Sedimentation, Erosion, and Water

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22 Φεβ 2014 (πριν από 3 χρόνια και 4 μήνες)

68 εμφανίσεις

Nancy Rybicki, Edward Schenk

Support cast: Julie Baldizar, Al Lombana

Sedimentation, Erosion, and Water
Quality at a Seagrass Transplant Site and
Reference Site (Virginia)

Eelgrass (
Zostera marina
), Justin Reel, 2003


Background:


A SAV mitigation project was created due to the
anticipated, and actual, destruction of SAV beds
around the Woodrow Wilson Bridge (I
-
495)
construction project.



The consultant selected six sites in the lower
Potomac for mitigating the impact of the
construction project (PCC 2001). The sites were
selected using Fred Short’s seagrass transplant
model (Short et al. 2000), Nancy Rybicki’s
propagule flux study (methods: Rybicki et al.
2001), and avoidance of potential conflicting
use (e.g. oystering).


Seagrass variables under Short et al. model


Preliminary Transplant Suitability Index
(PTSI)


Historical SAV distribution


Current SAV distribution


Water depth


Water quality


Sediment


Proximity to natural bed


Shoreline configuration

USGS work

March to November, of 2003 and 2004, at both sites we measured habitat
variables monthly, including sedimentation and erosion, shoot length and
burial, salinity, and light attenuation.

We monitored one seagrass transplant site (Judith Sound) and a reference
site (Dameron Marsh), in VA. The transplant site was one of six SAV
transplant sites in the Potomac that have only been marginally successful
since transplanting began in 2000 (written communication, Justin Reel)

.


Methods



Small grid
construction


Four 9 m
2

grids at
each site.


Five long (75 m)
transects at Judith
Sound in 2004.





Surveying method

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Methods




Plant characteristics


Measured by a diver by extending a handful of shoots within each grid,
ignoring the tallest 20% of leaves, and measuring the height of the majority
of leaves. Plant burial was measured by diver from top of sand to rhizome
mat using a ruler.


Sediment characteristics


Sediment cores (10 cm deep, 4 cm diameter) were collected near the grids
and in the vegetation at both sites to determine particle size, percent
moisture, percent carbon, and phosphate and ammonium concentration
(n=4 per site and date).


Light characteristics


Simultaneously photosynthetically active radiation (PAR) measurements
were made just below the surface of the water at 0.05, 0.55, and 1m depth
with two 4


LiCOR® 185B underwater radiometers to determine light
attenuation and then percent light at one meter.


Water Quality


Salinity was measured using a handheld YSI unit at 0.3m water depth.




Date
6/03
8/03
10/03
12/03
2/04
4/04
6/04
8/04
10/04
Elevation (cm) relative to initial elevation
-24
-22
-12
-10
-8
-6
-4
-2
0
2
4
6
Plant height (cm)
0
10
20
30
40
50
60
Judith Sound (Exposure = East, Northeast)
Dameron Marsh (Exposure = South)
JS plant height
DM plant height
Judith Sound no eelgrass
Dameron Marsh no eelgrass
Accretion, erosion

Hurricane Isabel

Mean site elevation (n=36
per site and date, n=18 at
Judith Sound: 6/23/04)

Accretion, erosion (cont’d)

-60
-40
-20
0
20
40
60
80
100
120
0
20
40
60
80
A
B
C
D
E
Elevation (cm) relative to benchmark
Distance from Shore (m)
Transect line
Transect Data
May Elevation
June Elevation
July Elevation
Aug. Elevation
Sept. Elevation
(2004)

Seagrass

DM eelgrass
Apr 03
Jul 03
Oct 03
Jan 04
Apr 04
Jul 04
Oct 04
Height (cm)
0
10
20
30
40
50
60
70
Mean rhizome burial (cm)
Mean plant height (cm)
JS eelgrass
Apr 03
Jul 03
Oct 03
Jan 04
Apr 04
Jul 04
Oct 04
Height (cm)
0
10
20
30
40
50
60
70
Mean rhizome burial (cm)
Mean plant height (cm)
Eelgrass was stressed at both sites in 2003, this is indicated by the
absence of eelgrass at either site beyond July.

Sediment Nutrients

0
2
4
6
8
10
12
14
16
Spring 03
Spring 04
P205_P,
m
g g
-1
JS
DM
0
2
4
6
8
10
12
14
16
Spring 03
Spring 04
NH4_N,
m
g g
-1
JS
DM
Is nitrogen limiting growth?

