Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake

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1 Lake and Reservoir Management ISSN: (Print) (Online) Journal homepage: Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake K. R. Reddy, M. M. Fisher, Y. Wang, J. R. White & R. Thomas James To cite this article: K. R. Reddy, M. M. Fisher, Y. Wang, J. R. White & R. Thomas James (2007) Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake, Lake and Reservoir Management, 23:1, 27-38, DOI: / To link to this article: Published online: 23 Jan Submit your article to this journal Article views: 274 View related articles Citing articles: 24 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 30 November 2017, At: 07:10

2 Lake and Reservoir Management 23:27-38, 2007 Copyright by the North American Lake Management Society 2007 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake K.R. Reddy*, M.M. Fisher, Y. Wang, J.R. White 1 and R. Thomas James 2 University of Florida-Institute of Food and Agricultural Sciences Gainesville, FL 1 Department of Oceanography and Coastal Sciences Louisiana State University Baton Rouge, LA 2 South Florida Water Management District West Palm Beach, FL Abstract Reddy, K.R., M.M. Fisher, Y. Wang, J.R. White and R.T. James Potential effects of sediment dredging on internal phosphorus loading in a shallow, subtropical lake. Lake and Reserv. Manage. 23: Long-term phosphorus (P) loading to lakes has resulted in accumulation of P in sediments. Internal nutrient loading from sediments of shallow lakes such as Lake Okeechobee, Florida, has become a major concern in restoration programs. The objectives of this study were to determine (1) the potential impact of dredging on dissolved reactive P (DRP) flux out of sediments and (2) the equilibrium P concentration (EPC w ) of post-dredge sediments. Intact sediment cores from one location representing P-laden mud sediments of the lake were obtained. Four simulated dredging treatments were implemented: control (no dredging-current conditions); top 30 cm; 45 cm; and 55 cm sediment removal. Phosphorus release/retention characteristics of sediments were determined at water-column DRP concentrations of 0, 0.016, 0.032, 0.064, and mg/l. The water column in each core was replaced at approximately 60-day intervals, for a period of 1.2 years, with fresh lake water spiked with respective P concentrations. Significant decreases in water column DRP were observed only in sediment cores with 0-30 cm dredging. At ambient water column DRP levels, the P fluxes during the first 32 days were 0.4, 0.1, 0.4 and 0.2 mg P/m 2 /day for the 0, 30, 45, and 55 cm dredging treatments, respectively, and accounted for 11-38% of total P released during the 431 day study. At the end of the 1.2-year study, estimated EPC w were on the order of 0.033, 0.008, 0.022, and mg P/L for 0, 30, 45 and 55 cm dredging treatments, respectively. Dredging the top 55 cm sediments would result in the removal of approximately 123 g P/m 2, as compared to 80 and 108 g P/m 2 for 30 and 45 cm dredging, respectively. Laboratory experiments suggest that dredging can reduce internal P loading. However, further evaluation is needed to determine the extent to which the controlled laboratory experiments can be used to predict fluxes in the lake under natural conditions, and the long-term sustainability of improving water quality by dredging. Key Words: eutrophication, sorption, internal load, water quality, Lake Okeechobee Lakes are often the final recipients of nutrients discharged from adjacent uplands and wetlands. Since many lakes are phosphorus (P) limited, loading of this nutrient is of particular concern to environmental managers. Nonpoint sources of P Contribution from Wetland Biogeochemistry Laboratory, Soil and Water Science department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL * Corresponding author: krr@ufl.edu dominate eutrophication processes of many lakes. Thus, in many situations, alternative land use management practices in the watershed have been implemented in an effort to reduce the overall load to receiving water bodies. For example, best management practices around Lake Okeechobee, Florida, have significantly improved water quality of the lake (Steinman et al. 1999). This shallow subtropical lake (Fig. 1) may be moving from a naturally eutrophic state to a hyper-eutrophic state due to P loading from the surrounding watershed 27

3 Reddy, Fisher, Wang, White and James (Havens et al. 1996). Recent hurricane events in South Florida resulted in substantial resuspension of bottom sediments. This has resulted in high turbidity levels and substantial increase in total P concentration of the water column. In eutrophic lakes much of the dissolved reactive P (DRP) is rapidly assimilated by algae. Both total P content and lability tend to be higher in recently deposited sediments, and in many cases this reflects increased anthropogenic P loading. Concentration of total P and fractions of P are generally higher in recent sediments, related to recent P loading. Although accretion of sediment-bound P suggests that P flux is downward (i.e., from the water column to sediments), the DRP flux is upward (i.e., from sediments to the water column) in response to concentration gradients across the sedimentwater interface (Lijklema 1994, Moore