19 Artificial Circulation 19.1 INTRODUCTION Artificial circulation, also referred to as destratification, and hypolimnetic aeration/oxygenation (Chapter 18) are two general techniques for aerating lakes. Circulation has been achieved by pumps, jets, and diffused air. Complete lake circulation is usually the objective, and in the majority of cases examined either stratification was prevented or destratification occurred. Unlike hypolimnetic aer- ation/oxygenation, the temperature of the whole lake is raised with complete circulation; the greatest increase in temperature occurs at depths that were previously part of the cooler hypolimnion. The principal improvements in water quality caused by complete circulation are oxygenation and chemical oxidation of substances in the entire water column (Pastorak et al., 1981, 1982). Similar to hypolimnetic aeration, its main benefit is enlarging the suitable habitat for aerobic animals. Complete circulation may reduce internal loading of P, if the principal P-release mechanism was due to iron reduction in anoxic profundal sediments (Chapter 18). Complete circulation may also reduce algal biomass by increasing the mixed depth, thereby reducing available light, and by subjecting mixed algal cells to rapid changes in hydrostatic pressure (Lorenzen and Mitchell, 1975; Fast, 1979; Forsberg and Shapiro, 1980). Although reduced internal P loading and decreased phytoplankton biomass may be reasonable expectations, other factors such as nutrient availability in the photic zone, may be more important to P availability, and actually be enhanced with circulation. In some instances, phytoplankton biomass and P content either did not change or were increased following circulation. Artificial circulation has been employed as a management technique since at least the early 1950s (Hooper et al., 1953). Initially it was used to prevent winter fish kills in shallow, ice-covered lakes (Halsey, 1968). Although not discussed here, refinements to winterkill prevention were proposed recently (McCord et al., 2000; Miller et al., 2001; Miller and Mackey, 2003). Nearly all of the reported applications of the technique to control eutrophication effects and to improve water quality occurred later than the mid 1960s. Complete circulation has been the most frequently used technique to improve water quality (except for algicides and herbicides). 19.2 DEVICES AND AIR QUANTITIES Introduction of compressed air through a diffuser or perforated pipe located at depth employs the air-lift method of circulating lakes and reservoirs, in which water is welled up by the rising plume of air bubbles (Pastorak et al., 1981, 1982). Although techniques using pumps and water jets have been used successfully to circulate lakes, the air-lift method, through diffusion of compressed air, is apparently the least expensive and is easiest to operate (Lorenzen and Fast, 1977). However, high efficiencies of oxygenation have been reported from pumped jets in some cases (Stefan and Gu, 1991; Michele and Michele, 2002). If the lake is already stratified, mixing is usually achieved only above the depth of air injection. If the lake is not stratified however, injection near the surface can prevent stratification (Pastorak et al., 1981, 1982). The effect of an unconfined rising plume of air bubbles on water circulation in an already stratified lake is illustrated in Figure 19.1. As the plume rises, the mixture becomes heavy, upward water flow ceases and the water plume spreads laterally or sinks to a neutral L1625_C019.fm Page 475 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis TABLE 19.1 Lakes Receiving Treatment by Artificial Circulation with Associated Characteristics Lake Depth Volume (10 6 /m 3 ) Area (ha) Q Air/m 3 /min Q Air/m 3 × 10 6 Q Air/km 2 ReferenceMax. Mean Device Clines Pond, OR 4.9 2.5 4.9 0.003 0.13 0.028 a 10.2 21.6 Malueg et al., 1973 Parvin, CO 10.0 4.4 10.0 0.849 19.0 2.1 a 2.5 11.18 Lackey, 1972 Section 4, MI 19.1 9.8 18.3 0.110 1.1 2.21 a 20.0 200.0 Fast, 1971a Boltz, KY 18.9 9.4 18.9 3.614 39.0 3.17 a 0.88 8.17 Symons et al., 1967, 1970; Robinson et al., 1969 University, NC 