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Ecological Engineering 64 (2014) 360–366

Contents lists available at ScienceDirect

Ecological Engineering

journa l h om epa ge: www.elsev ier .com/ locate /eco leng

iota and hydrology influence soil stability in constructed wetlands

tephanie G. Prellwitz, Anita M. Thompson ∗

epartment of Biological Systems Engineering, University of Wisconsin—Madison, 460 Henry Mall, Madison, WI 53706, USA

r t i c l e i n f o

rticle history:eceived 13 August 2013eceived in revised form9 December 2013ccepted 1 January 2014vailable online 29 January 2014

eywords:tormwaterrosion resistance

a b s t r a c t

The impact of hydroperiod and vegetation on soil stability was investigated in wetland swales that treaturban stormwater. Critical shear stress was measured as a proxy for soil stabilization using a CohesiveStrength Meter in three parallel wetlands. Despite efforts to create three replicate wetlands by uniformconstruction and identical planting, each developed a distinct hydroperiod (low, intermediate, and highwater-recession rates) and vegetation (varied biomass of cattails [Typha species]). Critical shear stress (�c)was highest in the high-recession, fast-draining wetland (7.8 Pa), followed by the intermediate-recessionswale (6.1 Pa) and the low-recession, inundated swale (4.1 Pa). These values correlated with differentialdevelopment of moss and algal mats, both of which were highly resistant to erosion (�c of 8.6 and 7.4 Pa,respectively). These epibenthic mats were patchy and developed primarily in the wetlands with high and

ryophyteascular plantsritical shear stress

intermediate water-recession rates but were limited in the low-recession wetland, with cattail shade,anaerobic conditions, and substrates consisting of organic matter (�c of 5.6 Pa), bare soil (3.0 Pa), and muck(1.8 Pa). Small inflows sustained ponding and associated cattails, which promoted shade and destabilizedthe surface soil of the low-recession system, as well as subareas within the better-drained wetlands.Epibenthic mats played an unexpected, disproportionate role in soil stabilization compared to vascularplants.

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. Introduction

Constructed wetlands are becoming a common ecologicalption for stormwater treatment, in part because the range ofcosystem services they can provide make them preferable to con-entional treatment systems. When used to treat urban stormwaterunoff, constructed wetlands are subjected to intermittent flowshat vary in depth and velocity. This variance in flow regimempacts the hydroperiod, or the duration and frequency of inun-ation. This may, in turn, influence the treatment performance ofetlands, including sediment and nutrient removal, water reten-

ion, vegetation diversity support, and soil stability (Kadlec andnight, 1996; Miller and Zedler, 2003; Greenway et al., 2007;

enkins and Greenway, 2007; Boers and Zedler, 2008; Moustafat al., 2011, 2012; Doherty et al., in revision).

The stabilization of soil is a balance between the hydro-ynamic mechanisms that cause erosion and the forces that resist

t (Grabowski et al., 2011). Vascular plants stabilize soil and pre-

ent erosion via: (i) belowground biomass which aggregates soil,rovides cohesion, and enhances microbial growth through a net-ork of plant roots and root hairs (Gyssels et al., 2005; De Baets

∗ Corresponding author. Tel.: +1 608 262 0604; fax: +1 608 262 1228.E-mail address: amthompson2@wisc.edu (A.M. Thompson).

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925-8574/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2014.01.010

© 2014 Elsevier B.V. All rights reserved.

t al., 2007) and (ii) aboveground vegetation which slows the flowf water, thereby reducing the erosive forces acting on the soil sur-ace (Gyssels and Poesen, 2003). Sediment detaches when the sheartress from flowing water exceeds a critical value that is influencedy the resistance of soil and plant structures.

Complex biological and physical components interact on aicroscopic scale to create significant spatial variability in the

roperties (e.g. stability or erodibility) of cohesive soil and sedi-ents (Tolhurst et al., 2006; Grabowski et al., 2011). Epibenthicats of moss (bryophytes) and algae (various filamentous anducilage-producing forms) utilize an extensive combination of

hysical, biological, and chemical processes to resist erosionPaterson et al., 2000; Whitehouse et al., 2000; Andersen, 2001;undkvist et al., 2007), which may make them effective soil stabi-izers (Tolhurst et al., 2006). In particular, mosses increase erosionhresholds by buffering the soil surface from direct shear stress ofowing water, and algae stabilize sediments by excreting extracel-

ular polymeric substances (EPS) that bond soil particles (Hoaglandt al., 1993). An increase in diatom biofilms has been shown toncrease erosion thresholds (Sutherland et al., 1998; Tolhurst et al.,008). Differences in erosion thresholds within intertidal mudflats

orrelate with the presence or absence of epibenthos (Andersen,001; Tolhurst et al., 2006).

