# Invasion of a Widespread, Non-Native Grass Causes Downstream Reductions in Bioavailable Silica

Laura Triplett, Michal Tal, Zachary Wagner, Karin Kettenring

► To cite this version:

Laura Triplett, Michal Tal, Zachary Wagner, Karin Kettenring. Invasion of a Widespread, Non-Native Grass Causes Downstream Reductions in Bioavailable Silica. Journal of the American Water Resources Association, 2020, 56 (5), pp.810-819. 10.1111/1752-1688.12868 . hal-03153035

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# Invasion of a Widespread, Non-Native Grass Causes Downstream Reductions in Bioavailable Silica

Laura D. Triplett, Michal Tal, Zachary Wagner, Karin M. Kettenring

Geology Department (Triplett, Wagner), Gustavus Adolphus College, Saint Peter, Minnesota, USA; CEREGE-Europole l'Arbois (Tal), Aix-Marseille Université, Aix-en-Provence, FRA; Department of Watershed Sciences and Ecology Center (Kettenring), Utah State University, Logan, Utah, USA (Correspondence to Triplett: triplett@gustavus.edu).

**Research Impact Statement:** This work demonstrates that managing vegetation in riparian zones of rivers can change the amount of bioavailable silica transported to downstream estuaries and coastal oceans.

**ABSTRACT:** This study shows that riverine riparian (shoreline) zones—and plant management decisions made therein—can measurably influence the availability of silica delivered to lower reaches of rivers, estuaries, and some coastal oceans. The aims were to determine how much silicon is sequestered by riparian vegetation in a river, and what processes govern that effect. We used the invasion and rapid spread of the non-native grass *Phragmites australis* (common reed) in the Platte River (central United States) to quantify the biochemical and physical mechanisms by which silica is removed from the river flow and stored in sediments. Specifically, we measured bioavailable silica in sediment, native and non-native vegetation biomass, and river water over a period of 4

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1752-1688.12868
years. The robust biomass production and high silica content of *Phragmites* led to biochemical
production of amorphous silica particles (ASi) that was 1.5x higher than native willow and 2x
higher than native *Phragmites*. And, the denser growth habit of non-native *Phragmites* contributed
to more rapid physical sequestration of ASi than with other vegetation. Altogether, the rapid
vegetation change due to this invasion caused sequestered silica to increase 1.5x in this reach, which
is ~7% of the Platte's annual silica load. Subsequent *Phragmites* eradication efforts, wherein the
plants were sprayed with herbicide and left physically intact, allowed rapid silica remobilization but
it was highly variable.

(KEYWORDS: Platte River; *Phragmites*; sediment; riparian ecology; nutrients; planning; silica.)

38 INTRODUCTION

The global biogeochemical silica cycle is recognized as an important control on coastal
marine ecosystems, where silicon is an essential nutrient for major groups of phytoplankton like
diatoms and radiolaria (e.g., DeMaster 1981, Durr et al. 2011, Ragueneau et al. 2009, Tréguer et al.
1995). Dissolved silica (DSi) and amorphous particulate silica (ASi) together comprise the most
bioavailable forms of silica, which is a critical micro-nutrient for diatoms, radiolarian, and other
organisms. Declines in riverine input of bioavailable forms of silica (hereafter Si), in combination
with increases in riverine discharge of phosphorus and nitrogen, have been shown to limit diatom
growth and lead to cascading effects on estuarine and coastal marine food webs (Garnier et al. 2010,
Lane et al. 2004, Officer and Ryther 1980, Smayda 1990). Therefore, understanding the processes
that reduce silica transport through rivers is vital if we are to protect or restore aquatic ecosystems.