Blue = Judith Sound (Transplant)

Red = Dameron Marsh (Reference)

Light

0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
6/03
8/03
10/03
12/03
2/04
4/04
6/04
8/04
10/04
Secchi depth (m)
Judith Sound
Dameron Marsh
SAV habitat requirement
6/03
8/03
10/03
12/03
2/04
4/04
6/04
8/04
10/04
% light at 1m depth
10
20
30
40
50
60
70
Judith Sound
Dameron Marsh
Habitat Requirement (>22% light)
Growing season median
percent light met the habitat
requirement at both locations
in 2004 (transplant site failed
in 2003).



The growing season median
percent light was lower in
2003 than in 2004.

2003
2004
Judith Sound
21%
28%
Dameron Marsh
23%
33%
Median percent light at 1 m
Date
1/2003
3/2003
5/2003
7/2003
9/2003
11/2003
1/2004
3/2004
5/2004
7/2004
9/2004
11/2004
Monthly Precipitation (cm)
0
5
10
15
20
30
Salinity (ppt)
0
5
10
15
20
25
30
35
40
45
50
55
60
52-yr. Average Precipitation (cm)
Actual Precipitation (cm)
Transplant Site Salinity
Reference Site Salinity
Eelgrass Lower Salinity Limit
Precipitation and salinity during 2003 and 2004 at the transplant site and
reference site. Precipitation data from Norfolk International Airport.


1)
Both sites were above the median 22 percent light level
in 2004.

2)
The percent of shoot that was buried was greater at the
transplant site (22 percent) than at the reference site (11
percent), although both percentages were below levels
shown to be detrimental to eelgrass survival (Vermaat et
al. 1997).

3)
Monitoring data support the conclusion that salinity not
erosion or accretion, were the primary factors
responsible for the decline of eelgrass at both sites.

4)
Low sediment fertility may also play a role in lack of
success at the eelgrass transplant site.

5)
The Dameron Marsh reference site was invaluable for
variable comparison with the transplant site.

Revising Short et al. site suitability index for
the Chesapeake Bay


Preliminary Transplant Suitability Index (PTSI)


Historical SAV distribution


Current SAV distribution


Water depth


Water quality


Sediment


Proximity to natural bed


Shoreline configuration


Future revisions and research needs


Salinity variation?


Include salinity variation versus target species


Have a ready supply of a diverse selection of transplant species tolerant
to local salinity fluctuations.


Sediment stability?


Sediment nutrients?


Susceptibility to high temperature, anoxic environment?


Water residence time, local hydrology



Monitoring some of these characteristics in healthy natural beds can
provide meaningful standards for determining transplant sites or
determining the source of transplant failures.

References

Batiuk, R., Heasley, P., Orth, R., Moore, K., Stevenson, J.C., Dennison, W., Staver, L.,
Carter, V., Rybicki, N.B., Hickman, R.E., Kollar, S., Bieber, S., and Bergstrom, P.,
1992, Chesapeake Bay submerged aquatic vegetation habitat requirements a
nd
restoration goals: a technical synthesis: U.S. EPA CBP/TRS 83/92.

Potomac Crossing Consultants, 2001, Woodrow Wilson Bridge Project: Identifying sites for
submerged aquatic vegetation, preliminary transplant suitability index: Report to
Potomac Crossing Consultants; Alexandria VA.

Cerco, C.F. and K. Moore. 2001. System
-
W
ide Submerged Aquatic


Vegetation Model for the Chesapeake Bay January, 2001. Chesapeake


Bay Program Office, Annapolis, MD.
http://www.chesapeakebay.net/modsc.htm

Rybicki N.B., McFarland D.G., Ruhl H.A., Reel J.T., and Barko J.W., 2001, Investigations
o
f the Availability and Survival of Submersed Aquatic Vegetation Propagules in the
Tidal Potomac River.
Estuaries
v. 24, 407
-
424.

Short F.T., Davis R.C., Kopp B.S., Short C.A., and Burdick D.M., 2002, Site
-
selection
model for optimal transplantation of eel
grass
Zostera marina
in the northeastern US.
Marine Ecology

Progress Series
227: 253
-
267.

Vermaat, J.E., Agawin, N.S.R., Durarte, C.M., Enriquez, S., Fortes, M.D.,

Marba, N., Uri,
J.S., and van Vierssen, W., 1997, The capacity of seagrasses to survive in
creased
turbidity and siltation: the significance of growth form and light use:
Ambio
, v. 26, p.
499
-
504.



Contact Information

Nancy Rybicki


Edward Schenk

703
-
648
-
5728


703
-
648
-
4746

nrybicki@usgs.gov



eschenk@usgs.gov