Lake Okeechobee Research Articles

Climatic Influences on Autochthonous and Allochthonous Phytoplankton Blooms in a Subtropical Estuary, St. Lucie Estuary, Florida, USA

The St. Lucie Estuary, located on the southeast coast of Florida, provides an example of a subtropical ecosystem where seasonal changes in temperature are modest, but summer storms alter rainfall regimes and external inputs to the estuary from the watershed and Atlantic Ocean. The focus of this study was the response of the phytoplankton community to spatial and temporal shifts in salinity, nutrient concentration, watershed discharges, and water residence times, within the context of temporal patterns in rainfall. From a temporal perspective, both drought and flood conditions negatively impacted phytoplankton biomass potential. Prolonged drought periods were associated with reduced nutrient loads and phytoplankton inputs from the watershed and increased influence of water exchange with the Atlantic Ocean, all of which restrict biomass potential. Conversely, under flood conditions, nutrient loads were elevated, but high freshwater flushing rates in the estuary diminished water residence times and increase salinity variation, thereby restricting the buildup of phytoplankton biomass. An exception to the latter pattern was a large incursion of a cyanobacteria bloom from Lake Okeechobee via the St. Lucie Canal observed in the summer of 2005. From a spatial perspective, regional differences in water residence times, sources of watershed inputs, and the proximity to the Atlantic Ocean influenced the composition and biomass of the phytoplankton community. Long water residence times in the North Fork region of the St. Lucie Estuary provided an environment conducive to the development of blooms of autochthonous origin. Conversely, shorter residence times in the mid-estuary limit autochthonous increases in biomass, but allochthonous sources of biomass can result in bloom concentrations of phytoplankton.

Technology adoption and mitigation of invasive species damage and risk: application to zebra mussels

Using a bio-economic model of zebra mussels (Dreissena polymorpha), we examine the expected economic value of prevention, control and eradication alternatives for the freshwater mussel in Lake Okeechobee (Florida, USA). We include two emerging technologies for zebra mussel (ZM) control: (1) a natural pesticide called Zequanox, and (2) hot wash stations at boat ramps. We employ water management district data, user data collected via a phone survey, and mitigation expenditures from infested locations elsewhere to estimate the potential damage from the introduction of zebra mussels in Florida. Methods used include static cost transfer estimation, econometric cost estimation, and stochastic-dynamic simulation. We use our bio-economic model to compare costs and risks with and without the emerging technologies. We also consider the impact of technology adoption rates by anglers, management policy efficacy, and opportunity costs associated with ZM control. Results indicate that, without investment in prevention, there is a very high probability that Florida waterways will be infested with zebra mussels by year 2025, and expected environmental damages and management costs are high. Slow response due to poor detection methods or insufficient control efforts will lead to a moderate probability of a significant infestation. Rapid reaction and enhanced prevention efforts are expected to greatly reduce the probability of ZM infesting Lake Okeechobee by 2025, and to generate much higher expected net benefits.

Estimation of significant wave height in shallow lakes using the expert system techniques

Significant wave height is an important hydrodynamic variable for the design application and environmental evaluation in coastal and lake environments. Accurate prediction of significant wave height can assist the planning and analysis of lake and coastal projects. In this study, the Genetic Algorithm (GA) is used as the optimization technique to better predict model parameters. Also, Kalman Filtering (KF) is used for prediction of significant wave height from wind speed. KF technique makes predictions based on stochastic and dynamic structures. The integrated Geno Kalman Filtering (GKF) technique is applied to develop predictive models for estimation of significant wave height at stations LZ40, L006, L005 and L001 in Lake Okeechobee, Florida. The results show that the GKF methodology can perform very well in predicting the significant wave height and produce lower mean relative error and mean-square error than those from Artificial Neural Network (ANN) model. The superiority of GKF method over ANN is presented with comparisons of predicted and observed significant wave heights.

