WETLANDS, Vol. 25, No. 2, June 2005, pp. 279–288
q 2005, The Society of Wetland Scientists
NEW ENGLAND SALT MARSH POOLS: A QUANTITATIVE ANALYSIS OF
GEOMORPHIC AND GEOGRAPHIC FEATURES
Susan C. Adamowicz1,3 and Charles T. Roman2,4
1
Graduate School of Oceanography
University of Rhode Island
South Ferry Road
Narragansett, Rhode Island, USA 02882
2
3
USGS Patuxent Wildlife Research Center
University of Rhode Island
Narragansett, Rhode Island, USA 02882
Present address and corresponding author:
U.S. Fish and Wildlife Service
321 Port Road
Wells, Maine, USA 04090
E-mail: [email protected]
4
Present address:
National Park Service
University of Rhode Island
Narragansett, Rhode Island, USA 02882
Abstract: New England salt marsh pools provide important wildlife habitat and are the object of on-going
salt marsh restoration projects; however, they have not been quantified in terms of their basic geomorphic
and geographic traits. An examination of 32 ditched and unditched salt marshes from the Connecticut shore
of Long Island Sound to southern Maine, USA, revealed that pools from ditched and unditched marshes had
similar average sizes of about 200 m2, averaged 29 cm in depth, and were located about 11 m from the
nearest tidal flow. Unditched marshes had 3 times the density (13 pools/ha), 2.5 times the pool coverage
(83 m pool/km transect), and 4 times the total pool surface area per hectare (913 m2 pool/ha salt marsh) of
ditched sites. Linear regression analysis demonstrated that an increasing density of ditches (m ditch/ha salt
marsh) was negatively correlated with pool density and total pool surface area per hectare. Creek density
was positively correlated with these variables. Thus, it was not the mere presence of drainage channels that
were associated with low numbers of pools, but their type (ditch versus creek) and abundance. Tidal range
was not correlated with pool density or total pool surface area, while marsh latitude had only a weak
relationship to total pool surface area per hectare. Pools should be incorporated into salt marsh restoration
planning, and the parameters quantified here may be used as initial design targets.
Key Words:
marsh pools, ditching, salt marsh, New England
INTRODUCTION
Able 1994, Erwin 1996, Wolfe 1996) have concluded
that pools provide important fish and wildlife habitat,
no research has been conducted in New England that
quantifies basic geomorphic and geographic pool features.
Salt marsh pools are significant because they provide habitat for numerous bird species. In Massachusetts, for example, Clarke et al. (1984) noted that
shorebirds, wading species, terns, swallows, and crows
were strongly attracted to salt marsh pools. In southern
New Jersey, Master (1992) documented that various
Salt marsh pools are soft-bottomed depressions that
hold water throughout a tidal cycle and do not dry as
shallower pannes often do (Figure 1; Chapman 1960).
Despite the extensive literature on salt marshes, research specific to marsh pools is quite limited. Harshberger (1916), Miller and Egler (1950), and Redfield
(1972) have provided the most extensive descriptions
of pools but no quantitative analyses. While several
authors (Daiber 1982, Clarke et al. 1984, Smith and
279
280
WETLANDS, Volume 25, No. 2, 2005
Figure 1. Small salt marsh pool in southern Maine (Wells, ME). Pool is 3–4 m in diameter and is located within a Spartina
patens-dominated high marsh.
shore and wading birds aggregated at pools to feed.
Erwin (1996) supported the conclusion that salt marsh
pools are vital year-round habitat for dabbling ducks,
shorebirds, and wading birds. Pools are valuable to
birds because they provide food items including fish
and macrophytes, specifically Ruppia maritima L.
(Daiber 1982).
Historically, New England salt marshes have been
extensively ditched for mosquito control and salt hay
farming, resulting in lowered water-table levels and
drainage of the marsh surface, often including salt
marsh pools (Miller and Egler 1950, Redfield 1972).
Salt hay farming has been a fairly common practice
since colonial times (Rozsa 1995), but the most extensive and widespread ditching occurred in the early-tomid 1900s for mosquito control (Daiber 1986). Bourn
and Cottam (1950) stated that, by 1938, approximately
90% of coastal marshes from Maine to Virginia had
been ditched in an effort to reduce breeding habitat for
the salt marsh mosquito (Ochlerotatus sollicitans
[Walker]). In New Jersey and Rhode Island, it is reported that pools were specifically targeted for drainage (Cottam 1938, Price 1938).
