Sentinel Sites

Overview

Grand Bay NERR’s Sentinel Site Program (SSP) is a system-wide effort to understand the effects of changing water levels and tidal dynamics on the composition and distribution of marsh plant communities. We are part of a larger network of reserves that are in various stages of implementing infrastructure and monitoring in order to be recognized as a “Sentinel Site”.  The Grand Bay SSP is based off of NOAA’s nation-wide Sentinel Site Program. More detailed information about how the two programs are integrated can be found in the NERR’s Sentinel Site Program Guidance Document.

Concern

Predicted increases in sea level will lead to more frequent and longer periods of marsh inundation. Tidal dynamics play a large role in shaping the estuarine ecosystem and shifts in inundation may have drastic effects on marsh habitats. Factors such as increasing storm frequency and intensity, local changes in marsh elevation due to subsidence and/or accretion, and sea level rise related to climate change affect water level and are expected to influence changes in plant communities throughout Grand Bay NERR. The transition of one marsh species to another may alter the marsh’s ability to withstand stress and decrease its capacity to absorb wave energy and provide a buffer from open water to upland areas and coastal communities. Marshes also act as a natural filtration system by removing nutrients from water draining into the estuary.  A loss of marsh habitat or change in marsh plant communities may alter an estuary’s capacity to absorb nutrients.  Estuaries provide many services to communities along the Gulf Coast where weather events and human activities can threaten the safety and livelihoods of people who call the coast home.  The main objectives of the Grand Bay NERR Sentinel Site Program are based on three general questions:

  1. What are current vegetation community distributions with respect to elevation and tidal range?
  2. How do these communities respond to long term changes in local water level and inundation patterns?
  3. How does sediment elevation respond to changes in water level and inundation patterns?

The field work and monitoring efforts described below contribute to our goal of understanding and detecting the effects of climate change on the Grand Bay NERR estuary.

Grand Bay NERR’s Vertical Control Network

Current horizontal and vertical positions were integrated into the components of Grand Bay’s survey control network in January 2013. Four new benchmarks were installed and the transfer of elevations was completed from US HWY 90 to the Grand Bay tide gauge using digital leveling. The Stewardship staff also conducted a 3 week GPS campaign with the help of Mississippi’s NOAA National Geodetic Survey state advisor to assign elevations to the Reserve’s 15 surface elevation tables (SETs) and other benchmarks.

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Detecting Accretion and Subsidence

 

Surface elevation tables consist of a steel rod driven far into the ground and equipped with a cemented attachment head for the measuring arm. The rod and concrete block ensure that the measuring head is stable and secure. Once the measuring arm is in place, pins are dropped down to the marsh surface and the height of each rod above the arm is measured. These measurements are taken quarterly and over time will be used to detect any increase (accretion) or decrease (subsidence) in the surface of the marsh. Since the SET measuring arm detects both accretion and subsidence, marker horizons are used to measure accretion only. This helps us determine the amount of accretion and/or subsidence happening at each site. Accretion is measured as the height of detritus and sediment accumulated on top of the white horizon layer.

 

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Creating Fine-Scale Elevation Maps

Reserve staff spent several months collecting highly accurate elevations at the sites surrounding our surface elevation tables. Using a Real-time Kinematic GPS receiver, staff assign coordinates and elevations to points in the marsh with less than a few inches of error. By repeating these measurements across the marsh at points in a pre-determined grid, we can use Geographic Information System (GIS) software to create a continuous topographic surface also called a digital elevation model (DEM) at each site. Combining this topographic surface with vegetation measurements and water level data, creates a detailed picture of marsh composition and distribution that will act as a baseline to compare changes to. Repeating elevation, vegetation, and water level measurements over time, will allow staff to model the long term viability of the marsh ecosystem in the face of rising water levels.

 

 

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Monitoring Marsh Communities

In addition to water depth recorded by our SWMP instruments (link to http://grandbaynerr.org/system-wide-monitoring), we also collect water depth data at locations close to our five SET sites.  This allows us to see if there are differences in the height and duration of tidal flooding among the sites. Currently, staff are establishing marsh monitoring plots within each SET site. The locations of the plots will be stratified by vegetation and determined with the help of the digital elevation models described above. Species percent cover will be estimated in a 1 m2 plot, with a 0.25 m2 portion subsampled for stem density. Biomass cores will be taken to determine above- and below-ground biomass. Water salinity at each vegetation plot will be measured from a PVC well inserted into the marsh sediment.

 

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