Abstract

Geographic Information System (GIS) spatial analysis methods was used to evaluate salinity variations found in peat soil across the mangrove-marsh interface (ecotone) adjacent to the Harney River estuary, Everglades National Park from April 1997 through March 2003. Water specific conductance (mS/cm) and temperature were sampled periodically (typically bi-weekly) for six years (n=170). Across a 300 m fringe mangrove N-S forest transect soil pore water was sampled at 30 cm and 60 cm depths at five (SW1...SW5) equally spaced monitoring sites. Each of the five sampling sites have three replicate sampling tubes for 30 cm and 60 cm. Surface water was included (when present) in the sample collection. Each sample data period was spatially analyzed (ArcView, ESRI Inc) by developing grid contours using an Inverse Distance Weighted (IDW) algorithm. Included in the contour grid are ancillary salinity data from adjacent Harney River (HR), Shark 4 and Shark 5 (shallow groundwater wells) monitoring stations. Seasonal variations of water salinity were observed a function of tidal exchange, storm events, local rain fall and overland freshwater flows. Spatial salinity contours provided a synoptic picture of the salinity gradients across the horizontal gradient and vertical profile. Over the study period, we observed the near river sites had the greatest maximum aggregate salinity's (30-35 PSU), both at the depth of 30 cm and 60 cm, while the interior sites indicated lower aggregate salinity values (14-16 PSU). The data also indicated a 1-2 month time lag at the outset of the wet season between the 30 cm and 60 cm salinity at all sites.

Hydrodynamics of the tidal cycle, freshwater inflows and local rainfall are the principal physical forces that determine the marsh-mangrove interface or ecotone in the southwestern coastal region of Everglades National Park

Study evaluates salinity (mS/cm) spatial and temporal variations across the mangrove-marsh ecotone near the Harney River estuary. Temperature data was found significantly less variable and not used for this analysis (avg SW1 30 cm, 26.4 C high; SW3 30 cm, 24.54 C)

In April 1997, soil pore water salinity and temperature bi-weekly sampling was started. Periodic sampling is still on-going.

Samples were collected along the 250 m boardwalk transect, located near the Harney River.

Study has a sampling design of five pore water sample sites: SW1, SW2, SW3, SW4 and SW5, regularly spaced from the river across the ecotone (figure 1).

To assist with sampling design, each soil pore water sampling site has three replicate tubes at both 30 cm and 60 cm depths (figure 2).

Ancillary salinity data from HR (NPS), SH4 and SH5 (USGS) gages. Rain data from HR. (permanent adjacent monitoring gages).

Figure 1. Study Area [larger image]

Figure 2. [larger image]

Typical pore water sample site

At each soil pore water site, three water samples at 30 cm and 60 cm were extracted and tested for salinity and temperature.

The sample site replicate salinity samples were aggregated in day site sample.

Data was subdivided into four salinity data groups: Surface water (SW), 30 cm (30) and 60 cm (60), and groundwater (GW). For each sampling day sample, four input text files were created. The input text files in order of data fields: HR, SW1, SH4, SW2, SW3, SW4, SW5 and SH5. Each field salinity day sample is spatially represented by UTM coordinates.

The maximum possible sample fields for surface water is eight; six for 30 cm and 60 cm and three for groundwater. There were six years of sample day periods (n=170) for each four sampling subgroups (total n=680 sample files). Sampling subgroup files were imported into GIS software (Arc View ESRI, Inc) and each file was converted into grid contour file utilizing an Inverse Distance Weighting (IDW) algorithm.

Contour grid boundaries were set at the riverbank and excluded the river from the transect contouring (except for surface water). The width of the contour was set by the actual transect width to create a narrow contoured corridor. After the contour were created, they were exported in a .jpeg format files to create a graphical animation format

Chart 1: Correlation of Harney River salinity with sample sites along mangrove-marsh transect [larger image]

Table 1: Summary sample statistics [larger image]

Chart 2: Monthly sample salinity (mS/cm) and Rainfall (HR) [larger image]

Chart 3: Soil Salinity (mS/cm) change at soil depths of 30 cm and 60 cm, April to October 2002. A. chart of soil water fluctuation; B. Horizontal salinity transect contour at 30 cm soil depth; C. Horizontal salinity transect at 60 cm soil depth. [click on each of the images below to view a larger version]
A.	SW1	SW2	SW3	SW4	SW5
	April 2002	June 2002	July 2002	August 2002	October 2002
B. 30 cm
C. 60 cm
	SW1 30/60 r2=0.81	SW2 30/60 r2=0.65	SW3 30/60 r2=0.54	SW4 30/60 r2=0.57	SW5 30/60 r2=0.37

Seasonal rainfall is the principal physical forcing function to moderate soil salinity. Yearly annual rainfall exhibits a bi-seasonal weather pattern (wet = June-October; dry = November to May). Yearly average rainfall for study area (Apr-Mar) was 1587 mm (Y99-00:max 2020 mm; Y98-99: min 1292 mm). June and September had four largest monthly rainfall totals (n=72) over six years (Sept 99 recorded 571 mm) and the greatest monthly impact on soil pore water salinity.

