTX/PDCR 1830 Depthand Level Pressure Sensors were deployed from aninstalled measuring platform at different positions along a43.5 m long cross-shore transect. This transect, starting atthe seaward side on the bare mudflat, first ran through auniform S. mariqueter stand over a distance of 16.5 m.After that, the vegetation abruptly changed to a uniformstand of S. alterniflora (Fig. 1d). Along this transect thepressure sensors were deployed in different configurations,due to the limited number of sensors. The sensor on thebare mudflat was fixed during the five measuring days (i.e.,reference sensor), whereas the two other sensors changedpositions from day to day (Table 1), in order to be able tomeasure waves at different distances within the two plantcommunities.

Starting from the mudflat sensor (2.75 m above LAT,Lowest Astronomic Tide), the following distances were usedfor wave measurements: 5 m (inside the S. mariquetervegetation, 2.77 m above LAT), 12.5 m (inside the S.mariqueter vegetation, 2.79 m above LAT), 16.5 m (insidethe S. mariqueter vegetation, just in front of the S. alternifloravegetation, 2.80 m above LAT), 20.5 m (corresponds to 5 minside the S. alterniflora vegetation, 2.82 m above LAT),30 m (corresponds to 13.5 m inside the S. alternifloravegetation, 2.86 m above LAT), and 43.5 m (corresponds to27 m inside the S. alterniflora vegetation, 2.89 m aboveLAT). As noticed from the elevations, a slight elevationgradient was present, but an abrupt change in elevation didnot occur at the transition of mudflat to salt marsh.

High frequency (20 Hz) pressure measurements weretriggered upon submergence of the sensors from a platformbuilt at the mudflat–salt marsh transition zone. These highfrequency records were split into series of 5 min andprocessed using Delft-Auke-PC software developed by WL| Delft Hydraulics (Deltares, Delft, The Netherlands).Summary statistics were calculated after de-trending of thepressure time series to remove any low-frequency tidalcomponent present. The data presented are based on theestimates of the significant wave height. Data on windspeed and direction were collected from the SheshanGauging Station, 20 km seaward from our study site(Fig. 1b).

Wave attenuation is caused by the energy dissipationgenerated by the physical structure of the plants, andKobayashi et al. (1993) assumed wave height decayedexponentially with distance of propagation through thevegetation. To a first approximation, wave attenuation as a

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function of distance in the vegetation can be approximatedby an exponential decay function of the form:

Hx ¼ H0 » exp �k»xð Þwhere Hx (m) is wave height at distance x in the vegetation,H0 (m) is wave height at distance 0, x (m) is the distanceinto the vegetation, and k (m-1) is the decay constant. Whendifferent distances, x, have to be compared, use of thedecay constant k is much more consistent than division ofrelative wave attenuation over distance x as (Hx-H0)/(Hxx).The latter will always change as x changes, even if thedecay is a perfectly exponential function. When severalmeasurements are available, k can be estimated from anexponential regression of relative wave height (Hx/H0)against distance. With only two measurements, k iscalculated as �ln Hx=H0ð Þ=x.

Plant Measurements

Vegetation characteristics of S. alterniflora and S.mariqueter were investigated at the locations wherehydrodynamic measurements were made. Density (num-ber of stems per m2) was determined in the surrounding ofthe different sensor positions using 25×25 cm (S.mariqueter, n=11) and 50×50 cm (S. alterniflora, n=4)quadrats, respectively. Smaller quadrats were used for S.mariqueter because of its much higher density. To derivegeneral insights in wave attenuation across the twodifferent vegetation types, we estimated the verticaldistribution of biomass and the number of structures foreach vegetation type. The vertical structure (length anddiameter of stems and length and position of leaves andseeds) was analyzed in six S. mariqueter quadrats usingdestructive harvest. After the measurements, S. mariqueterplants were cut into different 5 cm vertical layers and dry

