U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2016–1169

Environmental and Eelgrass Response to Dike 

Removal: Nisqually River Delta (2010–14)

By Renee K. Takesue
Cover. Photo of an environmental sensor measuring turbidity and temperature in an eelgrass bed near a restored tidal 

channel in the Nisqually River Delta, Puget Sound, Washington. Photo by Renee Takesue, U.S. Geological Survey.
Environmental and Eelgrass Response to Dike 

Removal: Nisqually River Delta (2010–14)

By Renee K. Takesue

Open-File Report 2016–1169

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Takesue, R.K., 2016, Environmental and eelgrass response to dike removal—Nisqually River Delta 

(2010–14): U.S. Geological Survey Open-File Report 2016–1169, 17 p., 

http://dx.doi.org/10.3133/ofr20161169. 

ISSN 2331–1258 (online)
iii

Acknowledgments

The Nisqually National Wildlife Refuge provided permission to work in Refuge lands; Daniel Hull, Director of the 

Nisqually Reach Nature Center, provided field and logistical support; and volunteers Anna Davenport, Michael 

Grilliot, Kyle Murphy, and Ginger Shoemaker provided field assistance. Sandy Wyllie-Echeverria made suggestions 

that improved this report.

The U.S. Geological Survey (USGS) Washington Water Science Center provided lab facilities, Jessie Lacy 

processed data logger records, Alex Cress performed lab calibrations of data loggers, Leticia Hallas provided lab 

assistance, and Isa Woo and Guy Gelfenbaum made suggestions that improved this report. This work was funded 

by the USGS Coastal and Marine Geology Program through the Coastal Habitats in Puget Sound Project.
iv

Contents

Abstract ......................................................................................................................................................... 1

Introduction.................................................................................................................................................... 1

Site Description.......................................................................................................................................... 2

Methods......................................................................................................................................................... 4

Environmental and Eelgrass Properties..................................................................................................... 4

Results........................................................................................................................................................... 4

Turbidity ..................................................................................................................................................... 4

Prerestoration Sediment Texture From Sediment Cores ........................................................................... 4

Postrestoration Changes in the Surface Layer........................................................................................... 5

Postrestoration Changes in the Rhizosphere............................................................................................. 6

Eelgrass Shoots......................................................................................................................................... 6

Eelgrass Rhizomes .................................................................................................................................... 8

Eelgrass Biomass .................................................................................................................................... 10

Summary ..................................................................................................................................................... 11

References Cited......................................................................................................................................... 13

Appendix 1. Field and Lab Methods ........................................................................................................ 14

Water Column Turbidity ........................................................................................................................... 14

Sediment Collection ................................................................................................................................. 14

Sediment Pore Water Analyses ............................................................................................................... 14

Sediment Grain-Size Determinations....................................................................................................... 15

Eelgrass Properties.................................................................................................................................. 15

Appendix 2. Example of Sensor Data....................................................................................................... 16

Appendix 3. Cross-Shore Z. marina Characteristics................................................................................. 17

Figures

1. Satellite images of the Nisqually River Delta, Washington. A, The delta in mid-2009 with the old 

dike. B, The delta in mid-2015 with the new dike. C, Locations of monitoring sites at Luhr N and 

Luhr S. D, Approximate locations of eelgrass transect by year......................................................... 3

2. Graph showing variation with depth of the sediment fine fraction in sediment cores collected in 

May 2010 at Luhr N and Luhr S........................................................................................................ 5

3. Graphs showing year-to-year changes in the sediment fine fraction in the active surface layer and 

rhizosphere and sediment pore water redox values, also known as Eh, in the rhizosphere ............. 6

4. Graphs showing year-to-year changes in mean shoot density, canopy height, and the approximate 

leaf area index at Luhr N and Luhr S................................................................................................ 7

5. Eelgrass rhizomes from Luhr Beach. A, Photograph of eelgrass rhizome with segmented growth; 

B, mean rhizome internode lengths of plants collected annually at Luhr N; and C, mean rhizome 

internode lengths of plants collected annually at Luhr S................................................................... 9

6. Graph showing year-to-year changes in the mean maximum rhizome internode lengths at Luhr N 

and Luhr S...................................................................................................................................... 10

7. Graphs showing year-to-year changes in mean dry biomass of shoots (AG), roots plus five 

rhizomes (BG5), and the AG:BG5 ratio at Luhr N and Luhr S ........................................................ 11
v

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

cubic meter (m3) 264.2 gallon (gal) 

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

liter (L) 61.02 cubic inch (in3) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

meter per second (m/s) 3.281 foot per second (ft/s) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as °C = (°F – 32) / 1.8.
vi

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Abbreviations

CI confidence interval

μM micromolar

mV millivolts

NNWR Nisqually National Wildlife Refuge

ppm parts per million

rpm revolutions per minute

USGS U.S. Geological Survey
Environmental and Eelgrass Response to Dike Removal: 

