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. StruckIndiana 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
BridgeSampling Points
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
New RiverHigh transect
Height 14.7m
Width 10.7
H/W Ratio High 1.36
Low transect
Height 7.3 m
Width 10.7
H/W Ratio 0.68Spartina alterniflora
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 IslandHeight 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
Per cent Ambient Light Under Bridges
0
20
40
60
80
100
120
Wrig
htsvi
lle
NR Low
New B
ern
Salte
r.C.
NR Hig
h
Oce
an.I.
Bogue.I.
% L
igh
t U
nd
er B
rid
ge
0
5
10
15
20
25
Hei
gh
t (m
)
% Light
Height (m)
Per cent Ambient Light Beneath Cedar Island Bridge
0
20
40
60
80
100
Se
c.1
Se
c.2
Se
c.3
Se
c.4
Se
c.5
Se
c.6
Se
c.7
Se
c.8
Se
c.9
Se
c.1
0
Se
c.1
1
Se
c.1
2
Se
c.1
3
Se
c.1
4
Se
c.1
5
Se
c.1
6
Am
bie
nt
Lig
ht
(%)
024681012141618
Hei
gh
t (m
)
Percent Light Height (m)
Effects of Shading on Above and Below-Ground
Biomass
Aboveground Biomass (g/m2)
0200400600800
1000120014001600
Wrig
htsvi
lle
NR Low
New B
ern
Salte
r.C.
NR Hig
h
Oce
an.I.
Bogue.I.
(Gra
ms
per
m s
q.)
0
5
10
15
20
25
Hei
gh
t (m
)
Under
Outside
Height (m)
*
*
Below-Ground Biomass
02468
10121416
Wrig
htsvi
lle
NR Low
Salte
r.C.
NR Hig
h
Oce
an.I.
Bogue.I.
Bio
mas
s (g
/co
re)
0
5
10
15
20
25
Hei
gh
t (m
)
Under
Outside
Height (m)
*
*
Stem Heights
0
50
100
150
200
250
300
Wrig
htsvi
lle
NR Low
New B
ern
Salte
r.C.
NR Hig
h
Ocean
.I.
Bogue.I.
0
5
10
15
20
25
UnderOutsideHeight (m)
*
*
Effects of Shading on Carbon and Nitrogen
Pools
Soil Carbon
01
2345
67
Wrig
htsvi
lle
NR Low
Salte
r.C.
NR Hig
h
Oce
an.I.
Bogue.I.
So
il C
arb
on
(%
)
0
5
10
15
20
25
Hei
gh
t (m
)
Under
Outside
Height (m)*
*
Soil Nitrogen
00.050.1
0.150.2
0.250.3
0.350.4
0.45
Wrig
htsvi
lle
NR Low
Salte
r.C.
NR Hig
h
Ocean
.I.
Bogue.I.
So
il N
itro
gen
(%
)
0
5
10
15
20
25
Hei
gh
t (m
)
Under
Outside
Height (m)
*
*
Effects of Shading on Infauna
Spe
cies
Ric
hn
ess
(#
of T
axa
/ m
2 )S
peci
es R
ichn
ess
(#
of T
axa
/ m
2 )
0
2
4
6
8
10
12
14
16
18Under Bridge
Outside Bridge
Bridge Height
0
5
10
15
20
25
Invertebrate Species Richness Invertebrate Species Richness
Wrig
htsville
Beach
Wrig
htsville
Beach
New Rive
r -Low
New Rive
r -Low
New Bern
New Bern
Salter's
Creek
Salter's
Creek
New Rive
r -High
New Rive
r -High
Ocean Is
le
Ocean Is
le
Bogue Inlet
Bogue Inlet
Heig
ht
(m)
Heig
ht
(m)
*
*
Mea
n In
vert
ebra
te D
ensi
ty (
1000
org
anis
ms/
m2 )
Mea
n In
vert
ebra
te D
ensi
ty (
1000
org
anis
ms/
m2 )
0
50
100
150
200
0
5
10
15
20
25
Invertebrate Density (2000 Data)Invertebrate Density (2000 Data)250
Hei
ght
(m)
Hei
ght
(m)
Under Bridge
Outside Bridge
Bridge Height
Wrig
htsville
Beach
Wrig
htsville
Beach
New Rive
r -Low
New Rive
r -Low
New Bern
New Bern
Salter's
Creek
Salter's
Creek
New Rive
r -High
New Rive
r -High
Ocean Is
le
Ocean Is
le
Bogue Inlet
Bogue Inlet
**
*
0
50
100
150
200
0
2
4
6
8
10
12
14
Density
Richness A
B
CC
a
b
cc
C
c
C
c
5 10 15
Bridge SamplesBridge Samples Natural Marsh Samples
Natural Marsh Samples
Cedar Island 2001 Invertebrate Density and Richness
Cedar Island 2001 Invertebrate Density and Richness
Mea
n D
ensi
ty (
1000
org
anis
ms/
m2 )
Mea
n D
ensi
ty (
1000
org
anis
ms/
m2 )
Bridge Height (m)Bridge Height (m)
Tax
a R
ichn
ess
Tax
a R
ichn
ess
Evaluation of Marsh Plant Seedling Growth in Pots Transplanted Under the Cedar Island Bridge at Various Bridge Heights
BridgeCedar Island Planting Layout
•3 Species in each randomized block (Spartina alterniflora, s. patens, Juncus roemerianus)
•3 Blocks per bridge section
•Replicated at varying heights
Plantings
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