Summary We wanted to determine if changes in algae in the Everglades were due to increased phosphorus (P) loading. Epiphytic algae, water chemistry, and surface sediment chemistry were characterized from 32 sloughs along a P gradient in the Everglades and changes in the algal assemblages along the P gradient were compared with those along an experimental P gradient of in situ mesocosms. The sloughs are the wettest open water habitats characterized by floating and submerged aquatic plants in the Everglades. Algal species composition was much more sensitive to P concentration than algal biomass. The diatom species variance among sloughs, captured by 1st ordination axis, was more highly correlated with total P (TP) in surface sediments (r = ‐ 0.79), than soluble reactive P (SRP) (r = ‐ 0.08) and TP (r = ‐ 0.48) in the water column. Algal biomass (µg chl a cm‐2) was not significantly correlated with P (SRP: r = 0.22, TP: r = 0.19, sediment TP: r = 0.07) along the P gradient in the Everglades. Cluster analysis classified diatom species assemblages in 32 sloughs into three groups (TWIN I, II, III), which corresponded to three zones along the P gradient. Dominant diatom species shifted from Mastogloia smithii (40.3%), Cymbella scotica (22.3%), and Fragilaria synegrotesca (21.8%) in TWIN I to Nitzschia amphibia (22.4%) and C. microcephala (12.4%) in TWIN III. TP in surface sediments and TP in epiphyton assemblages increased 4‐ and 5‐fold from TWIN I to TWIN III, respectively. Patterns in epiphytic assemblages along the experimental P gradient in the mesocosms were very similar to those along the Everglades P gradient. Shannon diversity indices and species richness significantly increased along both P gradients. TN : TP ratio in epiphyton assemblages significantly decreased as sediment TP increased along both P gradient. Ordination analysis showed that diatom assemblages in the impacted zone (TWIN III) were ordinated closely to the assemblages from the highest P treatments in the mesocosms. The assemblages from the less impacted zone (TWIN I) were ordinated closely to the assemblages from controls in the mesocosms. Concurrence between results of our survey and experiments suggest that changes in epiphytic assemblages along the P gradient in the Everglades are caused by increases in P concentrations.

The Florida Everglades, a wetland of international importance, has been undergoing a significant shift in its native flora and fauna due to excessive total phosphorus (TP) loadings (an average of 147 t per annum from 1995to 2004) and an elevated mean TP concentration (69 microg L(-1) of TP in 2004) from agricultural runoff and Lake Okeechobee outflow despite the use of 16000 ha of stormwater treatment areas. Here, we present a Bayesian changepoint analysis of long-term experimental research and show that exceeding a surface water geometric mean TP threshold concentration of 15 microg L(-1) causes an ecological imbalance in algal, macrophyte, and macroinvertebrate assemblages as well as slough community structure. A phosphorus threshold for all trophic levels may be more realistic and protective when presented as a threshold zone (12-15 microg L(-1)) because estimates of uncertainty must be utilized to accurately define TP thresholds, which change with seasons and water depths. Most interior areas of the Everglades are currently at or below this threshold zone, but the exterior areas near inflow structures (except for the Everglades National Park) are presently receiving double or triple the proposed threshold. Our Bayesian approach, used hereto address ecological imbalance along nutrient gradients, is applicable to determining thresholds and stable states in other aquatic ecosystems.

Long‐term studies along a 30‐yr nutrient‐enrichment gradient in the northern part of the subtropical Everglades fen allowed us to assess the effects of nitrogen (N) and phosphorus (P) additions on plant community structure and chemical qualities of wetland plants. Areas in the highest P‐enriched zones (>1000 mgP/kg), once dominated by open‐water sloughs and surrounding monocultures of sawgrass (Cladium jamaicense, a stress‐tolerant low‐nutrient‐status species), are now dominated by cattail (Typha domingensis, a competitive, high‐nutrient‐status species). Areas of moderate (750–500 mg/kg) and low (<500 mg/kg) P soil concentrations have maintained their original plant composition. Analysis of nutrient‐use efficiency indicates that sawgrass is highly efficient in nutrient resorption and nutrient proficiency, but this efficiency decreases at high soil P concentrations. Both indices indicate that suboptimal concentrations of P, which limit growth and optimize retention of P within the plant, exist in the Everglades. This information, when coupled with the low levels of P found in the plants, soil, and pore water of the unenriched portions of the Everglades, indicates that this ecosystem is extremely P limited. High molar N:P leaf ratios suggested P limitations for sawgrass at unenriched sites, but not in highly enriched areas. Intermediate P loadings resulted in either N or P limitations. Increased leaf P concentrations correspond to decreased phenolic content of sawgrass leaves, suggesting that this species’ metabolic growth response follows the carbon–nutrient balance hypothesis of decreased production of phenolics with increased nutrient supply. Concomitant with decreased foliar phenolic content at high P enrichment was an increase in herbivory and fungal infections.