A substantial portion of a river's silica load may be sequestered in floodplains and riparian
zones, but those values have rarely been quantified. Such gaps in knowledge lead to our inability to
model silica dynamics on regional or continental scales, as demonstrated by Lauerwald et al.
(2013). One reason that floodplain and riparian processes are poorly understood is that, although
we know many plants create siliceous phytoliths in their biomass (e.g., Cary et al. 2005, Struyf et al.
2012, Conley 2003), we know little about how and whether those phytoliths accumulate in the oft-
saturated soils of riparian zones. Another challenge in quantifying riverine silica sequestration is
that riverine sediments are notoriously difficult to date because riverine sedimentation is spatially
and temporally discontinuous. Without a reliable chronology, establishing the fine-scale residence
time of silica in floodplains is challenging.
A rare opportunity to study the interplay between silica, sediment, and vegetation emerged in the Platte River of the central United States when a Eurasian lineage of *Phragmites australis* (Cav.) Trin. ex Steud (common reed) invaded (Larson, Galatowitsch, and Larson 2011). The Platte River was historically a sand-bed braided river, but twentieth century reductions in flow for crop irrigation led to increased vegetative stabilization of the floodplain (Randle and Samad 2003, Tal et al. 2004, Horn, Joeckel, and Fielding 2012). In 2002, when the invasive lineage of *Phragmites* began to rapidly spread in this area, this non-native wetland grass had ample opportunity to occupy riparian zones on islands, bank-attached islands, and shorelines. Within 9 years, *Phragmites* had colonized over 100 km² of channel and near-channel areas (R. Walters, personal communication, 2010). That rapid vegetation change over a discrete and documented time period allowed us to measure how much silica was sequestered by those physical and biological drivers.

The physical restructuring of the river was dramatic. *Phragmites* imposes strength or ‘cohesion’ to non-cohesive bed and bank material through root and rhizome (underground stems) reinforcement, and protection of underlying substrate by stem and leaf litter deposits (Pollen-Bankhead, Thomas, and Simon 2014, Rooth, Stevenson, and Cornwell 2003). Sandbars increased in height and became more permanent. Channels narrowed and islands aggraded higher above the riverbed due to the cohesive effect of *Phragmites* roots and rhizomes (Pollen-Bankhead, Thomas, and Simon 2012, Johnson 1994, 1997). At the channel scale, those processes suppressed braiding by slowing down bank erosion, choking off weak channels, and assisting in bank accretion. At local scales, *Phragmites* interfered with water flow and decreased the velocity of (or drag exerted by) the water (see Pollen-Bankhead, Thomas, and Simon (2014) for a detailed discussion).

The biochemical restructuring of the river is less well understood. Historically, a native subspecies of *Phragmites australis* (subspecies *americanus*) was widespread along the Platte according to herbarium records (Larson et al. 2011). Today, it is effectively absent in the study reach, in part due to the widespread invasion of the non-native subspecies. Non-native *Phragmites* grow in more dense stands than native assemblages of riparian vegetation including native *Phragmites*, resulting in a higher biomass per area (Pollen-Bankhead, Thomas, and Simon 2012). *Phragmites* must obtain its DSi and other requisite nutrients from river water (including sediment porewater), and Struyf et al. (2007) showed that *Phragmites* has a relatively high rate of silica uptake. Furthermore, *Phragmites* produces abundant leaf litter that decomposes slowly, in part because of its high silica content (Windham and Meyerson, 2003, Triplett et al. 2014). Therefore, we hypothesized that the rapid increase in *Phragmites* biomass in the Platte River may have led to at least a temporary reduction in downstream nutrient transport. Furthermore, the new sediment This article is protected by copyright. All rights reserved
92 sequestered by *Phragmites* contained more ASi than unvegetated or native willow (*Salix* spp.)

93 sediments (Triplett et al. 2014), although the mechanisms by which the sediment was enriched have
94 not been fully explored until now.

95 A final twist in the story is that aggressive eradication efforts substantially reduced the
96 abundance of living *Phragmites* by the year 2012. *Phragmites* was causing a significant decline in
97 bird habitat in the Platte River region. Specifically, three threatened or endangered birds—least tern
98 (*Sternula antillarum*), piping plover (*Charadrius melodus*), and whooping crane (*Grus
99 americana*)—require more open nesting and roosting space. Furthermore, Kessler et al. (2011)
100 predicted that “the contraction of common reed [*Phragmites*] could increase sandhill crane [*Grus
101 canadensis*] habitat availability by 50%”. Also, *Phragmites* was constricting the channels and
102 reducing the floodplain’s ability to accommodate floodwaters. Beginning in 2008, The Nature
103 Conservancy and the Platte River Recovery and Implementation Project coordinated a multi-year
104 campaign of spraying imazapyr (an aquatic-approved herbicide) and using heavy machinery to kill
105 and uproot the plants. While the intention was to eliminate the non-native subspecies, if there was
106 any native *Phragmites* still present it was too difficult to distinguish from non-native, so all was
107 targeted. At sites where *Phragmites* had been killed, we were able to measure silica remobilization
108 into the downstream flow.