weight was determined on these intervals. In the S. alterni-flora vegetation, the vertical structure, length and diameter ofstems and length and position of leaves) was analyzed in twoquadrats using destructive harvest. After the measurements,S. alterniflora plants were cut into different 10 cm verticallayers and dry weight was determined on these intervals. Thefraction >140 cm was pooled into one sample. The numberof stems and leaves at height z (n(z)) above the soil surfacewas calculated for each plant following Bouma et al. (2005),considering a mean leaf angle of 15° for S. mariqueter and18° for S. alterniflora. This vertical distribution of plantmaterials was used to relate stem density (stems per m2) to aquantitative vertical distribution of occupied space that offersresistance to water movement. Below-ground biomass wasdetermined by digging out 0.25×0.25 quadrats to a depth of30 cm, after destructive harvest of the above-ground parts.Samples were rinsed in the laboratory to remove mud, soil,and fine particulate matter, dried in the oven to a constantmass, and weighed.

Results

Tidal and Wind Conditions

During the first 3 days (5–7 September), due to a combinedeffect of the spring tide and the strong winds blowing fromthe sea, water levels increased quickly during the flood tide,with a water depth of 1 m reached in less than 1 h and hightide reached in about 2 h time (Table 1). The ebb tide lastedabout 3 to 31/2 h. During the last 2 days (8–9 September)winds were weaker, a water depth of 1 m was reached in1½ hours, and the duration of the flood and ebb tide weresimilar. Wind direction was relatively constant over the5 days with north and northeasterly winds prevailing, and

Table 1 Positions of the three sensors on each measuring day. Forsensor 1, fixed on the bare mudflat, the maximum water depth andmaximum significant wave height Hs for each measuring day are

given. Average maximum wind speed is calculated based on hourlyrecorded maximum wind speeds during the measuring period

Date Average max.wind speed

Sensor 1 Sensor 2 Sensor 3

Position Max. waterdepth (m)

Max. waveheight Hs (m)

Position + distancefrom mudflat

Position + distancefrom mudflat

5-Sep-05 13.2±0.7 Mudflat 1.58 0.58 S. mariqueter (5 m) S. mariqueter(12.5 m)

6-Sep-05 13.5±0.8 Mudflat 1.86 0.64 S. mariqueter (5 m) S. mariqueter(16.5 m)

7-Sep-05 9.1±1.1 Mudflat 1.40 0.50 S. mariqueter(16.5 m)

S. alterniflora(30.0 m)

8-Sep-05 5.9±0.2 Mudflat 1.19 0.21 S. alterniflora(21.5 m)

S. alterniflora(43.5 m)

9-Sep-05 6.0±1.3 mudflat 1.19 0.18 S. mariqueter(16.5 m)

S. alterniflora(21.5 m)

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visual inspection during the measurements showed wavesentering more or less perpendicular to the marsh edge.

Plant Characteristics

Plant characteristics of both species differed in manyaspects. The density of S. mariqueter stems was signifi-cantly higher as compared to S. alterniflora (Table 2). Theheight of the S. mariqueter vegetation was on average38 cm, with a maximum height of 72 cm. Spartinaalterniflora reached an average height of 84 cm, includingyoung shoots, with maximum height observed being226 cm. Almost 60% of the stems were taller than 50 cm,and about 40% of the stems were >100 cm. The averagestem diameter of S. alterniflora (5.2 mm) was more thandouble compared to S. mariqueter (2.2 mm) (Table 2). Totalabove-ground biomass and below-ground biomass wasalmost seven and five times higher, respectively, for S.alterniflora than for S. mariqueter.

The vertical distribution of biomass showed a decreasingtrend with increasing height for both species (Fig. 2). For S.mariqueter a strong linear decrease was observed, with 75%of the biomass situated in the 0–25 cm height range. For S.alterniflora, biomass change with height was less steep, with50% of the biomass situated in the 0–50 cm height range.Compared to S. mariqueter, S. alterniflora has three timesmore biomass in the 0–50 cm height range. Whenconsidering the number of stem and leaves per m2, S.mariqueter showed an increase (1.75×) in the number ofstructures up to a height of 12 cm, after which a steep declinewas observed Fig. 2). For S. alterniflora, the number ofstructures increased more than 2.5 times up to a height ofabout 75 cm, after which a strong decline was observed.

