The Florida Everglades wetland ecosystem is subject to changes in hydroperiod and nutrient loading, resulting in soil P enrichment and changes in vegetation communities. The objectives of this study were to: (i) quantify the forms of inorganic and organic P in soils from four hydrologic units of the Everglades, and (ii) develop empirical relationships among various soil P forms. Soil samples from selected hydrologic units, including the Water Conservation Areas (WCAs) and the Holey Land Wildlife Management Area (HWMA), were obtained at various locations along transects perpendicular to each nutrient input source, while selected field sites were sampled in the Everglades Agricultural Area (EAA). Spatial distribution of total P in the surface 0-to 10-cm soil depth showed distinct gradients in the WCAs and HWMA soils, with high total P in soils closer to sources (canals and inflow structures) than in interior, unimpacted areas. Soil ash content and bulk density were also altered as a result of soil subsidence (for EAA soils), hydrology, and nutrient loading (for the WCAs and the HWMA soils). Influence of P loading was primarily confined to the top 30-cin soil depth, with about one-third of the P stored in the inorganic pool (primarily as Ca-and Mg-bound P), and the remainder present as organic P. Inorganic P content was higher in surface soils and decreased with depth. Soil P enrichment indicated that for approximately 5 km from the inflow structures or canals, soils have been impacted by nutrient loading. Empirical relationships developed in this study should be useful for estimating soil P forms at the landscape level, using total P data available for a large number of sites throughout the Everglades region. M OBILITY AND REACTIVITY OF P in wetlands under variable hydrologic conditions are controlled by the chemical composition of P in soil and water, relative sizes of various P pools in the soil, interactions of soluble fractions with solid phases, and decomposition of soil organic matter. Phosphorus is present in both organic and inorganic forms, with organic forms present as the dominant pool in many wetlands. Forms of inorganic P (Pi) in soils are usually determined by sequential extractions with acid and alkaline reagents, as proposed by Chang and Jackson (1957) and later modified by others for soils and sediments (Psenner et al., 1988; Ruttenburg, 1992; Olila et al., 1994). A modification of this scheme has been adopted for wetland soils (Quails and Richardson, 1995; Reddy et al., 1995). These schemes typically identify P in the following groups: (i) labile P; loosely adsorbed; (ii) P; associated with Fe and Al; (iii) Pi associated with Ca and Mg; (iv) alkali-extractable organic P (fulvic-and humic-bound P); and (v) residual organic P. Forms of organic P (P 0) have also been distin

Total P is increasing over time in the waters of Lake Okeechobee, Florida, but the concentrations do not correlate with external loads. The objectives of this study were to determine: (i) the P flux from various sediment types within the lake, (ii) the factors that control direction and magnitude of P flux, and (iii) the amount of P associated with various inorganic P phases within the sediment. Phosphorus flux was measured from intact sediment cores taken from eight sites that represent major sediment types and major inflows of Lake Okeechobee at four time periods in 1989–1990. At the same location‐times, dissolved reactive phosphorus (DRP) in porewater was determined using porewater equilibrators and/or sediment cores. Results indicate that P flux from sediments is very sensitive to changes in O2 status of the overlying water, with anaerobic conditions promoting large P fluxes. Despite steep porewater DRP gradients in sediments (varying from 0.1 mg P L−1 at the sediment/water interface to more than 1 mg P L−1 at lower depths), P flux was not regulated by such gradients. Such lack of dependence of P flux on DRP gradients highlights the role redox reactions (involving Fe) can play in P chemistry in the top few centimeters of the sediment. Internal P loads (i.e., flux from bottom sediments) were found to be approximately equivalent to external P loads (≈1 mg P m−2 d−1).

Bottom sediments in shallow lakes can play a major role in releasing nutrients to the overlying water column during wind induced sediment resuspension or by constant flux due to diffusion. Internal nutrient loads due to these processes may be equal to or higher than external loads. Laboratory and field experiments were conducted on Lake Apopka, a shallow, hypereutrophic subtropical lake located in central Florida. Ammonium (NH+4) and soluble reactive P (SRP) flux during sediment resuspension were measured under laboratory conditions using intact sediment cores. Ammonium N and SRP flux due soley to diffusion were assessed using in situ porewater concentrations. Average diffusive flux from sediment to the overlying water was estimated to be 25 mg NH4‐N m−2 d−1 and 1 mg P m−1 d−2. Resuspension fluxes of NH+4 and SRP were higher than diffusive flux. Soluble reactive P profiles of porewater showed distinct profile differentiation, with the surface 0 to 8 cm sediment depth acting as a P‐depletion zone, and the underlying sediment displaying steep gradients in porewater SRP. These results suggest that dissolved NH+4 and SRP transport from the surface 8 cm of sediment was due to sediment resuspension, while below this depth, upward mobility of NH+4 and SRP was regulated by diffusion. Although dissolved N and P flux is upwards (from sediment to water column), during extended periods (annual cycle) the lake is functioning as a net sink for N and P by transforming inorganic pools of nutrients into organic forms and depositing them on the sediment surface.