et al. 1998, Steinman et al. 2004, Fisher et al. 2005). The net transport of particulate P is from the water column to sediments. Diffusion of DRP into the overlying water can occur if the concentration of interstitial P exceeds that of the overlying water. This is especially true if surface sediments are anaerobic, reducing the ability of iron to bind to inorganic P, thus increasing the amount of interstitial P that can be released to the water column (Moore et al. 1998). Relative importance of P transfer due to diffusive flux and resuspension flux must be quantified to accurately estimate annual P flux from internal sources (Sheng 1999). This internal load (a result of historical external loads) can extend the time required for lake recovery once external loads are curtailed (Marsden 1989, Sas 1989). Internal dissolved P fluxes, measured in 1988 in Lake Okeechobee, were equivalent to external loads (Moore et al. 1998, Malecki et al. 2004, Fisher et al. 2005). Similarly, the significance of the internal P loads in regulating eutrophication has been demonstrated in other shallow lakes (Anderson and Ring 1999, Graneli 1999, Sondergaard et al. 1999, Watts 2000, Steinman et al. 2004). Therefore, internal P fluxes from benthic sediments to the water column can offset any water column responses to external P load reductions. The P-rich mud sediment in Lake Okeechobee covers an area greater than 80,000 ha and has a volume of approximately 200 million m 3 (Fisher et al. 2001). Compared to other lake restoration projects (Cooke et al. 1993), removal of mud sediments from Lake Okeechobee will require an order of magnitude greater effort. However, dredging of surface sediments alone will not reverse eutrophication, unless external loads are also curtailed (Kleeberg and Kohl 1999). Major questions related to dredging are: As older buried sediments are exposed from dredging, what is the impact on sediment water interactions? Will the P sorptive capacity of sediments improve or degrade? Will loads of DRP from the sediments increase or decline? To answer these questions, this study was conducted to estimate the impact of dredging Figure 1.-Map of Lake Okeechobee showing sediment types and location of sampling site. on flux of DRP out of the sediments and to determine the equilibrium P concentration of dredged sediments. Materials and Methods Study Site Lake Okeechobee, Florida (26 58 N, W), is the largest lake by area (1,732 km 2 ), in the southern United States (Fig. 1). This shallow (average depth 2.7 m) eutrophic lake is used for agricultural and residential water supply, flood control, commercial and sport fishing, and supports wildlife habitat for native species such as the American alligator and endangered species such as the snail kite. There is an extensive littoral area on the western side of the lake adjacent to a sand bottom area in the shallow pelagic region of the lake. A large area of peat sediments exists on the south end of the lake with a limestone rock outcrop abutting just to the north. The deep pelagic region has an extensive area of flocculent mud sediments. The top 10 cm of these sediments contain an estimated 28,600 metric tons of phosphorus (P) (Reddy et al. 1996, Fisher et al. 2005). Because these sediments are a source of DRP to the water column of Lake Okeechobee (Moore et al. 1998, Fisher et al. 2005), one possible management technique to improve water quality in the lake is dredging these mud sediments from the lake. 28

4 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake Sampling of Sediment Cores Intact sediment cores were taken by either SCUBA divers or a piston corer using acrylic tubes (Fisher et al. 1992) at one location, M-9, representing the mud zone in Lake Okeechobee (Fig. 1). Station M-9 is typical of the mud zone in the lake (Fisher et al. 2001). Sixty-two intact sediment cores were collected from station M-9 ( N, W) on October 21-22, The core material was 0.3 cm wallthickness acrylic, measuring 6.8 cm internal diameter by 1 m in length. The average sediment depth was 57.4 cm (±5.9 SD), ranging from 43 to 69 cm. The cores were hand-driven by a SCUBA diver, through the soft sediment until reaching the underlying marl or sand layer. Low visibility at the site necessitated the establishment of a reference system to avoid sampling in regions that had been disturbed by the coring operation. The system consisted of a 30 m line, weighted on each end to keep the line stretched taut against the sediment surface. A second short baseline (1 m) was attached to this reference line with a sliding clip. Groups of four to five cores were taken in front of the baseline, after which the baseline was advanced 1-2 m along the 30 m line. This ensured that cores were obtained only from undisturbed regions. Each core was stoppered on the top, then gently lifted out of the sediment column. A #13.5 stopper was inserted into the base of the core, and the core was returned to the laboratory for processing. Laboratory Methods Intact sediment cores were returned to the University of Florida Wetland Biogeochemistry Laboratory. One set of sediment cores was sectioned into 4-cm depth increments at selected depths (0-4, 30-34, and cm) and analyzed for loss on ignition (LOI), total carbon (C), total nitrogen (N), and total P (White and Reddy 2000). Another set of