Correspondence of historic salinity fluctuations in Florida Bay, USA, to atmospheric variability and anthropogenic changes

Florida Bay is a highly dynamic estuary that exhibits wide natural fluctuations in salinity due to changes in the balance of precipitation, evaporation and freshwater runoff from the mainland. Rapid and large-scale modification of freshwater flow and construction of transportation conduits throughout the Florida Keys during the late nineteenth and twentieth centuries reshaped water circulation and salinity patterns across the ecosystem. In order to determine long-term patterns in salinity variation across the Florida Bay estuary, we used a diatom-based salinity transfer function to infer salinity within 3.27 ppt root mean square error of prediction from diatom assemblages from four ~130 year old sediment records. Sites were distributed along a gradient of exposure to anthropogenic shifts in the watershed and salinity. Precipitation was found to be the primary driver influencing salinity fluctuations over the entire record, but watershed modifications on the mainland and in the Florida Keys during the late-1800s and 1900s were the most likely cause of significant shifts in baseline salinity. The timing of these shifts in the salinity baseline varies across the Bay: that of the northeastern coring location coincides with the construction of the Florida Overseas Railway (AD 1906–1916), while that of the east-central coring location coincides with the drainage of Lake Okeechobee (AD 1881–1894). Subsequent decreases occurring after the 1960s (east-central region) and early 1980s (southwestern region) correspond to increases in freshwater delivered through water control structures in the 1950s–1970s and again in the 1980s. Concomitant increases in salinity in the northeastern and south-central regions of the Bay in the mid-1960s correspond to an extensive drought period and the occurrence of three major hurricanes, while the drop in the early 1970s could not be related to any natural event. This paper provides information about major factors influencing salinity conditions in Florida Bay in the past and quantitative estimates of the pre- and post-South Florida watershed modification salinity levels in different regions of the Bay. This information should be useful for environmental managers in setting restoration goals for the marine ecosystems in South Florida, especially for Florida Bay.

Microbial degradation of microcystin in Florida's freshwaters.

Presence of microcystin (MC), a predominant freshwater algal toxin and a suspected liver carcinogen, in Florida's freshwaters poses serious health threat to humans and aquatic species. Being recalcitrant to conventional physical and chemical water treatment methods, biological methods of MC removal is widely researched. Water samples collected from five sites of Lake Okeechobee (LO) frequently exposed to toxic Microcystis blooms were used as inoculum for enrichment with microcystin LR (MC-LR) supplied as sole C and N source. After 20days incubation, MC levels were analyzed using high performance liquid chromatography (HPLC). A bacterial consortium consisting of two isolates DC7 and DC8 from the Indian Prairie Canal sample showed over 74% toxin degradation at the end of day 20. Optimal temperature requirement for biodegradation was identified and phosphorus levels did not affect the MC biodegradation. Based on 16S rRNA sequence similarity the isolate DC8 was found to have a match with Microbacterium sp. and the DC7 isolate with Rhizobium gallicum (AY972457).

Impacts of past climate and sea level change on Everglades wetlands: placing a century of anthropogenic change into a late-Holocene context

We synthesize existing evidence on the ecological history of the Florida Everglades since its inception ∼7 ka (calibrated kiloannum) and evaluate the relative impacts of sea level rise, climate variability, and human alteration of Everglades hydrology on wetland plant communities. Initial freshwater peat accumulation began between 6 and 7 ka on the platform underlying modern Florida Bay when sea level was ∼6.2 m below its current position. By 5 ka, sawgrass and waterlily peats covered the area bounded by Lake Okeechobee to the north and the Florida Keys to the south. Slower rates of relative sea level rise ∼3 ka stabilized the south Florida coastline and initiated transitions from freshwater to mangrove peats near the coast. Hydrologic changes in freshwater marshes also are indicated ∼ 3k a. During the last ∼2 ka, the Everglades wetland was affected by a series of hydrologic fluctuations related to regional to global-scale fluctuations in climate and sea level. Pollen evidence indicates that regional-scale droughts lasting two to four centuries occurred ∼ 1k a and∼0.4 ka, altering wetland community composition and triggering develop- ment of characteristic Everglades habitats such as sawgrass ridges and tree islands. Intercalation of mangrove peats with estuarine muds ∼1 ka indicates a temporary slowing or stillstand of sea level. Although sustained droughts and Holocene sea level rise played large roles in structuring the greater Everglades ecosystem, twentieth century reductions in freshwater flow, compartmentalization of the wetland, and accelerated rates of sea level rise had unprecedented impacts on oxidation and subsidence of organic soils, changes/loss of key Everglades habitats, and altered distribution of coastal vegetation.