To evaluate the effects of land use on P transport and transformation in the Atlantic coastal plain, we examined the distribution and fractionation of P in forested and agricultural soils of the Rhode River watershed and their export rates. Nearly 70% of P in forest soils was bound in organic forms. Fractionation studies reveal that cultivation and fertilizer P application increased total and inorganic P content and decreased organic P content of the soils. About half of the increase in the inorganic P content of the agricultural soils is due to the mineralization of organic P during cultivation and the rest is probably due to fertilizer P application. The inorganic phosphates in the forest and agricultural soils are chiefly associated with Fe either as iron phosphates or as phosphates adsorbed on the iron oxide surfaces. The agricultural watershed had a higher P export (46.3 g P ha~' wk~') compared to the forested watershed (5.96 g P ha~* wk~'). Particulate P dominated the P export from both watersheds. Variations in the P flux from the watersheds was correlated to changes in water discharge. The discharge pattern of organic and inorganic P forms from the watersheds are highly related to their soil P composition.

Macrophyte species distribution in the marsh and slough areas of the northern Everglades were examined along a surface water and soil P gradient to characterize the effects of nutrient enrichment on the macrophyte community changes in the Everglades ecosystem. Macrophyte species distribution and frequency examined at 51 sites located in the Water Conservation Area (WCA‐2A) revealed distinct trends in species changes with P enrichment. Marshes in the unenriched and enriched areas were dominated by Cladium jamaicense and Typha domingensis, respectively. Open water areas were characterized by Eleocharis sp., Utricularia sp., Chara zeylanica, and Nymphaea odorata in the unenriched areas and by floating plants and Polygonum sp. in the enriched areas. We compared the macrophyte species distribution with the soil P contour map of WCA‐2A and derived six major categories of P enrichment (labeled 0–5) that may be used to adequately describe macrophyte species distribution in the WCA‐2A. Soil and surface water P values that correspond to the different P‐enrichment categories and the number of macrophyte species in each category have been estimated. Examination of the relative abundance of the macrophyte species in the six P‐enrichment categories showed a decline in characteristic slough macrophyte species with P enrichment. Increase in P status of the system was also accompanied by the advent and gradual establishment of macrophytes such as Mikania scandens, Sarcostemma clausum, Polygonum sp., Acrostichum danaeifolium, and a variety of floating plants. Characteristic slough macrophyte species such as Utricularia purpurea, Eleocharis elongata, and Eleocharis cellulosa serve as indicators of the unenriched P conditions.

We examined the P sorption characteristics of northern Everglades peat soils along a eutrophication gradient to understand the P retention efficiency of the soils. The amount of phosphate adsorbed on the soils (Q) and the zero equilibrium phosphate concentration supported by the soils (EPCo) exhibited a linear decrease with distance from the inflow structures supplying agricultural drainage (r2 = 0.74 and 0.71, respectively; P < 0.05; n = 18). Values of Q and EPCo for the enriched soils are an order of magnitude higher than for unenriched soils. Estimated Q values of the soils compare well with the exchangeable P fraction determined by KCl and HCO3 extraction. Comparison of soil EPCo and pore water PO4‐P values with the surface water PO4‐P concentrations suggests that soils may serve as an additional internal source of P to the overlying water column in nutrient‐enriched areas. Linear phosphate adsorption coefficient (K) of the alkaline Everglades soils is higher than that of acidic pocosin bog peat soils of North Carolina but lower than wetland soils and sediments with a high mineral content. Phosphorus sorption characteristics of the Everglade soils are most likely regulated by CaCO3.

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