Measuring the resilience of salt marshes used in Living Shorelines and other nature-based efforts to protect

coastal infrastructureCarolyn Currin & Jenny DavisNOAA NCCOS, Beaufort, NC

NOAA Beaufort Lab Harkers Island, NC

• 50% of wave energy reduced within 5 m (15’) of marsh edge; >90% over 25 m of marsh (S. alterniflora)

• Belowground biomass binds sediments (and stores carbon)

• Wave energy reduction increases with plant biomass

• Linear Relationship between wave energy or wave power and marsh erosion over large scales, other factors important locally and regionally

• Wave energy reduction decreases as inundation depth exceeds canopy height

Salt marshes effectively attenuate wave energy and reduce erosion

Research reviewed in Currin et al. 2017 Response of salt marshes to wave energy provides guidance for successful living shoreline implementation.. In CRC Press The Science and Management of Nature-based Coastal Protection

Modified from: Cahoon, DR., J.W. Day, Jr., and D. J. Reed. 1999.

Oyster Reefs Can Keep Up with SLR in some settings

Worldwide 58% of salt marshes were adding elevation at rate > local SLR (Cahoon 2015)

Rodriguez et al (2014) showed NC oyster reefs can grow >1 cm yr-1

Cahoon, D. 2015 Estuaries & Coasts 38:1077-1084 ; Rodriguez, A. et al. 2014. Nature Climate Change DOI: 10.1038/NCLIMATE2216

Sediment supply is crucial parameter

Salt marshes and oyster reefs are resilient ….. and vulnerable… to sea level rise

Marsh transgression in response to SLRMove LANDWARD

• Landward transgression of salt marsh determined by topography and absence of development

• May preserve marsh habitat acreage even with accelerated SLR

Kirwan et al. 2016

New Marsh Drowned

Marsh

SLR

(a) Landward migration

Wave Energy, Cost, Permitting Time

Hard

enin

g

NWP 54-compliant LS

NOAA Defined LS

NNBF

Living Shorelines

Living Shoreline

• What are LS design impacts on resilience?• Does increasing resilience to SLR and erosion alter

ecosystem services provided by marsh habitats?

Measuring marsh elevation change in NC Living Shoreline Sites

Digital Elevation ModelsSurface Elevation Table

8 mGainLoss

10 m

0 NAVD88SET

•Surface elevation increase greater in Sill marshes than Natural at both upper and lower edges (p<0.025)

•Surface elevation change in Natural marshes significantly different at Upper marsh than Lower marsh edge

RTK GPS

4 Marsh-Sill and 4 Natural Fringing Marsh Sites

Date

1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 -40

-20

0

20

40

60

80

-40

-20

0

20

40

60

80

100

PINCMMPKSHI

Sill Lower

Sill Upper

Natural Lower

Date1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15

-140-120-100-80-60-40-20

020

Natural Upper

Surfa

ce e

levat

ion ch

ange

(mm

)

-140-120-100-80-60-40-20

020

PINCMMPKSHI

SET Results Fringing Salt Marshes Carteret County, NCSurface elevation change mm / year

Surface elevation change in Living Shorelines is dynamic

Long-term SET data collection is difficult to maintain

aa

b

b

Sill sediment accretion results in increased Spartina biomass at lower edge, loss of Spartina habitat at upper edge

Marsh vegetation in Living Shoreline Sites

(m) landward of shoreline

S. a

ltern

iflor

aBi

omas

s (g/

m) 2011

20 m510

15-1 0

20 m510

15-1 0

PLOT -1 m PLOT 0 m PLOT 5 m

Loss of vegetation at lower edgeMaintained interior vegetation

Increase in vegetation at lower edgeMaintained interior vegetation

Permanent Vegetation Plots 2006 -2016

Mean S. alterniflora Stem Density

Live

stem

s m-2

Marsh transgression in response to SLR

SLOPE

2 4 6 8 10 12 14-505

1015

Sea Level Rise Rate (mm yr-1)

Mar

sh e

xpan

sion

rate

(m y

r-1)

0.00050.0010.0021.0

Scenario with human barriers to

migration

Slope and SLR rate determine marsh area expansion

Kirwan et al. 2016 GRL

Habitat Changeleads toChanges in Ecosystem Services

Stone Sills

• Reduce/eliminate shallow subtidal

• Reflect wave energy• Non-native hard substrate;

Invasives

• Fish habitat• Oyster settlement• Increase sediment trapping

Low Marsh

• Less SLR resiliency• Lower plant diversity

• Absorb wave energy• Faunal utilization• Denitrification• Sediment trapping• C sequestration

High Marsh

• Less faunal utilization• Reduced denitrification• Reduced Sediment trapping• Lower C sequestration

• Greater SLR resiliency• Greater plant biodiversity

----

+++

New Marsh

Accumulated Sediment

SLR

New Marsh

Drowned Marsh

SLR

(a) Landward migration

Drowned Marsh

SLR

(b) Coastal squeeze

New Marsh

Accumulated Sediment

SLR

(c) Delayed squeeze

A longer view…

Using Living Shorelines to protect property and Infrastructure

Thin Layer Application of Dredged Sediment to Vulnerable Salt MarshesTwo pilot projects on Marine Corps Base Camp Lejeune, North Caroline

Raising elevation of low-lying salt marshDredged sediment added to ponded areas in fragmented marsh, Spartina

planted

Spartina alterniflora biomass : elevation distribution

Resilience to Sea Level Rise

Fishery Habitat, Denitrification,

Biom

ass

Elevation

Thin LayerAddition

Thin Layer application and Ecosystem Services

Will thin layer addition impact C burial?

Greater input?Greater turnover?

Can we quantifyhabitat trade offs?

Long-term Monitoring is needed to understand ecosystem impacts of Living Shorelines and TLA• Choose parameters wisely (easy, cheap, meaningful)• Use control and reference sites• Form partnerships• Use citizen scientists

Landward migration is crucial for maintaining future salt marsh habitat. • Living Shoreline placement and design need to accommodate this function• Avoid ‘Delayed Squeeze’

Adding marsh resiliency by increasing surface elevation alters habitats and ecosystem services• Be clear about temporal and spatial scales when measuring ecosystem service changes

SUMMARY