Occupancy and abundance of wintering birds in a dynamic agricultural landscape

AbstractAssessing wildlife management action requires monitoring populations, and abundance often is the parameter monitored. Recent methodological advances have enabled estimation of mean abundance within a habitat using presence–absence or count data obtained via repeated visits to a sample of sites. These methods assume populations are closed and intuitively assume habitats within sites change little during a field season. However, many habitats are highly variable over short periods. We developed a variation of existing occupancy and abundance models that allows for extreme spatio‐temporal differences in habitat, and resulting changes in wildlife abundance, among sites and among visits to a site within a field season. We conducted our study in sugarcane habitat within the Everglades Agricultural Area southeast of Lake Okeechobee in south Florida. We counted wintering birds, primarily passerines, within 245 sites usually 5 times at each site during December 2006–March 2007. We estimated occupancy and mean abundance of birds in 6 vegetation states during the sugarcane harvest and allowed these parameters to vary temporally or spatially within a vegetation state. Occupancy and mean abundance of the common yellowthroat (Geothlypis trichas) was affected by structure of sugarcane and uncultivated edge vegetation (occupancy = 1.00 [${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 0.96–1.00] and mean abundance = 7.9 [${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 3.2–19.5] in tall sugarcane with tall edge vegetation versus 0.20 [${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 0.04–0.71] and 0.22 [${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 0.04–1.2], respectively, in short sugarcane with short edge vegetation in one half of the study area). Occupancy and mean abundance of palm warblers (Dendroica palmarum) were constant (occupancy = 1.00,${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 0.69–1.00; mean abundance = 18,${\rm 95\%}\,{\rm C\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over I} }$ = 1–270). Our model may enable wildlife managers to assess rigorously effects of future edge habitat management on avian distribution and abundance within agricultural landscapes during winter or the breeding season. The model may also help wildlife managers make similar management decisions involving other dynamic habitats such as wetlands, prairies, and even forested areas if forest management or fires occur during the field season. © 2011 The Wildlife Society.

Water quality effectiveness of ditch fencing and culvert crossing in the Lake Okeechobee basin, southern Florida, USA

Ditch fencing and culvert cattle crossing Best Management Practice (BMP) was evaluated in this study with regard to phosphorus (P) and nitrogen (N) load reductions and economic feasibility in the Lake Okeechobee (LO) basin. The BMP was implemented at a 170m section of a drainage ditch within a ranch in the LO basin and flow and concentration (N and P) data at the upstream and downstream of the ditch were collected for one pre-BMP (June–October, 2005) and three post-BMP (June–October, 2006–2008) periods. During the pre-BMP period, downstream total P (TP) load was 20% (67.0kg) higher than the upstream, indicating the cattle crossing to be a source of P. Downstream loads of TP in 2006 and 2008 (post-BMP periods) became 26% (14.7kg) and 11% (85.9kg) lower than the upstream loads, respectively indicating that the BMP reduced the P loads. The site was a sink for N for all periods except the 2007. Unusual dry conditions during 2007 resulted in the addition of P and N at the BMP site, probably due to the release of P and N from soil and plants. Average of three post-BMP period load showed a 10% reduction of TP loads at the downstream (251.8kg) compared to the upstream (281.0kg) location. To consider potential P contributions from the soil and plant, two scenarios, conservative and liberal, were considered to estimate P load reductions due to the BMP. For the conservative scenario, P contribution from soil and plant was considered, while for liberal it was not. Reductions in P loads for conservative and liberal scenarios were 0.35 and 0.44kg/day, respectively. Phosphorus removal cost for the conservative scenario was $12.61/kg of P, which is considerably less than the cost of other P reduction strategies in the basin. Overall, results show that the BMP can reduce P concentration and loads from ranches without causing adverse impact on cattle production.