Wave Attenuation

Table 1 summarizes the wave conditions encountered at thebare mudflat during the five consecutive measuring days,before waves entered into the vegetation. Due to the strongwinds during the first 3 days, high waves were recorded witha maximum of 64 cm. During these days a very strongrelation between water depth and wave height was observed

(up to r2=0.98) (Fig. 3b–f). As an example, water depths andsignificant wave heights measured during 7 September areshown in Fig. 3a. During the last 2 days of the measurementsthe wind speeds dropped, resulting in waves with a maximumheight of 21 cm. The relation between wave height and waterdepth was weaker during these days, but still clearly present(r2=0.81 and 0.65 on 8 and 9 September, respectively).

All measurement cycles showed decreasing wave heightswith distance traveled within the vegetation. During the firstday of observations (5 September), when sensors werepositioned in the S. mariqueter vegetation and high waveswere present, wave attenuation by this species was verylimited (Fig. 4). At 5 m inside the vegetation, no waveattenuation was observed, whereas at 12.5 m inside thevegetation only limited wave attenuation was observed atlow water depths. At 5 m inside the vegetation, someshoaling effect was observed (i.e., negative values for waveattenuation), resulting in higher waves compared to thewaves on the mudflat. During the second day (6 September),with similar wave conditions, the sensor at 5 m inside the S.mariqueter vegetation recorded similar limited wave attenu-ation as during the first day, with shoaling effect at low waterdepths. The sensor positioned at 16.5 m inside the S.mariqueter vegetation showed larger wave attenuationcompared to the 12.5 m sensor, but also at higher waterdepths (>1 m) wave attenuation was <20%. On 7 September,wave heights entering the vegetation were lower than duringthe initial days. Sensors were positioned in the S. mariquetervegetation (16.5 m inside), and in the S. alternifloravegetation (30 m from mudflat sensor, 13.5 m inside the S.alterniflora vegetation). Both sensors showed high waveattenuation (80–90%) at low water depths, but waveattenuation dropped much faster with increasing water depthin S. mariqueter compared to S. alterniflora vegetation. Evenat the highest water depth the wave attenuation was larger inthe S. alterniflora vegetation. On 8 September, maximumwater depth and significant wave height dropped due to acombination of neap tide and calmer wind conditions. Thesensor positioned furthest inside the S. alterniflora vegeta-tion (43.5 m from mudflat sensor, 27.5 m inside the S.alterniflora vegetation), showed a continuously high waveattenuation during the whole inundation period, whereas the

Table 2 Plant characteristics ofScirpus mariqueter and Spartinaalterniflora in the study site.Averages and standard devia-tions are given, with betweenbrackets the number of quadrats

Plant characteristics Scirpus mariqueter Spartina alterniflora

Number of stems (n m−2) 2352±355 (n=11) 334±12 (n=4)

Height (cm) 38±4 (n=6) 84±63 (n=3)

Average stem diameter (mm) 2.2±0.14 (n=2) 5.2±1.70 (n=3)

Total above-ground biomass (g m−2) 327±7 (n=2) 2300±403 (n=3)

Total below-ground biomass (g m−2) 448±10 (n=2) 2755±326 (n=3)

Percentage of flowering plants (%) 72% 36%

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sensor positioned only 5 m inside the S. alternifloravegetation showed a decreasing attenuation of waves withincreasing water depths, as was observed during the previousdays. During 9 September, under similar tide and windconditions as 8 September, sensors were positioned in the S.mariqueter vegetation (16.5 m inside) and in the S.alterniflora vegetation (21.5 m from mudflat sensor, 5 minside the S. alterniflora vegetation). In both communitieswave attenuation was high at low water depths, but similar tothe previous days, wave attenuation was much less in S.mariqueter vegetation when the water depth increased.