Given their ecological benefits, restoring pool habitats should be an important salt marsh restoration objective. To enhance the success of such work, welldocumented guidelines for pool creation are necessary.
The objective of this paper is to begin addressing this
need by providing a quantitative baseline analysis of
geomorphic (e.g., pool depth and size) and geographic
(e.g., density, spatial arrangement, and coverage) attributes of salt marsh pools for ditched and unditched
salt marshes located throughout New England. By
studying a gradient of marshes from unditched to intensely ditched, this study also evaluates the relationship between ditch density and geographic factors such
as pool density and pool surface area.
METHODS
Salt marsh pools were assayed using both field investigations and aerial photographic surveys coupled
with geographic information system (GIS) analysis.
The complete effort extended from the Long Island
Sound shoreline of Connecticut to mid-coast Maine,
USA. A total of 32 sites were surveyed by either or
both methods (Table 1). Several criteria were used to
select marsh study areas. All sites were broad as opposed to fringing systems (e.g., average width over
150m). Impounded, filled, and tide-restricted marsh
sites were not sampled in order to minimize variability.
All surveyed locations were in public ownership. Unditched sites were very uncommon; only eight such
locations were included in this study, mostly in Massachusetts and Maine. Some marshes had discrete areas of ditched and unditched marsh that were considered separate study areas for the purposes of this work.
Adamowicz & Roman, NEW ENGLAND SALT MARSH POOLS
281
Table 1. Location of ditched and unditched marsh study sites, summary of survey method employed at each site, and determination of
pool density (number of pools/km of transect).
Latitude
Longitude
Survey
Method1
Pool Density
(no. of pools/km)
DITCHED MARSHES
Hammonasett State Park, Point, Clinton, Ct
Hammonasett State Park, Rotary, Clinton, CT
Grass Island 1, Guilford, CT
Grass Island 2, Guilford, CT
S. B. McKinney NWR, Hammonck Dock, West Site, CT
S. B. McKinney NWR, Hammonck Dock, East Site, CT
Great Island, Old Lyme, CT
Unnamed Island, Old Lyme, CT
Watts Island, Niantic, CT
Barn Island Management Area (1), Stonington, CT
Barn Island Management Area (3&4), Stonington, CT
Sachuest Point NWR, Middletown, RI
Sapowet Wildlife Management Area, Tiverton, RI
Coggeshall Marsh, NBNERR, Portsmouth, RI
100 Acre Cove, Barrington, RI
Felix Neck, Martha’s Vineyard, MA
Water Street, Essex, MA
Annisquam River Marshes, Gloucester, MA
Corn Island, Essex, MA
Moody Marsh, Wells, ME
Wells NERR, North Area, Wells, ME
Granite Point, Biddeford, ME
Morse River, Phippsburg, ME
Squirrel Point, Arrowsic, ME
Average Ditched Marshes (SE)
N41815.3859
N41815.3859
N41816.1009
N41816.3639
N41816.7309
N41816.7909
N41817.2139
N41817.1159
N41818.0259
N41820.3309
N41820.2499
N41828.6839
N41835.0129
N41839.0469
N41845.9859
N41824.9679
N42837.9529
N42837.9689
N42839.4359
N43815.8849
N43820.4109
N43824.9809
N43845.1309
N43849.4109
W72832.3979
W72832.3979
W72839.3269
W72839.4439
W72828.5709
W72828.2909
W72819.8109
W72819.3159
W72813.1939
W71852.3909
W71852.0139
W71814.6229
W71812.3199
W71820.6359
W71818.2449
W70833.5619
W70846.2519
W70841.0739
W70844.8029
W70835.5239
W70832.3409
W70823.3869
W69849.4709
W69847.4509
F/D
F/D
F/D/C
D/C
D
D
D
D
D
F/D/C
F/D/C
F/GPS
F/GPS
F/GPS
F/D
D
D
D
D
F/C
C
F/AP
D
D
1.7
0.2
0.0
0.6
0.0
1.9
1.1
17.6
2.1
3.8
1.8
1.8
7.1
1.8
7.4
4.0
0.0
6.2
11.3
19.8
10.7
18.7
5.5
0.4
5 (6)
UNDITCHED MARSHES
S. B. McKinney NWR, Bridgeport, Ct
Nauset Marsh, Eastham & Orleans, MA
Annisquam River Marshes, Gloucester, MA
Parker River NWR, Newbury, MA
Wells NERR, Harbor Area, Wells, ME
Scarborough Marsh, Scarborough, ME
Spurwink River, Pleasant Hill, ME
Reid State Park, Georgetown, ME
Average Unditched Marshes (SE)
N41809.3769
N41849.3489
N42838.6639
N42843.3239
N43819.0509
N43833.6939
N43835.2649
N43846.7279
W73808.2979
W69957.4709
W70841.0379
W70847.6569
W70934.2909
W70819.8289
W70815.7679
W69844.2679
D
F/D
D
F/D
C
F
F
F/D
4.8
21.3
14.6
23.1
7.5
13.8
31.3
24.3
18 (9)
Marsh Type and Location
1
Survey methods included georectified true color aerial photographs (AP), color infrared images (C), digital orthophotographic images (D), field survey
(F), and field-generated GPS maps (GPS).