Nitrogen dynamics in Lake Okeechobee: forms, functions, and changes

Total nitrogen (TN) in Lake Okeechobee, a large, shallow, turbid lake in south Florida, has averaged between 90 and 150 μM on an annual basis since 1983. No TN trends are evident, despite major storm events, droughts, and nutrient management changes in the watershed. To understand the relative stability of TN, this study evaluates nitrogen (N) dynamics at three temporal/spatial levels: (1) annual whole lake N budgets, (2) monthly in-lake water quality measurements in offshore and nearshore areas, and (3) isotope addition experiments lasting 3 days and using 15N-ammonium (15NH4+) and 15N-nitrate (15NO3−) at two offshore locations. Budgets indicate that the lake is a net sink for N. TN concentrations were less variable than net N loads, suggesting that in-lake processes moderate these net loads. Monthly NO3− concentrations were higher in the offshore area and higher in winter for both offshore and nearshore areas. Negative relationships between the percentage of samples classified as algal blooms (defined as chlorophyll a > 40 μg l−1) and inorganic N concentrations suggest N-limitation. Continuous-flow experiments over intact sediment cores measured net fluxes (μmol N m−2 h−1) between 0 and 25 released from sediments for NH4+, 0–60 removed by sediments for NO3−, and 63–68 transformed by denitrification. Uptake rates in the water column (μmol N m−2 h−1) determined by isotope dilution experiments and normalized for water depth were 1,090–1,970 for NH4+ and 59–119 for NO3−. These fluxes are similar to previously reported results. Our work suggests that external N inputs are balanced in Lake Okeechobee by denitrification.

Sulfate threshold target to control methylmercury levels in wetland ecosystems

Sulfate contamination has a significant environmental implication through the stimulation of toxic hydrogen sulfide and methylmercury (MeHg) production. High levels of MeHg are a serious problem in many wetland ecosystems worldwide. In the Florida Everglades, it has been demonstrated that increasing MeHg occurrence is due to a sulfate contamination problem. A promising strategy of lowering the MeHg occurrence is to reduce the amount of sulfate entering the ecosystem. High surface water sulfate concentrations in the Everglades are mainly due to discharges from the Everglades Agricultural Area (EAA) canals. Water and total sulfur mass balances indicated that total sulfur released by soil oxidation, Lake Okeechobee and agricultural application were the major sources contributing 49,169, 35,217 and 11,775 mtons year − 1 , respectively. Total sulfur loads from groundwater, levees, and atmospheric deposition contributed to a lesser extent: 4055; 5858 and 4229 mtons year − 1 , respectively. Total sulfur leaving the EAA into Water Conservation Areas (WCAs) through canal discharge was estimated at 116,360 mtons year −1, and total sulfur removed by sugarcane harvest accounted for 23,182 mtons year − 1 . Furthermore, a rise in the mineral content and pH of the EAA soil over time, suggested that the current rates of sulfur application would increase as the buffer capacity of the soil increases. Therefore, a site specific numeric criterion for sulfate of 1 mg L − 1 was recommended for the protection of the Everglades; above this level, mercury methylation is enhanced. In parallel, sulfide concentrations in the EAA exceeded the 2 μg L − 1 criterion for surface water already established by the U.S. Environmental Protection Agency (EPA).

A simple model of internal loading of phosphorus in Lake Okeechobee

Models of nutrient dynamics in Lake Okeechobee have not adequately simulated the lake's reduced capacity, over time, to remove phosphorus. These models lack processes to either simulate phosphorus saturation in sediments or internal loading to the water column from the sediments. We developed a relatively simple mass balance model that incorporates nonlinear Langmuir sorption dynamics previously used by Pollman (1983) to describe the partitioning of inorganic phosphorus between sediment aqueous and solid phases. This partitioning resulted in progressive saturation of sediment phosphorus exchange sites. The resulting model represents an improvement over earlier efforts but still yielded only modest agreement between observed and predicted annual values (r2= 0.38). Model residuals were significantly correlated with lake–water column calcium concentrations; therefore, the model was refined to include a predictive submodel for calcium, based on ambient loadings and precipitation to, or dissolution from, the sediments. The latter process was modeled kinetically, based on the deviation of water column concentrations from a calibrated equilibrium value. The submodel assumed that as particulate calcium dissolves it releases attached soluble reactive phosphorus. Although this additional source of phosphorus (P) to the lake was small (∼11% compared to external inputs), it was significant during years when appreciable calcium dissolution occurred. As a result, the revised model performance was improved (r2= 0.48). The model indicates that internal fluxes of P exceed external inputs on average by a factor of 2.6. This internal flux will compromise the efficacy of load reduction measures taken to improve the water quality of Lake Okeechobee.

