This scope of work outlines those
tasks necessary to provide an estimate of impacts to tidal freshwater wetlands within the
Savannah National Wildlife Refuge that would result from the proposed deepening of the
Savannah Harbor. The goal of this scope of
work is to develop a prototype wetland impact prediction model that would be ready for use
by the end of 2000. To accomplish this goal
the following detailed scope of work has been developed as a cooperative effort between
the U.S Fish and Wildlife Service and Applied Technology and Management, Inc. The scope outlines a comprehensive study of those
physical and biological factors that determine the capacity for change within the overall
tidal wetland system, as well as the relative resource value of the various wetland types
within the system. The scope is provided in
three sections: (1) ATM and USFWS Activities,
(2) Model development, and (3) Resource utilization studies.
This task will provide for continued monitoring of all established vegetation study plots, including the 10 FWS transect areas (seven marsh sites, as per methods of Latham, 1991 and three tidal forest sites using point-centered quarter transect techniques) and the 8 quadrats established during the 1997 ATM study. Data on additional Rapid Assessment Plots will also be collected. Transects and quadrats will be sampled during both the early and late growing season. Given the plant community dynamics in these marshes, it is also necessary to establish an intensive monthly sampling regime at a representative transect in each of the 7 FWS marsh transect sites to document temporal changes and provide a perspective for the seasonal comprehensive samplings and timeline sequences for subsequent succession models.
In order to more directly define the relationship among the various community types within the gradient of marsh types comprising the Savannah National Wildlife Refuge and in-situ salinity regimes, it is necessary to conduct experimental reciprocal transplanting among the various communities and marsh types. This task is intended to translocate and transplant intact cores of plant/substrate units in such a fashion as to cross transplant within and between and among tidal marsh types (fresh, intermediate and brackish) while documenting subsequent vegetation responses, if any over time. In addition to confirming or clarifying correlational results of the other field studies, this technique was found to be extremely valuable to previous studies (Pearlstine et al. 1990) for providing time-lines for vegetation responses as well as indications of ecological community structural changes due to hydrologic alterations. These core plots will be designed and the spatial protocol implemented by March 2000. The sites will be monitored semi-annually as per the above task.
Except for the salinity monitoring conducted during
the summer of 1999, there has been an absence of any long-term salinity monitoring
subsequent to the removal of the time gate. However,
even the summer 1999 monitoring did not capture seasonal or annual salinity dynamics in
the marshes. Data regarding these dynamics
are critical to defining the ecological functioning of the marsh. This task will continue
the marsh salinity monitoring initiated during the summer of 1999 by permanently
installing a YSI 6000 XLM salinity meter in each of the 10 FWS permanent study plots.
Salinity in the marsh substrates is one of the
principal factors controlling the tidal wetland community structure. Meters in the ten
transect sites (above) provide ideal information on the temporal dynamics of salinities at
specific sites. To maximize the value of this
information it is necessary to complement this long-term spatially limited information (10
sites) with short-term, spatially intense synoptic sampling (approximately 100 sites) to
resolve spatial distribution of salinity across and up and down the floodplain gradient. This sampling would be conducted by grabbing
samples at pre-selected grid-point locations across the wetland complex from a roving
airboat outfitted with GPS navigation. It
would be scheduled to co-incide with the 2 major wetland plant samplings that correspond
to the early and late growth season time points.
This task will provide detailed data on hydrologic regimes within the specific vegetation associations identified by the mapping and vegetation monitoring tasks. Water level monitoring instrumentation will be installed in various locations to determine hydrologic regimes within each vegetation association. In addition, the instrumentation will be used to determine to what extent the floating vegetation mats rise and fall with the tide. The instrumentation will be moved to various locations for the duration of the project to cover the range of vegetation associations found within the project area. For subsequent use in the marsh succession model, the water level data will be related to marsh surface elevations as determined by GPS survey.
A key component of the spatial aspect of the marsh
succession model will be marsh topography. Survey
grade elevations will be made at each of the 10 FWS vegetation monitoring sites along the
floodplain. These marsh elevations will be
tied to the results of the water level study to define hydrologic signatures. The topographic survey will be conducted using GPS
survey equipment. A permanent GPS base
station control point has already been installed at a central location to facilitate the
marsh survey.
One of the critical issues of concern regarding marsh conversions is impact to the total production of seed and mast for wildlife. At this point in time no studies have comprehensively measured and compared the seed production along a gradient of marsh types spanning tidal fresh to tidal subsaline conditions in a river delta. It is important to document the total production of edible seeds and mast across the marsh gradients in order to assess impacts of marsh conversions to this critical wildlife value function of the Savannah marsh complex.