Nisqually River Delta (2010–14)

By Renee K. Takesue

Abstract

Restoration of tidal flows to formerly diked marshland can alter land-to-sea fluxes and patterns 

of accumulation of terrestrial sediment and organic matter, and these tidal flows can also affect existing 

nearshore habitats. Dikes were removed from 308 hectares (ha) of the Nisqually National Wildlife 

Refuge on the Nisqually River Delta in south Puget Sound, Washington, in fall 2009 to improve habitat 

for wildlife, such as juvenile salmon. Ecologically important intertidal and subtidal eelgrass (Zostera 

marina) beds grow on the north and west margins of the delta. The goal of this study was to understand 

long-term changes in eelgrass habitat and their relation to dike removal. Sediment and eelgrass 

properties were monitored annually in May from 2010 to 2014 at two sites on the west side of the 

Nisqually River Delta along McAllister Creek, a spring-fed creek near two restored tidal channels. In 

May 2014, the mean canopy height of eelgrass was the same as in previous years in an 8-ha bed 

extending to the Nisqually River Delta front, but mean canopy height was 20 percent lower in a 0.3-ha 

eelgrass bed closer to the restored marsh when compared to mean canopy height of eelgrass in May 

2010, 6 months after dike removal was completed. Over 5 years, the amount of eelgrass leaf area per 

square meter (m

2

) in the 8-ha bed increased slightly, and surface-sediment grain size became finer. In 

contrast, in the 0.3-ha bed, eelgrass leaf area per m2 decreased by 45 percent, and surface sediment 

coarsened. Other potential stressors, including sediment pore water reduction-oxidation potential 

(redox) and hydrogen sulfide (H2S) concentration in the eelgrass rhizosphere, or root zone, were below 

levels that negatively affect eelgrass growth and therefore did not appear to be environmental stressors 

on plants. Eelgrass biomass partitioning, though less favorable in the 8-ha eelgrass bed compared to the 

0.3-ha one, was well above the critical above-ground to below-ground biomass ratio of 2:1 for Z. 

marina, an indication that these plants were not at risk of a carbon deficit during low-light conditions. 

After 5 years, nearshore changes associated with the restoration of tidal flows to formerly diked marshes 

of the Nisqually River Delta appeared to have little impact on the large eelgrass bed extending from

Luhr Beach to the Nisqually River Delta front; however, restoration appears to be contributing to the 

decline of a small eelgrass bed closer to the restoration area. 

Introduction 

Nearshore habitat restoration in the U.S. Fish and Wildlife Service Nisqually National Wildlife 

Refuge (NNWR) in the Nisqually River Delta (also referred to as “the delta”) began in September 2009 

to return tidal flow to 308 hectares (ha) of the delta that was diked off in the early 1900s. By November 

2009, 7 kilometers (km) of dikes were removed around freshwater marshes in the NNWR (fig. 1A, B). It 

was hypothesized that sediment and organic matter from the former freshwater marsh would be 

mobilized and transported to the nearshore region by ebb tidal currents, and that some of this material 
2

would be deposited in eelgrass beds along McAllister Creek. Potential increases in suspended sediment 

concentration would decrease water clarity, and this, coupled with the accumulation of fine-grained 

terrestrial sediment and organic matter in nearby eelgrass (Zostera marina) beds was of concern because 

both these factors can negatively affect eelgrass growth. Turbidity decreases the amount of 

photosynthetically active radiation (PAR) needed by eelgrass for growth (Zimmerman and others, 

1991), and an increase in the accumulation of fine sediment and organic matter can lead to the build-up 

of sulfides in the eelgrass rhizosphere (root zone) to levels that are toxic to plants (Holmer and others, 

2006). Long-term monitoring of environmental and eelgrass characteristics began in May 2010 near 

restored tidal channels on the west side of the delta to quantify the response to dike removal, identify 

long-term trends, and infer whether changes were associated with restoration activities. 

Site Description

McAllister Creek is a spring-fed creek on the west side of the Nisqually River Delta into which 

restored tidal channels drain (fig. 1B). Monitoring was conducted at two sites on the west bank of 

McAllister Creek, 200 meters (m) north and 300 m south of the Luhr Beach boat ramp, designated Luhr 

N and Luhr S (fig. 1C). North (seaward) of the boat ramp, McAllister Creek crosses intertidal mud flats 

and flows to the delta front, and a continuous 8-ha eelgrass meadow extends along its west bank (fig. 