Sediment and nutrient management solutions to improve the water quality of Lake Okeechobee

Two separate models, the Internal Loading Phosphorus Model (ILPM) and the Lake Okeechobee Water Quality Model (LOWQM) were used to predict outcomes of 3 alternatives to reduce internal sediment loads: (1) the baseline simulation, which reduced external phosphorus (P) loads over time to meet the total maximum daily load of 140 metric tons to the lake by 2015; (2) the baseline plus a large-scale chemical treatment, which used alum to bind sediment P; and (3) the baseline plus a large dredging project, which removed P-laden sediments. The ILPM and LOWQM differed greatly in their approach to simulate water quality, but their predictions for each scenario were similar. The baseline showed progressive improvements of water quality that approached the in-lake restoration goal of 40 μg P/L within 40 years; chemical treatment predicted this goal would be reached in 15 years; and dredging would potentially meet the goal within 30 years. Increasing or reducing the effectiveness of each treatment resulted in higher or lower predicted total phosphorus concentrations, respectively, but little change in the overall time line. An economic analysis found that in 2002 USD, chemical treatment would cost $500 million and dredging would cost $3 billion. Based on the model results, the economic analysis and other comprehensive technical evaluations of the scenarios, the baseline simulation was recommended as the preferred plan, and the management focus will remain on reduction of external P loads.

Hurricane effects on the planktonic food web of a large subtropical lake

Hurricanes Frances and Jeanne passed over Lake Okeechobee, Florida, in September 2004 and Hurricane Wilma in October 2005. The storms created large waves, strong currents, high wind seiches and uplifted over 3 million metric tons (collectively) of sediments into the water column. Suspended solid concentrations increased five-fold and there were substantial changes in the plankton. Unlike previously documented effects of hurricanes in the open ocean and estuaries, where increased nitrogen inputs stimulate primary productivty the hurricanes resulted in substantial reductions in biomass of bacteria, phytoplankton and phototrophic nanoflagellates, both in pelagic and near-shore habitats. Increases in macro-zooplankton biomass were observed in both habitats. There were sustained large increases in dissolved inorganic nitrogen and phosphorus in the water column after the hurricanes, coincident with large declines in mean irradiance in the mixed layer. Further, results from laboratory bioassays that exposed the phytoplankton to nutrient additions and a controlled light gradient indicate that the community shifted from being frequently nitrogen limited to most commonly light limited after the storms. The results confirm that the major driver of plankton food-web dynamics in this system is light availability, and that the primary mechanism of change caused by hurricanes is an accentuation of light limitation via greatly increased sediment re-suspension. There additionally was evidence of food-web-mediated effects where the loss of submerged vegetation and increased turbidity reduced the density and efficiency of visually feeding fishes, leading to a significant increase in biomass of macro-zooplankton.

Hurricanes Affect the Sediment and Environment in Lake Okeechobee

Three hurricanes (Frances and Jeanne in 2004 and Wilma in 2005) passed over Lake Okeechobee causing a number of changes in its sediment and environment. The unconsolidated sediment atop the mud sediments in the central region of the lake became thicker due to the settling and deposition of resuspended sediments after the hurricanes passed. The increased unconsolidated sediment layer was more easily resuspended, and resulted in sustained higher solids and nutrient concentrations and lower light transparency in the water column for more than 4 years after the hurricanes passed. Hurricanes Frances and Jeanne disturbed consolidated sediments to a depth of 7–15 cm from the surface while Hurricane Wilma extended the disturbance to a depth of 25 cm. Concentrations of total suspended solids (TSS) in the water column increased four to sixfold after the hurricanes. The high concentration of TSS was continuous and maintained 2–4 times higher than a normal situation in the lake until present. Hurricanes contributed to a decrease of submerged aquatic vegetation (SAV) density from 4–34 g dw/m2 to 0.1–4 g dw/m2 primarily through scouring. SAV recovery was very slow due to the increasing turbidity and reduced light transparency.