Field Survey
Field surveys were conducted at 16 salt marshes
throughout New England during the 1999 and 2000
growing seasons (Table 1). At each salt marsh, a study
area was identified as a discrete region bounded by
major creeks, roads, upland, or similar features. Selected study areas did not include tidal channels greater than 10 m wide, while all natural channels less than
10 m wide (‘‘creeks’’) and all ditches were included.
Large channels were excluded because they were representative of open water or aquatic habitat (Dame et
al. 1992, Heck et al. 1995, Bertness 1999).
Within each study area, line transects were used to
identify sample pools for a number of measures as
described below. The initial transect was randomly located along the main tidal channel or shoreline, with
subsequent transects set at 100-m intervals. Each transect extended perpendicularly from the edge of the
marsh to the upland border and was measured using a
metric survey tape. Only pools in direct contact with
transect lines were included in the field study. The
number of pools located on the transects was used to
calculate a measure of pool density (number of pools/
km transect). Adapting the standard line-intercept
282
method used in vegetation ecology (Mueller-Dombois
and Ellenberg 1974), pool coverage was defined as the
linear distance of pool falling directly on the transect
line (m of pool/km transect). The total transect distance sampled ranged from 293 m at the Sapowet
Marsh (RI) study area to 2,177 m at Hammonasett
State Park (CT).
For each individual pool encountered along the transects, pool surface area was estimated by measuring
the major and minor axes of regularly shaped pools
(Ingolfsson 1994) and applying the formula for the
area of an ellipse. Irregular pools were subdivided into
simple geometric shapes, with appropriate data taken
to calculate surface area. For each pool, geographic
parameters such as distance to nearest neighboring
pool and distance to nearest tidal flow (either a ditch
or creek) were also measured. Pool depth was based
on an average of at least three measurements of the
distance from the surface of the surrounding marsh
surface to the pool bottom. Water depth (measured as
surface of water to pool bottom) was recorded for use
in comparison with other studies, but it is less consistent than pool depth since the depth of water alone is
variable depending on tides, rainfall, and other factors.
To categorize qualitatively the location of pools
along a gradient from the frequently flooded low
marsh zone to the less frequently flooded high marsh,
the vegetation type adjacent to each pool was recorded
based on visual inspection. Three such types were distinguished: one dominated by Spartina alterniflora Loisel (tall form), one dominated by S. patens, and a third
dominated by S. alterniflora of intermediate height.
Spartina alterniflora (tall form) characterized more
frequently flooded (low) marsh areas, while the other
vegetation types characterized less frequently flooded
(high) marsh regions
Aerial Photography and Geographic Information
System Analysis
Digital orthophotographic quadrangle (DOQ) images were used to increase the number of marshes
studied. Black and white DOQ images (Multi-resolution Seamless Image Database ‘‘MrSID’’ compression) were downloaded from World Wide Web sites
for Connecticut (0.5 m per pixel resolution) and Massachusetts (1.0 m per pixel resolution). DOQs at the
same resolution were obtained from the University of
Rhode Island Environmental Data Center and the United States Fish and Wildlife Service Gulf of Maine Program for Rhode Island and Maine, respectively. In addition, color infrared (CIR) images were available for
a limited number of sites in Connecticut (1:12,000,
August 1995) and Maine (Wells NERR 1:12,000, August 1998; Moody Marsh 1:8,000, June 1992). A true
WETLANDS, Volume 25, No. 2, 2005
color photograph was available for Granite Point
Marsh in Maine (1:8000, May 1986). A total of 21
ditched and 6 unditched marshes were surveyed with
ArcView 3.2 geographic information system (GIS)
software applied to the DOQs and CIR photographs.