While biological and physical components are responsible foroil stability, we know of no study that investigates how hydrologic

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S.G. Prellwitz, A.M. Thompson / Eco

egime and vascular vegetation together impact substrate devel-pment and stability in a constructed wetland. Soil texture andoisture content, presence or absence of vegetation, and hydrope-

iod have been shown to impact biological, physical, and chemicalechanisms in terrestrial applications and also impact stormwater

reatment in constructed wetlands (Greenway, 2004). The abilityf vegetation and epibenthos to stabilize soil, prevent erosion andesuspension of deposited sediment, and ultimately to improveater quality in constructed wetlands designed to treat urban

unoff remains understudied.Our goal was to understand how hydrology and vegetation

evelopment influence substrate stability in constructed wetlands.e studied three experimental wetlands (swales) that were con-

tructed identically but developed different hydrologic regimesue to subsurface substrate heterogeneity. We hypothesized thati) soil stability at the swale-scale would differ with hydrologicegime and (ii) swale vegetation and physical parameters wouldiffer in their ability to explain erosion thresholds at the sub-meter0.25 m2) scale.

. Material and methods

.1. Study site

The project site is located within the University ofisconsin—Madison Arboretum in Madison, Dane County,isconsin. A Stormwater Management Research Facility (SMRF;

atitude = 43◦2′11.2′ ′N, longitude = 89◦25′28.5′ ′W) was con-tructed to treat stormwater runoff from a 45.7-ha contributingrban watershed. Construction of the SMRF began in 2008 andas completed in November 2009. The site was mass-graded and

overed with 15 cm of silt loam topsoil salvaged from the siteuring excavation.

The SMRF is one component of a treatment train which consistsf a forebay and retention pond system that discharges into fourrapezoidal wetland swales (each swale is 96-m long, 8.7-m widet inlet end and 14.7-m wide at outlet end; slope: 0.06 cm m−1)eparated lengthwise by 0.3 m high earthen berms. Flow from allour wetland swales (Swale 0, I–III) discharges into a 7.6-m widey 0.6-m deep collection swale which outlets to a trapezoidal con-rete flume (runs parallel to the swales) that discharges into aecond stormwater retention pond. A diversion structure with sto-logs allows incoming stormwater to be diverted in varied volumesoward the SMRF, or to bypass the system altogether and flow downhe concrete flume toward the retention pond. The stoplogs were

odified to limit incoming flow volumes to ensure that the swalesere hydrologically isolated (did not overflow their berms) during

he study period. Stormwater was directed into the swales on 1arch 2011. Prior to this date, all stormwater was diverted toward

he concrete flume and retention pond.Swales I–III were seeded in November 2009 with identical seed

ixtures from a collection of 27 native wet prairie species at a ratef 590 seeds m−2 (Prellwitz, 2013). Each swale was divided alonghe length of the swale into 16 equal area “sections”, each seededith a subset of 3 or 9 plant species. Swale 0 was subdivided and

eeded in a different manner for another experiment and was notonsidered in this study. Water levels in the swales were measuredith pressure transducers at the inlet and at the outlet of each swale

Doherty et al., in revision).

.2. Critical shear stress measurements

As a proxy for soil stability, we measured critical shear stress�c), or the shear stress required to initiate particle detachment,n situ with a Cohesive Strength Meter (CSM, Model MKIV 60 psi,

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l Engineering 64 (2014) 360–366 361

artrac of Glasgow, United Kingdom). To date, CSM researchas been limited primarily to estuarine and intertidal sedimentsTolhurst et al., 2000, 2003; de Deckere et al., 2001; Defew et al.,002; Friend et al., 2003; Watts et al., 2003; Chen et al., 2012). TheSM utilizes infrared optical sensors within a testing chamber toeasure water transparency after the soil surface is subjected toater pulses at increasing pressures and correlates these values to

ediment concentration and �c (see Tolhurst et al., 1999). The CSMs easily transported for in situ measurements, which are superioro estimates of �c based on soil samples brought back to the labora-ory for flume experiments (Wilcock and McArdell, 1997; Houwing,999; Vousdoukas et al., 2011).

