ECOLOGICAL ENGINEERING ELSEVIER Ecological Engineering 5 (1995) 183-207 Phosphorus dynamics in selected wetlands and streams of the lake Okeechobee Basin * K.R. Reddy *, O.A. Diaz, L.J. Scinto, M. Agami Soil and Water Science Department, 106 Newell Hall, Box 110510, Uniuersity of Florida, Institute of Food and Agricultural Sciences, Gainesuille, FL 32611, USA Abstract Lake Okeechobee is becoming increasingly eutrophic, presumably due to P loading from numerous dairy operations in the Lake's northern drainage basin. Phosphorus released from this basin is transported through canals, streams, and wetlands before its discharge into the lake. This paper summarizes the results of several studies on P dynamics in wetlands and stream sediments in the Lake Okeechobee Basin with primary focus on P interaction with soil/sediment-water column and vegetation. Stream sediments and wetland soils in the basin were characterized for labile and non-labile pools of P. The labile inorganic P (Pi) pool (KCt-extractable) accounted for 0.1 to 2.3% and 0.1 to 0.7% of the total P in sediments and wetland soils, respectively. The NaOH extractable Pi, representing the P associated with Fe and AI oxyhydroxides, was the dominant Pi in both stream sediments and wetland soils (accounting for up to 71 and 43% total P, respectively). The NaOH-Po (humic and fulvic acid associated organic P) is considered resistant to biological breakdown and accounted for 6 to 56% of total P. Stream sediments showed higher buffer intensity for P sorption than wetlands. Phosphate sorption capacity (Smax) and buffer intensity (Kd-adsorption coefficient) were highly correlated with oxalate extractable [Fe + AI] and total organic carbon (TOC) suggesting P sorption is associated with amorphous and weakly crystalline forms of Fe and AI, and/or complexed with organic matter. Phosphorus assimilation in vegetation was found to be short-term and dependent upon plant species, P loading, and wetland hydrology. Decomposition of detrital tissue resulted in rapid release of P into the water column. Phosphorus release was rapid during decomposi- tion of floating macrophytes, as compared to herbaceous vegetation. Paper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the South Florida Water Management Distirct and the University of Florida, Institute of Food and Agricultural Sciences. " Corresponding author. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD[ 0925-8574(95)00024-0 184 K.R. Reddy et al. / Ecological Engineering 5 (1995) 183-207 Phosphorus retention coefficients were positively correlated with oxalate extractable Fe and AI content of soils and sediments. The average EPC w (threshold P concentration in the water column where P retention = P release) for stream sediments was 0.10 mg P 1-1 and 0.42 mg P 1-1 for wetland soils. The stability of the P retained was regulated by the physico-chemical properties of the soils and sediments. Keywords: Phosphorus dynamics; Wetlands; Streams; Lake Okeechobee 1. Introduction Lake Okeechobee (surface area 1890 km 2) is the fourth largest lake in the United States of America. This shallow sub-tropical lake may be moving from a naturally eutrophic state to a hypereutrophic state as a result of increased phosphorus (P) loading from surrounding watersheds. For example, the Taylor Creek/Nubbin Slough watershed which provides 5% of the water input supplied approximately 28% of the total P, whereas the Kissimmee River Basin which provides 30% of the water input contributed 20% of the P loading (Federico et al., 1981a). The high P loading from these two basins was attributed to numerous dairies located in this region. Most dairy sites in the Okeechobee Basin are located on Myakka (a sandy, siliceous, hyperthermic Aeric Haplaquod) or Immokalee (a sandy, siliceous, hyperthermic Arenic Haplaquod) fine sand soils (Graetz and Nair, 1995). The poor P retention capacity and high water permeability of these sandy surface soils reduces the amount of particulate-bound P to negligible proportions (Knisel et al., 1985; Allen, 1987). The inefficient P removal by these sandy soils are caused by: low P retention capacities of the A and the highly eluted E horizons, and a lack of deep percolation of soluble P into Al rich lower spodic (Bh) horizons (Yuan, 1965; Allen, 1987). These conditions result in high soluble P concentrations in the drainage discharged from these basins. Therefore, soluble P is transported via overland flow or lateral seepage along eluted horizons above the spodic layer (Campbell et al., 1995). The route of P transport to the lake is by surface and subsurface flow from uplands to adjacent wetlands and streams, which feed into major inflows and finally discharge into the lake. About 400000 ha of wetlands are located in the Okeeehobee Basin, with > 80% in the Lower Kissimmee River Basin (Table 1). In addition, these wetlands are connected by canals and streams which transport water to Lake Okeechobee. Table 1 Wetland area in major drainage basin of Lake Okeechobee (Federico et al., 1981a) Basin Area (ha) % Land use in respective basin Lower Kissimmee River 320355 18 Taylor Creek/Nubbin Slough 8955 15 Fisheating Creek 48 963 42 Harney Pond/Indian Prairie 32827 36 Lake Istokpoga 5024 4 K.R. Reddy et al. / Ecological Engineering 5 (1995) 183-207 185 Rainfall ~ Atmosphere n Soil/sediment \ it I ! A : ~. -! ____.