Effects of Shading from
Bridges on Estuarine
Wetlands
S.W. Broome and M. D. SanClements
N.C. State University
C. B. Craft and S. D. Struck
Indiana University
Rationale
• The effects of shading from bridges
of different heights on marsh
productivity is unknown
• If the effects can be quantified,
ecosystem damage and costly
mitigation can be minimized
Objective
• The overall objective of the
research project was to determine
the effects of shading from
bridges, which span salt or
brackish-water marshes, on
ecosystem structure and function.
Specific Objectives were to:
1. Evaluate the effects of height and width of bridges on
marsh structure and function including emergent
vegetation, soils, and benthic invertebrates.
2. Directly assess light attenuation by bridges using
sensors to measure photosynthetic photon flux
density under and near the bridges.
3. Compare the relative effects of shading on growth of
the dominant species of salt marshes, Spartina
alterniflora (a C4 plant), with the growth of the
dominant species in brackish marshes Juncus
roemerianus (a C3 plant).
Variables Measured
• Plant Biomass- Measure of primary
production
• Soil C and N- Measure of change in
nutrient pools
• Light- Direct measure of bridge
shading
• Infauna- Indicator of secondary
production
Materials and Methods
Sampling
Points
Bridge
Materials and Methods
•
•
•
•
Biomass
Soils
Light
Infauna
Locations of Bridge Study
Sites
Bogue Inlet
Height 19.8 m
Width 11.1 m
H/W Ratio 1.78
Spartina alterniflora
Ocean Isle
Height 15.2 m
Width 9.9 m
H/W Ratio 1.55
Spartina alterniflora
High transect
Height 14.7m
New River
Width 10.7
H/W Ratio High 1.36
Low transect
Height 7.3 m
Width 10.7
Spartina alterniflora
H/W Ratio 0.68
Salter Creek
Height 11.6 m
Width 10.7 m
H/W Ratio 1.08
Juncus roemerianus
New Bern
Height 8.5 m, Width 16.7 m
H/W Ratio 0.51
Spartina cynosuroides
Wrightsville Beach
Height 5.85 m
Width 20.82 m
H/W Ratio 0.28
Spartina alterniflora
Cedar Island
Height Range 2.9-15.5 m
Width 9.9 m
H/W Ratio 0.29-1.57
Spartina alterniflora,
S. cynosuroides, S. patens,
Juncus roemerianus
Light Measurements
W
sv
i
N lle
R
Lo
N
ew w
B
e
Sa rn
lte
r
N .C.
R
H
ig
h
O
ce
an
.
B
og I.
ue
.I.
rig
ht
% Light Under Bridge
120
25
100
20
80
40
15
60
10
20
5
0
0
Height (m)
Per cent Ambient Light Under Bridges
% Light
Height (m)
Ambient Light (%)
100
18
16
14
12
10
8
6
4
2
0
80
60
40
20
0
Sec.16
Sec.15
Height (m)
Sec.14
Sec.13
Sec.12
Sec.11
Sec.10
Sec.9
Sec.8
Sec.7
Sec.6
Sec.5
Sec.4
Sec.3
Sec.2
Sec.1
Percent Light
Height (m)
Per cent Ambient Light
Beneath Cedar Island Bridge
Effects of Shading on Above
and Below-Ground Biomass
Aboveground Biomass (g/m2)
rig
ht
W
25
20
*
15
10
5
0
Height (m)
*
sv
i
N lle
R
Lo
N
ew w
B
e
Sa rn
lte
r
N .C.
R
H
i
O gh
ce
an
.
B
og I.
ue
.I.
(Grams per m sq.)
1600
1400
1200
1000
800
600
400
200
0
Under
Outside
Height (m)
W
sv
ill
e
N
R
Lo
Sa w
l te
r.C
.
N
R
H
ig
h
O
ce
an
.I.
B
og
ue
.I.
rig
ht
Biomass (g/core)
16
14
12
10
8
6
4
2
0
*
20
*
15
10
5
0
Height (m)
Below-Ground Biomass
25
Under
Outside
Height (m)
Stem Heights
*
300
250
200
20
*
150
100
15
10
5
50
0
Sa
lte
r.C
N
.
R
Hi
gh
O
ce
an
.I.
B
og
ue
.I.
er
n
B
ew
N
N
R
Lo
w
0
ill
e
rig
ht
sv
W
25
Under
Outside
Height (m)
Effects of Shading on
Carbon and Nitrogen Pools
Soil Carbon
W
*
25
20
*
.
.
.I.
li le ow r.C igh n.I
e
u
sv R L lte R H cea
g
t
o
h
N
O
N
B
Sa
ir g
15
10
5
0
Height (m)
Soil Carbon (%)
7
6
5
4
3
2
1
0
Under
Outside
Height (m)
Soil Nitrogen
rig
ht
W
25
20
*
15
10
5
0
Height (m)
*
sv
ill
e
N
R
Lo
Sa w
lte
r.C
.
N
R
H
ig
h
O
ce
an
.I.
B
og
ue
.I.
Soil Nitrogen (%)
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Under
Outside
Height (m)
Effects of Shading on Infauna
Invertebrate Species Richness
Species Richness (# of Taxa / m2)
18
16
14
12
Under Bridge
*
Outside Bridge
*
10
25
8
20
6
15
4
10
2
5
0
0
Invertebrate Density (2000 Data)
250
*
Mean Invertebrate Density
(1000 organisms/m2)
200
Under Bridge
Outside Bridge
*
150
*
25
100
20
15
50
10
5
0
0
Cedar Island
2001 Invertebrate Density and Richness
14
200
C
150
C
c
Density
A
a
0
10
5
Bridge Height (m)
c
8
6
4
b
50
10
C
c
c
B
100
C
Richness
2
0
15
Taxa Richness
Natural Marsh
12
Samples
Bridge Samples
Evaluation of Marsh Plant Seedling
Growth in Pots Transplanted Under
the Cedar Island Bridge at Various
Bridge Heights
Cedar Island
Planting Layout
Bridge
•3 Species in each
randomized block (Spartina
alterniflora, s. patens,
Juncus roemerianus)
•3 Blocks per bridge
section
Plantings
•Replicated at varying
heights
Planting
Conclusions
• Taller bridges greatly reduce the negative effects
associated with shading.
• Lower and wider bridges reduce light to point that
marsh productivity is affected.
• In general, one lane bridges greater than 9 m (29.5 ft)
in height were found to have no effect on marsh
productivity
• The Height/Width ratio takes into account the two
main bridge characteristics that determine the degree
of shading.
• Bridges with Height/Width ratios greater than 0.7 had
little effect on marsh productivity.
• Stated simply, bridges with heights greater
than 70% of their widths have little effect on
marsh productivity.
• A conservative generalization might be that
when heights of bridges are equal to or greater
than widths, there will be no shading effects
on marsh ecosystems