In 2010, CSM measurements were taken between 13 Octobernd 18 October. The swale lengths were divided into four equiv-lent zones (four sections per zone). One section was randomlyelected per zone per swale and measurements were taken withinhe extents of two randomly selected 0.25 m2 quadrats in thatection. In 2011, CSM measurements were taken between 15eptember and 21 November. In Swales I and II, measurementsere taken within two randomly selected quadrats in every other

ection (starting with the furthest upstream section). Due to homo-eneity of vegetation and substrate in Swale III, a reduced numberf sections representing the upstream-, middle-, and downstream-ections were measured for �c. Measurements were taken aftereak productivity or vegetation senescence, which provided anntire growing season of root development between the 2010 and011 measurements.

For each CSM test, moderately loose surface particles wereently brushed from the measurement site. Particles that adheredo the soil surface were not disturbed. In particularly dry conditionshat prohibited the insertion of the optical sensor head into the soil,he surface was hand-sprayed with distilled water to wet the soilurface enough to allow insertion (approximately 20 sprays). Dis-illed water was slowly injected via syringe into the optical sensoread prior to each measurement to initialize the light transmissioneading. A CSM default test (S1–S19) was selected for appropriatencremental and maximum water pressure within the sensor head.he CSM measurements with an initial beam transmission readingess than 70% were discarded since this was indicative of surfacearticle disturbance prior to a test.

Multiple measurements (n ≥ 2) were taken per 0.25-m2 quadrato characterize critical shear stress. Representative soil substratesas described in Section 3.1 of Section 3) visually observed withinach quadrat were documented. At least two CSM measurementsere taken per quadrat, with at least one within each represen-

ative soil substrate observed within the quadrat. Measurementsere avoided in cracks, as this prohibited the optical sensor head

rom retaining water for the test duration. Samples were locatedpproximately between vascular plants. Volumetric soil moistureontent was measured near each CSM measurement site using aH2O Soil Moisture Meter with HH2 Moisture Meter readout unit,±2%; Dynamax Inc., Houston, TX).

Pressure and beam transmission data were uploaded from theSM, and multiple beam transmission measurements were aver-ged for each incremental pressure value (Black, 2007). Verticallypplied jet pressures were converted to an equivalent horizon-al bed shear stress using the equation developed by Tolhurstt al. (1999) and transmission values were converted to suspendedediment concentration using the equation by Black (2007). Bothorizontal bed shear stress and suspended sediment concentrationere plotted with respect to time.

The resultant two-series plot of sediment concentration versusime and horizontal shear stress versus time typically produced

soil erosion profile with three distinct regions: (1) an ini-ial portion before sediment detachment occurred and particle

362 S.G. Prellwitz, A.M. Thompson / Ecological Engineering 64 (2014) 360–366

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oncentration was approximately 0 g L−1 (light transmission70%), (2) a sloped portion after sediment detachment was ini-

iated, and (3) a final portion where sediment concentration wast an approximate maximum (light transmission ≈0%). A linearegression was fit separately to data in regions (1) and (2) and theorizontal shear stress corresponding to the intersection of the twoegression lines represented �c (Black, 2007). In some instancesf highly resistant surfaces, sediment did not detach even at theaximum eroding pressure of 9.12 Pa. In these instances, �c was

ssigned a minimum value of 9.12 Pa (similar to Friend et al., 2003nd Tolhurst et al., 2006).

In all cases, CSM measurements were taken in quadrats whereegetation was sampled. In 2010, shoot biomass (g) and speciesichness (n) were measured (Boehm, 2011). In 2011, several metricsere measured (Doherty and Zedler, 2013) with the following

ubset used for the present study: shoot biomass (g); speciesichness (n); Bryophyte cover (cm per 40-cm transect); above-round biomass (AGB; g); belowground biomass (BGB; g); BGB tohoot biomass (BGB:shoot biomass); Leaf Area Index (LAI; m2 m−2);ercentage of AGB as Typha (%AGB as Typha); and soil moisturem3 m−3).