1A). South (landward) of the boat ramp, the channel of McAllister Creek is confined between a bluff to 

the west and marsh platforms to the east (fig. 1C). Eelgrass occurs in discrete beds and patches in the 

landward portion of McAllister Creek. The eelgrass bed monitored at Luhr S was 0.3 ha in area. 
3

Figure 1. Satellite images of the Nisqually River Delta, Washington (Google Earth). A, The delta in mid-2009 

with the old dike; Zm is the 8-ha Zostera marina bed. B, The delta in mid-2015 with the new dike; 1 and 2 are 

restored tidal channels that drain to McAllister Creek. C, Locations of monitoring sites at Luhr N and Luhr S; rm are 

remnant marsh platforms. D, Approximate locations of eelgrass transect by year.
4

Methods

Environmental and Eelgrass Properties

Environmental and eelgrass properties were measured annually from 2010 to 2014 in the month 

of May, when above-ground (shoot) biomass of intertidal Puget Sound eelgrass peaks (Phillips, 1972). 

Z. marina leaves persist on shoots from several weeks to more than a month, whereas rhizomes persist 

in the sediment for several years. Their growth patterns provide insights into environmental conditions 

over different time scales. Intertidal eelgrass beds were accessed during a 4- to 5-day window around 

the lowest low tides in May, which varied from early to late in the month. Eelgrass morphology and 

growth (shoot density, leaf length and width, number of leaves, above-ground (AG) and below-ground 

(BG) biomass, and rhizome internode lengths) were measured in 10 quadrats across a 100-meter-long 

shore-parallel transect at each site, when possible (see details in appendix 1). The initial alongshore 

transect at Luhr N (May 2010) was approximately midway between the intertidal and subtidal edges of 

the eelgrass bed (fig. 1D). In 2011, the substrate was too muddy to allow reoccupation of this site, and 

the transect was moved 20 m closer to the intertidal edge of the eelgrass bed, where the ground was 

firmer (fig. 1D). In May 2013, Z. marina characteristics were additionally determined in the cross-shore 

direction at Luhr N to quantify how plant characteristics varied with tidal depth across the tidal flat (fig. 

1D). Water column turbidity, surface and subsurface sediment grain-size distribution (sediment texture), 

subsurface reduction-oxidation potential (redox) of pore waters, and subsurface hydrogen sulfide (H2S) 

concentration of pore waters were measured in short sediment cores collected in the eelgrass beds at 

Luhr N and Luhr S. Detailed descriptions of the methods are given in appendix 1. 

Results 

Turbidity

When the turbidity sensors (SLOBS) were recovered after deployment, they were found to be 

fouled by drifting macroalgae (Ulva species) that interfered with backscatter measurements and

produced erroneously high readings at one or both sites during each deployment (see appendix 2). In 

2011–14, macroalgae was manually removed from the SLOBS during each morning or early afternoon 

low tide of the 4–5 day deployment to allow an accurate backscatter measurement on the following 

flood tide. It may be possible to estimate minimum turbidity levels over the Luhr N and Luhr S eelgrass 

beds; however, high, rather than low, turbidity levels would be more likely to impact eelgrass growth. 

Prerestoration Sediment Texture From Sediment Cores

Surface sediment was not collected at the monitoring sites prior to dike removal, so 

prerestoration sediment grain-size distributions were determined from records in sediment cores. 

Rooted, mature eelgrass plants (>1 year old, see section on eelgrass rhizomes) were present when cores 

were collected in May 2010. Therefore, sediment in and below the root zone, or rhizosphere, was in 

place for at least 1 year, meaning it was deposited before dike removal. Below the rhizosphere (>6 

centimeters [cm]), sediment texture varied between 10 to 16 percent fines at Luhr N and averaged 7

percent fines at Luhr S (fig. 2). In the rhizosphere (2–6 cm), sediment texture became finer toward the 

surface at Luhr S (fig. 2) and was variable at Luhr N. Prerestoration sediments in the rhizosphere and 

deeper at Luhr N and S had a coarser texture than that of the active surface layer (0 to 2 cm). The 

overall sediment texture was finer at Luhr N than at Luhr S (fig. 2). 
5

 

Figure 2. Graph showing variation with depth of the sediment fine fraction (percent fines) in sediment cores 

collected in May 2010 at Luhr N and Luhr S. Dashed lines show the approximate depths (in centimeters) of the 

active surface layer, rhizosphere, and prerestoration intervals.