The relative wave height at the different positions,averaged over a complete tidal cycle, is shown in Fig. 5.Some temporal variability was observed, which may becaused by differences in wave forcing, but also by differ-ences in water height due to different tidal phase and/ordifferent experimental setup. However, over the period, aconsistent picture of relative wave height on the transect isseen. We fitted an exponential decay of relative wave height,separately for S. mariqueter and S. alterniflora. The decayconstant in the S. mariqueter vegetation (0.015 m−1) wassignificantly different from the decay constant in the S.alterniflora vegetation (0.038 m−1). The 2.5 factor differencebetween the constants expresses the stronger wave dampingby S. alterniflora. The decay constant also changed with

water depth. A comparison between the decay constant in theS. mariqueter and S. alterniflora communities, respectively,on 7 (large waves) and 9 September (small waves) clearlyshows that when water depth is below 50 cm, the decayconstant by S. alterniflora is about 1.5× to 2.0× times highercompared to S. mariqueter (Fig. 6) With increasing waterdepth, the difference between both species increases, althoughthe wave decay constant decreases for both species. Between50–125 cm water depths, when S. mariqueter was fullysubmerged and S. alterniflora was to a large extent emerged,the decay constant per m by S. alterniflora was about 3 to 5times higher than in the S. mariqueter vegetation. At thehighest water depth, this decreased to 3 to 4 times, becauseof a further drop in the decay constant by S. alterniflora as itbecame increasingly submerged. When comparing 7 and 9September, the decay constant observed for S. alterniflorawas higher on the calm day with relatively small waves (9September) compared to the day with higher waves (7September), especially at higher water levels.

Discussion

Results from the wave measurements carried out at easternChongming Island showed differences in the capacity of

Fig. 2 Top figures: Vertical distribution of above-ground biomass ofScirpus mariqueter (left, n=2 plots of 25×25 cm)) and Spartinaalterniflora (right, n=3 plots of 50×50 cm). Note the different axisranges in both figures. Bottom figures: Schematic representation of the

vertical distribution of structures (number of stems and leaves) withinthe S. mariqueter vegetation (left, n=6 plots of 25×25 cm) and the S.alterniflora vegetation (right, n=2 plots of 50×50 cm). Values aremeans ± SD

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Fig. 3 a Significant wave heights (Hs) and water depth observedduring one tidal cycle on 7 September 2005. One pressure sensor waspositioned on the bare mudflat at the edge with the vegetation, onesensor at 16.5 m distance from the mudflat sensor (inside Scirpus

mariqueter vegetation) and one sensor at 30 m from the mudflatsensor (inside Spartina alterniflora vegetation). b–f Linear relationbetween water depth and wave height at the mudflat site during thefive measuring days

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Fig. 4 Wave attenuation in relation to water depth observed during the 5 days (a–e) of observations with sensors positioned at different distancesfrom the mudflat edge and in different vegetation types (Scirpus mariqueter and Spartina alterniflora)

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attenuating waves by the two co-occurring but verydifferent salt marsh species, the native Scirpus mar-iqueter and the invasive Spartina alterniflora. Althoughthe experimental setup was limited both in space and time,our results indicate that S. alterniflora is able to modify its

physical environment by reducing hydrodynamic energyfrom waves to a larger extent than S. mariqueter, andtherefore has a larger ecosystem engineering capacity. Thefact that the S. mariqueter vegetation was shoreward fromthe S. alterniflora vegetation and therefore measurementswere done along a vegetation gradient could haveinfluenced the observations to some extent. Slightly lowerwaves were entering the S. alterniflora vegetation afterpassing through the S. mariqueter vegetation, which couldresult in a relatively larger attenuation effect in S.alterniflora because of a higher attenuation rate withsmaller waves, especially at the higher water levels. Butthe apparent differences in wave attenuation between thetwo plant species, as demonstrated by the large differencein decay constants, are most likely related to the differentgrowth forms between the two species.