109 In this study, we determined the mass of ASi in the biomass and sediments of four site
110 types—live *Phragmites*, killed *Phragmites*, native willow, and unvegetated (Fig. 1)—and quantified
111 those silica pools over the course of four years. We addressed three questions: (1) Can a single
112 plant species, in this case the non-native, invasive grass *Phragmites australis*, substantially change
113 the amount of silica sequestration in a river’s floodplain? We expected the answer to be yes, given
114 previous studies that documented the high silica content of *Phragmites* (Struyf et al. 2007, Struyf et
115 al. 2005). (2) Do physical or biochemical processes drive the sequestration? We speculated that
116 both processes would be important, but no previous studies have addressed this question. (3) What
117 happens to stored silica when vegetation is rapidly killed over a large area? We expected to see
118 some remobilization, based on a similar study of a hardwood forest (Conley et al. 2008), but again
119 we had no directly analogous studies to draw from.

120 The research findings can inform understanding of the short- and long-term impacts of plant
121 invasions (specifically *Phragmites*) on riverine/floodplain wetlands while also improving modeling
122 of geochemical cycles on a local and regional scale. In addition, we anticipate these findings to be
123 of interest to managers of downstream receiving waters (reservoirs, estuaries, and coastal waters)
124 where diminished silica concentrations can stress ecosystems and ultimately fisheries.

This article is protected by copyright. All rights reserved
Sixty-nine sampling locations were distributed across ~200 kilometers of the Platte River over a variety of geomorphic settings in the channel including mid-channel bars, side-channel bars and streambanks (Table 1; Fig. 2). No major tributaries enter the Platte in this reach between Brady and Grand Island, Nebraska (NE), but the Platte is an anastomosing river so the “active channel” could be comprised of several small channels at any given time. Minimum criteria for an acceptable sampling location were that: (1) the location was within the active channel and would be in contact with the flow on an approximately annual basis and (2) the vegetation stand (or unvegetated condition) be at least 3 meters in diameter to collect a single sediment core for silica analysis, or at least 15m² when triplicate cores were collected to test for intra-site variability. Locations were identified and chosen through in-person visits to public land and privately-held land where we had acquired landowner permission.

Eighteen sites were sampled in two or three years—2010, 2012, and/or 2013—in order to measure change in sediment silica content over time (Table 1). Fifty-one other sites were sampled in a single year to provide a more robust characterization of the silica content of each site type. Due to a large flood in 2011 and aggressive *Phragmites* management efforts in 2011 and 2012, many sites were dramatically altered or eliminated entirely from one year to the next.

At each location, distance from flowing water and height above current river elevation were measured using line levels and basic surveying techniques.

We used the observed history of vegetation in the study area to constrain silica sequestration rates to a specific time period, rather than attempting to establish sedimentary chronologies from this diverse and episodically inundated channel. Assumptions we made to estimate chronologies:

(1) Non-native *Phragmites* first became a problem in this region in 2001, so it had only been occupying our sampling sites for 11 years or fewer.

(2) Willow sites were certainly occupied by willow for a longer time period, because it is native to the area and has been widespread since the 1950s (Johnson 1994). We confirmed the historical presence of willow with satellite photos from the 1970s-1990s, but could not determine when each site was first occupied by willow. So, while we could not establish the precise length of time willow had occupied the sampling sites, the
study design is conservative in that willow occupied its sites for as long or longer than
did *Phragmites*.

(3) Unvegetated sites only exist where managers have removed encroaching vegetation, so we know the length of time that they have been treated.

In addition, weekly water samples were collected by the Nebraska Department of
Environmental Quality at Grand Island from January through September, 2012. Samples were
immediately refrigerated and shipped overnight to the Gustavus Adolphus College laboratory,
where they were filtered to isolate total suspended solids (TSS) containing particulate silica from
the filtrate.

Vegetation and Sediment Sample Collection

Vegetation stand surveys were conducted using 1-m² quadrats in each sampled vegetation stand. Three quadrats were placed within stands to represent the variation within a stand (i.e., the stand was visually divided into thirds and a single quadrat was sampled in the middle of each of the thirds) while also avoiding edge effects (quadrats were placed at least 1m from patch edge). Stems of live or dead *Phragmites* or willow were counted, diameters were measured 10cm above the ground, and 10 randomly selected stems per quadrat were selected for collection for biomass measurements. After *in situ* stem diameter and height measurements, plant samples were clipped at the base and stored in paper bags for later biomass assessments. In the lab, plants were oven-dried at 105°C then net leaf and stem weights were measured and silica analysis conducted (see below).
For each quadrat, a “net stem perimeter” was calculated by adding together all stem perimeters (as determined from the measured diameters, assuming stems are circular). The net stem perimeter is a metric to describe how much stem surface is in contact with water flow, inducing drag and thus reducing flow velocity.