cores was sectioned at 0-10, 10-20, 20-30, and cm intervals. The sediment porewater was extracted under anaerobic (glove bag) conditions and analyzed for selected physico-chemical parameters. At the time of core collection, 180 L of lake water was retrieved from site M- 9, filtered through 0.45 µm membrane filters and used to reflood sediment cores through a drip line causing minimal sediment disturbance. Sediment removal to mimic dredging was performed on October 24-25, 2000 as follows: Control (no dredging-current conditions) 30-cm surface mud removal 45-cm surface mud removal 55-cm surface mud removal Dredging was accomplished after the overlying water was removed using a screw-type device. The device was screwed into the sediment at approximately 4-cm intervals, and a plug of sediment was removed from the core. After the dredging procedure, the core walls were wiped clean with wet paper towels. At the end of dredging, sediment cores were flooded with filtered lake water and spiked with predetermined DRP concentrations. The water column was adjusted to cm to equal 0.75 L of water. The subsurface sediments that remained after dredging to various depths are referred as postdredge sediments. Five water-column DRP concentrations (0.00, 0.015, 0.03, 0.06 and 0.12 mg/p/l) were established in triplicate for each dredging treatment. The dredge and spike treatments were randomly arranged in light-proof boxes. The water column in all treatments was aerated to maintain dissolved oxygen levels >4 mg/l. All sediment cores were incubated in the dark at approximately 22 C. Temperatures were continuously recorded with a Campbell Scientific CR10 data-logger (Campbell Scientific, Logan UT). At the end of 30 days the water column of each core was replaced with filtered lake water spiked with the same P concentrations as described previously. To determine the longterm P flux from sediments, the water column was exchanged with filtered lake water at 32, 99, 156, 218, 281 and 345 days, and the experiment was concluded after 431 days. The total length of incubation was long enough to allow six loading events. This time was sufficient to investigate the decrease in short term benefits from exposing sediments during dredging in an effort to find a longer term equilibrium. Water samples of 10 ml each were removed at predetermined intervals from the center of each water column, filtered through 0.45 µm membrane filters and analyzed for DRP. The amount of water removed was replaced with filtered lake water. At selected sampling periods, water column ph, dissolved oxygen and electrical conductivity (EC) were measured. At the end of each exchange period, water samples also were analyzed for total P. DRP and total P were analyzed with a Technicon AutoAnalyzer (Tarrytown, NY) using EPA method At the end of the experiment, sediment cores were sectioned into predetermined depth increments under anaerobic conditions (N 2 atmosphere). Subsamples of these sections were measured for water content and bulk density, and subsequently extracted with 1 M HCl (sediment to solution ratio of 1 to 50; w/v basis) after a 3-hr equilibration period. Filtered solutions were analysed for Ca, Mg, Fe, and Al using inductively coupled argon plasma spectrometry (USEPA 1983). A second subsample of each section was transferred, under a N 2 atmosphere in a glove box, into 50-mL centrifuge tubes, and a sequential fractionation scheme was performed to determine KCl-extractable P (labile inorganic P), NaOH extractable inorganic P (Fe and Al-bound P or nonapatite P), NaOH organic P (humic- and fulvic-bound P), HCl extractable P (Ca- and Mg-bound P), and residual P (Olila et al. 1994). DRP and NH 4 -N were analyzed with a Technicon 29

5 Reddy, Fisher, Wang, White and James AutoAnalyzer (Tarrytown, NY) using EPA methods and respectively. Sulfate was determined with a Dionex Series 4500i ion chromatograph (Sunnyvale, CA). Standard methods were used to determine the sediment chemical parameters (APHA 1989). Calculations Flux calculations were based on DRP increase over time (Malecki et al. 2004). The amount of P removed during sampling and the amount of P in the filtered lake replacement water were determined. At the end of each reflooding interval, net P retention or release by bottom sediments was calculated based on changes in mass P removed per unit surface area: P r = ([(C t - C t-1 ) V t + (C t-1 - C r ) V s ] / A) [1] where C r is the DRP concentration (mg/l) of the filtered lake replacement water; V t is the total volume of the water column (L); C t is the DRP concentration (mg/l) at time t (days); C t-1 represents DRP concentration (mg/l) at sampling period before time t days; V s is the sample aliquot volume (L); A is the cross-sectional area (m 2 ); and P r denotes the P release or retention by sediments (mg/m 2 ) for one reflooding interval. Positive P r values represent P release from sediments, while negative P r values represents P uptake by sediments. The P r values were averaged for all water exchange periods, for a total of seven exchange periods over 431 days. Long-term average P flux (P avg ) was expressed as the average of P retention or release normalized for one year, with units of mg P/m 2 /year. Phosphorus retention (P avg ) was related to both the initial water column DRP concentration (C o ) and DRP loading