Metadata

Description
This study was conducted in Everglades National Park, Florida, USA along the southeastern boundary of Shark River Slough, the largest drainage basin in the southern Everglades. The coastal Everglades range along a gradient from freshwater sawgrass ridges and sloughs to coastal mangrove forests. We chose two sites for our study: a brackish marsh that was already experiencing saltwater intrusion and a freshwater marsh that, to our knowledge, had not experienced elevated salinity. The brackish marsh (25°13’13.17” N, 80°50’36.96” W) was dominated by Cladium jamaicense (sawgrass) sparsely interspersed with Conocarpus erectus (buttonwood). The site was non-tidal and characterized by distinct wet-dry hydrologic regimes in which the site was flooded for ~8 months out of the year (mean since 2000, Everglades Depth Estimation Network (EDEN) at station NMP). The freshwater marsh (25°26’07.77” N, 80°46’51.50” W) was co-dominated by sawgrass and Eleocharis cellulosa (spikerush) but also contained other freshwater marsh plants such as Crinum americanum (swamp lily), Bacopa caroliniana (waterhyssop), and Panicum hemitomon (maidencane). The hydrologic regime at the site was characterized as long-hydroperi od, flooded nearly year-round (~11 months, mean since 2000, EDEN at station NP62) during a typical season. The soil properties of each site are given in Table 1.

Instrumentation
LI-COR 840

Description
In September 2014, 16 plots were established at each site along an 80-m long constructed boardwalk (Fig. 1). In twelve plots, we installed 1.4-m diameter, 0.4-m tall clear, cylindrical, polycarbonate chambers by inserting them 30-cm into the soil. We designated 4 additional plots as “no-chamber” controls, and these had no chamber installed around them. Each chamber had a movable collar with a series of 10-cm diameter holes that could be closed during application of dosing water but were open to natural flow at all other times. Six ambient-water addition (“+AMB”) plots were established upstream in the natural flow, while 6 treatment (+saltwater, “+SALT”) plots were established downstream to avoid salt contamination into the +AMB and “no-chamber” control plots. The 4 “no-chamber” controls, which were interspersed within the +AMB plots (Fig. 1), did not receive any water additions and were used only for C flux and redox potential measurements (see below). A 3-m “buffer zone” was established to avoid contamination between salt-dosing and control plots.

Instrumentation
HP 5890 Gas Chromatograph

Description
Experimental water additions began in October 2014 and were conducted monthly for 2 years (see Stachelek et al. (2018) for detailed methods). The volume and salinity of brine solution mixed to deliver our dose varied for each dosing month in order to reach porewater concentration targets. The volume and salinity of the brine solution was calculated based on both water height from soil surface and surface water salinity so that we could reach the target of twice ambient porewater salinity, 2-5 ppt at the FW site and 20 ppt at the BW site. Our brine solution during dosing ranged from 30.7-65.0 ppt at the FW site and 26.8-68.0 ppt at the BW site (Stachelek et al. 2018). The dosing solution was prepared using source water obtained at each study site (when the marsh was wet) or from a nearby canal (when the marsh was dry) with similar nutrient concentrations found in freshwater wetlands of the Everglades (C-111; 25°17'31.74" N, 80°27'21.59" W; Wilson et al. in review); source water was combined with a commercially available sea salt mix (Instant Ocean ® (Atkinson and Bingman 1997)). An equal volume of site surface water or canal water was added to the +AMB plots each month to account for the addition of water in the absence of salinity.

Description
The movable collar on the chambers was used to close the chambers while dosing to ensure that the dosing water remained within the chamber. Doses were delivered from elevated boardwalks running alongside each chamber using a submersible bilge-style pump (Xylem Inc, USA). The outlet hose was fitted with a spreader device that split the large output stream into six smaller streams. This design was intended to maximize mixing with ambient site water while minimizing disturbance to sensitive benthic periphyton. Emergent plants were briefly sprayed with freshwater following dosing to avoid potential damage from direct salt application. Chambers remained closed for 24 hours to allow the elevated-salinity water to penetrate into the porewater, then chambers were opened to prevent closure artifacts.

Description
Within each plot, one 0.5 x 0.5 m polycarbonate collar was permanently installed 10-cm into the soil and extended 5-cm above the soil surface for ecosystem C flux measurements. Each collar had eight 2.5-cm diameter holes at the soil surface level to allow for natural flow of water when measurements were not occurring. Plot-scale CO2 exchange was measured monthly using a transparent static chamber (0.25 m2 x 1.5 m; after Neubauer et al. 2000, Wilson et al. 2015). Prior to measurements being taken, each hole in the collar was plugged with a rubber stopper, the chamber was placed into a lip in the collar and sealed, and the system was allowed to equilibrate for 2 minutes. Measurements were then made for 3 minutes each in full light and in the dark, with the chamber lid removed in between each measurement to allow the chamber to return to atmospheric conditions (LI-COR 840, Lincoln, NE; Wilson et al. in press). All measurements made at each site were taken within ± 3 hours of solar noon and on the same day. Missing measurements occurred when there was either equipment failure (BW site, Jul – Aug 2016) or when water levels were higher than the boardwalk (FW site, Dec 2015 – Apr 2016), limiting access to the plots.

Description
Methane exchange measurements were conducted monthly from October 2014 – February 2016. After the dark CO2 exchange measurement was conducted, the chamber was kept in the dark and resealed for 20 minutes. Air from the chamber was continually pumped through a closed loop with a sampling port attached. Gas samples were taken at 0, 10, and 20 minutes using a 60-mL syringe to withdraw 25-mL of air from the sampling port placed in line with the chamber. The sample was then injected into a 20-mL evacuated vial. Methane concentrations were determined using a gas chromatograph (Hewlett-Packard 5890, Palo Alto, CA, USA), and the change in concentration over time was used to calculate the flux.