In the Loup River, a tributary of the Platte River near our study site, native and non-native *Phragmites* grow in close proximity to each other. In 2012, samples were collected from 37 stands of *Phragmites*—of then-unknown nativity—to ultimately determine whether non-native *Phragmites* had higher silica concentrations. In the Platte River, most native *Phragmites* known to exist historically (Larson, Galatowitsch, and Larson 2011) had been eliminated through non-native *Phragmites* eradication efforts or due to competition with non-native *Phragmites*. Native and non-native lineages were distinguished using restriction fragment length polymorphism (RFLP) genetic analyses following the standard methods of Saltonstall (2003). We do not directly compare Loup to Platte River plants because those rivers have different silica concentrations and water chemistries that may be reflected in the plant composition. But we report the relative difference of the silica This article is protected by copyright. All rights reserved
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contents between native and non-native *Phragmites* since the relative differences should hold in other systems.

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Subaerial sediment cores of 20–30cm in length were collected at each site. Cores of 8 cm width were collected using a parallel blade and shovel system, in a manner that ensured minimal sediment disruption. Cores were immediately sectioned at 10-cm intervals (all cores in 2010, and select cores in 2012 and 2013) or at finer resolution according to visual identification of horizons. Standard loss-on-ignition techniques (Dean 1974) were used to determine dry density and weight-percent of organic matter and carbonate in the sediments.

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### Silica Analysis

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The silica in all sediment, vegetation, and TSS samples was extracted following Triplett et al. (2008) to ensure complete dissolution of resistant phytoliths: dried sediments were digested in 0.2M NaOH at 85°C. For sediment and TSS samples, aliquots were removed at 15, 30, 45, 60, and 90 minutes. Sediment samples collected in 2010 were analyzed for Si on a Cary-50 UV-vis spectrophotometer as molybdate reactive silica, as reported in Triplett et al. (2014). Water, sediment, vegetation, and TSS samples collected in 2012 and 2013 were analyzed for Si using an inductively coupled plasma mass spectrometer (ICP-MS). Extracted solutions were pipetted into an ammonia buffer solution to maintain pH at 10, and the ICP-MS was operated with hydrogen as an interference gas. The UV-vis and ICP-MS methods were validated against each other using split samples and check standards compared as major axis regressions.

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Particle size was measured with a Microtrac 3500 SIA laser diffractometer. Wet sediment samples were suspended in a dilute solution of hexametaphosphate, and sonicated prior to analysis. An in-line imager was used to confirm that particles were completely disaggregated.

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### Statistical Analysis

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We used a one-way Analysis of Variance (ANOVA) in JMP (©130.0, SAS Institute Inc., 2016) with an α = 0.05 to compare variation in average Si sediment content and average particle size (separate analyses for each response variable) among the four site types (the fixed effect). We used the distribution of residuals, which was sufficient for assessing normality. Data transformations were not necessary to meet model assumptions of normality or homogeneity of variance. We then used Tukey's HSD for multiple comparisons of means at α = 0.10 (Tukey's HSD is overly conservative for its control of a Type I error, thus a higher α controls for this unnecessary loss of power). Pair-wise comparisons were made using independent t-tests to compare silica content in native and non-native *Phragmites* (1) stems and (2) leaves from the Loup River as well as between non-native *Phragmites* and willow for leaves and stems in the Platte River. This article is protected by copyright. All rights reserved
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Riverine fluxes were calculated using free FLUX32 Load Estimation Software (version 3.37, U.S. Army Corps of Engineers, 2016). We used Method 8 in the software to combine water discharge and discrete silica concentration to calculate the annual silica load at the water sampling location at Grand Island, NE (the downstream end of our study reach). Water discharge data came from the National Water Information System (U.S. Geological Survey. Accessed November 1, 2013, http://waterdata.usgs.gov/nwis.)