rate (mg P/m 2 /year) to estimate retention coefficients: P avg = - R c C o + P o [2] Where: P avg represents the P retention or release (mg/m 2 /year); R c is the P retention coefficient related to the water column concentration (L/m 2 /year); C o is the initial water column concentration (mg/l); and P o represents maximum P release potential of sediments (mg P/m 2 /year) at water column DRP levels near zero levels. The P avg values were also compared to DRP loading rates: P avg = - R l P l + P o [3] where: R l is the P retention coefficient related to P loading rate; and P l is the phosphorus loading (mg/m 2 /year). Critical initial concentrations (equilibrium P concentration in the water column, EPC w ), where P avg approaches zero can be estimated as follows: EPC w = P o /R c [4] Using intact sediment cores, we estimated both EPC w and R c for various water exchange periods involving the water column with several DRP concentrations. Critical P loading rates (CP l ), where P r approaches zero can be estimated from equation 3: CP l = P o /R l [5] Data Analysis Statistical analysis was performed using a two-way ANOVA. Linear regression analysis was performed among variables including: P concentration, P loading, and P removal or retention. A t-test was used to compare the influence of dredging treatments with DRP concentration of the water column. Lake Okeechobee Water Quality and Phosphorus Budget Water quality has been monitored on a monthly or biweekly basis since 1973 at eight locations in Lake Okeechobee (James et al. 1995b, Havens et al. 2003). Total and dissolved phosphorus loads to and from the lake were developed with a loading calculation program that uses daily flow and water quality data collected monthly to biweekly collected as grab and/or composite sample at each lake inflow or outflow site (James et al. 1995a). The data were summed yearly to develop annual budgets (James et al. 1995a, Havens and James 2005, James et al. 2006). Phosphorus loading from atmospheric deposition added an estimated 35 metric tons per year to the lake (Florida Department of Environmental Protection Annual assimilation of total and dissolved phosphorus to the sediments was determined using the equation: σ = M - (I - O) [6] where σ = annual assimilation of P into the sediments; M = change in mass of P in the lake from January to the next January; I = sum of all loads into the lake (including atmospheric deposition); and O = sum of all surface loads out of the lake. A negative σ represents accumulation of P in the sediments. Results Sediment Characterization The sediment was fine-grained silty mud, with little stratigraphic change with respect to depth. There were, however, the remains of small (2 mm) gastropods in very distinct bands, one at approximately 9 cm and another at 27 cm. There was another layer of large gastropods at a depth of approximately 53 cm. These layers could be used as data for future sediment accretion studies. A deposit of what ap- 30

6 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake Table 1a.-Selected physico-chemical characteristics of sediments (BD, bulk density; LOI, loss on ignition). Depth BD LOI Total P Total N Total C cm g/cm 3 % mg/kg g/kg g/kg 0-4 Mean SD Mean SD Mean SD Mean SD Table 1b.-Selected metal concentration in sediments. Depth HCl-Ca HCl-Mg HCl-Fe HCl-Al cm g/kg g/kg mg/kg mg/kg 0-4 Mean SD Mean SD Mean SD Mean SD Table 1c.-Selected physico-chemical characteristics of sediment porewater in the mud zone of Lake Okeechobee (n=3) (EC, electrical conductivity; DRP, dissolved reactive phosphorus; TP, total phosphorus; TKN, total Kjeldahl nitrogen; TOC, total organic carbon) cm cm cm cm cm Parameter Mean SD Mean SD Mean SD Mean SD Mean SD ph EC µs/cm NH 4 -N mg/l DRP mg/l TP mg/l TKN mg/l TOC mg/l SO 4 -S mg/l Ca mg/l Mg mg/l Fe mg/l Al mg/l peared to be well-decomposed peat was found at the base of the fine-grained sediment, overlying the marl. This material had lost any distinct fibrous structure. This layer was not removed for the 55 cm dredging treatment. Total P concentration in surface sediments was much higher than at the selected dredged depths (Table 1a). Similar trends were observed for total N and C, and organic matter content (expressed as loss on ignition, LOI). Extractable metals (Ca, Mg, Fe, and Al) were measured in the surface 4 cm sediment at the end of the study (Table 1b). Calcium and Mg concentrations were higher at depths cm and cm as compared to 0-4 and cm depths. These depths represent surface sediments after the dredging was implemented at 0, 30, 45, and 55 cm, respectively. Iron concentrations showed a distinct trend with high values in 0-4 cm and decreased with depth. Aluminum concentrations were higher in surface sediments and decreased with depth. The bulk DRP and TP concentrations of sediment porewater were higher at subsurface depths (40-55 cm) as compared to surface sediments (Table 1c). Ammonium N, Total Kjeldahl N (TKN), and Total Organic C (TOC) concentrations of sediment porewater also increased with depth. Low concentrations in surface sediments were due to diffusion of dissolved NH 4 -N, TKN and TOC from sediments to the overlying water column. Sulfate concentrations were higher in surface sediments and decreased to negligible levels in subsurface sediments, indicating highly reduced conditions (Table 1c). Calcium, Mg, Fe, and Al concentrations did not vary with depth. 31

