&EPA
United States
Environmental Protection
Agency
Office of Research
and Development
Washington, DC 20460
EPA/600/R-14/065
June 2014
www.epa.gov/ord
Pettaquamscutt Cove Salt Marsh
Environmental Conditions
and Historical Ecological Change
-------
Photographs on front cover
Background: Sunrise at high tide showing salt marsh pools, 7 Oct 2013, Pettaquamscutt Cove salt marsh.
Bottom left: Peeling biofilm on unvegetated marsh, 7 Oct 2013, Pettaquamscutt Cove salt marsh.
Bottle middle: Interface between short-form Spartina a/ternif/ora and Spartina patens at Pettaquamscutt Cove.
Bottom right: Salt marsh pool at Pettaquamscutt Cove.
All photos by E.B. Watson
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Pettaquamscutt Cove Salt Marsh
Environmental Conditions and
Historical Ecological Change
Elizabeth B. Watson
Cathleen Wigand
National Health and Environmental Effects Research Laboratory
Atlantic Ecology Division
Holly M. Andrews
University of Michigan
Dept. of Ecology and Evolutionary Biology
Ann Arbor, MI
S. Brad Moran
Graduate School of Oceanography
University of Rhode Island
Narragansett, RI
Prepared for
The Narrow River Restoration Committee
U.S. Environmental Protection Agency
Office of Research and Development
National Health and Environmental Effects Research Laboratory
Atlantic Ecology Division
Narragansett, RI 02882
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This report is contribution number ORD-007757 of the Atlantic Ecology Division,
National Health and Environmental Effects Research Laboratory, Office of Research and
Development, U.S. Environmental Protection Agency. Although the information in this
document has been funded by the U.S. Environmental Protection Agency, it does not
necessarily reflect the views of the Agency and no official endorsement should be inferred.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
We appreciate assistance from Roger Kelley with gamma spectroscopy, Clarice Esch,
Amy Fischer, Roxanne Johnson, Justen Skenyon, Nick Angelo, Earl Davey, and Alana Hanson
with field data collection, and Joseph Bishop with laboratory analyses. Doug McGovern,
Carol Pesch, and Mike Charpentier collected, collated, and digitized historic maps, and
historic aerial imagery was obtained from the Rhode Island CIS center at the University of
Rhode Island. We also acknowledge Roxanne Johnson, Earl Davey, Walter Berry, and Joe
LiVolsi for constructive feedback and helpful comments on an earlier draft of this report.
ii Pettaquamscutt Cove Salt Marsh
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Notice ii
Acknowledgements ii
Background 1
Focus Area 2
Historical Vegetation Change 3
Elevation and Vegetation Surveys 6
Water Levels 9
Marsh Soils and Accumulation Rates 11
Historical Changes in Salt Marsh Nitrogen Sources 13
Bibliography 14
Contents iii
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he Narrow River Restoration Committee, consisting of mostly federal and state
agencies including the U.S. Environmental Protection Agency, is charged with
identifying restoration priorities for the Pettaquamscutt Estuary, located in Rhode Island
with a focus on U.S. Fish and Wildlife Service (USFWS] refuge lands. Committee members
envision that a mosaic of estuarine communities of historic significance will be supported
by overall restoration and enhancement actions, which include shellfish and seagrass beds,
fisheries habitat, and salt marsh plant communities. At present, committee members are
principally concerned with the degradation of salt marsh habitat, resiliency of salt marsh
to current and future rates of sea level rise, and loss of associated services.
The formation of pools in areas of poor drainage has occurred over the past century at
Narrow River. These pools fluctuate in water level over spring-neap tidal cycles, and inter-
annually, as depressions support greater or less amounts of stunted short form Spartina
alterniflora. While pools are a natural and ecologically valuable component of salt marsh
landscapes (Adamowicz and Roman 2005], this loss of high salt marsh habitat has
implications for a bird species of concern, the salt marsh sparrow (Ammodramus
caudacutus), which prefers the high salt marsh species, Spartina patens, for nesting.