9.1 3.2 9.1 2.591 80.9 0.40 a 0.15 0.49 Weiss and Breedlove, 1973 Kezar, NH 8.2 2.8 8.2 2.008 73.0 2.83 a 1.41 3.88 Anon., 1971; Haynes, 1973 Indian Brook, NY 8.4 4.1 2.2 0.302 7.3 4.53 a 15.0 62.06 Riddick, 1957 Prompton, PA 10.7 3.7 10.7 0.193 112.0 4.53 a 1.08 4.04 McCullough, 1974 Cox Hollow, WI 8.8 3.8 8.8 1.480 38.8 2.04 a – 1.38– 5.26– Wirth and Dunst, 1967 4.08 2.76 10.53 Wirth et al., 1970 Stewart, OH 7.5 3.4 7.0 0.090 2.6 0.25 b 2.83 9.80 Barnes and Griswold, 1975 Wahnbach, 1961–1962 43.0 19.2 43.0 41.618 214.0 2.01 b 0.048 0.94 Bernhardt, 1967 West Germany 1964 5.95 b 0.143 2.78 Starodworskie, Poland 23.0 23.0 7.0 0.27 a 3.81 Lossow et al., 1975 Roberts, NM 9.1 4.4 9.1 1.233 28.3 3.54 a 2.87 12.5 USEPA, 1970 2.26 a 1.84 8.00 McNally, 1971 Falmouth, KY 12.8 6.1 12.8 5.674 91.0 3.26 0.58 3.58 Symons et al., 1967, 1970; Robinson et al., 1969 Test II, U.K. 10.7 9.4 10.7 2.405 25.4 2.01 a 0.84 7.92 Knoppert et al., 1970 Test I, U.K. 10.7 9.4 10.7 2.097 22.7 2.01 a 0.96 8.86 Knoppert et al., 1970 Mirror, WI 13.1 7.6 12.8 0.40 5.3 0.45 a 1.13 8.55 Smith et al., 1975; Brynildson and Serns, 1977 Växjosjön, Sweden 6.5 3.5 6.0 3.1 87.0 7.2 a 2.32 8.28 Bengtsson and Gelin, 1975 Buchanan, ON 13 4.9 13 0.42 8.9 0.28 a 0.67 3.17 Corbett, BC 19.5 7.0 19.5 1689 24.2 4.5 a 2.66 18.52 Halsey, 1968; Halsey and Galbraith, 1971 Maarsseveen, U.K. 29.9 14.0 19.0 8.018 60.7 2.49 a 0.31 4.10 Knoppert et al., 1970 29.9 Casitas, CA 82.0 26.8 39.0 308.0 1100.0 17.84 b 0.06 1.62 Barnett, 1975 55.0 Hyrum, UT 23.0 11.9 15.2 23.1 190.0 2.83 b 0.17 1.49 Drury et al., 1975 Waco, TX 23.0 10.7 23.0 128.0 2942.0 3.11 b 0.02 0.10 Biederman and Fulton, 1971 L1625_C019.fm Page 476 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis Catharine, IL 11.8 5.0 8.5 3.034 59.5 0.76 c 0.25 1.27 Kothandaraman et al., 1979 El Capitan, CA 1965- 62.0 9.8 21.3 17.99 183.9 6.09 b 0.34 3.31 Fast, 1968 1966 9.4 28.3 21.05 222.0 6.09 b 0.29 2.74 Calhoun, MN 27.4 10.6 23.0 18.01 170.4 2.83 b –3.54 0.16–0.20 1.66–2.08 Shapiro and Pfannkuch, 1973 Eufaula, OK 27.0 16.2 27.0 703.1 414.8 × 10 2 33.98 c 0.05 0.06 Leach et al., 1980 Pfaffikersee, Switzerland 35.0 18.0 28.8 56.5 325.0 6.0 b 0.11 1.85 Thomas, 1966; Ambuhl, 1967 Wahiawa, HI 26.0 8.0 2.7 1.7 20.0 2.4 b 1.4 12.0 Devick, 1972 Trasksjön, Sweden 4.0 3.0 4 0.365 12.1 a Karlgren and Lingren, 1963 Altoona, GA 1968–1969 46.0 9.4 42.7 453 4800 21.6 b –27.7 0.05–0.86 0.45–0.58 USAE, 1973; Raynes, 1975 27.7 b 0.06 0.58 Lafayette, CA 24.0 9.1 18.0 5.243 53 1.68 c 0.32 3.17 Laverty and Nielsen, 1970 Hot Hole, NH 13.3 5.7 13.3 0.733 12.9 0.59 a 0.80 4.57 NHWSPCC, 1979 Heart, Ontario 10.4 2.7 10.0 0.392 14.5 0.23 a –0.92 0.58–2.34 1.56–6.33 Nicholls et al., 1980; Nicholls d Clear, CA 15.0 10.2 14.0 115.9 1217 17 a 6.82 114 Rusk d Kremenchug, Poland 3.0 2.0 2.6 0.002 0.12 4.38 a 1750 3500 Ryabov et al., 1972; Sirenko et al., 1972 Tarago, Australia 23.0 10.5 14.0 27.6 360 3.0 c –9.0 0.08–0.24 0.83–2.50 Bowles et al., 1979 3.0 c –7.50 0.08–0.20 0.83–2.08 Silver, OH 12.0 4.22 10.0 1.68 38.44 3.37 b 2.01 8.77 Brosnan, 1983 East Sydney, NY 15.7 4.9 15 4.17 0.85 1.8 b 0.43 2.1 Barbiero et al., 1996a Crystal, MN 10.4 3.0 10 0.93 0.31 1.44 b 1.55 4.6 Osgood and Stiegler, 1990 King George VI, U.K. 16.0 14.0 10.0 20.0 142.0 Water jet c Ridley et al., 1966 Queen Elizabeth II, U.K. 17.5 15.3 17.5 128.0 Water jet c Ridley et al., 1966 Ham’s, OK 10.0 2.9 1.2 115.0 40.0 Axial-flow pump a Stichen et al., 1979; Toetz, 1977a,b Stewart Hollow, OH 7.6 4.6 7.6 0.148 3.2 Axial-flow pump a Garton et al., 1978 Cladwell, OH 6.1 3.0 6.1 0.123 4.0 Axial-flow pump b Irwin et al., 1966 Pine, OH 5.2 2.1 5.2 0.121 5.7 Axial-flow pump b Irwin et al., 1966 Vesuvius, OH 9.1 3.6 9.1 1.554 42.5 Axial-flow pump c Irwin et al., 1966 Arbuckle, OK 1975; 1977 24.7 9.5 6.0; 2.0 89.3 × 10 2 951.0 Axial-flow pump c Toetz, 1977a, b, 1979 West Lost, MI 12.8 6.2 11.9 0.089 1.4 Pump c Hooper et al., 1953 a Flow rate produced destratification. b Partly mixed. c Flow rate inadequate to destratify. d R.A. Pastorak, personal communication. Source: From Pastorak, R.A. et al. 1981; Pastorak, R.A. et al. 1982. Tech. Rept. No. E-82-3. U.S. Army