Site boundaries were delineated in the same manner
as in the field surveys. Transects were established on
the imagery following methods used in the field surveys, and density, distance measures, and coverage (m
of pool/km of transect) were then calculated. For each
individual pool encountered along the transects, surface area was calculated from digitized ArcView shape
files. Additionally, within the study boundary for each
site, all pools, creeks, and ditches were delineated with
ArcView shape files to calculate pool density and total
pool surface area per hectare of marsh study area.
Comparison of Survey Methods
The two survey methods (field survey transects and
DOQ transects) were compared using paired t-tests for
each pool variable (pool density, pool coverage, nearest neighbor distance, pool size, distance to tidal flow)
to determine if the two data sets for transects could be
combined. A total of nine sites (6 ditched, 3 unditched)
were used in this analysis, but because some surveys
(variously among the field and DOQ methods) did not
encounter pools, sample size was either 7 or 9 sites
for the 5 parameters of concern. It should be noted
that, while the same study sites were used in each survey method, initial transects were given independent,
random starts with the result that each survey provided
a statistical representation of that site.
Statistical Analyses
Based on results from the method comparison noted
above, pool densities (number of pools/km transect)
for field and digital orthophotographic quadrangle
(DOQ) transect studies were combined to test for differences attributable to marsh type (ditch vs. unditched) using a two-sample t-test. Relying on field
data alone, ANOVA was used to determine if the proportion of the total number of pools differed among
the three marsh zones [S. alterniflora (tall), S. patens,
S. alterniflora (intermediate)] or between ditched and
unditched marsh sites. Logistic regression was applied
to determine the environmental variable(s) (distance to
nearest neighboring pool, distance to nearest tidal
flow, pool surface area, log of pool surface area, and
pool depth) that best predicted pool location (SAS
1985). The effect of marsh type on a series of pool
variables measured from DOQs was determined by
two-sample t-test.
While it was instructive to examine ditched versus
Adamowicz & Roman, NEW ENGLAND SALT MARSH POOLS
283
Table 2. Average values (6 SE) for geographic measures of pools in ditched and unditched New England salt marshes based on digital
orthophotographic images. Data generated from transects placed on the imagery and digitizing techniques.
Geomorphic Measures
Transect Data
Pool Cover/Transect (m/km)
Individual Pool Size (m2)
Distance to Nearest Neighbor Pool
(m)
Distance to Tidal Flow (m)
Area Data
Density (number of pools/ha)
Total Pool Surface Area (m2/ha)
Ditch Length (m/ha)
Creek Length (m/ha)
Ditched Marshes
Unditched Marshes
t-value
df
p
31 6 37
227 6 305
83 6 47
205 6 113
22.85
20.47
24
21
,0.01
0.32
15 6 14
12 6 15
966
10 6 5
1.07
0.65
21
21
0.15
0.26
23.12
22.78
9.70
23.52
24
24
19
6
,0.001
,0.01
,0.0001
,0.01
4
229
217
38
6
6
6
6
5
242
100
44
unditched marshes, some marshes have been ditched
more intensively than others. Similarly, ditched and
unditched marshes also varied in the amount of natural
creek length that each possessed. Because ditches and
creeks both convey water to and from the marsh, these
parameters may be correlated with pool density and
the amount of pool surface area per hectare of marsh.
Therefore, separate regression analyses were used for
ditches and creeks to determine the relationship between these factors and pool density and total pool
surface area per hectare on data obtained from DOQ
sources. In addition, tidal range and latitude were each
regressed against pool density and total pool surface
area.
Paired Study
In order to examine the effect of ditching on salt
marsh pools more closely and to control for potential
effects of tidal range and latitude, ten total areas (five
paired sites) in five marsh systems were selected from
the DOQ data set in large marshes where a ditched
section was adjacent to an unditched portion of marsh
but separated by a hydrologic feature (e.g., tidal creek)
sufficient to consider the areas to be independent.
These pairings included sites from Old Lyme, CT;
Newbury, MA; Gloucester, MA; Essex, MA; and
Wells, ME. The same procedures were followed for
delineating study areas, ditches, creeks, and pools as
previously noted. Linear regressions and paired t-tests
were used to determine the effect of ditching on pool
density (number of pools/ha salt marsh) and total pool
surface area per hectare salt marsh (m2 pool/ha salt
marsh).