Spartina patens, also known as salt marsh hay, is a thin grass that grows 20-70 cm in
height. The thin blades of 5. patens blow over, creating a mosaic of tufts and cowlicks,
which provide habitat for sparrow nesting. Restoration actions under consideration
include options designed to protect and promote the expansion of high marsh habitat
through hydrological and elevation enhancements.
Investigations of salt marsh at Pettaquamscutt Cove were undertaken by the U.S.
Environmental Protection Agency (EPA] scientists as part of their activities designed to
elucidate the impacts of multiple anthropogenic stressors (i.e., climatic change and high
nutrient loads] on coastal habitats. As part of planning restoration activities, the Narrow
River Restoration Committee and the U.S. Fish and Wildlife Service have been requesting
a summation of research findings from active special use (i.e., research] permit holders.
This report is designed to summate research findings in a way that is most useful for
restoration planning purposes.
Background 1
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FOCUS AREA
eographical Extent. EPA conducted a series of investigations at Pettaquamscutt Cove
salt marsh at the Little Neck Cove during the summer of 2012 (Figure 1}. The purpose
of these investigations was to learn more about Pettaquamscutt Cove salt marshes and
identify any perceived changes in vegetation, ecology, and sustaining processes over time.
Using historic air photos and US coast survey maps, historic vegetation changes at
Pettaquamscutt Cove over the past 180+ years were identified. Marsh elevation and plant
surveys were conducted and compared with other salt marshes in Rhode Island and
neighboring states. Water levels within a salt marsh tidal channel at Pettaquamscutt Cove
were measured and used to develop an elevation/inundation frequency diagram. By
analyzing dated and undated salt marsh sediment cores, accumulation rates, soil
composition, and historic nitrogen sources were also reported. These data are intended
to provide information useful for ongoing restoration planning.
-41' 3D1
-41' 29'
I
Study
Area
-41' 25'
Conanicut
Island
N
A
0 1 2 3 km
Figure 1. The Narrow River Estuary, with area of investigation delineated.
2 Pettaquamscutt Cove Salt Marsh
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HISTORICAL VEGETATION CHANGE
1 rends in Salt Marsh Areal Extent. To establish trends in salt marsh extent, marsh area
(Civco et al. 1986; Smith 2009} at Little Neck Cove was digitized using an historic
survey map (USCS 1869} and air photographs (Figure 2}. Digitization used spatially
referenced imagery available via mapserver from the Rhode Island State Geographic
Information System Database and was conducted using ArcGIS, versionlO.
Figure 2. Data layers used for digitizing marsh extent over time for Little Neck Cove, Narrow River.
These data sources document the shift from a nearly completely vegetated salt marsh to a ponded marsh.
Early coast survey maps (USCS 1839} show the location and extent of salt marsh for
Narrow River depicting sinuous tidal channels. Salt marsh tidal channels depicted on later
series maps (USCS 1869; USCS 1871} also include numerous linear channels, presumably
recirculation ditches. The 1871 series map also shows a road and bridge traversing
Pettaquamscutt Cove, and Pettaquamscutt Cove salt marsh, and the position and general
width of subtidal channels for Pettaquamscutt Cove. The salt marsh extent was not
digitized in 1839 and 1871.
Historical Vegetation Change 3
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» Jl
1 QCLI
i yo i
•**. -!-.„.
Pond
1962
N
meters
I I I I I
100200300400500
Figure 3a. Historical salt marsh landscape change for the Little Neck Cove portion of Pettaquamscutt Cove.
Wetland hatching delineates areas of vegetated wetlands. Ponding first appears in this region on 1962 air
photos, although the 1951 series photos show ponding at other locations. For Little Neck Cove, ponding
makes up less than 10% of the landscape; however, for other portions of the estuary, ponds are larger
and dominate the landscape.