Soil samples were extracted from Swales I–III in November 2010nd November 2012. One section was randomly selected in each ofhe four equivalent zones per swale and soil samples were col-ected directly adjacent to CSM sampled quadrats in that section.oil samples were dried, ground, and submitted to the University ofisconsin—Madison Soil and Plant Analysis Laboratory for analysis

f organic matter and texture (percent sand, silt, and clay).

.3. Statistical analysis

Because there was an equipment-induced maximum erodingressure for our critical shear stress measurements, substrate

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e conditions encountered within the swales.

nd swale medians were compared using the non-parametricruskal–Wallis test followed by Wilcoxon Rank Sum Testith appropriate Bonferroni correction for p-values. Differences

etween medians were evaluated for significance at ̨ = 0.05.Non-parametric Spearman’s rank correlation coefficients (�)

ere generated to determine quadrat-scale correlations betweenritical shear stress and various physical and vegetation parame-ers at ̨ = 0.10 given the size of the dataset. Data from 2010 to 2011ere analyzed separately because (i) more extensive vegetationarameters were measured in 2011 and (ii) 2010 measurementsere made prior to stormwater entering the swales while 2011easurements were made after the first growing season, duringhich stormwater flowed through the swales. Data were combined

cross swales in each year. The Spearman analysis did not considerhe minimum assigned critical shear stress values of 9.12 Pa (whenediment did not detach at this maximum eroding pressure), as thisreated an artificial lack of variability in the data that preventedeaningful statistical comparisons and artificial correlations that

ended toward a y-intercept of 9.12 Pa. Therefore, the Spearman’statistical analysis represents a truncated dataset (i.e. more conser-ative �c values) due to the exclusion of critical shear stress valuesbove this threshold. All statistical analyses were performed usingAS 9.3 software (Cary, NC).

. Results

The swales differed in hydrologic regime and vegetation. Dis-ontinuities in the subsurface clay layer of the SMRF impactedater level recession rates over time. The resultant hydrologic

egimes in Swales I–III functioned with intermediate- (aver-ge = 1.7 cm day−1), high- (6.0 cm day−1), and low- (1.2 cm day−1)nter-storm recession rates, respectively, (Doherty et al., inevision) Hydrologic profiles in the swales largely influenced

S.G. Prellwitz, A.M. Thompson / Ecological Engineering 64 (2014) 360–366 363

Fig. 2. The frequency of occurrence (%) of the five soil substrates present within thequadrats was largely swale-dependent. In this bar graph, high resistance substrates(moss mat: MM, algal mat: AM; left axis and solid fill) and low to moderate resistancessY

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Fig. 3. Mean critical shear stress (�c; Pa) varied based on surface substrate in thefall of 2010 and 2011. Algal mat (AM), moss mat (MM), and organic matter (OM)were more effective soil stabilizers, while bare soil (BS) and muck substrates weremore prone to erosion. Minimum assigned critical shear stress values of 9.12 Pa(when sediment did not detach at this maximum eroding pressure) are includedin these means and can be particularly detected (average near 9.12 Pa with smallerror bars) in AM and MM. No instances of muck occurrence were detected in thefi

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ubstrates (bare soil: BS, muck, organic matter: OM; right axis and hatched fill) arehown. Soil substrate occurrence per swale totals 100%, though the scales on the-axes are offset to emphasize high- and low-resistance substrates.

egetation establishment and development (Doherty and Zedler,013). Swale III (low-recession rate) was dominated by cattailsTypha species, unplanted) and had the greatest plant biomass withhe lowest species richness, while Swales I and II (intermediate-nd high-recession rates, respectively) had lower plant biomassith greater species richness (Doherty and Zedler, 2013). Leaf Area

ndex (LAI), a metric of surface light penetration (lower values indi-ate more light reaches the surface), averaged 1.7 and 1.6 in Swales

and II, respectively, versus 2.6 in Swale III.Soils in Swales I–III over the study period were similar in organic

atter content (2.8%, 2.8% and 2.9% in Swales I, II and III, respec-ively) and texture (38.9% sand, 44.5% silt, 16.1% clay in Swale I;4.9% sand, 48.6% silt, 16.5% clay in Swale II; 37.8% sand, 45.6% silt,6.6% clay in Swale III). Surface soil moisture at the time of criticalhear stress measurements was lower in Swales I and II (0.39 and.33 m3 m−3, respectively) compared to Swale III (0.46 m3 m−3).