RESULTS
Pool Density, Size, and Other Geographic Measures
Transect data from field and DOQ evaluations
showed that pool density in unditched marshes was
13
913
0
171
6
6
6
6
7
489
0
89
more than three times that of ditched marshes (Table
1, t29 5 24.25, p,0.0001). Pool density ranged from
0 to 19.8 pools/km of transect in ditched sites and from
4.8 to 31.3 pools/km of transect in unditched marshes.
When calculated on an area basis from DOQ evaluations, pool density (number of pools/ha salt marsh)
again was three times greater in unditched marshes
compared to ditched marshes (Table 2). Other parameters, including total pool surface area per hectare (m2
pool/ha salt marsh), pool coverage (m pool/km transect), and creek length (m creek/ha salt marsh), were
all greater in unditched marshes (Table 2).
Somewhat unexpectedly, several pool measures
were the same between ditched and unditched marshes. Pools in ditched marshes were no further from the
nearest neighboring pools or from nearest tidal flow
than in unditched marshes. In both ditched and unditched marshes, pool size averaged about 200 m2 (Table 2). It should be noted that 100-Acre Cove, RI was
eliminated as an outlier from all but pool density calculations since pools there were up to 35 ha in size—
much larger than the next largest pool in the study
(0.56 ha, Scarborough Marsh, ME).
Influence of Ditching Intensity, Creeks, Tidal Range,
and Latitude
Along a gradient from unditched to highly ditched
marshes, both pool density (number of pools/ha salt
marsh) and total pool surface area (m2 pool/ha salt
marsh) decreased significantly (Figure 2). The weak
coefficients of determination reflect the high variability
in pool density and total pool surface area for marshes
with no ditches. There was also a negative relationship
between ditch length per hectare and creek length per
hectare (Figure 3), with the regression equation indicating that about 300 m of ditching per hectare of
marsh study area would result in little or no creek hab-
284
WETLANDS, Volume 25, No. 2, 2005
Figure 2.
Relationship of pool density and total pool surface area with ditching intensity (ditch length, m/ha).
itat, at least from the study sites in this investigation.
When pool parameters were regressed against natural
creek length per hectare (Figure 4), there was a positive relationship in each case; as creek density increased, so too did pool density and total pool surface
area.
Mean tidal range (F1,24 5 3.25, p50.08, R250.02)
and latitude (F1,24 5 2.04, p50.17, R250.08) were not
Figure 3.
related to pool density. However, there was a weak
relationship between total pool surface area per hectare
of marsh and both tidal range (F1,24 5 6.02, p50.02,
R250.20) and latitude (F1,24 5 4.69, p50.04, R250.16).
While there were no or only weak relationships between tidal range or latitude with pool density and
total pool surface area, the fact that the marsh study
areas were distributed along a continuum of tidal range
Relationship of creek length and pool length per hectare of salt marsh.
Adamowicz & Roman, NEW ENGLAND SALT MARSH POOLS
Figure 4.
285
Relationship of pool density and total pool surface area with natural creek intensity (creek length, m/ha).
and latitude could influence the significant relationship
between ditching and pool density and total pool surface area (Figure 2). To eliminate possible confounding influences of tidal range and latitude on pool parameters, five pairs of DOQ salt marsh study areas (1
pair 5 1 ditched site adjacent to an unditched site),
located along the continuum, were analyzed. Obviously, the ditched and unditched sites of each pair were
located at the same latitude and were influenced by the
same tidal regime. Once again, unditched marshes had
greater pool density (paired t-test, t4 5 23.15, p 5
0.02) and total pool surface area (paired t-test, t4 5
22.97, p 5 0.02) than ditched sites. Ditched marshes
had 4.3 pools per ha 6 2.5 (SD) and 249 m2 6 292
m2 total pool surface area per marsh hectare; unditched
marshes had 27 pools per ha 6 20 and 1063 m2 6 270
m2 total pool surface area per marsh hectare. Similarly,
pool density and total pool surface area per hectare of
salt marsh decreased with increasing total ditch length
per hectare of salt marsh (linear regression; n 5 10,
R25 0.42, p,0.001; R25 0.78, p,0.05; respectively,
Figure 5).
Figure 5. Relationship of pool density and total pool surface area with ditching intensity based on paired ditched and
unditched marshes.
