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