.1. Composition of swale substrate

The swale soils developed patchy surfaces that included aiverse variety of vascular plants and assemblages of mosses andlgae, i.e., epibenthos. Surface substrates were characterized as:lgal mat (AM; primarily cyanobacteria and diatoms), noted forts plated surface and curled edges in dry conditions and thick,aste-like surface in wet conditions; moss mat (MM), with a thickurface layer of what appeared to be Mnium sp.); organic mat-er (OM), a surface of lightly-adhered vegetation debris or otherrganic matter, including dominant root structures; muck, a highlyrosive surface present in saturated conditions with no stable soiltructure; and bare soil (BS), with no noticeable or dominant coverFig. 1).

Epibenthic mats differed by inundation regime (Fig. 2); theyourished in the more open canopies of Swales I and II (lower LAIalues), which had higher water recession rates. Moss cover in 2011veraged 5.3 and 10.5 cm along a 40 cm transect for Swales I andI, respectively, but only 0.3 cm for Swale III (Doherty and Zedler,013). Likewise, Swales I and II had 17.0% and 32.0% frequen-ies, respectively, of algal mat occurrences in randomly sampleduadrats over the study period, compared to 8.1% in Swale III.

Swale III had nearly undetectable moss mats and abundant

rganic matter. Less organic matter accumulated in the swales with

prevalence of moss and algal mats (Swales I and II) and wherehere was bare soil (Swale I). Organic matter substrate occurredn 62.2% of measured quadrats in Swale III versus 24.5% in Swale

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all of 2010. Error bars indicate SE; different letters within each substrate categoryndicate significantly different medians (p < 0.05).

and 28.0% in Swale II. Swale III was unique in its developmentf muck, defined here as sediment with extremely low cohesion,hich occurred within 18.9% of randomly sampled quadrats in

wale III, while it was not found within Swales I and II. Swale IIhighest water-level-recession rate) had the least frequent occur-ence of bare soil (4.0%) compared to 26.4% and 10.8% in Swales Ind III, respectively.

.2. Erosion threshold of surface substrates

The erosion threshold (critical shear stress, �c) rankings for theve substrates were consistent over the course of the study (com-ined data for 2010 and 2011 shown in Fig. 3). Bare soil had the

owest �c, regardless of year. Muck, only detected in 2011, had aimilar �c to bare soil. The �c for organic matter was mid-range withhe exception of Swale II. Epibenthic mat cover corresponded to theighest �c, regardless of year. Significant differences (p < 0.05) in �c

mong swales were only observed for organic matter.Combined data across all swales and years show significant dif-

erences (p < 0.05) in erosion thresholds for the various surfaceubstrates (Fig. 4). Epibenthic mats of moss and algae had the high-st �c (mean = 8.6 and 7.4 Pa, respectively,; median = 9.1 and 9.1 Pa,espectively), which did not differ significantly from each otherp = 0.74), but were significantly greater than �c for organic mattermean = 5.6 Pa; median = 5.5 Pa; p < 0.001 and < 0.05, respectively),are soil (mean = 3.0 Pa; median = 2.7 Pa; p < 0.001 and < 0.001,espectively) and muck (mean = 1.8 Pa; median = 2.1 Pa; p < 0.001nd < 0.05, respectively). The �c for OM was significantly greaterhan bare soil (p < 0.05) and, although not significant, �c for OMas also greater than muck (p = 0.12). Bare soil and muck did notiffer significantly (p = 1.0). Mean and median �c values for epiben-hic mats are conservative; as noted in the Methods, no detachmentccurred at the maximum eroding pressure of 9.12 Pa for several ofhe tests. Additionally, �c values for muck are likely high because

everal muck measurements were discarded (initial transmission70%) and the CSM repositioned to less vulnerable muck sites.

