Possible Geomorphic EffectsOf Invasive Spartina alterniflora

in the San Francisco Estuary

Joshua N. Collins, Ph.D

San Francisco Estuary Institute

Josh@sfei.org

Acknowledgments

• US EPA, California Coastal Conservancy• Debra Smith, Katy Zaremba, Shannon Klohr

of the Spartina Project• Donna Morton, SFEI

Laurel Collins, Watershed Sciences• Debra Ayers and Donald Strong, UC Davis• Andrew Cohen, SFEI

Karl-Malamud-Roam, CCMAD

Train of Study Assumptions

1. Function follows form.

2. Form of the intertidal zone is a consequence of interactions between water supply and sediment supply, as mitigated by vascular vegetation.

3. Vascular vegetation is a major structural component of the intertidal zone of the Estuary.

Study Assumptions (cont’d)

4. The geo-ecological effects of plants vary with their architecture through space and over time.

5. NIS Spartina (S. alterniflora and hybrids) has different architecture than any intertidal native plants.

6. Dominance by NIS Spartina will alter the form and function of the intertidal zone.

Fundamental Study Questions

Where does NIS Spartina dominate the intertidal zone?

Based on its dominant location, what might be its effects on geomorphic form?

Given these effects on form, what might be effects on function (questions to answer)?

Synthesis of Study Assumptions and Questions

Interactions between water, sediment, and plants comprise a dynamic

physical template for geo-chemical and ecological processes.

Study Approach

• Map the distribution of NIS Spartina throughout the Estuary.

• Map its distribution relative to other dominant plants along salinity gradients.

• Map its vertical intertidal distribution.

Regional Distribution and Abundance

Regional Abundance and Distribution of

NIS Spartina by Patch Size Class

Distribution Relative toAqueous Salinity:Basic Methods

• Choose local creeks near verified NOS benchmarks.

• Conduct surveys of minimum elevations of NIS Spartina and co-dominant plants throughout local salinity gradient.

• Verify genotype for each NIS Spartina patch surveyed.

Distribution of Low Marsh Plants Alameda Flood Control Channel

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

-500 0 500 1000 1500 2000 2500 3000 3500

Distance Upstream (ft)

Tid

al E

lev

atio

n (

ft b

elo

w L

oca

l M

HW

)

Spartina Hybrids Spartina foliosa Scirpus robustus

Scirpus acutus/californicus Typha sp.

Salicornia

Scirpus robustus

Atriplex Scirpus acutus/californicus

Typha

"Saline" "Brackish" "Fresh"

Distribution of Low Marsh PlantsSan Leandro Creek

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Distance Upstream (ft)

Tid

al E

leva

tion

(ft b

elow

Loc

al M

HW

)

Spartina Hybrids Spartina foliosa Scirpus acutus/californicus Scirpus robustus Typha sp

Salicornia

Scirpus robustus

Distichlis

Scirpus acutus/californicus

Typha

"Brackish" "Fresh"

"Saline"

Extent of aggradation upstream of

cement apron

Degree of Hybridization in Relation to Salinity Regime as Indicated by Distance Downstream from Head of Tide

R2 = 0.39

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000 3500 4000

Distance from Head of Tide (ft)

Deg

ree

of

Hy

bri

diz

atio

n

(% a

lter

nif

lora

)

Hybrids Spartina foliosa Spartina alterniflora Linear (Hybridization Trend)

“Fresh” “Brackish” “Saline”

Minimum elevations of NIS Spartina relative to MHW for fresh-brackish tidal reaches of

three creeks of South San Francisco Bay

SiteElevation Relative to

Local MHW (ft)Colma Creek

n = 23 -4.24 0.9

Alameda Flood Channel n = 14 -3.85 0.7

San Leandro Creekn = 13 -3.31 0.6

Distribution vs Aqueous Salinity:Basic Results

• Minimum elevation of NIS Spartina intersects creek bed at about Mean Tide Level.

• NIS Spartina grows lower than other plants throughout salinity gradient.

• Pure S. alterniflora is restricted to fresher conditions near the head of tide.

Vertical Distributionin Saline Intertidal:

Basic Methods

• Choose sites with verified NIS Spartina and NOS benchmarks.

• Map distribution of NIS Spartina relative to intertidal physiography.

• Randomly survey minimum elevations of NIS Spartina.