## RESULTS

### Vegetation Characteristics and ASi

The ASi concentration in non-native *Phragmites* stems and leaves ranged from 9.3 to 25.4 mg g⁻¹ and 27.2 to 53.5 mg g⁻¹, respectively. The mean stem and leaf concentrations were on average 2× higher than native willow stems and leaves (Table 2), to a significant level according to an independent t-test (both had p<0.001). Willow stands had more dry biomass because willow stems are woody and have more biomass, but they are also relatively low in silica. In the Loup River, native and non-native *Phragmites* (as determined by genetic testing) were both dispersed throughout the study reach. Non-native *Phragmites* leaves and stems (n=6) had over twice as much ASi as native *Phragmites* (n=31; Fig. 3) and were statistically different (p<0.005 for stems, p<0.001 for leaves).

Beyond ASi content, the vegetation stands had other key differences. Live non-native *Phragmites* grew in tightly-packed, dense stands and had far more stems per square meter than did stands of dead (sprayed) *Phragmites* and stands of native willow (Table 3). Willow stems are on average larger but sparser. Thus, live *Phragmites* had higher net stem perimeter intercepting river flow with the potential to create drag and reduce flow velocity (Table 3).

### Sediment Characteristics and ASi

Sediments accumulating under non-native *Phragmites* in the Platte River had on average 2.5× more ASi by volume than unvegetated sediments, a statistically significant difference (df = 3, F = 3.4, p = 0.02; Fig. 4). Live *Phragmites* sediment had slightly more ASi than dead *Phragmites*, although the difference was not statistically significant at α = 0.05. Also, *Phragmites* sediments on average had finer particle sizes and more organic matter than the other site types, although the differences were only significant for live *Phragmites* vs. unvegetated sediments (df = 3, F = 2.9, p=0.04; Fig. 4).
Six locations were sampled as live *Phragmites* in 2010 and as dead *Phragmites* in 2013, after spraying. Four of the sites experienced a loss in sediment ASi after the *Phragmites* was killed (Fig. 5). And, one of the sites that was killed in 2012 shows the most ASi loss over the study period.

## Riverine Silica Fluxes

During 2013, the year for which we have the most comprehensive data, the DSi concentration in the Platte River ranged from 14.6–30.4 mg/L and was relatively invariable and independent of flow stage (Fig. 6). Conversely, ASi concentration was more variable ranging from <1.0 to 12.0 mg/L, and generally was highest in the spring. The annual flow-weighted means (DSi: 20.6 mg/L, ASi: 6.9 mg/L) show that about one-fourth of the river’s annual bioavailable silica load is as ASi. For 2013 the annual loads of DSi and ASi were 19,400 t/year and 9,800 t/year ASi, respectively, for a total of 29,200 t/year total bioavailable Si.

## DISCUSSION

Non-native *Phragmites australis* in the Platte River caused the sequestration of more silica in just a few years than did native vegetation over what was likely decades of establishment and growth, substantiating our first hypothesis. The concentrations of ASi in *Phragmites* biomass and sediments are higher than in unvegetated or native willow vegetated areas. And, non-native *Phragmites* has about 2× the ASi than native *Phragmites* (according to the comparative Loup River dataset), so we can infer that it also sequestered far more ASi than did any native *Phragmites* that may have previously occupied this reach.

The mechanism for Si sequestration in this fluvial system was a combination of biochemical and physical processes, as hypothesized. *Phragmites* produced abundant ASi particles, as demonstrated by its high ASi concentration relative to native willow (42±12 mg g⁻¹ vs 11±3 mg g⁻¹ in leaves, respectively). Those phytoliths were shed with the annual senescence of biomass and became hypothetically available to stream transport. However, the dense growth habit of *Phragmites* (average net perimeter in contact with flow was 1920 cm m⁻² vs. 720 cm m⁻² for willow) made it an effective inhibitor of stream power, allowing both autochthonous and allochthonous ASi particles to be deposited and retained at the site. Thus, we demonstrated how *Phragmites* is a powerful force for sequestering silica, similar to its strong effect on other nutrient cycles like nitrogen (e.g., Windham and Meyerson 2003, Windham and Ehrenfeld 2003).
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A first order calculation of the magnitude of *Phragmites* silica sequestration can be made using *Phragmites* eradication efforts. By 2013, The Nature Conservancy had sprayed 109 km² of *Phragmites*, which at the average measured biomass and sediment concentrations contained approximately 56,000 (± 26,000) t of sequestered bioavailable silica. Compared to a hypothetical 109 km² of unvegetated river sediments, which would have contained an estimated 10,000 (±10,000) t of sequestered silica based on our measurements, *Phragmites* had increased silica sequestration by about 5×, or 46,000 t. The difference between silica sequestered by *Phragmites* and native willow (43,000 ± 31,000) is large, but the errors overlap because of multiplying uncertainties.