While the ranking of erosion thresholds among substrates wasonsistent over the study period, the thresholds of some individual

364 S.G. Prellwitz, A.M. Thompson / Ecological Engineering 64 (2014) 360–366

Fig. 4. Mean critical shear stress (�c; Pa) varied based on surface substrate withdata combined for all years and swales. Minimum assigned critical shear stressvalues of 9.12 Pa (when sediment did not detach at this maximum eroding pres-sure) are included in these means. Error bars indicate SE; different letters indicatesignificantly different medians (p < 0.05).

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Table 1Spearman’s � coefficients and corresponding p-values for correlations between crit-ical shear stress and vegetation/physical parameters for 2010 data (n = 34). Boldvalues are significant (p < 0.10).

Parameter Spearman’s � p Value

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ubstrates did vary with time. Values of �c decreased from the fall of010 to the fall of 2011 for both algal and moss mat (p < 0.05). Valuesf �c for bare soil increased over this period (p < 0.05), while �c didot change significantly for organic matter substrate (p = 0.45).

Despite differences in the erosion thresholds of some substratesver time, swale-scale �c did not vary from the fall of 2010 tohe fall of 2011 (Swale I: p = 0.48; Swale II: p = 0.52; Swale III:

= 0.89). Moreover, swale-scale erosion thresholds (Fig. 5) differedy inundation regime. Combining data for 2010 and 2011, �c wasighest for Swale II (mean = 7.8 Pa; median = 9.1 Pa; high-recessionate and most open canopy), which was significantly greater thanwale I (mean = 6.1 Pa; median = 6.4 Pa; intermediate-recession

ate; p < 0.001) and Swale III (mean = 4.1 Pa; median = 2.5 Pa; low-ecession rate and most dense canopy; p < 0.001). Critical sheartress for Swale I was significantly greater than for Swale IIIp < 0.01).

ig. 5. Mean critical shear stress (�c;Pa) varied based on swale. Minimum assignedritical shear stress values of 9.12 Pa (when sediment did not detach at this maxi-um eroding pressure) are included in these means. Error bars indicate SE; different

etters indicate significantly different medians (p < 0.05).

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Shoot biomass 0.172 0.33Species richness 0.053 0.77Soil moisture −0.259 0.14

.3. Correlations between critical shear stress andegetation/physical parameters

In 2010, prior to stormwater entering the swales, there were noignificant correlations between �c and the parameters evaluatedrow number, which indicates distance down the swale or treat-

ent length; shoot biomass; species richness; and soil moisture;able 1; p = 0.14 to 0.77). While not significant (p = 0.14), criticalhear stress did decrease as soil moisture increased. In 2011, �c wasegatively correlated with AGB (p < 0.10), shoot biomass (p < 0.01),nd %AGB as Typha (p < 0.10). While not significant, �c was posi-ively correlated with moss cover (p = 0.11) and BGB:shoot biomassp = 0.12). No other correlations were significant (Table 2; p = 0.33o 0.97).

. Discussion

The combination of water-level recession rate and vegetativeesponse contributed to differences in the establishment of surfaceoil conditions with different erosion thresholds. The swales wereonstructed identically, yet subsurface substrate heterogeneityesulted in different hydrologic responses and water-level reces-ion rates for the three swales. The swales were planted with theame plant species at the same rate, yet hydrologic regimes sup-orted the invasion and dominance of cattails only in Swale III andubareas with ponding in Swales I and II.

Surface conditions within swales were patchy at the small scale.ome 0.25-m2 quadrats had three of the five soil surface typesbserved overall: moss mat, algal mat, organic matter, bare soil,nd muck. In Swale III, algal mats were the least prominent com-ared to the other swales and moss development was very limited,ith no patches large enough to measure critical shear stress. Sim-

lar patchiness has been observed in intertidal mudflats due to bothiotic and abiotic variability (Tolhurst et al., 2006).

Moss and algal establishment in Swale III appeared to be hin-

ered by frequent and prolonged inundation, as well as shadingy dense cattails and leaf litter. Anoxic conditions likely developedith little diffusion of oxygen into the soil (Miller and Zedler, 2003).

n contrast, the more open canopies of Swales I and II increased

able 2pearman’s � coefficients and corresponding p-values for correlations between crit-cal shear stress and vegetation/physical parameters for 2011 data (n = 42). Boldalues are significant (p < 0.10).