4 Pettaquamscutt Cove Salt Marsh
-------
N
meters
I I I I I I
0 100200300400500
Figure 3b. Historical salt marsh landscape change for the Little Neck Cove portion of Pettaquamscutt Cove.
From 1869-2011, area of marsh vegetation for this region has declined 18%; loss rates analyzed using linear
regression suggests that the loss rate is linear and sums to 1.5% per decade (/?2=0.81; p<0.01). Loss is from
a combination of ponding and edge loss. For the upper portion of the cove, ponds are ephemeral.
Historical Vegetation Change 5
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ELEVATION AND VEGETATION SURVEYS
r\ ispersion of Plant Species. Vegetation and elevation surveys were performed at
201 points along 10 transects at Little Neck Cove, with measures performed every 5 m
along transects. Vegetation was described using the point intercept method (as described
by Roman et al. 2001} using a 25 point/sq m grid. Nomenclature follows treatment in the
Flora of North America. Topographic surveys were performed using a CST/Berger Self-
Leveling Exterior Rotary Laser, with reported accuracy of ±1.5 mm at 30 m and a range of
up to 800 m. To convert relative elevations to orthometric heights, 2-4 hour static post-
processed GPS surveys were performed at three temporary benchmarks using a Trimble
4700 survey-grade GPS receiver (Figure 4}. To produce elevation solutions, GPS data was
post-processed using OPUS (Online Positioning User Service, National Geodetic Survey}.
Surveys were conducted to minimize elevation uncertainties by surveying under two
separate satellite configurations, away from potential sources of multipath errors, during
clear conditions free of magnetic disturbances, and using a fixed height antenna.
Figure 4. Topographic surveys were performed by differential leveling. Temporary benchmarks were
established; orthometric heights for these marks were measured using a survey grade GPS receiver,
post-processed using data from the National Geodetic Survey CORS network.
We found thatSpartina patens was not associated with significantly higher elevations than
short form Spartina alterniflora, which was an unexpected result (Figure 5}. For other
Narragansett Bay salt marshes for which we have data, 5. patens is located in the upper
marsh (Watson, unpublished data}. For instance, we have found that 5. patens, on average,
grows at marsh elevations 18.7 cm higher than 5. alterniflora at Hundred Acre Cove
(Barrington, RI}, and at 11.7 cm higher in elevation at Mary Donovan Marsh (Little
Compton, RI}. This suggests that the primary control on 5. patens distribution at Narrow
River is not elevation. Examination of air photos taken during the growing season shows
areas of light green reflected light, presumably 5. patens, found adjacent to the shoreline
and tidal channels, suggesting that drainage may instead determine distribution of this
species at Narrow River. To test this hypothesis, we computed for our survey stations,
6 Pettaquamscutt Cove Salt Marsh
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0.60-
0.50-
oo 040
Q
I 0.30
E
^ 0.20
o
1 0.10
Tall form
Spartina alterniflora
Short form
Spartina alterniflora
LU
-0.10
rr
F
Distichlis spicata ~\ Schoenoplectus
amencans
0 20 40 60 0 10 203040 50 0 10 20 30 40 0 20 40 60 0 20 40 60
frequency (%)
Figure 5. Distribution of salt marsh plant species relative to elevation at Little Neck Cove, Narrow River.
Elevations are relative to the NAVD88 datum, GEOID12A. Surveys were conducted in 2012.
a metric of weighted channel proximity (Sanderson et al. 2001], where the cumulative
inverse squared distance (CISD} function was calculated for each point q:
CISD (q) = I(w0 * cT2)
where q is a particular location in the salt marsh, d is the distance from the location to
a potentially influential channel that is less than 50 m away, and w0 is a weighting factor
(1,10, and 100 in this example] based on channel size. The CISD was then used as an input
in a logistic regression model to predicts, patens presence. The model was significant
(X2=14.3, df=l, p=1.57 x 10"4}, and plotting the CISD for locations where S.patens was found
to be present and absent demonstrates that channel proximity, and presumably drainage,
significantly influences distribution of 5. patens (Figure 6}.