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Was the silica sequestration caused by *Phragmites* significant in terms of the Platte River's silica load? In 2013, the river's load was calculated as 19,400 t of DSi and 9,800 t of ASi, or 29,200 t of total bioavailable Si. (Again, ASi is considered to be bioavailable to downstream aquatic ecosystems because it is readily dissolved.) These numbers are reasonable given that our DSi measurements, ranging from 15-30 mg L⁻¹ through the year, closely match DSi data from the U.S. Geological Survey in 1993-95 (Survey 2013), which ranged from 10-36 mg L⁻¹. And, our DSi and ASi values are within the range of river data reported elsewhere (e.g., Beusen et al. 2009, Durr et al. 2011). So, if *Phragmites* sequestered 46,000 (±16,000) t of silica during its 10 years of occupation, then about 16% of the river's load was sequestered in that time.

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Similar to results from a previous small-scale, short-term study (Triplett 2014), these data show that the reduction in silica flux down-river was substantial. But, it was still not large enough to lead to silica limitation for phytoplankton in the Platte, because ambient concentrations are so high. Also, this series of events on the Platte may not have led to a noticeable reduction in the Mississippi River's silica delivery to the Gulf of Mexico because the Platte is a relatively small tributary of that larger river. Nevertheless, this study demonstrates the importance of vegetative control on silica cycling in riparian zones of rivers, as has previously been demonstrated for estuaries and wetlands (Carbonnel et al. 2009, Jacobs et al. 2013, Weiss, Amann, and Hartmann 2013). Quantifying silica pools through the discrete vegetative phases of the Platte helps constrain the magnitude of sequestration that may result from vegetation change in other places.

*Reversing Sequestration*

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By 2012, the third year of this study, the campaign to rid the Platte of *Phragmites* had made tremendous progress: 3,018 hectares had been sprayed with herbicide. We had hypothesized that when *Phragmites* was sprayed (but not bulldozed), silica stored in plant biomass and sediments would gradually be released back into the river flow. This effect was observed by Conley et al. This article is protected by copyright. All rights reserved
(2008) in a terrestrial environment; when trees were killed and removed, DSi flux to the watershed's stream increased. The geochemical mechanism invoked was that ASi in the soil and biomass dissolves. Without a living plant there to re-uptake the DSi, it leaves the site through diffusion and dispersion. That geochemical mechanism certainly occurs in the Platte, in addition to a physical mechanism that is likely absent in terrestrial systems: With the gradual decomposition of the extensive *Phragmites* root and rhizome system, sediment—and its associated ASi particles— would be fluvially transported away from the sites.

Building on results from our previous research (Triplett, 2014), here we show that release of silica occurs shortly after vegetation is killed. Ironically, the scale of the *Phragmites* eradication effort meant that only six of our *Phragmites* sites still had standing *Phragmites* through the four years of this study. But, the data from those sites do support the hypothesized remobilization of silica. The data also speak to the variability in site response. For example, variability in sediment ASi in live *Phragmites* sites increased from 2010 to 2012. Sites that were still alive in 2012 had increased their sequestered ASi, which is supported by the mechanism of more phytolith formation and accumulation occurring in that time. But, the magnitude of increase varied between sites.

One explanation for the variability is that in 2011 the Platte River experienced a large flood that substantially reorganized parts of the braided channel. Some of our long-term sites were entirely washed away (not pictured in Fig. 5), while others experienced deposition of new riverine sediment—likely including some allochthonous ASi. Another likely explanation is that the rates of standing biomass decomposition and ASi dissolution vary between sites. In future studies, a robust approach would be to monitor sediment ASi over more years to determine longer-term trends and reduce the impact of episodic variation.

The *Phragmites* invasion of the Platte demonstrates the rapidity with which physical and biochemical mechanisms can sequester—or release—elements from soils and sediments. Silica sequestration by non-native *Phragmites* is likely also occurring in other heavily-infested areas of North America like Chesapeake Bay, the Great Lakes region, the Gulf Coast, and the Great Salt Lake (Kettenring, de Blois, and Hauber 2012). These impacts may also occur from other species of wetland plants that can sequester substantial amounts of ASi when they have the characteristics of high ASi production and dense growth habit like non-native *Phragmites*. For marine ecosystems already under pressure from a variety of human-induced environmental changes, the effect of reduced Si delivery by rivers could be very damaging. Coastal fisheries and natural resource managers should determine whether silica is a limiting factor in their waterways, and if so, could
347 investigate whether changes in upstream river management might promote more silica delivery. As
348 shown in this study, riparian vegetation can reduce riverine fluxes and transport of silica to oceans.