Wetland soils play a key role in the cycling of nutrients within an ecosystem. Since soils are potentially a source or a sink for inorganic nutrients, it is important to quantify their influence on overlying water quality in order to understand their importance in overall ecosystem nutrient budgets. Laboratory and field studies were performed in the northern Everglades (WCA-2A) to determine the magnitude of phosphorus (P) flux between the soil and the overlying water column, under various redox conditions. The P flux was estimated using three techniques: intact soil cores, in situ benthic chambers, and porewater equilibrators. There was reasonable agreement between the P flux estimated using intact soil cores and benthic chambers; however, P flux estimates using the porewater equilibrators were considerably lower than the other two techniques. Models of solute flux, based solely on soil physico-chemical characteristics, may substantially underestimate soil-water nutrient exchange processes. Phosphorus flux measured with the intact soil cores varied from 6.5 mg m(-2) d(-1) near nutrient inflow areas to undetectable flux 4 km away from the inflow. Oxygen consumption varied from 4 mg m(-2) d(-1) near the inflow to a constant 1 to 2 mg m(-2) d(-1) at a distance of 4 km from the inflow. Rate of consumption of NO3- -N and SO4(2-) showed no significant trend with respect to distance from inflow. Nitrate N and SO4 consumption rates averaged 120 and 130 mg m(-1) d(-1), respectively. Consumption of O2 was correlated with P flux, whereas NO3- -N and SO4(2-) consumption were not.

Increased nutrient loading to the northern Everglades from the nearby Everglades Agricultural Area (EAA) has raised concerns of eutrophication of this oligotrophic wetland. A field study was conducted to determine the influence of nutrient loading on spatial distribution of P, N, C, and related physico-chemical parameters in the peat soils (Histosols) of Water Conservation Area 2A (WCA-2A) in the northern Everglades. Field sampling of the top 30 cm of soil was performed at 74 sites across WCA-2A. Isarithmic plots of N and P forms based on geostatistical analyses revealed widespread enrichment of P, especially in areas proximal to surface inflows importing nutrient-laden water from the EAA. Enrichment of less magnitude was shown for soil N, while spatial variability of C, bulk density, ash content, and pH were minimal. Concentration of soil porewater NH/-N was typically in the 1.5 to 2.5 mg L" 1 range in the interior (less impacted) region of WCA-2A, compared with 4 to 8 mg N L' 1 near surface inflows at the northern end of WCA-2A. In contrast, soluble reactive P in the porewater varied from = 100 ug L" 1 or less in the interior marsh to >1000 ug L' 1 near inflow structures. Mean soil total P at a depth of 0 to 10 cm was 473 mg kg ~' in the interior marsh, compared with 1338 mg kg" 1 in the areas adjacent to inflows. Results of this study show that WCA-2A soil has served as a net storage for the increased load of P in nutrient-laden surface inflows. Much of the additional soil P is available for plant uptake, based on the magnitude of P enrichment across a large area. Nutrient enrichment in the soil corresponded with the occurrence of cattails (Typha domingensis Pers. and T. latifoUa L.) in areas previously dominated by sawgrass (Cladium jamaicense Crantz). T HE EVERGLADES represent a unique and complex composite of ecosystems forming a vast wetland covering a large portion of southeastern Florida. The original Everglades, which encompassed an area of about 10000 km 2 , extended southward approximately 160 km from the southern shore of Lake Okeechobee to Florida Bay, with a width of up to 65 km (Davis, 1943). Surface hydrology was characterized by a seasonal north-south sheet flow driven by rainfall and pulsed loading from Lake Okeechobee. Currently, the Everglades consist of two major regions: WC As to the north and the Everglades National Park to the south (Fig. 1). The three WCAs were created in the early 1960s for the purposes of water supply, flood control, wildlife habitat, and recreation. The principal vegetational communities found in the northern Everglades are sawgrass marsh, wet prairies, sloughs, and tree islands (also known as bayheads). The sawgrass