Study sites will be set up in major vegetative communities in each of the 3 marsh types (fresh, intermediate, and brackish). Within each site, key edible seed producing species will be monitored for seasonal phenological development. Individual plants of each of the selected species will be tagged and carefully monitored through the season for flowering event timing and intensity. Seeds will be carefully counted on the flowering heads of each individually tagged stem The intent is to monitor numbers and biomass of ripe seeds cumulatively produced over the season for each tagged stem. This is turn will be translated in to aerial production within the species stands and communities by using percent cover occupied by the key species in each type.
Also fundamental to any discussion of marsh succession is the availability and viability of seeds within the marsh sediments. This study will determine what seeds are found at what locations within the sediments of the study area and under what salinity conditions they will germinate and grow. This study will be conducted in a greenhouse using sediments collected from a number of locations within the study area. The study design will provide for salinity treatments ranging from fresh to brackish.
The tree community structure at various sites within the floodplain is a reflection of past and historic hydrologic conditions. Accordingly, this task will provide data that will be important for documenting major hydrologic events and trends in the past several decades. Two study methodologies will be used. First, a tree “gap” study will be conducted within the tidal forested transect sites previously established and monitored for breeding bird surveys. The intent is to document the relationship between tree canopy species and the sapling/seedlings in the various regeneration layers. This analysis will distinguish how present hydrologic regimes may compare to the previous conditions under which the canopy trees were established. These sites are unlikely to occur on the transects proper and will require finding tree-fall sites in the immediate vicinity of the transects or as proximal as feasible.
The second method to be utilized in the tidal swamp study will provide for a tree growth study. Seeds will be collected from selected tree species during fall 1999. In a greenhouse-based study, the collected seeds will be germinated and growth rates measured under various salinity treatments.
In addition to the sediment sampling initiated previously at each of the 10 FWS permanent study sites, a synoptic series of regularly spaced grid sites will be sampled seasonally along the upstream/downstream gradient. These data will include: bulk density, organic content, conductivity, total dissolved solids, extractable phosphorus, iron, sodium, ammonium and nitrate-nitrogen. The information will be used to create a substrate characterization set of GIS layers to define transition and breakpoint regions representing the spatial boundaries of marsh zones from fresh to subsaline conditions. This task is essential to developing a spatial characterization of marsh substrate types across the various gradients of hydroperiods, salinities, riverine inputs, and tidal influences (tidal subsidies). This information is essential to the implementation of spatially-based vegetation succession modeling.
The data to date strongly indicate that vegetation distribution is heavily influenced by sediment characteristics. Two primary sediment characteristics will be investigated in this task: (1) the locations of sediments that support floating vegetation mats, and (2) the locations of sediments that support the production of hydrogen sulfide gas or methane gas.
Some investigators hypothesize that floating vegetation mats are able to support relatively high plant diversities because, since they float, they are flooded less often than plants that are rooted into a firm substrate. The marshes of highest plant diversity within the study area exist as floating vegetation mats. This task will map the areas of floating mats for incorporation into the marsh succession model. Field mapping will be conducted in conjunction with collection of Rapid Assessment Plot data and other ongoing field work. Ground penetrating radar (GPR) will be used to assist in mapping substrate types in selected locations. GPR data will be verified by extraction of sediment cores.
Flooded sediments typically produce gases as a result of the decomposition of organic matter within the substrate. Sediments with an input of sulfate, such as from seawater during an incoming tide, will produce hydrogen sulfide gas. Sediments that receive no sulfate input typically produce methane. Hydrogen sulfide is potentially toxic to a number of wetland plant species and may be an additional factor limiting the distribution of plants within the study area. In addition, production of hydrogen sulfide or methane gas may be a very sensitive indicator of areas that are receiving inputs of saline water during incoming tides. This task will provide for collection of sediment samples at various locations throughout the study area. These samples will be analyzed in the lab for their potential for producing either hydrogen sulfide or methane gas.
This task will be conducted by the hydrodynamic modeling group and will provide data regarding historic water levels in the study area. It is anticipated these data will be generated by both compilation and review of historic records and through modeling. An estimate of long-term sea level rise over the past 150 years and a prediction of continued rise for the next 50 years are to be produced. Water level predictions are to be accompanied by estimates of changes in salinity. Specific factors to be considered in this task are water level changes and discharge volumes associated with the construction the upstream dams, the effect of the tide gate on water levels (including the residual effect of the remaining concrete structure), and tidal surge and post-storm river discharges resulting from Hurricane David in 1979.
This task will continue a study already initiated by the USFWS based on other sources of funding. The task will be based on analysis of aerial photography and satellite imagery to determine changes in vegetation signatures over time. A primary aspect of the analysis will focus on the changes in the forested shrub/scrub swamp located in the transition zone between the tidal forest and the tidal freshwater marsh. Some field observations have indicated that the shrub cover has been increasing in this area. Comparisons between historical imagery and current imagery will be made to confirm and quantitate this trend.