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350

ACKNOWLEDGEMENTS

351 This study was funded by the U.S. National Science Foundation Division of Earth Sciences, award
352 #1148130 and the John S. Kendall Center for Engaged Learning at Gustavus Adolphus College.

353 Also, this research was supported by the Utah Agricultural Experiment Station, Utah State
354 University, and approved as journal paper number XXXX. We thank Rich Walters (The Nature

355 Conservancy), Jason Farnsworth (Platte River Recovery and Implementation Program), and the
356 Audubon Society's Rowe Sanctuary for site access and logistical support. Dr. Julie Bartley, Dr. Jeff

357 Jeremiason, and Bob Weisenfeld (Gustavus Adolphus College) generously provided ideas and
358 technical assistance. Students Carson Smith, Emily Seelen, Zach Van Orsdel, Emily Ford, Rachel

359 Mohr, Tara Selly, Todd Kremmin and Russell Krueger (Gustavus Adolphus College) gave
360 substantial assistance to this work. Zach Wagner was in residence at the Large Lakes Observatory,

361 University of Minnesota-Duluth, Duluth Minnesota, USA during final preparation of this
362 manuscript.

363

LITERATURE CITED

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460

461 **Table 1** Vegetation type at each Platte River site over three years. LP is live non-native *Phragmites*, DP is dead non-native *Phragmites*, W is native willow (*Salix*), and U is unvegetated.

462 Sites 1-18 were sampled in more than one year, sites 19-69 were sampled in only one year. In 2012, three cores were collected at each location to determine intra-site variability.

<table><thead><tr><th rowspan="2">Site #</th><th colspan="3">Year</th><th rowspan="2">Location</th></tr><tr><th>2010</th><th>2012</th><th>2013</th></tr></thead><tbody><tr><td>1</td><td>LP</td><td>DP</td><td>DP</td><td>40.69129 -98.891</td></tr><tr><td>2</td><td>LP</td><td>LP</td><td>DP</td><td>40.65842 -99.08579</td></tr><tr><td>3</td><td>LP</td><td>DP</td><td>DP</td><td>40.65727 -99.086</td></tr></tbody></table>
<table><caption>**Table 2** Amorphous silica in Platte River vegetation. Differences in silica concentrations (mean ± 1 standard error) between live non-native *Phragmites* and willow</caption><tr><td>4</td><td>LP</td><td>LP</td><td>DP</td><td>40.82561 -98.37489</td></tr><tr><td>5</td><td>LP</td><td>LP</td><td>DP</td><td>40.75057 -98.58345</td></tr><tr><td>6</td><td>LP</td><td>DP</td><td>DP</td><td>40.7502 -98.58012</td></tr><tr><td>7</td><td>DP</td><td>DP</td><td>DP</td><td>40.75018 -98.58014</td></tr><tr><td>8</td><td>LP</td><td>LP</td><td>DP</td><td>40.75058 -98.58332</td></tr><tr><td>9</td><td>LP</td><td>DP</td><td>DP</td><td>40.68369 -99.32483</td></tr><tr><td>10</td><td>W</td><td>W</td><td>W</td><td>40.71605 -98.74106</td></tr><tr><td>11</td><td>W</td><td>W</td><td>W</td><td>40.716 -98.74084</td></tr><tr><td>12</td><td>W</td><td>W</td><td>W</td><td>40.69139 -98.89097</td></tr><tr><td>13</td><td>W</td><td>W</td><td>W</td><td>40.6915 -98.89093</td></tr><tr><td>14</td><td>W</td><td>W</td><td>W</td><td>40.65835 -99.08589</td></tr><tr><td>15</td><td>W</td><td>W</td><td>W</td><td>40.82559 -98.37504</td></tr><tr><td>16</td><td>W</td><td>W</td><td>W</td><td>40.82562 -98.37486</td></tr><tr><td>17</td><td>U</td><td>U</td><td>U</td><td>40.7513 -98.57959</td></tr><tr><td>18</td><td>U</td><td>U</td><td>U</td><td>40.87805 -98.27902</td></tr><tr><td>19-26</td><td>LP</td><td>LP</td><td>DP</td><td>multiple</td></tr><tr><td>27</td><td>LP</td><td>LP</td><td>DP</td><td>40.87439 -98.27905</td></tr><tr><td>28-37</td><td>LP</td><td>LP</td><td>DP</td><td>multiple</td></tr><tr><td>38-49</td><td>LP</td><td>LP</td><td>DP</td><td>multiple</td></tr><tr><td>50</td><td>W</td><td>W</td><td>DP</td><td>40.65852 -99.0863</td></tr><tr><td>51</td><td>U</td><td>U</td><td>DP</td><td>40.6584 -99.0863</td></tr><tr><td>52-54</td><td>U</td><td>U</td><td>DP</td><td>multiple</td></tr><tr><td>55-69</td><td>U</td><td>U</td><td>DP</td><td>multiple</td></tr></table>