7 Reddy, Fisher, Wang, White and James Dissolved Reactive Phosphorus Concentration in the Water Column Water column DRP concentrations were influenced by sediment dredging. During the first and second exchange period, the DRP levels increased up to 0.07 mg/l. These effects were more pronounced during the first seven months after dredging, followed by very little or no effect during the remaining six months of the study. For all water exchange periods, DRP levels in the water column were significantly lower (p < 0.01) in the 30-cm sediment dredging, as compared to other dredging treatments (Fig. 2). In contrast, dredging sediments to 45 and 55 cm depth showed very little or no effect on water column DRP concentrations. The initial increase in water column DRP concentrations was due to rapid release from underlying sediments (Table 1b). The DRP gradients decreased with time as sediment porewater DRP diffused into the overlying water column. In all post-dredge treatments, water column DRP concentrations generally stabilized after 7-14 days with minimal changes thereafter (Fig. 3). Changes in DRP concentration of the water column for all water exchange periods were expressed as a fraction of initial concentration (C o ; Fig. 3.) The C t /C o values were significantly different with time and dredging treatments (p < 0.01). At ambient levels of P, a 10-fold increase in DRP occurred during the first 30 days in sediment cores with no dredging, as compared to a two-fold increase in 30-cm dredging treatment and a five to 8-fold increase in 45 and 55 cm dredging treatments, respectively (p < 0.01). At the end of the study period, DRP levels in the water column of all treatments reached ambient levels. Phosphorus Retention or Release by Sediments Phosphorus release or retention was evaluated at various water column P concentration treatments and for several dredging treatments. The initial water column DRP concentrations of 0.007, 0.023, 0.039, 0.071, and mg/l equated to loadings of 9.4, 29, 49, 88, and 150 mg P/m 2 /year, respectively. Cumulative P retention (-) or release (+) was determined on a mass per unit area at the end of each water exchange period (Fig. 4 and 5). At low P loading rate (9.4 mg P/m 2 /year), net P release from sediments occurred in all dredging treatments (Fig 4a). A linear increase in P release occurred during the first 156 days, followed by very little or no change during the remainder of the study. At this loading rate (9.4 mg P/m 2 /year) the P release rate was significantly lower in 30-cm dredging treatment, as compared to other treatments. At ambient water column DRP levels, P flux during the first 32 days were 0.4, 0.1, 0.36, and 0.16 mg P/m 2 /day for 0, 30, 45, and 55 cm dredging treatments, respectively. These fluxes represent approximately 11-38% of total P released in <8 % of the total incubation time of the 431 day study. The P fluxes during the first 99 days (21% Figure 2.-Water column P concentration as sediment dredging. 0 cm refers to no dredging and 30 cm refers to dredging surface 30 cm sediments. Water column was exchanged 7 times with filtered lake water during 431 day study. Figure 3.-Changes in water column dissolved reactive P (DRP) concentration at days, expressed as a function of initial concentration. Sediment dredging treatments are: 0, 30, 45 and 55 cm depth. 32

8 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake Figure 4.-Influence of sediment dredging on dissolved reactive P (DRP) on cumulative P retention or release by sediments. Negative values represent P retention. Initial DRP concentration represent annual P loading of 9.4, 29, 49, 88 and 150 mg P/m 2 /year for initial P concentrations of 0.007, 0.023, 0.039, and mg P/L, respectively. of incubation time) accounted for approximately 49-71% of total P released, and through day 156 (31% of incubation time) this flux accounted for % of the total. The P flux in the final 275 days of the study accounted for 0-34% of the total P released. Table 2.-Regression equations showing the relationship between phosphorus retention or release (P avg, mg P/m 2 /year) and phosphorus loading (P l, mg P/m 2 /year). R l denotes phosphorus retention coefficient related TP loading rate and P o is the maximum P release potential of sediments under ambient water column conditions. All R 2 values are significant (p < 0.01). At a P loading rate of 29 mg P/m 2 /year (0.023 mg/l), net P release occurred only from 0 and 55 cm dredged sediments, while 30 and 45 cm dredged sediments had net P retention. At a P loading rate of 49 mg P/m 2 /year (0.039 mg/l), net P release occurred only during the first 156 and 280 days for the 0 and 55 cm dredged sediments, respectively, followed by net P retention during the remainder of the study. At P loading rates of 88 and 150 mg P/m 2 /year (0.07 and mg/l), all dredged sediments retained P (Fig. 4d and e). Estimated CP r (critical P loading rates) values were 26.7, 5.3, 13.4 and 34.1 mg P/m 2 /year for the four dredging treatments, for 0, 30, 45, and 55 cm, respectively (Table 2). Below these loading rates sediments functioned as a net source of P, and above these loading rates sediments functioned as net sinks. Increased P loading not only increased net P retention, but Dredging treatment P avg = - R l P l + P o R 2 0 cm P avg = 0.75 P l cm P avg = 0.85 P l cm P avg = 0.76 P l cm P avg = 0.67 P l it also increased the water column P concentration. High P retention coefficient (R l ) and low maximum P release from sediments (P o ) are ideal for maintaining low water column P concentration. High R l represents strong buffering capacity of sediments to retain P, and high P o represents the potential 33