Wave attenuation in vegetation depends on thegeometrical (number of stems, diameter, branching, andheight) and biophysical characteristics (stiffness andbuoyancy) of the vegetation canopy present, as well ason the hydrodynamic conditions including water depth,wave period, and wave height. In our study, plantcharacteristics differed greatly between S. mariqueter andS. alterniflora. First, S. alterniflora, reaching heights ofover 2 m, is a much taller species compared to S.mariqueter, typically reaching heights of 50–60 cm.Vegetation reaching the water surface and above (i.e.,emergent structures) is more effective in reducing waveheight than submerged vegetation (Augustin et al. 2009).This explains why, with increasing water depths, weobserved a much sharper decline in wave attenuation forS. mariqueter as compared to S. alterniflora. Our resultsthus confirm earlier results showing that water depth is animportant factor in determining wave attenuation inmarshes (Möller et al. 1999). Second, above-groundbiomass was on average seven times higher for S.alterniflora. Within the height range of S. mariqueter (0–50 cm), S. alterniflora has three times more biomasscompared to S. mariqueter. For shallow systems, standingbiomass has been shown to be a good indicator of thepotential reduction in wave heights over vegetated bedswith similar shoot lengths (Bouma et al. 2010). Our studysupports this finding to some extent, but the difference invegetation height makes it impossible to obtain the truebiomass effect. Within a 0 to 50 cm water depth and undersimilar wave conditions, S. alterniflora had a 1.5× to 2×higher wave attenuation rate (or exponential decayconstant) compared to S. mariqueter. The number ofstems was not a good indicator of wave attenuation, as thenumber of stems was much higher for S. mariqueter. It isimportant not only to consider the number of stems, butinclude all structures (i.e. branching pattern, leaves, fruits)of a plant that cause friction to waves (Bouma et al. 2005;

Fig. 6 Exponential decay constant per meter in Scirpus mariqueterand Spartina alterniflora vegetation in relation to the measured waterdepth (at the mudflat site) during two different days (7 September:large waves, 9 September: small waves)

Fig. 5 Exponential decay of (tide-averaged) relative wave height (theratio of wave height at the marsh site to the wave height at the mudflatsite), separately for Scirpus mariqueter and Spartina alternifloravegetation at different distances from the mudflat sensor

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Möller 2006; Feagin et al. 2011). We did not include plantflexibility in this study, although there were apparentdifferences in flexibility between the two species, with S.mariqueter being much more flexible and S. alterniflorabeing more stiff. Bouma et al. (2010) showed that for stiffSpartina anglica and flexible Puccinellia maritima whosebiomass was comparable, wave attenuation was remark-ably similar for these flexible and stiff species, indicatingflexibility might be less important for wave attenuationcompared to the standing biomass.

Our study shows that salt marsh vegetation can reduceaverage wave heights up to 80% over a relatively shortdistance (<50 m). The attenuation of salt marsh vegetationin coastal regions has been investigated in several studies(Knutson et al. 1982; Möller et al. 1999; Möller andSpencer 2002; Möller 2006). Moller et al. (1999) studiedwave transformation over saltmarshes through field andnumerical modeling studies. Field measurements indicatedthat wave energy dissipation rates over the saltmarsheswere significantly higher (an average of 82%) than over thesand flat (an average of 29%) due to the saltmarshes havinggreater surface friction compared to the unvegetated tidalflat. In a similar period as our observation period, Yang etal. (2008) showed a decrease of wave height by 11% over a185 m long unvegetated mudflat transect. This correspondsto a decay constant of 0.00064 m−1, about 20–50 timeslower than the constants found in the salt marshes of thisstudy (Fig. 5). Möller et al. (1996, 1999) observed similardecay constants of 0.00077–0.00084 m−1 over a 197 mmudflat transect in the UK. Möller and Spencer (2002)showed that the reduction in significant wave height waslargest over the first 10 m of salt marsh vegetation (1.1–2.1% per m for typical Hs=30 cm and water depths of 1–3 m), with lower attenuation over the mudflat and saltmarshas a whole (Möller et al. 1996, 1999). This range ofattenuation corresponds to an exponential decay constantbetween 0.0117 and 0.0237 m−1 in the first 10 m, verysimilar to the values obtained in this study (0.014 m−1 for S.mariqueter, 0.039 m−1 for S. alterniflora). Studies on S.alterniflora in the USA by Wayne (1976) and Knutson etal. (1982) showed high decay constants of 0.062 m−1 and0.094 m−1 over short distances of 20 and 30 m, respec-tively, corresponding with wave height reductions of 71%and 94%. These high decay constants are most likely dueto the fact that field measurements in the USA were donein microtidal, relatively low wave energy environments.This points to the importance of the physical conditionspresent in the system (tidal range, incident wave energy)to evaluate the capacity of plants to attenuate incomingwaves. In our study, both tidal range and wave heightswere relatively high, with mesotidal to macrotidal con-ditions and significant wave heights up to 0.64 m, whichexplains the somewhat lower decay constants observed for

