Materials and methods

SET theory

The Surface Elevation Table (SET), based on the design of Boumans and Day (1993), allows for precise measurements of soil surface elevation (± 1.4 mm total error; Cahoon et al. 2002a). The SET consists of a mechanical arm that is attached to a benchmark and leveled, establishing a fixed measuring point. Typically each SET has four fixed measurement locations (directions), where nine measuring pins are lowered to the soil surface to obtain a relative soil elevation. The elevation is the mean of 36 measuring pin readings per benchmark. SETs have been successfully used to monitor changes in elevation in a number of wetland environments (Cahoon et al.1999). They have been used to monitor mangrove vertical accretion and subsidence (Cahoon and Lynch 1997) and to follow the response of soil elevation to season (Childers et al. 1993), water management (Boumans and Day 1994, Hensel et al. 1999), vertebrate herbivores (Ford and Grace 1998) and hurricane disturbance (Cahoon et al. 2003).

New SET designs have recently been described that measure the change in soil elevation of specific parts of the soil profile (e.g. root zone, below the root zone; Cahoon et al. 2002b). At the Shark River, the shallow-rod surface elevation table (Shallow-RSET) benchmarks were installed to a depth that measures elevation change in the majority of the active root zone (top 0.35 m of the soil profile). The deep-rod surface elevation table (Deep-RSET) benchmarks were driven to bedrock and thus measure the full soil profile. The original design SET (Original-SET) benchmarks used by Smith and Cahoon (2003) were driven to ~ 4 m (Fig. 1). Further information on the design and accuracy of the original SET and RSET can be found in Cahoon et al. (2002a; b). By using a combination of SET designs at a single study site it is possible to partition changes in soil elevation among specific parts of the soil profile, such as the shallow root zone and deeper soil zones (Fig. 1). By determining the absolute change for each depth zone we can calculate expansion and contraction for each zone (surface [accretion/erosion; above 0 cm], shallow [active root; 0-0.35 m], middle [0.35-4 m], and bottom [4-6 m]) of the profile.

Site description

Figure 1. Profile of the substrate showing Original, Deep, and Shallow-RSETs, groundwater well and relative depth of each benchmark at Shark River mangrove site. (Adapted from Cahoon et al. 2002b with permission of the author). Drawing at 1:24 scale. [larger image]

Vegetation

The study site, SH3 of Smith and Cahoon (2003), is located near the mouth of the Shark River (Zone 17 N 2805254 mE. 0492112 mN., UTMs WGS 1984) in a mature mixed mangrove riverine forest comprised of Rhizophora mangle L. (red mangrove), Laguncularia racemosa (L.) Gaertn. (white mangrove), and Avicennia germinans (L.) Stearn (black mangrove). The site has a sparse understory. The canopy ranges in height from 13 to 17 m. The site has mixed tides. During the study period the Shark River had a daily average conductivity of 40 mS cm ^-1 and varied between a low of 25 mS cm ^-1 to a high of 51 mS cm ^-1. Shark River discharge was greatest at the end of the wet season, from September to November for 2002.

Soil profile

The soil profile of this site was determined from the well drilling log (G. Anderson, unpublished, Fig 1). The mangrove peat was 5.5 m in depth. The peat matrix lay directly on top of limestone, into which the well was drilled 1.8 m. The transition between the peat matrix and limestone was rapid. The limestone-peat interface was difficult to drill but had softer material below it. Otherwise, the entire peat layer was of similar constituency. No clay deposits were encountered during the drilling.

Cohen (1968) described the stratigraphy of the mangrove soil column at the mouth of the Little Shark River, a location approximately 2.5 kilometers away from SH3. He found that the mangrove peat was 3.81 m in depth and the total depth to bedrock at the site was 3.86 m. The peat types did not have recognizable petorgraphic constituents. All of the peat types were marine or brackish and dominated by R. mangle. There was a general increase of fine granular debris at the top and bottom of the profile. Fine granular debris comprised approximately 35 % of the sample at the top and the bottom of the core. At the top of the core it was suggested that an increase in fine-grained marine carbonates were responsible for this high number. The increase in fine granular debris at the bottom of the core may be due to greater amount of degradation of the organic constituents of the peat. Pyrite content was relatively high (2 % to 18%) throughout the core suggesting reducing conditions. Fusinite only occurred at the bottom of the core and comprised a small percentage of the constituents. There were no clays reported from this core.