Corps of Engineers; with additions. L1625_C019.fm Page 477 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis buoyancy level. However, the bubbles continue to rise with increased buoyancy having expanded due to reduced hydrostatic pressure at shallow depth, repeating the water-entrainment process, until they reach the surface. Assuming air flow is adequate, the process continues until the density difference above the diffuser is zero (Zic and Stefan, 1994; Sahoo and Luketina, 2002). The overall effect is that water is pulled from the hypolimnion into the epilimnion, breaking up the thermocline, producing generally homothermous, completely mixed conditions near the plume. As mixing and entrainment continue, erosion of the thermocline proceeds away from the plume so long as the energy applied through the airlift system exceeds the energy of resistance due to thermal (density) stability. Injection of compressed air at maximum depth usually affords the greatest rate of mixing, because flow of the entrained water is a function of depth of release and air-flow rate. Lorenzen and Fast concluded that an air-flow rate per lake surface area of 9.2 m 3 /km 2 per min (1.33 ft 3 /acre per min) should provide adequate surface reaeration and other benefits of circulation. However, the areal air-flow rates approached or exceeded that critical value in only 42% of the cases cited in Table 19.1. Effectiveness of that flow rate is substantiated by the cases in Table 19.1 where before and after temperature data were provided (Pastorak et al., 1982). Figure 19.2 is a plot of the degree of destratification (percent reduction in Δt in the water column) related to air-flow rate per unit area. Except for three observations, areal air-flow rates approaching or exceeding 9.2 m 3 /km 2 per min produced complete mixing, or 100% decrease in the surface to bottom Δt. In two of the three exception lakes to the right of the line in Figure 19.2, the final Δt was < 3°C, which was used as the criterion for satisfactory destratification (Pastorak et al., 1982). In 30 of the 45 cases cited for the airlift technique, where temperature data were available, the presented air-flow rates were adequate to destratify or prevent stratification (Table 19.2). The Lorenzen and Fast areal air-flow rate criterion has been more reliably followed in more recent commercially installed systems. The average areal air-flow rate for 21 systems installed by General Environmental Systems in reservoirs and lakes > 23 ha during 1991–2002 was 7.8 m 3 /km 2 per min (Geney, personal communication). Delivering the air to as much of the deep area of the water body as possible is also important to attain and maintain destratification (Geney, 1994). The basis for the areal air-flow rate criterion of 9.2 m 3 /km 2 per min is a relationship among air- flow rate, depth, and flow rate of up-welled water above an orifice (Lorenzen and Fast, 1977; Pastorak et al., 1982): FIGURE 19.1 The process of destratification as a result of entrainment of water by a rising plume of air bubbles. Cooler, hypolimnetic waters from elsewhere replace the volume entrained near the plume, ultimately eroding away the thermocline. (From Davis, J.M. 1980. Water Serv. 84: 497–504. With permission.) L1625_C019.fm Page 478 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis (19.1) where Q w (X) = water flow rate in m 3 /s, C = 2V o + 0.05 m 3 /s, X = height above orifice in m, V o = air flow in m 3 /s at 1 atm, h = depth of orifice in m, and μ b = 25V o + 0.7 m 3 /s. Using this estimated water flow rate, the effect of various air-flow rates on hypothetical lake and reservoir morphometry was studied (Chen and Orlob, 1975). Results from 38 airlift cases over a range of lake reservoir areas, volumes, and depths, indicated an air-flow rate approaching or greater than the 9.2 m 3 /km 2 per min level (midpoint of a range 6.1–12.3 m 3 /Km 2 per min) consistently achieved destratification (Table 19.1). The diffuser should be a pipe with multiple orifices, usually located at the deepest point in the lake, but suspended sufficiently well off the bottom (1 to 2 m) to minimize sediment entrainment. Orifice spacing should be about 0.1 times the depth of air release, because the rising water plume will spread horizontally at 0.05 m/m of rise (Lorenzen and Fast, 1977). Another approach to designing an air-lift system to destratify lakes and reservoirs was described in detail by Davis (1980). This approach requires the following information/steps: 1. Obtain surface area and volume as function of depth. 