Distribution of Spartina patens
relative to a metric of channel influence
absent
present
Figure 6. Air photo on left (July 2008) taken during the growing season shows dispersion of Spartina patens
(light green) relative to tidal channels for Narrow River. On right, the CISD metric for locations where
Spartina patens is present and absent for survey stations.
Elevation and Vegetation Surveys 7
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Elevation. The concept of elevation capital has been used by Reed (2002} and Cahoon and
Guntenspergen (2010} to describe the elevation of a salt marsh relative to its potential
range, with high elevation salt marshes possessing more elevation capital than a marsh
situated lower in the inter-tidal range. To determine the elevation capital of Narrow River
salt marshes, we compared heights to other sites in Rhode Island for which we have
geodetic survey data, and to 38 other sites in Rhode Island and bordering states (Long
Island, Connecticut, and southern Massachusetts} where we derive elevation from digital
elevation models. Finally, we compare our geodetic survey heights to LiDAR (Light
Detection And Ranging} elevations at Narrow River (after converting to the appropriate
geoid model} to test cross-comparability (Figures 7 and 8}.
15% -i
0%
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
10% -I
8% •
>s
% 6% -I
| 4%
2%
0%
I Northeastern States
I Narrow River
ffffffff
9?
Elevation (cm NAVD88)
Figure 7. Comparison of salt marsh elevations for Narrow River to other locations in Rhode Island for which
we have data, and for other locations in the northeastern US, suggests that Narrow River salt marshes are
found at the low-mid part of the distribution. Mean heights for Narrow River are 31.7 cm; the mean height for
all of Rhode Island (for which we have data) is 41.2 cm, and the mean height for the northeast is 52.3 cm
NAVD88.
70
c 60 -
•| 50 -
jj 40 -
LU
01 30 -
9 20 -
10 -
0
y=0.4175x + 31.703
R2 = 0.3461
tu/o •
35% •
£ 30% •
S1 25% •
§ 20% •
§f 15% •
"- 10% •
5% •
0% •
• geodetic survey
• LiDAR elevations
J
I.
0 20 40
Geodetic Elevation (cm NAVD88)
60
-505 101520253035404550556065
Elevation (cm NAVD88)
Figure 8. Comparison of geodetic survey elevations with LiDAR survey elevations shows that plot level
correlations are fair for Narrow River (R2= 0.35). However, LiDAR elevations are overestimates (by ~10 cm),
and under-report elevations below 32 cm, showing that LiDAR did not penetrate water at this location.
8 Pettaquamscutt Cove Salt Marsh
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WATER LEVELS
idal Range. Water levels were measured in a salt marsh tidal channel at Little Neck
Cove (41.44288°, -71.45775°} from 19 July 2012 to 15 October 2012 at five-minute
intervals using a Solonist Model 3001 Levelogger Edge. Values were barometrically
compensated using pressure measures from a Solonist Barologger. Tidal datums were
estimated using the modified-range-ratio method (Table 1], with Newport, RI, as the
control station. Such a conversion is associated with accuracy on the order of 2-3 cm
(Swanson 1974; NOAA 2003}. Water levels were converted to the NAVD88 datum using
measures of water level height made on monthly sampling trips relative to field
mesocosms, which were themselves part of elevations surveys (Figure 9}.
Figure 9. Water levels measured for the second two weeks of September, 2012, at the Narrow River
salt marsh channel and at the Newport NOAA tide station.
Table 1. Tidal datums during the calibration period, and preliminary computed values for the National
Tidal Datum Epoch (1983-2001) reported in meters relative to the NAVD88 datum. MHW refers to mean high
water, MLW to mean low water, MTL to mean tide level, or the arithmetic mean of MHW and MLW, and MN to
mean range of tide, or the difference in height between mean high water and mean low water.