Additionally, preliminary sampling of the mangrove peat hydraulic conductivity (at a site 4 km away) yielded relatively low values (hydraulic conductivity field saturation method (Guelph permeameter) = k_fs = 1.87 m day ^-1, see Hughes et al. 1998), which suggest slow water transmittance through the surface layer of the peat (Anderson et al. 2001).

SET installation

I installed three groups of SETs within 18 m of each other and 45 m of the Shark River. All groups were within 15 m of a United States Geological Survey (USGS) hydrological monitoring station (USGS station # 252149081044301, described below). Each group included one Shallow-RSET, one Original-SET, and one Deep-RSET along with four feldspar marker horizons (Cahoon and Turner 1989). The three Original-SETs, used in the Smith and Cahoon study (2003), were installed on July 16, 1998. Three Shallow-RSETs and three Deep-RSETs were installed on February 28, 2002 (Table 1). On March 18, 2002, four separate layers of feldspar (0.5 - 3 mm deep) were laid as marker horizons with each group for a total of twelve new marker horizons. Shallow-RSETs benchmarks were installed to a depth of 0.35 m. The original-SET benchmarks (76 mm (3") diameter aluminum pipe (1 mm thick wall) were driven approximately 4 m deep. The Deep-RSET benchmarks (1.43 cm (9/16") diameter stainless steel rods) were driven to approximately 6 m deep (Table 1). All SETs and feldspar markers were measured monthly from March 18, 2002 to March 21, 2003. Measurements were taken during low tide exposure on the same day. Two sampling events occurred with minimal water (a few puddles) present on the soil surface. On November 9, 2002 and on February 10, 2005 we surveyed the elevation of only the group number 3 Shallow-RSET, Original-SET, and Deep-RSET with standard survey methods (± 3 mm). There was no movement of the SET devices, over this period of 2 yrs 4 mos, in relation to an established benchmark, suggesting that the assumption of a stable datum (Childers et al. 1993, Cahoon et al. 1995, Cahoon and Lynch 1997, Cahoon et al. 2002b) was valid during the study (Table 1).

Hydrological data

The hydrological conditions investigated were (1) daily rate of change in groundwater piezometric pressure and (2) river stage. Groundwater head pressure was collected from a USGS station installed at the site in 1996 (Anderson and Smith 2005, Fig. 1). A piezometer recorded groundwater head pressure of the shallow coastal aquifer in a layer of limestone (hereafter referred to as groundwater). The 7.33 m piezometer consisted of threaded 7.62 cm diameter PVC pipe that was screened (0.20 slot PVC) from 5.7 m to 7.2 m depth. The slotted part of the well was entirely within the limestone. The well was sealed with "formation packer" at 5.5 m depth, the interface of the limestone and the peat layer to prevent vertical flow. Piezometric head pressure measurements were collected at hourly intervals. The pressure transducer was located at the depth of the well screen (further details see Anderson and Smith 2005).

Shark River stage data were obtained from the "Shark River" hydrological monitoring station of Everglades National Park located 2.37 km downstream from SH3. This station records tidal influences as well as seasonal changes in river discharge for the area. Tidal flooding occurred at the site when the Shark River stage was above 0.07 m (Fig 2b). Shark River stage data were collected hourly. The groundwater piezometric head pressure and the Shark River stage were reported in North American Vertical Datum (NAVD) 88 datum (Geiod 99) (Fig 2). Hourly Shark River stage and groundwater head pressure for the interval from December 13, 2002 to January 9, 2003 is included in Fig. 2b and 2d. I used daily averages of the above parameters in order to remove the diurnal tidal signal. The daily averaged signal of these parameters shows the monthly lunar influences on the tide (Provost 1973), annual change in sea level (Provost 1973), and the seasonal changes in water level due to regional wet season (Fig 2a and c). The hourly tidal signal was assumed to have minimal impact upon my SET measurements because elevation data were always collected at low tide. Sensor malfunction resulted in the loss of daily groundwater piezometric head pressure data from October 7, 2002, to November 8, 2002, an interval that included the October 10 SET sample measurement.

Figure 2. Hydrograph of (A) Daily averaged Shark River stage, (B) Hourly Shark River stage interval from December 13, 2002 to January 9, 2003, (C) Daily averaged groundwater piezometric head pressure and (D) Hourly groundwater [click on images above for larger version]