Parameter Spearman’s � p Value

Row 0.022 0.89Species richness −0.045 0.78Bryophyte cover 0.249 0.11AGB −0.262 0.09Shoot biomass −0.378 0.01BGB 0.007 0.97BGB:AGB 0.072 0.65BGB:shoot biomass 0.241 0.12LAI −0.156 0.33%AGB as Typha −0.271 0.08Soil moisture −0.099 0.63

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ight penetration to the surface (Timofeev, 1959) and supportedhe establishment of algae and moss. Neither algal nor moss matsersisted on the elevated berms between swales indicating thatome threshold of soil moisture or other input was needed forhese communities to establish. The accumulation of organic mat-er was prominent in Swale III because of frequent inundation (Daynd Megonigal, 1993) and decomposing Typha spp. litter. The com-ination decreased soil cohesion and led to the development ofuck.The patchiness in soil substrate types within the swales resulted

n spatially variable soil stability with distinct differences betweenubstrates with no visible moss or algal mats and epibenthos.reas with no visible moss or algal mats (bare soil, organic matter,uck) were less stable (lower resistance to erosion) than areas with

pibenthic moss or algal mats. Areas without epibenthic mats relyn physical mechanisms to stabilize the surface, mostly throughhysical properties such as water content and particle size distribu-ion (Lundkvist et al., 2007). Similarly, mosses act as high strengthuffers that protect the soil surface from the direct shear stress ofowing water. Moss and algal mats within the swales were moreesistant to erosive forces as evidenced by higher average ero-ion thresholds. In addition to physio-chemical factors, sedimenttabilization may result from establishment of bacterial and/oricroalgal biofilm that can increase the erosion threshold, decrease

rosion rates, and promote deposition (Lundkvist et al., 2007).xtracellular polymeric substances (EPS) are the principal compo-ent of biofilms that form on or within sediment and its stabilizing

nfluence on sediment has been widely reported (Grabowski et al.,011).

When dry, the algal mats typically had plated edges that curledp; when moistened through precipitation and stormwater, thelated edges uncurled to form a mat on the soil surface. While theseaised edges may be more susceptible to erosion from shear flows,nce wetted (through precipitation or the slow flowing runoff),he structural transformation of the algal mats reduced their sus-eptibility to erosion. Similar phenomenon have been observed inntertidal diatom mats (Tolhurst et al., 2009).

The frequency of occurrence of the five substrates and the cor-esponding �c values led to swale-scale differences in soil stability.he high recession rate and open canopy of Swale II facilitated theevelopment of high-strength moss and algal mats and deterredhe development of low-strength substrates without epibenthic

ats. Light plays a major role as a forcing factor in biostabilizationf cohesive sediments (Lundkvist et al., 2007). The low recessionate of Swale III, on the other hand, facilitated the development andncreased the frequency of occurrence of low-strength substrates

ithout epibenthic mats. Similarly, Swale III had nearly continuousnundation with high cattail litter biomass that formed a dominantrganic substrate. These factors created surface conditions withinhe swales that were susceptible to erosion at varying applied sheartresses and, combined, resulted in different erosion thresholds forach of the three swales.

The erosion thresholds for both algal and moss mats decreasedrom 2010 to 2011, while the threshold increased for bare soil. Asiving substrates, moss and algae respond to local environmentalonditions. A decrease in rainfall or an increase in temperatureould reduce the development of moss and algae cover. While theverage monthly temperatures for Apr–Oct were similar in 201017.1 ◦C) and 2011 (16.0 ◦C), the total precipitation for the sameime period was higher in 2010 (99.4 cm) than in 2011 (56.7 cm).he drier conditions in 2011 may have affected the development

f the epibenthic mats. As the swales developed over time, smallicrobial populations, moss rhizoids (root-like structures), andacrophyte roots may have enhanced soil cohesion (Gyssels et al.,

005; De Baets et al., 2007), thereby decreasing �c of bare soil.