Postrestoration Changes in the Surface Layer

Year-to-year changes of sediment properties of the active surface layer (0–2 cm) were compared 

to gain insight about temporal changes following dike removal. Surface sediment texture at both sites 

coarsened between 2010 and 2012, then became finer in 2013 (fig. 3, left column). From 2010 to 2013, 

year-to-year changes in surface sediment grain-size distributions varied in the same direction, either 

coarser or finer, at both sites. This pattern diverged in 2014 when surface sediment at Luhr N became 

considerably finer, while that at Luhr S changed little. Surface sediment at Luhr N had a higher 

proportion of fines than at Luhr S in all years except 2012, when the two were similar. Over the 5-year

period, the mean percentage of fine sediment in the active surface layer was higher at Luhr N and lower 

at Luhr S compared to May 2010 (fig. 3, left column). In all years, surface sediment was well 

oxygenated (redox potential, also known as Eh, was 0 to 200 millivolts [mV], not shown), light brown 

in color, and had no detectable H2S (not shown). 
6

Figure 3. Graphs showing year-to-year changes in the sediment fine fraction (percent fines) in the active surface 

layer (left column) and rhizosphere (middle column) and sediment pore water redox values, also known as Eh, in 

the rhizosphere (right column). Top row, Luhr N; bottom row, Luhr S. Values are averages over the specified 

sediment intervals. Red numbers in parentheses indicate negative values. Error bars show ±1 σ. [cm, centimeters; 

unk, value unknown; mV, millivolts.] 

Postrestoration Changes in the Rhizosphere 

Year-to-year changes of sediment properties of the rhizosphere (2–6 cm) were compared to gain 

insight about temporal changes following dike removal. Prior to 2014, sediment pore water in the 

rhizosphere of the Luhr N eelgrass meadow was oxygenated to slightly reducing (Eh > –30 mV; fig. 3, 

right column) and had no measurable H2S (not shown). The Luhr S rhizosphere was also oxygenated, or 

slightly reducing, except in 2011, when pore waters were moderately reducing (Eh = –150 mV; H2S = 

2.2 parts per million [ppm], or 63 micromolar [μM]), which was consistent with a large fraction of fine 

sediment in the substrate (26 percent fines). In 2014, a new Eh electrode was used, and it measured a 

five times larger range of Eh values in pore water (whose H2S content was three times lower in 2014 

than in 2010–13) compared to the previous electrode, suggesting it had a different sensitivity to Eh. 

Multipoint calibrations of the two electrodes, which would be needed to cross-calibrate the sensitivities 

of the electrodes, were not performed at the time of the study, therefore it is not known whether Eh 

values measured in 2014 were comparable to those from previous years. The 2014 Eh data will not be 

discussed further. 

Eelgrass Shoots

Between May 2010 and 2014, mean eelgrass shoot densities at Luhr N increased from 110 to 

146 shoots per m2

, but this change was not statistically different at the 95 percent confidence level (fig. 

4). The mean increase did not appear to be related to the lower tidal depth at which the shoots were
7

counted in 2010 because shoot densities were no different at this depth than at the depth of the 2011–14 

shoot counts (appendix 3). Canopy heights of eelgrass measured in May 2010 were not directly 

comparable to those in subsequent years because there was a tidal depth dependent variation of leaf 

length (appendix 3). Measurements in the cross-shore direction showed that eelgrass canopy heights 

were up to 24 percent higher at the tidal depth of the 2010 measurements than at the tidal depth of the 

2011–14 measurements. If the mean canopy height measured in 2010, 117 ±10 cm (95 percent

confidence interval [CI], n=10), were reduced by 24 percent to 89 cm to allow comparison with 2011–

14 data, then there did not appear to be any change in mean canopy heights at the 95 percent confidence 

level between 2010 and 2014 at Luhr N (fig. 4). Mean leaf widths did not change from 2010 to 2014 

(not shown), and as a result, there was a very small but not statistically significant increase in available 

eelgrass leaf area from 55 to 60 m2 of leaf per m2 of seabed at Luhr N (fig. 4). 

 At Luhr S, there was a 20 percent decrease in mean canopy height and a 35 percent decrease in 

mean shoot density between 2010 and 2014 (fig. 4). This resulted in a large decrease in eelgrass leaf 

area between 2010 and 2014. The large 95 percent CIs of mean shoot densities at Luhr S reflected the 

presence of bare or sparsely vegetated patches in the eelgrass bed. The narrowing of CIs over 5 years 

(fig. 4) reflects infilling of bare or sparsely vegetated patches, resulting in more uniform shoot densities 

across the bed. 

Waypoints taken by hand-held GPS (Garmin 78sc, 3-m accuracy) showed that the north edge of 

the Luhr S eelgrass bed extended 10 m and 30 m further alongshore in 2013 and 2014, respectively, 

compared to 2010. 

Figure 4. Graphs showing year-to-year changes in mean shoot density, canopy height (leaf length), and the 

approximate leaf area index (LAI) at Luhr N (top row) and Luhr S (bottom row). Error bars show the ±95 percent

confidence interval (CI); n=10 except for Luhr N in 2012 (n=11) and Luhr S in 2013 (n=9). [#, number; m2, square 

meter; cm, centimeter; act, actual value; est, estimated value.]
8

Eelgrass Rhizomes

Z. marina rhizomes are multisegmented and laterally branching (fig. 5A). The beginning and end 

of a rhizome segment is marked by a node that forms when a new leaf emerges (Phillips, 1984). The 

distance between two rhizome nodes is referred to as the internode length. Nodes and leaves of Z. 

marina form approximately every 14 days in temperate latitudes (Phillips, 1984; Lee and others, 2006), 

and the internode length can vary with the season (Phillips, 1972). In winter, when irradiance and the 

length of the photoperiod decreased, rhizome internode lengths of Nisqually eelgrass were short

(December/January minima, fig. 5B, C). In spring, when light was more plentiful, internode lengths 

increased and peaked in May of each year (fig. 5B, C). 