9 Reddy, Fisher, Wang, White and James Table 3.-Regression equations showing the relationship between phosphorus retention or release (P avg, mg P/m 2 /year) and phosphorus concentration of the water column (C o, mg P/L). R c denotes phosphorus retention coefficient related to water column concentration (L/m 2 /year) and P o is the maximum P release potential of sediments under ambient water column conditions. All R 2 values are significant (p < 0.01). Dredging treatment P avg = - R c C o + P o R 2 0 cm P avg = C o cm P avg = C o cm P avg = C o cm P avg = C o Figure 5.-Influence of sediment dredging on dissolved reactive P (DRP) on P retention or release by sediments at the end of 431 days. Negative values represent P retention. Initial DRP concentration represent annual P loading of 9.4, 29, 49, 88 and 150 mg P/m 2 /year for initial P concentrations of 0.007, 0.023, 0.039, and mg P/L, respectively. capacity (or P present in labile pool under ambient conditions) of native sediments to release P, prior to any loading. The P retention coefficient (R 1 ) and equilibrium P concentration (EPC w ) are influenced by sediment dredging treatments (Table 3). The EPC w is the ratio between P o (mg P/m 2 /year) and R c (L/m 2 /year). Sediments with low P o /R c ratios maintain low DRP levels in the water column. At the end of the 431-day study, estimated EPC w were on the order of 0.033, 0.008, and mg P/L for 0, 30, 45 and 55 cm dredging treatments, respectively. For 0 and 55 cm dredging treatments, high EPC w values were noted during the first 30 days, followed by a steady decrease during the remainder of the study (Fig. 6). The EPC w values were consistently lower in the 30 and 45 cm dredged sediments for all water exchange periods. Sediment Phosphorus Forms Surface sediment (0-4 cm) P concentrations were highest in the no dredged (control) sediments and decreased in the order of 30, 45, and 55 cm dredged sediments (Table 1a). Total P concentrations of 4-10 cm sediments were approximately in the same range as 0-4 cm sediments (Fig. 7). Figure 6.-Influence of sediment dredging on equilibrium P concentration (EPC w ). At water column dissolved P concentrations above these values, sediments function as sinks and below these values sediments function as source. Phosphorus loading to the water column had no significant effect on total P concentrations or P forms of surface sediments (White et al. 2004, White et al. 2006). The highest P loading to the water column during 431 days of study was 176 mg P/m 2, <4% of the total P storage in surface sediments. The amount of this added P taken up by sediments at the end of 431 days was <2% of the total P storage in surface sediments. Thus it is not surprising that P pool sizes in surface sediments were not altered significantly during 431 days of laboratory incubation. Inorganic P was the dominant pool in sediments, with 79-90% of the total P in 0-4 cm, and 88-91% of total P in 4-10 cm sediments (Fig. 7). Labile inorganic P (estimated by KCl extraction) accounted to <1% of total P. Non-apatite P (Feand Al-bound P) accounted for 4-6% of total P in surface sediments and 3-5 % of total P in 4-10 cm sediments. Low Fe- and Al-bound P was found in surface sediments with 55 cm dredging treatment. These sediments have much larger proportion of inorganic P as apatite P (Ca- and Mg-bound P). Alkali extractable organic P (fulvic- and humic acid-bound P) accounted for <2% of total P. Phosphorus not extracted 34

10 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake Figure 7.-Influence of sediment dredging on sediment phosphorus forms. KCl-Pi = labile inorganic P extracted with neutral salt; NaOH-Pi = Fe and Al bound P or non-apatite P; HCl-Pi = Calcium and Mg bound P or apatite P; NaOH-Po = Humic and Fulvic bound P; Residue P = resistant P that was not extracted either by alkali or acid. by either NaOH or HCl was considered resistant P and is not biologically available. This fraction accounted for 9-18% of total P in 0-4 cm sediments and 8-10% of total P in 4-10 cm sediments. Labile inorganic P storage in the top 10 cm sediments was 69, 67, 81 and 103 mg P/m 2 for 0, 30, 45 and 55 cm dredging treatments, respectively (Fig. 8). At the annual P fluxes measured, these storage amounts represent approximately a 3-year supply for 0, 45 and 55 cm dredged sediments, and 28 years for 30 cm dredged sediments. Phosphorus in the labile pool is replenished through desorption and dissolution of Fe- and Al-bound P and Ca- and Mg-bound P. Mineralization of organic P also can replenish the labile inorganic P pool. Measured P flux had no significant relationships with sediment P forms. Lake Okeechobee Water Column Phosphorus Concentration and Loads Over the past three decades, total P in the water column of Lake Okeechobee has increased from a five-year average of mg/l in the mid-1970s to a five-year average of mg P/L from (Fig. 9a). The overall increase of P in the past decades has resulted in many changes in the lake including increased frequency of algal blooms and an increasing abundance of nitrogen fixing cyanobacteria (Havens et al. 1996). The increased TP has been attributed Figure 8.