EFFECTS OF PHOSPHATE AVAILABILITY AND REDOX INTENSITY ON GROWTH AND NUTRIENT UPTAKE OF RHYNCHOSPORA TRACYI, A WET PRAIRIE SPECIES IN THE EVERGLADES Hongjun Chen 1 , Irving A. Mendelssohn 2 , Bent Lorenzen 3 , Hans Brix 3 , and ShiLi Miao 4 1 Fort Lauderdale Research and Education Center University of Florida 3205 College Avenue Fort Lauderdale, Florida, USA 33314 E-mail: hjchen@ufl.edu 2 Wetland Biogeochemistry Institute and Department of Oceanography and Coastal Sciences School of the Coast and Environment Louisiana State University Baton Rouge, Louisiana, USA 70803 3 Department of Plant Ecology University of Aarhus Nordlandsvej 68 DK-8240 Risskov, Denmark 4 South Florida Water Management District 3301 Gun Club Road West Palm Beach, Florida, USA 33416 Abstract: Growth and nutrient responses of Rhynchospora tracyi (Tracy’s beak-rush), a wet prairie species in the Florida Everglades, to redox intensity and phosphate availability were examined under controlled redox intensities (Eh: 2150, +150, and +600 mV) and phosphate levels (P: 10, 80, and 500 mgP 1 21 ) for 60 days. Root length, total biomass, and photosynthesis of R. tracyi were higher at Eh +600 than at Eh 2150 . Growth variables (e.g., shoot length, biomass, relative growth rate, and root porosity) significantly increased with phosphate availability, while R. tracyi allocated more biomass to shoots and rhizomes than to roots at the higher P levels. Tissue P concentration increased 5–6 times when phosphate availability increased from 10 mgPl 21 to 500 mgPl 21 , while P use efficiency and molar N:P ratio significantly decreased with P availability. Net P accumulation was significantly higher at P 500 than at P 10 across the three redox intensities. Tissue N concentration was significantly higher at Eh +600 than at Eh 2150 , and higher at P 500 than at P 10 . In terms of plant performance at the three redox intensities, we conclude that R. tracyi is not well-adapted to strongly reduced conditions that would result from lengthened hydroperiods. Altered hydroperiods may be a major reason why wet prairie species, such as R. tracyi, have decreased in abundance in some areas of the Everglades. Key Words: Cladium jamaicense, Eleocharis cellulosa, flood tolerance, growth response, hydrologic regimes, hydroperiods, nutrient enrichment INTRODUCTION Due to nutrient enrichment and altered hydrolo- gy, the vegetative communities of the Florida Everglades have undergone dramatic changes (Davis et al. 1994, White 1994). The critical ecological linkages between the wet prairies and multiple ecosystem stressors in the Everglades are not well understood (White 1994, Ogden 2005). Wet prairies, known as flats, are a dominant plant community in the Everglades and are characterized by aquatic emergent plants such as Eleocharis spp., Rhyncho- spora spp., and Panicum hemitomon (Loveless 1959). The wet prairies are saturated approximately 90% of the year in the Everglades and occur on low, relatively flat, poorly drained depressions (Loveless 1959, Wood and Tanner 1990). Wading birds and the Everglades snail kite forage in wet prairies for fish, aquatic invertebrates, and apple snails (Lodge 1994). Wet prairies primarily include two types: those dominated by Eleocharis spp., which have long WETLANDS, Vol. 28, No. 1, March 2008, pp. 151–163 ’ 2008, The Society of Wetland Scientists 151