S. alterniflora compared to the studies of Wayne (1976)and Knutson et al. (1982).

Only at 5 m inside the S. mariqueter vegetation was anincrease in significant wave height observed at low waterdepths compared to the incoming waves at the mudflatsensor. Although no sharp elevation change was present,the vegetated area was slightly higher compared to the baremudflat. This change in elevation and the presence of theplants might lead to a shoaling effect, i.e., an increase inwave height due to a sudden decrease in water depth. Theeffect was unimportant above 40–50 cm water height, andlimited to 20% increase of wave height. Under theseconditions, wave attenuation may be slightly underesti-mated since the maximal wave height at 5 m from themudflat was higher than at the mudflat, and the range overwhich attenuation took place was shorter.

Reduced hydrodynamics within the vegetation also hasimportant implications for sedimentation and marsh pro-gradation. The plants trap sediment by modifying thehydrodynamic forces from waves and currents and therebyenable them to maintain or expand their habitat (Morris etal. 2002; Bouma et al. 2007). Potentially, the superior waveenergy attenuation of S. alterniflora alters other propertiessuch as a greater sediment deposition that may facilitate itscolonization and spread in this habitat and in other habitatswhere it has invaded. But also other biological traits (e.g.,fast growth, well-developed belowground structures, highsalt tolerance, great reproductive capacity through bothclonal growth and sexual reproduction, timing of germina-tion) make S. alterniflora a successful invader (Callawayand Josselyn 1992; Li et al. 2009; Schwarz et al. 2011).

Our field measurements support the hypothesis that saltmarsh vegetation can act as an efficient wave energy buffer.In this era of global change with increasing coastal erosionproblems due to sea level rise and human interferences, thepotential role of tidal wetlands for coastal protection isincreasingly recognized (Koch et al. 2009; Borsje et al.2011). At the same time, it is recognized that the ecosystemservice of coastal protection by vegetation is non-linear anddynamic (Barbier et al. 2008; Koch et al. 2009). Althoughthere are general commonalities in wave attenuationprocesses among plant communities, it is important torecognize that there are many other factors, such as plantdensity and location, species type, tidal regime, season, andlatitude, that influence the patterns of non-linearity observed(Koch et al. 2009). Knowledge of the feedback mechanismsby which vegetated marsh surfaces achieve energy dissipa-tion, and the thresholds at which these dissipative controlsare exceeded, is a key factor in understanding the morpho-logical response of marshes to sea-level rise and their role forcoastal protection. Although limited in space and time, ourstudy demonstrates that knowledge about the engineeringproperties of salt marsh species is important to further our

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understanding of wave attenuation by tidal wetlands andtheir possible role in coastal protection.

Acknowledgments We thank the Programme Strategic Alliancesbetween the People’s Republic of China and the Netherlands for fundingthis research (PSA 04–PSA–E–01; 2008DFB90240). We also want tothank especially our Chinese cooperation partners of the SKLEC researchinstitute at the East ChinaNormal University in Shanghai for their supportduring the field work. We acknowledge the EU-project THESEUS forsupporting SKLEC and NIOO for research on the application of saltmarshes for coastal defense. This is Netherlands Institute of Ecology(NIOO-KNAW) publication number 5078.

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