-Remaining phosphorus storages in various sediment P pools in sediment cores with no dredging (0-55 cm) and sediment cores with 30 cm dredging (30-55 cm). Values in boxes are the storages (mg P/m 2 ), and the arrows indicates fluxes (mg P/m 2 / year). to excessive external P loads to the lake (Fig. 9b and 9c), which have ultimately accumulated in mud sediments in the center of the lake (Fig. 9d). The dramatic increase in P after 2003 is attributed to the impacts of three hurricanes (Frances, Jeanne and Wilma) that moved directly over the lake. These hurricanes caused increased resuspension of P-laden sediments and increased loading via increased inflows to the lake. Currently, there are nearly 30,000 metric tons of P that have accumulated in the upper 10 cm of these mud sediments (Fisher et al. 2001). This accumulation has reduced the ability of sediments to assimilate P (Fig. 9b). In the past two years ( ), based on nutrient budgets, the sediments were a source of P not a sink. This change was caused by the combination of increased outflow to reduce lake levels and the associated high levels of P from resuspended sediments during large storm events. Discussion In this study we evaluated the influence of simulated sediment dredging and P loading on DRP flux from sediments to the 35

11 Reddy, Fisher, Wang, White and James Figure 9.-a) Annual mean Total and Dissolved Reactive Phosphorus by date, b) Annual total phosphorus load export and assimilation, c) Annual Dissolved Reactive Phosphorus load, export, and assimilation, and d) P accumulation in sediments since water column. Dredging the top 30, 45 and 55 cm sediment had a variable effect on P flux at ambient water column DRP concentrations. Maximum P flux occurred during the first 32 days of incubation. Phosphorus flux with no dredging was 0.4 mg P/m 2 /day. In studies conducted 10 years ago, maximum P flux measured for mud sediments were in the range of mg P/m 2 /day (Moore et al. 1998, Fisher et al. 2005). These results suggest that during the last 10 years internal P load from sediments has not changed appreciably. Dredging the top 30 cm of sediments significantly decreased P flux into the overlying water column based on the cores collected from the mud zone site of the lake. Much of this dredged sediment is the result of recent loads (Brezonik and Engstrom 1998). These sediments are enriched in both organic and inorganic forms of P. The porewater P concentrations in the top 30 cm of sediments were in the range of mg P/L, suggesting sharp gradients between sediments and the overlying water column (Table 1c). These sharp gradients can increase the potential DRP flux from sediment porewater to the overlying water column. Dredging the top 55 cm of sediments resulted in removal of approximately 123 g P/m 2, as compared to 80 and 108 g P/m 2 for the 30 and 45 cm dredging, respectively. Approximately 66 and 88% of the P stored in the top 55 cm sediments was removed by 30 and 45 cm dredging. Post 1980 P accumulation rates in the mud zone were estimated at 0.85 g P/m 2 /year (Brezonik and Engstrom 1998). Assuming similar P accu- mulation occurred prior to 1980, removal of the top 30 cm sediment accounts for approximately 94 years of P loading, as compared to 127 and 145 years of P loading for the 45 and 55 cm dredging, respectively. Assuming P accumulates only in mud sediments, which accounts for 44% of the lake bed (Fisher et al. 2001), estimates from P budgets (Fig. 9d) indicate that 0.63 g P/m 2 /year accumulated from 1973 to At this rate of P accumulation, the top 30 cm sediment accounts for 128 years of P loading. Both biotic and abiotic processes regulate DRP concentration of the water column of lakes. In eutrophic lakes such as Lake Okeechobee, much of the DRP inputs are rapidly assimilated by algae. Dead algal cells, along with the particulate inorganic and organic solids accrete on the bottom as sediment. Sediment bound P (particulate P) accretion rates increase with P loading. For example, in Lake Okeechobee, P accretion rates have increased about 4-fold since the 1900s (from about 0.25 g P/m 2 /year before 1910 to 0.85 g P/m 2 /year in the 1980s; Brezonik and Engstrom 1998). Concentrations of total P and several P fractions were generally higher in recent sediments and decreased with depth, suggesting the influence of recent P loading. Although accretion of sediment-bound P suggests that particulate P flux is downward (i.e., from the water column to sediments), the DRP flux is upward (i.e., from sediments to the water column) in response to concentration gradients established at the sediment-water interface. 36