- V V &,o%.~=~, 7 In/out | -91 I1,.- l Upland , i, .... Fig. 1. Schematic showing phosphorus exchange processes in upland-wetland-stream system. Thus, P removal by wetlands in the basins has been suggested as a means to reduce P loading to Lake Okeechobee (Federico et al., 1978; Davis, 1982; Goldstein, 1982). However, wetlands were shown to function as sinks or sources for P depending on water column and sediment physico-chemical properties (Richard- son, 1985). To evaluate the potential of wetlands and stream sediments for P assimilation, it is critical that we determine the roles of various components of these systems (soils/sediments, vegetation, water) in regulating the water quality. Thus, the key cluestions often asked by managers and regulatory agencies.involved in water management are: (1) how much P can be stored in wetland soils and stream sediments? (2) how stable is the stored P and under what conditions will it be released back into the water column? (3) are wetlands and stream sediments sources or sinks for P? and (4) what is the long-term P assimilatory capacity of wetlands and stream sediments? These questions are very complex and involve a comprehensive study of biogeochemical processes regulating P retention/release by soils or sediments, and vegetation of the ecosystem. These processes include: adsorption/desorption reactions, precipitation, mineralization, exchange processes between soil/sediment and the overlying water, and plant uptake and release (Fig. 1). In this paper, we examined P behavior in soil/sediment-water-plant compo- ,nents of selected wetlands and streams in the Lake Okeechobee drainage basin. In 186 K.R. Reddy et al. /Ecological Engineering 5 ~1995) 183-207 Table 2 Flow weighted mean phosphorus concentration for tributary inflows (Federico et al., t981b and SFWMD unpublished results) Basin 1973-1979 1991 1992 SRP Total P SRP Total P mg/1- t Kissimmee River (S-65E) 0.092 0.107 Taylor Creek (S-133) 0.258 0.341 0.327 0.437 Nubbin Slough (S-191) 0.749 0.906 0.613 0.753 Fisheating Creek 0.161 0.235 0.104 0.196 Harney Pond Canal (S-71) 0.185 0.261 0.186 0.238 Indian Prairie Canal (S-72) 0.139 0.217 0.160 0.253 Rainfall 0.041 0.061 0.05 particular, we summarize the results of a comprehensive project on the biogeo- chemistry of P in these systems. Specific topics include: (1) forms of labile and non-labile pools of P in soils and sediments, (2) P sorption capacity of selected stream sediments and adjacent wetlands, (3) mobility of P in sediment, soil and the overlying water column, (4) chemical precipitation of P in the water column, (5) P storage and release capacity of selected aquatic macrophytes in streams and adjacent wetlands and (6) long-term P storage in wetlands and stream sediments. Detailed results of these studies are presented in Reddy et al. (1992a); Reddy et al. (1992b); Reddy et al. (1993); and Diaz et al. (1995). 2. Site description The Lake Okeechobee drainage basin covers more than 4600 square miles. Inflows into the lake include: rainfall, the Kissimmee River, Fisheating Creek, Taylor Creek/Nubbin Slough, Harney Pond and Indian Prairie Basins, and the Everglades Agricultural Area. The total P concentration of the major inflows has changed little during the past 20 years (Table 2) with a major portion of the inflow water P (64 to 83% of total P) in soluble form. The P concentration of Taylor Creek/Nubbin Slough inflow was higher than other inflows. Approximately 50% of the total P loading to the lake is from Kissimmee River and Taylor Creek/Nub- bin Slough inflows (Federico et al., 1981a). The sampling sites (Fig. 2; Table 3) used in the study were selected in Taylor Creek/Nubbin Slough Basin, Lower Kissimmee River Basin (S-65D, S-65E, S-154 Basins), Istokpoga Basin (Bates Canal), Fisheating Creek Basin, and Harney Pond/Indian ~Prairie Basin (C-41 Basin). At each site, stream sediment and adjacent wetland soil and overlying water samples were obtained. For the purpose of this paper, the physico-chemical characteristics of surface soil or sediment to a depth of 15 cm were used. Selected physico-chemical characteristics of the stream K.R. Reddy et al./ Ecological Engineering 5 (1995) 183-207 187 River S-135 i? ¸¸ ii i!!ii :. :.:: I,~ke Okeechobee Tabor Creek/NubblnSlough ~ Im t~okpo~ e,,s~ Fig.