The longest rhizomes of plants collected in May 2010 had 31 (Luhr N) and 37 (Luhr S) 

internodes. By multiplying an average 14-day interval for internode formation by the numbers of 

internodes on Luhr N and Luhr S rhizomes, whole-rhizome ages of 14 to 17 months, respectively, were 

derived, indicating that eelgrass plants collected in May 2010 were growing in situ before and during 

dike removal. There was little difference in winter and spring internode lengths in the months following 

dike removal compared to subsequent years. The maximum elongation (longest internode length) of 

Luhr N rhizomes, which is one measure of eelgrass growth rate, varied slightly over 5 years but without 

a consistent trend (fig. 6). At Luhr S, maximum elongation was on average 14 percent lower in 2012–14 

compared to 2010–11; however, this amount was not statistically different at the 95 percent confidence 

level. In summary, there were no significant trends in eelgrass rhizome growth patterns during the 5 

years following tidal restoration in the Nisqually River Delta.
9

Figure 5. Eelgrass rhizomes from Luhr Beach. A, Photograph of eelgrass rhizome with segmented growth; B, 

mean rhizome internode lengths (in millimeters) of plants collected annually at Luhr N; and C, mean rhizome 

internode lengths (in millimeters) of plants collected annually at Luhr S. Colors vary with the year that rhizomes 

were collected. Numbers, shown for rhizomes collected in May 2010 (blue) and May 2011 (red), indicate the 

internode number starting with the most recently formed internode. For internodes 1–11, n=10 except for Luhr N in 

2012 (n=11) and Luhr S in 2013 (n=9). For internodes 12 and higher, 2≤n≤10. The shaded region shows the period 

when dike removal occurred.
10

 

Figure 6. Graph showing year-to-year changes in the mean maximum rhizome internode lengths (in millimeters

[mm]) at Luhr N and Luhr S. Error bars show ±95 percent confidence interval (CI); n=10 except for Luhr N in 2012 

(n=11), Luhr S in 2013 (n=9). 

Eelgrass Biomass

Mean biomass of eelgrass shoots (AG) and roots plus five rhizome internodes (BG5) at Luhr N

both initially decreased for 3 years following dike removal, and these decreases were proportionate, 

resulting in little change in the AG:BG5 ratio from 2010 to 2012 (fig. 7). Mean AG biomass at Luhr N 

did not change in 2013 and 2014, while BG5 biomass increased, resulting in lower mean AG:BG5 ratios 

in 2013 and 2014 (fig. 7). At Luhr S, mean AG biomass decreased for 4 years following dike removal, 

while overall mean BG5 biomass decreased over 5 years, with the exception of a one-time increase in 

2013 (fig. 7). Compared to May 2010, the mean AG:BG5 ratio of Luhr S eelgrass was 21 to 44 percent 

lower in the subsequent 4 years, but this difference fell within the 95 percent CI of the 2010 mean 

because of the large variability that year. Mean AG:BG5 ratios were much greater than the critical value 

of 2:1 for Z. marina (Hemminga, 1998) in all years at both sites. When AG:BG ratios are less than 2:1, 

the respiratory carbon demand of the plant can exceed the photosynthetic carbon production, resulting in 

a carbon deficit that can negatively impact plant growth (Hemminga, 1998). 
11

Figure 7. Graphs showing year-to-year changes in mean dry biomass (in grams [g]) of shoots (AG), roots plus

five rhizomes (BG5), and the AG:BG5 ratio at Luhr N (top row) and Luhr S (bottom row). Error bars show the ±95

percent confidence interval (CI); n=10 except for Luhr N in 2012 (n=11), Luhr S in 2013 (n=9). 

Summary 

The monitoring of environmental conditions and eelgrass growth and morphology has been 

conducted every May since dikes were removed in the Nisqually River Delta in late 2009 in order to 

quantify changes, identify temporal trends, and assess whether changes were associated with restoration 

activities. Eelgrass beds along McAllister Creek are in close proximity to two restored tidal creeks, 

whereas delta-front eelgrass beds are over 2 km away and more likely to be marine-influenced. 

Therefore, environmental and eelgrass changes at McAllister Creek are not necessarily representative of 

deltawide changes. 