12 Potential Effects of Sediment Dredging on Internal Phosphorus Loading in a Shallow, Subtropical Lake Present internal loading was estimated two ways: (1) sediment cores were incubated for 431 days under laboratory conditions, with a water exchange period of approximately 60 days, and (2) short-term initial incubation of sediment cores for 30 days under aerobic conditions (equilibration period). These were measured in sediment cores dredged for 0, 30, 45 and 55 cm. This flux represents diffusive flux only and does not consider any potential bioturbation or sediment resuspension. Estimated annual P flux from sediments to the water column is 25, 2.4, 25 and 21 mg P/m 2 /year for no dredging, 30, 45 and 55 cm dredging treatments, respectively. These flux values are very low compared to those measured during short term incubations. In our opinion, these annual fluxes are not truly representative of field conditions in the lake, since the system did not receive any external inputs and lacks hydrodynamic events in the water column. Sediment P fluxes measured during short-term incubations may provide more realistic estimates of diffusive flux P into the water column. Under aerobic water column conditions, P fluxes were 0.41, 0.11, 0.36 and 0.16 mg P/m 2 /day for 0, 30, 45 and 55 cm dredging treatments, respectively. Phosphorus flux from sediments is regulated by the physical-chemical properties at the sediment-water interface. Phosphorus solubility in mud sediments of Lake Okeechobee is regulated by both iron and calcium chemistry (Moore and Reddy 1994). Surface sediment is usually aerobic to a depth of 1-2 cm, where ferric oxyhydroxide (FeOOH) regulates DRP solubility by forming ferric phosphates (Moore and Reddy 1994). This zone usually functions as a potential sink for DRP diffusing from the underlying anaerobic sediments. Thus, any physical-chemical changes such as inputs of organic matter and depletion of dissolved oxygen can result in reduction of ferric phosphates and release P. In our study, P flux was measured under aerobic water column conditions, which maintained an aerobic sediment-water interface. Under field conditions, inputs of organic matter and diel fluctuations in oxygen can result in anaerobic conditions at the sediment water interface and concomitant release of DRP. Pool sizes of inorganic and organic P forms can regulate P flux. Readily mobile and labile pools of P in sediment include DRP in porewater and an exchangeable P that is extracted with a neutral salt. However, P release from sediments cannot be explained by labile P alone, since other pools of P including Fe- and Al-bound P, Ca- and Mg-bound P, and organic P continuously replenish labile P at varying rates (Olila et al. 1994). Surface sediments are enriched by P derived from two sources: particulate organic and inorganic P settling from the water column, and DRP flux from anaerobic sediment layers into aerobic surface sediments, where it is precipitated with oxidized forms of iron (Moore and Reddy 1994, Watts 2000). Dredging surface sediments to a depth of 30 cm removed the accumulated equivalent of more than 100 years of P loading. Also, it is likely that the greater P retention capacity and lower annual flux observed in the 30 cm dredge treatment was due in part to exposure of deeper sediments to an aerobic water column. This newly exposed sediment interface was probably under-saturated with respect to both Fe- and Ca-bound P compared to P enriched surface sediments. In addition, this new surface sediment was probably very reactive when exposed to an aerobic water column, thus resulting in increased retention of P. Total P in the dredged sediment contained approximately 52-56% of P in the inorganic form and the remaining as organic P. Conclusions Laboratory experiments indicated that dredging the upper 30 cm of sediment of Lake Okeechobee might have a positive effect in reducing internal P loading for a short period of time. Laboratory studies consider only diffusive flux from sediment to the overlying water column and do not include repeated sediment resuspension events and inputs of organic material at the sediment water interface. Removal of the top 30 cm of sediment can remove approximately 65% of TP sediment storage. Implementation of P reduction goals such as dredging may have significant costs and economic impacts. Thus, lake management strategies should focus on reduction of external loads, which will ultimately have a positive effect in reducing the internal load. To determine the regulators of internal load, it is critical that we have a thorough understanding of the dynamics of physical, chemical, and biological processes at the sediment-water interface regulating the internal load within the lake. The key questions often asked are: (1) Will Lake Okeechobee respond to P load reduction? (2) If so, how long will it take for the lake to recover and reach its background condition? (3) Are there any economically feasible management options to hasten the recovery process? Release of the internal load (a consequence of past excessive external loads) can extend the time required for lake to reach its original condition. This lag time for recovery should be considered in developing management strategies for the lake. Acknowledgments This research was in part funded by a contract (C-11680) from the South Florida Water Management District, West Palm Beach, FL, to the University of Florida. The authors thank Karl Havens, South Florida Water Management District for his critical comments to improve the quality of this manuscript. References Anderson, F.O. and P. Ring Comparison of phosphorus release from littoral and profundal sediments in a shallow, eutrophic lake. Hydrobiologia 409:

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