Mineral Distribution and Weathering in the Greater Everglades: Implications for Restoration

The Greater Everglades encompasses two regions of mineralogical contrast—the Lake Okeechobee Basin (LOB) and the region south of the lake that includes the Everglades and Florida Bay. Lake Okeechobee is a transition zone with mineralogical similarities to both regions. Weathering in the LOB is mainly unidirectional and flow driven, whereas in the Everglades it tends to be cyclical and driven by diurnal and seasonal biogeochemical gradients. LOB soils formed in quartz-dominated marine sediments of various ages under diverse landscape, hydrology, vegetation, and weathering intensities. Calcite and quartz are dominant south of the lake, with quartz diminishing southward. Secondary calcite forms via algal photosynthesis to become a major constituent of marl soils and periphyton. Phosphorus immobilized via periphyton formation is mainly in microbial biomass, but calcite is essential to the process. Minerals of lesser abundance can have disproportionate ecological influence. These include phyllosilicates, oxides, sulfates, sulfides, and phosphates, which have roles in turbidity and biogeochemical processes affecting the fate of P, Hg, and possibly As. Mineral-related perturbations of ecological impact include inhibited precipitation of calcite in periphyton mats, enhanced Hg methylation via sulfide formation from agricultural sulfate, and entrainment of readily-suspended minerals via construction of canals. The nature and rate of Ca-P interactions bear on the rate of ecological recovery. Understanding the dynamics of mineral redistributions is critical for effective restoration.

Best Management Practices and Long-Term Water Quality Trends in the Everglades Agricultural Area

The Everglades Agricultural Area (EAA) in South Florida, part of the historical Everglades, was initially drained in the early 20th century for agriculture and flood protection. The organic soils have been subject to subsidence caused by organic matter oxidation. Soils are deeper east of Lake Okeechobee compared to soils south of the lake. The area is mostly planted to sugarcane and other crops such as rice, vegetables, and sod. Concerns about quality of water leaving the EAA led to a regulatory program for mandatory best management practices (BMP) since 1995 to reduce phosphorus (P) loads out of the EAA by 25% compared to historical levels. The program is highly successful, with 100% grower participation and exceeding P load reduction required by law. Trend analysis conducted on selected EAA farms, subbasins, and whole basin show, in general, decreasing trends in P concentrations, drainage flow, and P loads. Differences are noted between farms and subbasins due to factors that include rainfall distribution, water management practices, irrigation water quality, soil type/depth, and cropping systems. Water management practices were the dominant factors affecting P loads out of the EAA. Water management research that targets farms with deeper soils is recommended to achieve additional P load reductions. Other practices to improve BMP performance include minimizing generation and transport of sediments from farm canals. The quality of irrigation water from Lake Okeechobee is of concern of its impact on BMP performance.

Phosphorous Cycling in the Greater Everglades Ecosystem: Legacy Phosphorous Implications for Management and Restoration

Phosphorus (P) retention in wetlands is an important function of watershed nutrient cycling, particularly in drainage basins with significant nonpoint nutrient contributions from agriculture and urban sources. Phosphorus storage involves complex interrelated physical, chemical, and biological processes that ultimately retain P in organic and inorganic forms. Both short-term storage of P mediated by assimilation into vegetation, translocation within above- and below-ground plant tissues, microorganisms, periphyton, and detritus, and long-term storage (retention by inorganic and organic soil particles and net accretion of organic matter) need to be considered. Here, we review and synthesize recent studies on P cycling and storage in soils and sediments throughout the Greater Everglades Ecosystem and the influence of biotic and abiotic regulation of P reactivity and mobility as related to restoration activities in south Florida. Total P storage in the floc/detrital layer and surface soils (0–10 cm) is estimated to be 400,000 metric tons (mt) within the entire Greater Everglades Ecosystem, of which 40% is present in the Lake Okeechobee Basin (LOB), 11% in sediments of Upper Chain of Lakes, Lake Istokpoga, and Lake Okeechobee, 30% in the Everglades Agricultural Area (EAA), and 19% in the Stormwater Treatment Areas (STAs) and the Everglades. Approximately, 35% of the P stored is in chemically nonreactive (not extractable after sequential extraction with acid or alkali) pool and is assumed to be stable. Phosphorus leakage rates from LOB and EAA are approximately 500 and 170 mt P per year, respectively, based on long-term P discharges into adjacent ecosystems. The estimated reactive P in the LOB soils is 65% of the total P, of which only 10 –25% is assumed to leak out of the system. Under this scenario, legacy P in LOB would maintain P loads of 500 mt per year to the lake for the next 20– 50 years. Similarly, surface soils of the EAA are estimated to release approximately 170 mt P per year for the next 50–120 years. The role of the STAs in reducing loads to downstream regions is critical and requires effective management of P forms to ensure the P is stabilized in these systems by the addition of chemical amendments or by dredging of accumulated soils. Also, additional efforts to minimize leakage of the legacy P from the northern regions should also be evaluated to reduce external P loading loads to the STAs.