286
Pool Location within Marshes
Pools were unequally distributed among the three
marsh zones (ditched and unditched marshes combined), with the greatest proportion of pools consistently found in the high marsh compared to low marsh
or intermediate S. alterniflora zones (ANOVA, arcsine
square-root transformation of proportion data, F3,38 5
10.89, p,0.0001; least squares means post-hoc test,
p,0.0005). Marsh type (ditched vs. unditched) was
not a significant factor (ANOVA, F1 5 0.07, p50.79),
and there was no significant interaction between marsh
type and pool distribution across zones.
Distance to tidal flow was the best predictor for determining pool location along a flooding gradient when
given the three vegetation types (S. patens, S. alterniflora (intermediate), S. alterniflora (tall); logistic regression, Wald X2 5 11.19, 1df, p50.0008). As would
be expected, distance to tidal flow was greater for
pools in S. patens areas, 31.4 m 6 33.6 m (SD,
n5109), compared to tall S. alterniflora (6.9 m 6
10.3m, n514) or the intermediate S. alterniflora regions (13.9 m 6 14.8 m, n550) (least squares means
post-hoc test, p,0.003). When discriminating between
just low (S. alterniflora (tall)) and high marsh (S. patens) locations, distance to tidal flow and pool size both
were significant variables in predicting pool location
(logistic regression; distance to tidal flow, Wald X2 5
6.39, 1df, p50.01; pool size, Wald X2 5 4.92, 1df,
p50.03). Pool size was greatest in the high marsh (310
m2 6 817 m2, n5109) compared to the low marsh zone
(229 m2 6 794 m2, n514) (logistic regression, Wald
X2 5 4.92, 1df, p50.03; least squares means post-hoc
test, p,0.05).
Pool Depth and Water Depth
Pool depth was the same regardless of marsh type
(ditched 5 26.0 cm 6 12.7 cm, n598; unditched 5
30.2 cm 6 21.1 cm, n580) or pool location (high
marsh 5 27.3 cm 6 15.7 cm, n5109; intermediate S.
alterniflora 5 27.9 cm 6 18.0 cm, n550; low marsh
5 31.4 cm 6 23.0 cm, n514; F3,131 5 0.74, p50.53).
Similarly, water depth did not differ between marsh
types or marsh vegetation zone and averaged 19.1 cm
(range 1–65 cm) for ditched marshes and 18.1 cm
(range 1–71 cm) for unditched sites (F5,140 5 0.73,
p50.60). The shallowest depths were from pools that
were breached and drained almost completely at low
tide. The maximum water depth (71 cm) and maximum pool depth (92 cm) were both from pools in Nauset Marsh, MA (an unditched marsh).
Methodology Comparison
Information for this paper was generated from two
methods, field survey transects and DOQ transects.
WETLANDS, Volume 25, No. 2, 2005
Paired t-tests indicated no significant difference between the methods for pool density (number of pools/
km transect; t8 5 21.07, p50.16), pool coverage (m
pool/km; t8 5 20.46, p50.33), nearest neighbor distance (t6 5 1.63, p50.08), or distance to tidal flow (t6
5 0.55, p50.30). However, there was a significant difference in average individual pool size (m2) (log transformed data, t6 5 2.71, p50.02). In general, pools sampled in DOQ transects were larger. This may have
been due to the limited resolution of DOQs compared
to field surveys or merely due to the fact that DOQ
transects were located in slightly different positions
compared to those in the field, resulting in a different
set of pools actually being sampled. Throughout this
paper, field and DOQ transect data were only combined for the comparison of pool density (number of
pools/km transect) between ditched and unditched
marshes.
DISCUSSION
New England salt marsh pools are remarkably similar between ditched and unditched sites. Average values for pool size, depth, nearest neighbor distance, and
distance to tidal flow are similar (Table 2). Additionally, most pools are located in the high marsh zone of
salt marshes. Conversely, a recent quantitative comparison of ditched and unditched salt marshes in New
Jersey found marsh pools in an unditched marsh to be
larger than those of a ditched marsh, and further, unditched marsh pools had a lower nearest neighbor distance (Lathrop et al. 2000). Except to mention the difference in geographic location, we offer no explanation for the divergence of our findings.
