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
Surface water temperature and salinity were measured immediately in the field (YSI). Soluble reactive phosphorus (SRP) and total dissolved P (TDP) were analyzed at the South Florida Water Management District Analytical Research Laboratory on an Alpkem Flow Solution Analyzer (OI Analytical, College Station, TX, USA) following Standard Method 4500-P F (SRP) and Solorzano and Sharp (1980, TDP). Ammonium (NH4+), and dissolved inorganic N (DIN) were analyzed at the South Florida Water Management District Water Quality Laboratory on a Lachat Flow Injection Analyzer (Lachat Instruments, Loveland, CO, USA) following Standard Method 4500-NH3 H (NH4+) or Standard Method 4500-N C (DIN). Dissolved organic C (DOC) was analyzed using a Shimadzu TOC-L analyzer (Shimadzu Scientific Instruments, Columbia, MD, USA) following Standard Method 5310 B. Alkalinity and pH were determined using an automated titrator (Metrohm 855 Titrator, Herisau, Switzerland) following Standard Method 2320 B (Alkalinity) and a modification to Standard Method 4500 H+ B (pH). Chloride (Cl-) and sulfate (SO42-) were measured using a Metrohm 881 Compact IC Pro System (Metrohm, Riverview, FL, USA) following Standard Method 4110 B. Sulfide (HS-) was measured using standard methods (McKee et al. 1988).

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.

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
Porewater salinity and nutrient measurements were made from three sampling wells (“sippers”) placed randomly inside each chamber to a depth of 15-cm. Porewater salinity was measured 24 and 120 hours after dosing. Two sippers were installed 0.5-m outside the edge of each chamber to monitor any potential leakage of saltwater outside of the treatment plots. Samples for nutrient and carbon analyses were collected 24 h after dosing. From each sipper, a ~25-mL sample was extracted after purging the length of tubing, and temperature and salinity were measured immediately in the field (YSI Model 600 XL, Yellow Springs, OH). The porewater from each of the three wells was then combined into one sample per chamber (~75mL total), field-filtered (0.7 mm GF/F), transferred to new, single-use bottles, stored at 20°C, and analyzed within 21 d.

Description
Surface water salinity was collected from each plot during wet periods by collecting 140-mL of sample water and processing the same as porewater. Surface water temperature and salinity were measured immediately in the field (YSI). Soluble reactive phosphorus (SRP) and total dissolved P (TDP) were analyzed at the South Florida Water Management District Analytical Research Laboratory on an Alpkem Flow Solution Analyzer (OI Analytical, College Station, TX, USA) following Standard Method 4500-P F (SRP) and Solorzano and Sharp (1980, TDP). Ammonium (NH4+), and dissolved inorganic N (DIN) were analyzed at the South Florida Water Management District Water Quality Laboratory on a Lachat Flow Injection Analyzer (Lachat Instruments, Loveland, CO, USA) following Standard Method 4500-NH3 H (NH4+) or Standard Method 4500-N C (DIN). Dissolved organic C (DOC) was analyzed using a Shimadzu TOC-L analyzer (Shimadzu Scientific Instruments, Columbia, MD, USA) following Standard Method 5310 B. Alkalinity and pH were determined using an automated titrator (Metrohm 855 Titrator, Herisau, Switzerland) following Standard Method 2320 B (Alkalinity) and a modification to Standard Method 4500 H+ B (pH). Chloride (Cl-) and sulfate (SO42-) were measured using a Metrohm 881 Compact IC Pro System (Metrohm, Riverview, FL, USA) following Standard Method 4110 B. Sulfide (HS-) was measured using standard methods (McKee et al. 1988). Soil redox potential was measured using standard techniques (Faulkner et al. 1989); briefly, three platinum-tipped probes were inserted to 15-cm depth in each plot and allowed to equilibrate for 30 minutes before measurement. Soil bulk density was determined at the end of the experiment by taking one 2.4-cm diameter core per chamber down to 30-cm. Samples were dried at 60°C and weighed to calculate dry bulk density (g cm-3).