Long-Term Water Quality Trends in the Lake Okeechobee Watershed, Florida

A Seasonal Kendall Tau test was used to determine statistical significance of mean monthly total phosphorus (TP) and total nitrogen (TN) concentration trends during the time period of 1991–2007, by station and basin at 35 long-term water quality monitoring stations located within the northern Lake Okeechobee watershed. One drainage basin (S-154) had a significant decreasing trend for mean monthly TP concentrations during the analysis period. A significant increase in TN concentrations was detected for four basins during this same time period. The S-154 basin which had the significant decreasing trend for TP also had the highest percentage of implemented Best Management Practices (BMPs) and the most rigorous types of nutrient control projects. These findings emphasize the need for continued implementation of intensive P management strategies. The increasing trend in TN for this watershed signals the need for added focus on reductions of TN in future nutrient source control projects and BMPs.

Environmental Influences on the Spatial Ecology of Juvenile Smalltooth Sawfish (Pristis pectinata): Results from Acoustic Monitoring

To aid recovery efforts of smalltooth sawfish (Pristis pectinata) populations in U.S. waters a research project was developed to assess how changes in environmental conditions within estuarine areas affected the presence, movements, and activity space of this endangered species. Forty juvenile P. pectinata were fitted with acoustic tags and monitored within the lower 27 km of the Caloosahatchee River estuary, Florida, between 2005 and 2007. Sawfish were monitored within the study site from 1 to 473 days, and the number of consecutive days present ranged from 1 to 125. Residency index values for individuals varied considerably, with annual means highest in 2005 (0.95) and lowest in 2007 (0.73) when several P. pectinata moved upriver beyond detection range during drier conditions. Mean daily activity space was 1.42 km of river distance. The distance between 30-minute centers of activity was typically <0.1 km, suggesting limited movement over short time scales. Salinity electivity analysis demonstrated an affinity for salinities between 18 and at least 24 psu, suggesting movements are likely made in part, to remain within this range. Thus, freshwater flow from Lake Okeechobee (and its effect on salinity) affects the location of individuals within the estuary, although it remains unclear whether or not these movements are threatening recovery.

Phosphorus run-off assessment in a watershed

The Watershed Assessment Model was used to simulate the runoff volume, peak flows, and non-point source phosphorus loadings from the 5870 km(2) Lake Okeechobee watershed as a case study. The results were compared to on-site monitoring to verify the accuracy of the method and to estimate the observed/simulated error. In 2008, the total simulated phosphorus contribution was 9634, 6524 and 3908 kg (P) y(-1) from sod farms, citrus farms and row crop farmlands, respectively. Although the dairies represent less than 1% of the total area of Kissimmee basin, the simulated P load from the dairies (9283 kg (P) y(-1) in 2008) made up 5.4% of the total P load during 2008. On average, the modeled P yield rates from dairies, sod farms and row crop farmlands are 3.85, 2.01 and 0.86 kg (P) ha(-1) y(-1), respectively. The maximum sediment simulated phosphorus yield rate is about 2 kg (P) ha(-1) and the particulate simulated phosphorus contribution from urban, improved pastures and dairies to the total phosphorus load was estimated at 9%, 3.5%, and 1%, respectively. Land parcels with P oversaturated soil as well as the land parcels with high phosphorus assimilation and high total phosphorus contribution were located. The most critical sub-basin was identified for eventual targeting by enforced agricultural best management practices. Phosphorus load, including stream assimilation, incoming to Lake Okeechobee from two selected dairies was also determined.