MHW 7/1 9/201 2-1 0/1 5/201 2
MLW 7/1 9/201 2-1 0/1 5/201 2
MTL 7/1 9/20 12- 10/1 5/20 12
MN 7/1 9/201 2-1 0/1 5/201 2
MHW 1983-2001
MLW 1983-2001
MTL 1983-2001
MN 1983-2001
Narrow River
0.396
-0.102
0.147
0.497
0.232
-0.302
-0.035
0.534
Newport
0.619
-0.477
0.071
1.097
0.477
-0.699
-0.111
1.176
Water Levels 9
-------
Using water level data, we plotted a frequency-inundation curve for the inter-tidal zone at
Narrow River (Figure 10}. To further focus on the range of salt marsh elevations measured
along transects, we plot mean percent inundation for 5cm intervals where salt marsh
vegetation was found (Figure 11}.
0.8
00
00
a
E
c
o
re
j> -0.2 -|
LU
0.6 -
0.4 -
0.2 -
0 -
-0.4
10 20 30 40 50 60 70
Percent inundation
80
90 100
Figure 10. Estimates for intertidal inundation at Little Neck Cove as function of elevation for mid- and
late summer of 2012.
100% -i
80% -
re
c 60%
3
c
c
0)
o
L.
0)
Q.
40% -
20% -
0%
-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Elevation (m NAVD88)
Figure 11. Estimates of inundation for the range of elevations found for salt marsh at Little Neck Cove,
Narrow River. The elevation mode (~40 cm NAVD88) for salt marsh elevation is associated with an
inundation figure of 10.2%. Elevations of 25 cm were inundated 27.9% of the time, elevations of 30 cm were
inundated 21.1% of the time, elevations of 35 cm were inundated 15.4% of the time, and elevations of 45 cm
were inundated 5.8% of the time.
10 Pettaquamscutt Cove Salt Marsh
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MARSH SOILS AND
ACCUMULATION RATES
edimentation. A shallow sediment core (10cm diameter by 30 cm in length] was
removed from Little Neck Cove (41.44217° -71.45689°} in August of 2012. Gamma
activity for 214Pb, 210Pb, and 137Cs was measured on homogenized and sieved sediment sub-
samples (roots removed] to determine accretion rates. The depth associated with peak
concentrations of radiocesium (a product of nuclear testing] was assigned an age of 1962.
210Pb is a naturally produced decay product of the 238U decay series, has a half-life of
22 years, and is removed from the atmosphere by precipitation. 214Pb is a parent of 210Pb
in the 238U decay series; 214Pb decay ultimately produces the radon that decays into 210Pb.
Measuring 214Pb activity for each sample allows specific sample-by-sample estimates
of supported and unsupported 210Pb activity.
Accretion rates were estimated from profiles of excess 210Pb (Figure 12] following the
Constant Flux: Constant Sedimentation model (Nittrouer et al. 1979; Wheatcroft and
Sommerfield 2005]. In the following equation, A represents excess 210Pb activity, z is
depth, and lambda is the 210Pb decay constant of 0.0311 yr1. To calculate w, an
unweighted least squares regression was fit to the log transformed activity data, where
depth was the independent variable. To calculate the average accretion rate, the 210Pb
decay constant (0.0311] was divided by w. Confidence intervals (95%] were calculated for
this accretion rate by calculating the uncertainty of the slope estimate.
LnAz = LnAo - A/w * z
0.00
0.20
dpm/g
0.40
0.60
0.80
0.00
dpm/g
0.10 0.20
0.30
o
0
5
10
15
25
30
35
210Pbxs activity
Figure 12. Downcore profiles of excess 210Pb and 137Cs activity in disintegrations per minute per gram.
Based on the 210Pb profiles, sediment accumulation rates have averaged 0.17 cm/yr (95% Cl: 0.13-0.27) at
Narrow River for the past 180 years. Based on the 137Cs peak, accretion rates have averaged 2.1 mm/yr since
the 1960s.