Elevations of NIS SpartinaSan

Leandro Channel

Arrow- head

Marsh

Coyote Point

Marsh

Coyote Hills

Slough

Mean maximum elevation (ft) relative to local

MHHWn = 30

- 0.25 0.3

- 0.44

0.2

- 0.93 0.4

- 0.36

0.4

Mean maximum elevation (ft) relative to maximum elevation of native marsh plants (Grendelia zone)

n = 30

- 0.87 0.4

- 0.91

0.4

- 1.46

0.4

- 1.41 0.5

Mean minimum elevation relative to local MLLW

n = 30

+ 2.68

0.3

+ 2.89

0.3

+ 3.10 0.3

+ 3.26

0.4

Plan Form Details of Distribution

Distribution of NIS Spartina within Arrowhead Marsh

Arrowhead Marsh IR Photo

Arrowhead Marsh IR Photo

Plan Form Details:Basic Results

• Due to channel cross-section form and preferred elevation range, NIS Spartina colonizes channel beds of existing saline tidal marshes mainly in middle reaches of the drainage networks.

• Colonization of an existing marsh by NIS Spartina disconnects headward channels from the rest of the marsh drainage network.

Saline DistributionRelative to Tidal Datums

Mean Minimum Elevation of NIS SpartinaRelated to Mean Tide Range

y = 0.59x - 0.18R2 = 0.81

2.5

2.75

3

3.25

3.5

4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7

Local Mean Tide Range (ft) (MHW - MLW)

Mea

n M

inim

um E

leva

tion

(f

t abo

ve M

LL

W)

Vertical Distribution of NIS Spartina Relative to Tidal Datums and Distance South of Golden Gate

-6

-4

-2

0

2

4

6

Lattitude of NOS Tide Stations

Ele

vatio

n R

elat

ive

to L

ocal

M

ean

Tid

e L

evel

(ft)

Mean High Water Mean Lower Low Water Predicted Minimum Elevation of NIS Spartina

Predicted Minimum Elevation of NIS Spartina Relative to Cumulative Duration of Indundation For Three Tide Stations In San Francisco Bay

0

10

20

30

40

50

60

70

80

90

100-1

.25

-0.7

5

-0.2

5

0.2

5

0.7

5

1.2

5

1.7

5

2.2

5

2.7

5

3.2

5

3.7

5

4.2

5

4.7

5

5.2

5

5.7

5

6.2

5

6.7

5

7.2

5

7.7

5

8.2

5

8.7

5

9.2

5

9.7

5

10

.2

Mid-point of 0.5 ft Height Class

Cu

mu

lati

ve

Du

rati

on

of

Inu

nd

ati

on

(p

erce

nt)

Alameda MTR = 4.43 ft Redwood City MTR = 5.87 ft

Dumbarton Bridge MTR = 6.1 ft

Conclusions

The vertical distribution of NIS Spartina relates to the duration of tidal inundation, or conversely, the duration of exposure above the tides.

Conclusions

NIS cordgrass is likely to invade the upper half of the saline tidal flats and will tend to invade a smaller proportion of the tidal flats in Far South Bay than in South Bay or Central Bay, but plant evolution may change the rules.

Conclusions

NIS cordgrass will probably not dominate the saline high marsh above MHW, but plant evolution may change the rules.

Conclusions

The invasion of existing saline marshes will tend to isolate the headward reaches of their channel networks.

Conclusions

NIS cordgrass can cause the headward channels of marsh drainage systems to retrogress, thus shortening and simplifying intertidal channel networks and the shoreline of the Estuary as a whole.

Conclusions

NIS cordgrass can obstruct tidal flow and upland runoff in the upper tidal reaches of fluvial drainages.

Conclusions

NIS Spartina tolerates fresher conditions than native Spartina, grows lower upstream than downstream in local creeks, and grows lower than other intertidal plants along local salinity gradients.

Conclusions

The upper tidal reaches of local streams can serve as refugia for non-hybrid S. alterniflora and as sources of new recruits for continued invasion around San Francisco Bay.

Major Questions

How low will NIS Spartina grow in Suisun and perhaps the western Delta?

Will NIS Spartina meadows evolve into high marsh with native plants?

How will native plants and animals adjust to the Invasion by NIS cordgrass?

How will the simplification of the intertidal drainage system affect estuarine filtration and material flux?