Secondary physical factors include: tidal inundation (SW1 and SW2 most directly impacted, especially at spring tides); overland interior freshwater flow and drainage; soil type-red mangrove peat (SW1-SW4 3 m and SW5 sawgrass, 2.5 muck-mangrove peat) permeability (Hydraulic k: SW1 least and SW5 greatest), evapo-transpiration rate; stochastic events (T.S. Harvey Sept 1999); localized topography (slight berm between SW2 – SW3 and low areas between SW3-SW4 and SW5; figure 3).

Figure 3. Harney River Transect Elevation Profile [larger image]

Salinity Variability was observed at 2-D inter-site horizontal profile (SW1-SW5 transect), and 3-D intra-site vertical profile (0 cm, 30 cm, and 60 cm soil pore water).

Averaged site Salinity and variability decreases from the Harney River to coast marsh (Chart 1). Surface water salinity is strongly correlated with the Harney River at all sites. 30 cm soil pore water correlates at until SW3 and breaks down; 60 cm pore water poorly correlates with Harney River at all sites. (x<0.5). Highest average salinity at site SW1 30 cm (14.1 mS) and SW1 60 cm (14.2 mS) and lowest average salinity at SW4 cm (7.9 mS) and SW4 cm (8.3 mS). (Table 1).

SW5 shows largest average salinity difference between 30 cm and 60 cm (2.2 mS; SW1 0.02 mS). SW1, SW2 and SW3 (SW4 and SW5 less variable) show late spring separation between 30 cm and 60 cm. 30 cm salinities jumps sharply (April or May) and then falls sharply with the wet season rain pulse in June or July. 60 cm soil water salinity also rises in late spring, but it is dampened Chart 3-A. Salinities equalize in July and August (1-2 month lag period), then 30 cm salinity continue to drop through the wet season at a greater slope than the 60 cm salinity. In October-November salinities are roughly equalized again until next year's late spring (Chart 3-B). Scatter plots (Chart 3-C) indicate at each site, salinity between 30 cm/60 cm to have a decreased relationship across the transect from the river. (SW1 r²= 0.81; SW5 r²=0.37)

Study has collected six years of salinity and temperature data and has determined principal seasonal and site variability. Important to utilize these data with needs of Everglades science and restoration purposes and coastal estuary studies. Current Everglades projects that can benefit from pore water salinity study are mangrove soil water quality study (see Ilami et al poster); mangrove vegetation study (Smith); Hydraulic soil conductivity study (Anderson); LTER-Everglades related studies. (Twilley et al).

Anderson, G.H., S. Cleaves and T.J. Smith III. 1997. Horizontal Surface and Soil Water Salinity gradients across the mangrove/marsh ecotone. First Annual Conference of the Walt Dineen Society, Miami, Fl., 22-24 May 1997.

Harvey, J.W., and W.K. Nuttle 1995. Fluxes of water and solute in a coastal wetland sediment. 2. Effect of Macropores on solute exchange with surface water. Journal of Hydrology, 164, 109-125

Smith III, T.J., S. Cleaves, K. Whelan, and G. Anderson. 1998. Spatial and Temporal Patterns of Sediment Pore water Conductivity Variation Across a Mangrove Forest- Sawgrass Ecotone in the Everglades of South Florida 1998 AGU Spring Meeting: Supplement Publication of 28 April 1998 EOS. Boston MA., 26-29 May 1998.

Anderson, G.H., A.V. de Lockant, C. Walker, T.J. Smith III and T. Mullins. 2000. A Spatial Analysis of Seasonal Surface, Soil and Groundwater Salinity Variations from April 1997 to April 2000 Across the Coastal Mangrove-Freshwater Marsh Ecotone, near the Harney River in Everglades National Park. 2000 GEER Conference Naples, Fl. 11-15 Dec, 2000.

Anderson, G.H., T.J. Smith III. 2002. Hydraulic Conductivity of Riparian Mangrove Forest Peat Adjacent to the Harney River, Everglades National Park: A Comparative Field Study of Field Saturated and Saturated Hydraulic Conductivity Methods. 2002 AGU Spring Meeting. Washington D.C. 28-31 May, 2002

This poster was presented at the 2003 Estuarine Research Federation Conference in Seattle, October 2003. Financial support for this USGS research study was provided the USGS' Place-Based Studies and Global Climate Change Programs. Base funds were provided by the USGS Florida Integrated Science Centers - Center for Water and Restoration Studies. Field assistance was principally provided by by Christa Walker, Kevin Whelan, and other brave souls. Special recognition is given to Stephanie Cleaves for her initial development of the pore water sample sites. Ancillary ENP data was provided by Kevin Kotun and Stephanie Beeler. Technical help was provided by Troy Mullins (GIS script development).

The use of firm, trade and brand names in this poster presentation is for identification purposes only and does not constitute endorsement by the U.S. Government.

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Related information:

SOFIA Project: Dynamics of Land Margin Ecosystems: Historical Change, Hydrology, Vegetation, Sediment, and Climate