Marsh Soils and Accumulation Rates 11
-------
This shallow sediment core and three additional undated sediments cores collected from
two high marsh and one low marsh locations around Little Neck Cove were sub-sampled
and analyzed for sediment organic content and bulk density (Figure 13} using loss on
ignition methods (Dean 1974; Heiri etal. 2001}. Sediment lithic particle size distribution
was also measured for these three undated sediment cores. Samples were pretreated with
concentrated hydrogen peroxide to remove organic particulates (Gray et al. 2010},
dispersed with a deflocculant, and analyzed using a Malvern Mastersizer 2000S laser
granulometer (Figure 14}.
fraction
0 0.25 0.5 0.75 1
fraction
0 0.25 0.5 0.75 1
5 -
10 -
15 -
20 -
25 -
30 -
35
water content
0.75
Figure 13. Downcore profiles of sediment organic content, water content, and density for bulk soils.
Organic content ranges from 38-78%, with a mean of 72% of dry weight. Water content ranges from 58-72%,
with a mean of 70%. Dry bulk density ranges from 0.16 to 0.71 g cc"1, with a mean of 0.36. Supplemental
cores average 50% organic, water content averages 82%, and average bulk density averages 0.20 g cc'1.
nclay
• silt
nsand
o>
cr
o>
6
5
4
3
2
1
0
0.01 1 100 10000
particle size (|j,m)
Figure 14. Particle size distribution for sediments from Little Neck Cove, Narrow River. Little variation
is apparent with depth, or by location. Median particle size is 17.5 urn, mode is 18.7 urn. Using methods of
Folk (1980), particle size distribution is classified as poorly sorted with a symmetrical distribution.
12 Pettaquamscutt Cove Salt Marsh
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HISTORICAL CHANGES IN SALT MARSH
NITROGEN SOURCES
\ A/ ater Quality. Ground and surface waters at Narrow River are degraded by residential
and commercial land use activities, with both nitrogen and coliform bacteria as
known pollutants (RICRMC 1999}. While nutrient inputs may have negative affects on salt
marshes (Deegan etal. 2013], impacts may vary with geographic location, elevation, and
soil matrix. Soil stable isotope values were used to help determine whether Narrow River
salt marshes appear to be impacted by high nutrient loads.
Nitrogen occurs as two major stable isotopes (15N and 14N}, which have slightly different
physical properties. The ratio of these isotopes (615N} varies with nitrogen source (e.g.,
sewage effluent, groundwater, fertilizer] and can be used to help discriminate sources of
nitrogen in estuarine water bodies. Locations where wastewater makes up a large
proportion of dissolved inorganic nitrogen tend to have values for 615N in biota that are
above 5-8%o (Cole et al. 2004}. By measuring 615N in soils from a dated sediment core
collected from Little Neck Cove, we found low values of 615N, both for recent time periods
and historically (Figure 15}.
Compared to sediments with known heavy nitrogen loads from wastewater (e.g., Nguyen
and Peteet 2012; Wigand et al., 2014}, these values are extremely low and indicate dilution
of wastewater by tidal flushing. While preliminary, these results suggest that high nutrient
loads are unlikely to be a primary stressor on Narrow River salt marsh.
10
Ib
1.5
0.5 •
0 •
-0.5
1820
1840
1860
1880
1900
1920
Year
1940 1960
1980
2000
2020
Figure 15. Nitrogen stable isotope ratios for soils over time for a sediment core collected at Little Neck Cove,
Narrow River. Overall low values (815N<5) suggest that nitrogen source to salt marsh is primarily oceanic
nitrogen, rather than anthropogenic nitrogen. Although nuances in the 815N curve are relatively small in
magnitude, increases over time suggest enhanced wasterwater-N, with a potentially small recent drop
reflecting a decrease in sewage inputs with sewering and storm drain remediation. The drop corresponding
to 1940-1980 may reflect land use patterns or tidal flushing (a similar drop ca. 1938 was noted for salt marsh
at Block Island; Thomas and Varekamp 2013)
Historical Changes in Salt Marsh Nitrogen Sources 13
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