1. Project Description/Goals
The goal of this project is to understand and predict ecological and morphological
responses to sea-level rise. Development of a two-dimensional landscape model
for the Pamlico coastal wetlands is aimed at forecasting long-term changes
in plant community composition, sediment accretion and supply, and morphological
evolution in response to tidal forcing and sea-level rise.
Characterization of salt marsh macrophyte dynamics will help define the relationship
between inundation ratio and biomass, as well as competition among species.
Prediction of flow velocity is a surrogate for inundation time and advection
of sediments adjacent to tidal channels. Sedimentation patterns can be extracted
by documenting the role of turbulent energy and dissipation as a function of
biomass. The significance of drag from macrophyte stems, which effects the
rate at which water floods and drains salt marsh platforms, has yet to be appropriately
tested.
3. Background
Tidal marsh lands are sensitive to long-term climatic change, with survival
related to a balance between surface slope, sedimentation, and rates of sea
level rise. In general, the elevation of salt marsh platforms strongly influences
plant productivity, and this productivity has a positive feedback on the rate
of accretion of the marsh surface. Marsh surface vegetation, on the other hand,
controls flow (velocity and turbulence) within the canopy through drag and
enhanced sediment deposition, which ultimately determines change in elevation
(Fagherazzi et al., 2004).
Future changes along the marsh-terrestrial boundary are expected to be a consequence
of sea-level rise, and it is important to know how the geometry of the shoreline
will change as a result of intertidal marshes transitioning to subtidal habitat.
4. Research Design and Methods
This study will take advantage of existing long-term data, as well as establish
a comprehensive data set using field and experimental laboratory methods. See
milestone chart
5. Field Work
- Salt Marsh Macrophyte Dynamics
Biomass distribution on salt marsh
platforms is both directly and remotely observed with aerial photographs
and hyperspectral data. Establishment of several marsh planters provides
belowground production of Juncus in response to changes in relative elevation
and tidal forces.
- Sedimentation
Marsh elevation is monitored with the deep and shallow rod surface elevation
(SET), a portable, mechanical leveling device designed to attach to a benchmark
pipe driven into the ground (Cahoon & Turner, 1989). Vertical accretion
in fertilized and unfertilized marsh plots within the edge, middle, and interior
zones of the marsh platform are monitored with artificial soil marker horizons.
Sedimentation is being analyzed with radioisotope methods ( 210 Pb changes
and 137 Cs accumulations) within different plant communities and distances
from channel networks to determine subsidence due to sediment autocompaction,
constrain organic and inorganic deposition, and determine past rates of deposition.
Short-term (biannual) sedimentation and grain size is monitored using tile
plates for both Juncus and Spartina species at varying distances from the tidal
channel.
- Long-term and synoptic measurements of flow, suspended sediment and water-surface
elevation during ebb and flood cycles will be conducted over different stages
of growing seasons.
- Pressure transducers
Pressure transducers will record surface
water depths along transects within Spartina and Juncus .
At the completion of a growing season they will be moved to cover greater spatial
coverage, i.e. increased distance from the mouth of the estuary. The continuous
measurements will provide for a time-space data set that will for the first
time reveal the two-dimensional form of flooding-ebbing platform flow and associated
variations in water surface that drive this flow. Additionally, the rate of
flood-ebb propagation will be monitored over several tidal cycles to obtain
information about the spatial path of platform inundation.
- Suspended sediment concentrations (SSC)
Suspended sediment concentrations
(SSC) will be measured along transects parallel to flooding-ebbing flow away
from the tidal creek. SSC will be used to evaluate gravity-driven settling
and trapping by vegetation (which is necessary for the hydrodynamic-sedimentation
model).
- Flow velocity
Flow velocity measurements with an acoustic Doppler
velocimetry meter will be used to characterize turbulence intensities in
relation to vegetation structure. Such measurements will clarify the relationship
between sedimentation rates and suppression of turbulent kinetic energy by
vegetation.
- Field Work Products to Date:
- Water level recorders installed at both the Cedar Island and PKS aquarium
sites
- GPS with RTK the PKS aquarium site, June 2006
- Survey elevation of marsh platform at the PKS aquarium site, October 2006
- Collect sediment from sediment tiles from 1 st grain size analysis, October
2006
- Analyzed surface water data from July to October 2006 and created animation
of flood-ebb flow
- Dismantle and sample marsh plants, September 2006
- Model Formulation
The model that will be used to predict flow,
sedimentation, and vegetation dynamics in the Pamlico Sound is currently
under development, with several milestones having been achieved. The two-dimensional
flow routing algorithms are now functioning. We will calibrate the appropriate
drag coefficients and fluid flux laws that will govern flow on the marsh
by comparing model predictions with field measurements. In addition, the
model now has data structures in place that can incorporate arbitrary marsh
geometry; the geometry of the field sites will be incorporated into the
model upon processing of the GPS surveys recently collected in the field.
Over the next several months, we anticipate completion of the sedimentation
and biomass components of the model, as well as a module that tracks the
evolution of marsh stratigraphy.
- Expected products
The end products of this study will provide
a coupled physical and ecological model (1-D and 2-D) that demonstrate marsh
surface response to sea-level rise.
Empirical products will include:
- Above and belowground production of Juncus roemerianus and Spartina
alterniflora as a function of relative elevation (marsh organ experiments).
- Time series measurements of surface water elevations across the marsh
surface and subsequent drag coefficients.