Hood (2004) found changes in postdike tidal channel morphology in the Skagit River Delta (140 

km north of the NNWR) that were consistent with the theory that diking results in a tidal prism loss, 

reduced tidal flushing, and sedimentation in tidal channels seaward of the dike. If dike removal in the 

Nisqually River Delta resulted in a reversal of these processes, then the postrestoration system is 

hypothesized to have a larger tidal prism and increased tidal flushing than the prerestoration system. The 

coarser texture of prerestoration sediment in sedimentary records in McAllister Creek eelgrass beds 

compared to postrestoration sediment seems to suggest that tidal currents were stronger and more 

effective at winnowing away fine-grained sediment in the diked system, contrary to expectations. The 

age of prerestoration sediment intervals were not known, however, and could have been Holocene-age 

delta deposits (Barnhardt and Sherrod, 2006) rather than sediment deposited over historic time scales

when the delta was diked. One explanation for the finer texture of postrestoration sediment 

accumulating in McAllister Creek eelgrass beds is a shift in source of sediment to one that contained a 

larger fine fraction. This is consistent with the hypothesis that fine-grained sediment from the 

restoration area would be mobilized and deposited in McAllister Creek eelgrass beds following dike 
12

removal. Stable carbon isotope ratios of sedimentary organic matter are currently being analyzed to 

determine whether material accumulating in McAllister Creek eelgrass beds after restoration was more 

terrestrial in nature than before.

After the initial increase in 2010 in the amount of fine-grained sediment in McAllister Creek 

eelgrass beds, there was a subsequent decrease over the next 2 years that could have arisen from 

winnowing of the initial material or the accumulation of new material with a slightly coarser texture. It 

was not possible to distinguish between these mechanisms based on the parameters measured in this 

study. Despite a large increase in the fraction of fine sediment in the active surface layer at the north site 

in 2013 and 2014, eelgrass shoot densities and canopy heights varied little, suggesting that processes 

that contributed to the accumulation of fine sediment did not measurably impair shoot growth in the 

Luhr N meadow. At the south site, Luhr S, a general coarsening trend of the active surface layer over 5 

years was accompanied by a general decrease in the size of plants and the amount of eelgrass leaf 

habitat per square meter. Stronger tidal currents in McAllister Creek following dike removal could have 

coarsened the bed sediment texture at Luhr S and induced a shift in plant morphology to a shorterleaved form to reduce frictional drag, an adaptation than has been observed in energetic environments

(Peralta and others, 2006). The smaller plant size and sparser eelgrass bed at Luhr S could also be an 

adaptive response to a suboptimal light environment (Ralph and others, 2007), which is consistent with 

the hypothesis that turbidity would increased in response to dike removal. 

Over the range of values observed between May 2010 and 2014, the fraction of fine sediment in 

the rhizosphere of McAllister Creek eelgrass beds did not generally appear to be related to reducing 

conditions in sediment pore waters during midday low tides in May. One exception, at Luhr S in 2011, 

was characterized by a large fine fraction and moderately reducing pore waters that corresponded with 

measurable H2S and lower canopy height and biomass of eelgrass compared to the previous year. 

However, a causative relationship between pore water redox/H2S and plant growth is uncertain because 

the mean H2S concentration in the rhizosphere was below 150 micromolar (μM), the level found to 

impair Z. marina photosynthesis and respiration (Penhale and Wetzel, 1983; Goodman and others, 

1995; Holmer and Bondgaard, 2001). Furthermore, the sediment redox state can vary widely and rapidly 

depending on the tidal cycle, season, and oxygen exudation by eelgrass roots during photosynthesis. The 

timing of sampling was at a time of day and season when rhizosphere oxygenation by eelgrass should 

have been high and able to mitigate the build-up of H2S. The alteration of the pore water redox state by 

eelgrass could explain the lack of correlation between redox and the fine fraction of sediment. 

The amount of eelgrass leaf area per square meter along McAllister Creek was relatively stable 

over 5 years in the Luhr N (north) eelgrass meadow but declined in the Luhr S eelgrass bed. At Luhr S, 

there were fewer and smaller plants compared to 2010, but they were more evenly distributed across the 

bed, an indication that lateral shoot growth became more profuse with time. The downward shift of 

mean AG:BG5 ratios after 2010 at the Luhr S eelgrass bed imposed a greater respiratory carbon burden 

on plants. Nevertheless, eelgrass plants in the Luhr S bed had AG:BG5 ratios well above the value at 

which a carbon deficit can occur (Hemminga, 1998). Other stressors, such as disease or shading by 

epiphytes, were not evaluated in this study. The alongshore expansion of the Luhr S eelgrass bed in 

2013 and 2014 suggests that new substrate that could support eelgrass was made available after dike 

removal, likely owing to sediment deposition along the channel margin. Landscape-scale bathymetric 

maps could identify new habitat available to eelgrass arising from deltawide sedimentation patterns 

following restoration. 
13

References Cited

Barnhardt, W.A., and Sherrod, B.L., 2006, Evolution of a Holocene delta driven by episodic sediment 

delivery and coseismic deformation, Puget Sound, Washington, USA: Sedimentology, v. 53, no. 6, p. 