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l Engineering 64 (2014) 360–366 365

The presence of vegetation and structure of macrophyte rootstabilize the soil surface and increase the surface erosion thresh-ld. The stabilizing effect of plant roots on stream banks has beendentified (Wynn and Mostaghimi, 2006); however, research thatxplicitly measures the effects of aquatic plant roots on cohe-ive sediment erodibility is not available (Grabowski et al., 2011).yssels and Poesen (2003) found through cross-sectional analy-is that an increase in shoot or root density reduced erosion fromoncentrated flow. The positive relationship between critical sheartress and the ratio of BGB:shoot biomass, as well as the nega-ive relationships between critical shear stress and AGB and shootiomass could be due to (i) greater soil cohesion resulting fromore root development or (ii) lower frequency of moss and algalats with denser vegetation. The relative provisioning of biomass

o belowground resources may be important not only because ofhe stabilizing effect of root systems but also because the lessense canopy (less aboveground biomass) promotes developmentf epibenthos.

In other terrestrial erosion studies, both aboveground andelowground vascular plant productivity have been shown to

ncrease erosion thresholds. For example, increased AGB and theesultant enhanced canopy cover intercept raindrops that detachoil particles (Gyssels et al., 2005). Among other functions, BGBggregates soil, provides cohesion, and enhances microbial growthnd binding chemicals through a network of plant roots and rootairs (Gyssels et al., 2005; De Baets et al., 2007). A synthesis of2 terrestrial plant studies from 1954 to 2004 by Quijas et al.2010) concluded that an increase of BGB consistently increasesrosion control. Rickson and Morgan (1988) attribute increasedGB to decreased rates of rill erosion. While we did not investi-ate the effect of the vegetation to reduce stormwater flows, ouresults indicate decreased soil stability with increased AGB and noelationship between soil stability and BGB. Additionally, soil sta-ility increased as moss cover increased and as the %AGB as Typhaecreased. Combined, these results suggest that the decrease inhade, which promoted formation of algal and moss mats, con-ributed more to surface soil stabilization than increased roots.

Epibenthic mats played a very important role in stabilizing theurface of the wetland swales. Studied for the first time in con-tructed wetlands, these mats were functional in small patches andt the larger swale-scale. The patterns observed support the needo better understand and account for the role of biota in the erosionrocess. While several studies have concluded that sediment pro-esses are highly sensitive to biota, very few sediment transportodels account for these effects (Le Hir et al., 2007). Here, ponding

upported the establishment of dense cattails that trapped light,hile drainage promoted more open canopies that allowed lightenetration (Doherty and Zedler, 2013); the former prevented soiltabilization and the latter promoted growth of live mats of mossnd algae that biologically stabilized the soil.

Implications for design, assessment and future research: Design-ng a constructed wetland to drain between pulses of stormwaternflow could promote the establishment of epibenthic mats, help tommobilize sediment (and associated phosphorus), and reduce theransfer of suspended materials to downstream sources. Similarly,esigning constructed wetlands to limit periods of standing waterould allow vulnerable soil substrates to become stabilized andnhance stormwater treatment. However, the long-term ability ofoss and algae to persist while vascular vegetation continues to

evelop is unknown. Additionally, the stabilizing function of mossnd algae may change in systems designed to receive heavy pulses

f stormwater. Thus, we recommend further tests of the role ofpibenthic mats in contributing to water quality improvement.

Post-construction assessment of the performance of stormwa-er facilities often relies on vegetation cover and density, but such

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ndicators would have incorrectly characterized sediment stabi-ization in our swales, and the important roles of moss and algae

ould have been underestimated or entirely overlooked. Hence, weecommend direct assessments that include the characterization ofpibenthic mats and soil substrates.

. Conclusions

The fast-draining water-recession rates of Swales I and IImproved soil stability compared to the ponded Swale III. Dif-erential soil subsurface composition allowed for (1) fluctuatingeriods of wet and dry conditions that promoted the establish-ent of epibenthic and highly-erosion resistant moss and algalats; and (2) nearly-continuous ponding that contributed to the

ccumulation of highly-erodible substrates of organic matter anduck.

cknowledgements

We thank the US EPA Great Lakes Restoration Initiative (Awardo. GL-00E00647) for funding and R. Patel for oversight. We thank. Keuler (University of Wisconsin—Madison College of Agricul-

ural and Life Sciences Statistical Consulting Group) for assistanceith the statistical analysis, Z. Zopp for assistance with the critical

hear stress measurements, and J. Zedler for thoughtful feedbackn various drafts of this manuscript. We thank the two anony-ous reviewers for their helpful comments that strengthened thisanuscript.

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