While New England pools appear similar between
marsh types, ditched marshes have less pool habitat
than unditched marshes, a relationship that holds for
both pool density and pool coverage (m pool/km transect; Table 2). In fact, whether based on the number
of pools per kilometer of transect or the number of
pools per hectare of salt marsh, ditched marshes have
70% fewer pools than unditched marshes. Similar to
our findings, studies in New Jersey salt marshes found
that unditched sites had dramatically more pool habitat
than ditched sites (Lathrop et al. 2000, Lathrop and
Bognar 2001). Collectively, these findings seem to
support Redfield’s (1972) hypothesis that ditching results in decreased pool density. Further, our findings
(Figure 3) and those of Lathrop et al. (2000) would
support a hypothesis that ditching may influence the
density of natural salt marsh creeks.
The fact that marshes with an abundance of natural
creeks also have a high density of pools (Figure 4),
while intensively ditched marshes have few pools
(Figure 2) or creeks (Figure 3), suggests that ditches
Adamowicz & Roman, NEW ENGLAND SALT MARSH POOLS
and creeks drain marshes differently. Further study is
necessary to determine the precise hydrologic interactions among creeks, ditches, and pools. Of particular
interest is the manner in which ditches, intertidal
creeks, and subtidal creeks influence sub-surface
marsh drainage.
While it is clear that the unditched marsh sites in
this study have a greater density of pools and more
pool surface area than ditched marshes, there is considerable variation in these relationships pointing to
heterogeneity in marsh morphology. As noted, some
unditched marshes have many pools, while others have
few (Figure 2, Table 2), thereby contributing to habitat
variability. With ditching, this important variation in
habitat is reduced and marshes are more uniform, at
least with regard to pool geography.
Heterogeneity of unditched marshes included in this
study can be related to several factors. Our marsh
study areas extended from Long Island Sound to the
Gulf of Maine, an extensive geographic region with
considerable variation in climate, tidal range, sediment
supply and type, and other factors (Roman et al. 2000).
It is likely that processes of pool formation and maintenance vary in response to these factors. When considering all of the unditched and ditched sites, there
was a weak relationship between total pool surface
area and latitude, as well as tidal range, suggesting that
marshes to the north and marshes with a high tidal
range (typically also to the northern portion of our
study region) have a greater total pool surface area. It
is also noted that our study marshes were classified as
back-barrier, embayment, or river valley, and it is likely that pool formation and maintenance vary among
these types. Classic papers on New England salt marsh
geomorphology (Harshberger 1916, Miller and Egler
1950, Redfield 1972) offer hypotheses for pool formation, but additional studies are needed to describe
the role of tidal range, climate (particularly ice), classification type, and other factors influencing marsh
pool geography.
Implications for Restoration
The descriptive analysis of pools from unditched
New England marshes may provide some design targets as efforts to re-establish pool habitat are considered. On average, 9% (913 m2/ha) of an unditched
marsh is composed of pool habitat, with an average
density of 13 pools/ha. The range of pool size was
broad (3–3,786 m2), but large pools were rare, with
only 6% of all pools in the entire DOQ survey being
greater than 800 m2. Pool depth, measured as the average distance from marsh surface to pool bottom, was
about 30cm, with actual water depth usually less (i.e.,
pools are often not flooded to the marsh surface). Most
287
pools are found within the high marsh zone. In the
absence of site-specific, pre-alteration data to use as
restoration targets, managers may want to consider
these average pool characteristics, while incorporating
variation in pool size, depth, and density into restoration plans.
Restoration of marsh pool habitat is an important
resource management objective but should proceed
with caution. Pools are just one of many different habitat types integrated within the salt marsh mosaic, and
all pool restoration plans must carefully consider the
overall effects on marsh development processes, vegetation dynamics, fish and wildlife support functions,
and other marsh processes and roles.
ACKNOWLEDGMENTS
This study was supported by the U. S. Geological
Survey, administered through the USGS Patuxent
Wildlife Research Center, Coastal Field Station at the
University of Rhode Island. Brandi Bornt and Susan
Wilson (URI Coastal Fellows), Mike Morrison, and
interns from the Rachel Carson National Wildlife Refuge provided invaluable assistance with fieldwork.
Ron Rozsa (Office of Long Island Sound Programs),
Robert Houston, and Rick Schauffler (U. S. Fish and
Wildlife Service) provided aerial imagery. Lynn Carlson (Brown University) assisted with geo-rectification
of several aerial photographs.
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Manuscript received 23 September 2002; revisions received 3 October 2003 and 12 November 2004; accepted 2 February 2005.