1211–1228. 

Goodman, J.L., Moore, K.A., and Dennison, W.C., 1995, Photosynthetic responses of eelgrass (Zostera 

marina L.) to light and sediment sulfide in a shallow barrier island lagoon: Aquatic Botany, v. 50, no. 

1, p. 37–47. 

Hemminga, M.A., 1998, The root/rhizome system of seagrasses—an asset and a burden: Journal of Sea 

Research, v. 39, nos. 3–4, p. 183–196. 

Hood, W.G., 2004, Indirect environmental effects of dikes on estuarine tidal channels—thinking outside 

of the dike for habitat restoration and monitoring: Estuaries, v. 27, no. 2, p. 273–282. 

Holmer, M., and Bondgaard, E.J., 2001, Photosynthetic and growth response of eelgrass to low oxygen 

and high sulfide concentrations during hypoxic events: Aquatic Botany, v. 70, no. 1, p. 29–38. 

Holmer, M., Carta, C., and Andersen, F.O., 2006, Biogeochemical implications for phosphorus cycling 

in sandy and muddy rhizosphere sediments of Zostera marina meadows (Denmark): Marine Ecology 

Progress Series, v. 320, p. 141–151. 

Lee, S.Y., Kim, J.B., and Lee, S.M., 2006, Temporal dynamics of subtidal Zostera marina and intertidal 

Zostera japonica on the southern coast of Korea: Marine Ecology Progress Series, v. 27, no. 2, p. 

133–144. 

Penhale, P.A., and Wetzel, R.G., 1983, Structural and functional adaptations of eelgrass (Zostera 

marina L.) to the anaerobic sediment environment: Canadian Journal of Botany, v. 61, no. 5, p. 1421–

1428. 

Peralta, G., Brun, F.G., Perez-Llorens, J.L., and Bouma, T.J., 2006, Direct effect of current velocity on 

the growth, morphometry and architecture of seagrasses: a case study on Zostera noltii: Marine 

Ecology Progress Series, v. 327, p. 135–142. 

Phillips, R.C., 1972, Ecological life history of Zostera marina L. (eelgrass) in Puget Sound, 

Washington: Seattle, University of Washington, Ph.D. dissertation, 154 p. 

Phillips, R.C., 1984, The ecology of eelgrass meadows in the Pacific Northwest—A community profile: 

U.S. Fish and Wildlife Service FWS/PBS-84/24, 85 p. 

Ralph, P.J., Durako, M.J., Enriquez, S., Collier, C.J., and Doblin, M.A., 2007, Impact of light limitation 

on seagrasses: Journal of Experimental Marine Biology and Ecology, v. 350, nos. 1–2, p. 176–193. 

Zimmerman, R.C., Reguzzoni, J.L., Wyllie-Echeverria, S., Josselyn, M., and Alberte, R.S., 1991, 

Assessment of environmental suitability for growth of Zostera marina L. (eelgrass) in San Francisco 

Bay: Aquatic Botany, v. 39, no. 3–4, p. 353–366. 
14

Appendix 1. Field and Lab Methods

Water Column Turbidity

Water column turbidity was measured at the top of the eelgrass canopy using self-logging optical 

backscatter sensors (SLOBS) with added temperature and pressure sensors (Aquatech). The top of the 

eelgrass canopy was approximately 90 cm above the sediment surface. The sensors recorded in burst 

mode, taking readings once a second (1 hertz [Hz]) for 120 second-long bursts every 15 minutes. During 

the initial year of study, sensors were deployed for 12 days from May 17, 2010, to May 29, 2010, with 

automated wipers to prevent particles from settling on the sensor windows and interfering with 

backscatter readings. In subsequent years, SLOBS were deployed for 4–5 days without wipers because 

particle build-up was not significant over the shorter period. Surface sediment (0–2 cm) was collected in 

replicate below the SLOBS for lab calibrations to convert optical backscatter (volts) into sediment 

concentration (milligrams per liter [mg/L]). Data was processed using Matlab software.

Sediment Collection

Surface sediment (0–2 cm) was collected for sediment grain-size determinations at the 

beginning, midpoint, and end of each eelgrass transect. Data from the three samples were averaged to

obtain a mean sediment grain-size distribution for each site. The subsurface sediment column of the 

eelgrass bed was sampled using 3.5-inch-diameter (9-cm) push-cores with end caps. Two replicate short 

sediment cores (~20 cm in length) with overlying water were collected within 1 m of each other at each 

monitoring site, transported on ice, and stored refrigerated until processing the same day to minimize 

sedimentary biogeochemical reactions. Sediment cores were intentionally collected in different parts of 

the eelgrass bed each year to avoid resampling disturbed sediments. Sediment grain-size distributions 

from the replicate cores were averaged over depth intervals of interest to obtain mean values.