<table><thead><tr><th>Vegetation</th><th>ASi concentration<br>in leaves (mg g<sup>-1</sup>)</th><th>ASi concentration<br>in stems (mg g<sup>-1</sup>)</th><th>Total biomass<br>(g m<sup>-2</sup>)</th><th>ASi stored in standing<br>biomass (g m<sup>-2</sup>)</th></tr></thead><tbody><tr><td>Non-native<br><i>Phragmites</i></td><td>42 ± 12</td><td>18 ± 7</td><td>1570 ± 110</td><td>42 ± 14</td></tr></tbody></table>

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Willow 11 ± 3 5 ± 0.5 3000 ± 71 17 ± 9

470

471

472

<table><caption>**Table 3** Vegetation character. Vegetation densities and diameters (mean ± 1 standard error), and resulting net stem perimeters, from Platte River sites.</caption><thead><tr><th>Vegetation type</th><th>Number of stems per m<sup>-2</sup></th><th>Stem diameter (mm)</th><th>Net stem perimeter (cm m<sup>-2</sup>)</th></tr></thead><tbody><tr><td>Live<br><i>Phragmites</i></td><td>87 ± 30</td><td>7 ± 1</td><td>1920 ± 720</td></tr><tr><td>Dead<br><i>Phragmites</i></td><td>53 ± 2</td><td>7 ± 1</td><td>1100 ± 160</td></tr><tr><td>Live native<br>willow</td><td>16 ± 9</td><td>14 ± 3</td><td>720 ± 430</td></tr></tbody></table>

473

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<figure><img src="image_3.png" /><figcaption>**Fig. 1** Photos of different reaches of the Platte River: (a) non-native *Phragmites* (b) killed non-native *Phragmites* (c) native willow (*Salix*) on the left and non-native *Phragmites* on the right (d) unvegetated</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Fig. 2** Map of sample distribution along the Platte River in Nebraska, central United States. The river flows west to east toward its confluence with the Missouri River. Multiple samples were collected from each site marked on the map; green circles were live or dead *Phragmites*, yellow circles were live or dead willow, and brown squares were unvegetated. Native and non-native *Phragmites australis* were also sampled from the Loup River, a small tributary of the Platte River visible toward the northeast corner of this map. Inset map shows the location of the Platte River sub-watershed (shaded green) within the Missouri and Mississippi River watersheds. Base map courtesy of Google Earth
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Native and non-native *Phragmites* stem and leaf amorphous silica (ASi) concentrations</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4** Average Si concentrations and particle sizes in sediments at live *Phragmites*, dead *Phragmites*, willow, and unvegetated sites. Means denoted with different levels were statistically different based on Tukey's HSD means comparisons (α = 0.10; (Day and Quin, 1989)). Lowercase levels denote Si sediment differences and uppercase letters denote particle size differences by site type.</figcaption></figure>
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<figure><img src="image_7.png" /><figcaption>Fig. 8 A5i concentration in sediment at an site that initially contained are Laminated in 2010 and cored Plagenite (due to bermudic management) by 2015. Data for 2010 previously published (Tippitt et al. 2014)</figcaption></figure>

jawra_12868_f5.tif
<figure><img src="image_8.png" /><figcaption>**Fig. 6** Platte River discharge, and ASi and dissolved silica (DSi) concentrations in 2013. ASi peaked in the spring, consistent with a spring diatom bloom. DSi varied within a narrow range and shows no seasonal trend</figcaption></figure>
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