| Description: | Measurements were made in the Florida Everglades which are classified as subtropical, with a year-long growing season and distinct wet and dry seasons. Water enters the Everglades through local precipitation events (1380 mm y-1) and regional runoff. The majority of rainfall (70%) occurs during the wet season, which begins in May or early June with convective events and tropical depressions, e.g., thunderstorms and cyclones. Generally, surface water levels increase throughout the wet season and are highest in October, and decline to their lowest levels by the end of the dry season in May. Presently, the South Florida Water Management District uses a complex system of canals, levees, and pumping stations to control the system's hydrology.
There are two major drainage systems in ENP, Shark River Slough (SRS) and Taylor Slough (TS). Shark River Slough is the largest freshwater flow system in ENP (991 km2). It serves as an important area for water storage and recharge to the Biscayne aquifer, provides critical habitat for a diverse assemblage of marsh dwelling fauna and flora, and acts as a major source of freshwater input into the estuarine areas of ENP. Wetland ecosystems in Shark River Slough are generally inundated throughout the year, including long-hydroperiod freshwater marshes that are characterized by peat soils (~1 m thick) overlying limestone bedrock with ridge and slough microtopography. Taylor Slough is the second largest flow-way (409 km2) for surface water. TS originates along the eastern boundary of ENP and extends approximately 30 km to Florida Bay. Freshwater ecosystems in Taylor Slough include short-hydroperiod marl prairies that are inundated for 4 to 6 months each year (June to November) and are characterized by shallow (~0.15 m) marl soils overlying limestone bedrock.
Static chamber measurements of ecosystem carbon dioxide (CO2) and methane (CH4) fluxes were made at multiple locations throughout short-statured freshwater wetlands in Shark River Slough (n=83) and Taylor Slough (n=45) from 2017- 2019. In Shark River Slough, measurements were made at locations between the FCE-LTER research sites SRS-1 and SRS-2, on ridges and adjacent dead floating mats that were accessible by airboat (n=41). We included chamber measurement of ridge and slough from the FCE-LTER SRS-2 research site (n=42). In Taylor Slough, all measurement locations were at established sampling locations for FCE-LTER site TS/Ph-1 and representative of short-hydroperiod freshwater marl prairie communities (n=45). Dead floating mats were not present in Taylor Slough.
All flux measurements (CO2 and CH4) were conducted using a closed-path chamber system which was composed of a Los Gatos Ultraportable Greenhouse Gas Analyzer (ABB, Los Gatos Research, San Jose, CA) that measured at 1 Hz and vented polycarbonate chambers (1.2×0.5×0.5 or 0.8×0.5×0.5 m, depending on the vegetation height). At Shark River Slough, the chambers were placed on a floating foam-backed plywood platform with an opening matching that of the chamber (0.5 x 0.5 m, Table S1). At TS/Ph-1, the chamber was positioned on aluminum-reinforced polyethylene bases (0.5 x 0.5 x 0.2 m) permanently mounted in the soil that provided a seal between the chamber and the soil when water level was at or below the upper edge of the base. The chamber was allowed to equilibrate to a steady rate of change (~1-2 min) followed by a 3 min measurement period under ambient light to determine NEE. After the ambient light measurements, a custom fit black cover was placed over the chamber for a dark measurement to determine ecosystem respiration (Reco). During the measurements, temperatures inside of the chamber and in the water were recorded with a shaded thermocouple temperature probe. Photosynthetically active radiation (PAR) was recorded using a LI-COR LI-190S-1 quantum sensor with a LI-250Q sensor reader. Water level was measured with a meter ruler at each sample location. At TS/Ph-1, chamber volumes for flux calculation were adjusted by water volume in the chamber when water levels were above the soil surface. Measurements were taken between 10:00 am and 3:00 pm at ambient light. To calculate flux rates of CO2 and CH4, we used the R package FluxCalR Gross ecosystem exchange (GEE) was calculated as:
GEE = NEE - Reco Eq. 1
The net CH4 flux was measured at the same time as the CO2 measurements (both in light and dark). The gas fluxes are presented using atmospheric convention, where negative numbers denote uptake by the ecosystem and positive numbers indicate emissions from the ecosystem. We calculated the ratio of ambient light CH4 emissions to NEE (mol mol-1) or CH4:CO2. Values of CH4:CO2 greater than the greenhouse carbon compensation point for a 100-year time frame (-0.03) indicate that CH4 emissions were not offset by ecosystem productivity over a 100-year period, resulting in a warming effect to the climate. We examined changes in CH4:CO2 over the study period. |
