Hypothesis 1: The balance of fresh and marine water supplies influences microbially-mediated C and nutrient cycling in wetland soils through interacting effects on P availability, salinity, and water residence time, culminating in gains or losses in C storage.
Approach - We will conduct a suite of controlled six-month experiments in the mesocosm facility to understand how P, salinity, and water residence time and depth affect microbially-mediated C and nutrient cycling along the TS/Ph and SRS transects. Briefly, in year 1, we will establish a plant-soil system representative of the SRS mangrove ecotone. Tidal cycles of the field site will be recreated by controlling the input of fresh and marine source water, and inundation manipulated within each mesocosm by adjusting soil depth relative to mid-tidal height. Four treatments will be assigned across 12 mesocosms, including salinity and P controls, and salinity-enhanced, P-enhanced, and combined salinity and P treatments. Similar experiments will follow annually for the TS/Ph ecotone and TS/Ph and SRS marsh ecosystems, adjusting treatments to represent the conditions of adjacent downstream ecosystems. We will measure effects on soil CO2 and CH4 efflux (continuous), bacterial production, redox conditions, total and dissolved nitrogen (N), P, organic C concentrations, and the optical properties of water (weekly), soil bulk density, total C, N and P concentrations, microbial activity and composition (by molecular fingerprinting) (initial and final soils), and leaf and root decomposition rates (using litterbag techniques).
Hypothesis 2: The balance of marine and freshwater supplies of dissolved organic carbon (DOC) to Everglades estuaries will determine bioavailability for bacterioplankton and the microbial loop.
Approach - Assays will be performed with and without nutrient additions to quantify the coupling of bacterial production and respiration to phytoplankton, seagrass, and microphytobenthos primary production. We will expose producers to 13C-bicarbonate and quantify primary production on filtered samples or tissue, while filtrate will be incubated in the dark to quantify bacterial respiration. Bacterial production will also be measured using 3H-thymidine uptake. Lability of allochthonous C in the ecotone and estuary will be quantified by measuring changes in dissolved organic carbon over one-month incubations. Bacterial growth efficiency will be compared across FCE transects relative to biogeochemical drivers.