Sediment Pore Water Analyses

Sediment cores were split longitudinally in an oxygen-free glove bag (GlasCol) filled with highpurity nitrogen (N2) to prevent oxygen-sensitive reactions from occurring. Sediment pore water 

oxidation-redox potential (ORP, commonly called redox) was measured approximately every 1 cm 

down the center of each core using a combination platinum microelectrode (Lazar Research Labs) 

calibrated to a quinhydrone redox solution (+206 mV, Hanna Instruments). In 2014, a new redox 

microelectrode was used, and its calibration was 34 percent below the reference value. One-half of each 

core was sliced into 10 intervals 1–2 cm in thickness and packed into air-tight, acid-cleaned 

polypropylene 50-cc (cubic centimeter) centrifuge tubes with friction-fit closures and centrifuged at 

3,900 revolutions per minutes (rpm) for 15 minutes. The centrifuge was in ambient air and at room 

temperature. After centrifugation, the supernatant (pore water) was drawn off in the N2 glove bag and 

filtered through acid-cleaned polyethersulfone 0.2-μm pore-size syringe-tip filters (Whatman). Filtered 

pore waters were used for colorimetric determination of hydrogen sulfide (H2S) concentration using a 

prepackaged reagent kit with a minimum detection limit of 0.05 ppm (CHEMetrics). The remaining half 

of each core was subsampled into intervals 1–2 cm in thickness for sediment grain-size determinations. 
15

Sediment Grain-Size Determinations

Sediment was dried at 60 °C, disaggregated in an agate mortar and pestle, and split into 1- to 2-g 

subsamples. The subsample was dry-sieved for 12 minutes in a dual action rotating-tapping shaker 

(Gilson) into five size fractions with particle diameters of >500 μm, 250–500 μm, 125–250 μm, 63–125 

μm, and <63 μm. The material in each fraction was weighed using a precision balance accurate to 0.01 

mg (Mettler Toledo). Fine sediment is defined as material with diameter <63 μm and includes silt and 

clay. 

Eelgrass Properties

Eelgrass characteristics were measured at each site along fixed alongshore transects at a tidal 

height of –0.7 m (–2 ft) relative to mean lower low water (MLLW). The transect at Luhr N was 100 m 

in length, except in 2010 when only 50 m of the eelgrass meadow could be safely accessed. The transect 

at Luhr S was determined by the alongshore extent of the eelgrass bed and was 50 m in length in 2010–

12, 60 m in 2013, and 80 m in 2014. For each transect, plant metrics were determined in 10 0.25-m2

quadrats spaced evenly across the transect (that is, every 5 or 10 m at Luhr N and every 5 or 8 m at Luhr 

S). Shoot density (number per quadrat) was determined in each quadrat, and one plant was harvested 

from each quadrat and measured in the lab to determine leaf length (canopy height), leaf width, number 

of leaves, rhizome internode lengths, and above-ground (shoots) and below-ground (roots and rhizomes) 

biomass. Rhizome internode lengths were measured with a digital caliper with an accuracy of 0.1 mm. 

The age of the whole rhizome was calculated assuming a mean 14-day formation interval for each 

rhizome internode (Lee and others, 2006). Dry biomass was measured on individual plants after they 

were dried at 60 °C for 24 hours. Below-ground biomass was measured on the five most recently 

formed rhizome internodes (BG5). Five rhizome internodes were used because the formation of each 

rhizome node corresponds to the formation of each leaf, and there were generally five leaves on each 

shoot. An above-to-below-ground biomass ratio (AG:BG5) was calculated for each plant. Mean 

characteristics of eelgrass beds were calculated from individual measurements of 10 plants, except at 

Luhr N in 2012 (n=11) and Luhr S in 2013 (n=9). The mean leaf area (length × width), number of 

leaves, and shoot density were multiplied to give an estimate of eelgrass leaf area (m

2 leaf per m2

seabed) at each site, here called an approximate leaf area index (a-LAI). 
16

Appendix 2. Example of Sensor Data

Graphs show time series of water depth (top), 2-minute averages of suspended sediment 

concentration (SSC, middle), and 2-minute averages of water temperature (bottom) at Luhr N during 

May 2014. The high and variable SSC values likely reflect biofouling by macroalgae. 

 
17

Appendix 3. Cross-Shore Z. marina Characteristics

Graphs show eelgrass characteristics along a cross-shore depth transect at Luhr N in May 2013 

(fig. 1D). Data points represent single measurments (n=1). Vertical lines show where the 2010 

alongshore transect intersected the cross-shore transect (fig. 1D). The cross-shore transect started (0 m

mark) on the 2011–14 alongshore transect line and extended 80 m across-shore (fig. 1D).