2. Determine or assume temperature or density profile. 3. Existing stability and added heat input and theoretical energy required to overcome it are calculated. 4. Calculate free air-flow rate at the compressor. 5. Calculate perforated (diffuser) pipe length (50 m suggested as minimum). FIGURE 19.2 Percent destratification, based on surface to bottom temperature differences (Δt) before and after circulation, related to free air flow. (Data from Pastorak, R.A. et al. 1982. Environmental Aspects of Artificial Aeration and Oxygenation of Reservoirs: A Review of Theory, Techniques, and Experiences. Tech. RepT. No. E-82-3, U.S. Army Corps of Engineers, Vicksburg, MS; from Cooke et al. 1993. With permission.) % of destratification (Δt = 0) 100 80 60 40 20 0 5 1015202530 Critical limit, Lorenzen and Fast (1977) Free air flow, m 3 min −1 km −2 62 114 200 QX CX V X h w o b () . ( .) ln . =+ −− + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ 35 6 0 8 1 10 3 μ ⎜⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ ⎟ L1625_C019.fm Page 479 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis 6. Select diffuser pipe and hole diameters (0.8 mm suggested) and hole spacing (0.3 m suggested). 7. Determine internal pipe diameter and air pressure at compressor considering losses due to hydrostatic pressure, excess pressure at pipe end, friction in the pipe, the pipe bends, valves, etc 8. Recheck diffuser length, considering pressure losses and free air flow through a single hole. 9. Calculate anchor weight. Stability is calculated first, as the difference between the unmixed, existing density gradient, and the mixed condition: (19.2) where S = stability, joules (kg m 2 /s 2 ), g = acceleration due to gravity, m/s 2 , ρ i = density of layer i, kg/m 3 , V i = volume of layer i, m 3 , h i = height of centroid of layer i, m, m = mixed, and s = stratified. The energy required for destratification is calculated by (19.3) where S = stability, R = heat input, and W = wind energy, all in joules. Wind is neglected, as a conservative approach, so that mixing is possible without wind. R can be approximated as 5 J/m 2 per day. The required air-flow rate (Q) in L/s is (19.4) where E = energy input required; 20 times the theoretical level (that assumes isothermal conditions and bubble pressure slightly in excess of the hydrostatic head) is factored into the equation, T = time to achieve destratification, D = depth of diffuser in m, and 10.4 = depth of water equivalent to atmospheric pressure. The volume of water entrained by the air bubbles from a perforated pipe is recommended to be 2.5 times the volume of the lake or reservoir to be destratified and can be calculated according to (19.5) From Equation 19.4, and knowing the volume to be destratified (m 3 ) and the required air flow (L/s), the length of perforated pipe (diffuser) in m can be calculated: Sg Vh VH im i i i n is i i i n =− == ∑∑ ρρ 11 ESRW=+− Q E T D = + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 0 196 1 10 4 . ln . VLT gQ L D e = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + − 0 486 1 10 4 1 13 13 . . ln // DD 10 4. ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ L1625_C019.fm Page 480 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis (19.6) Pastorak et al. (1982) compared the calculated flow rates required by the two procedures, using an example from Davis (1980) for a body of water with a volume of 20 × 10 6 m 3 , a maximum depth of 20 m, and an area of 1.2 × 10 6 m 2 . The flow rate recommended by the Davis procedure would be 70 L/s (3.5 m 3 /km 2 per min). By the Lorenzen and Fast (1977) procedure the rate would be 6 m 3 /km 2 per min, or 120 L/s, nearly twice the Davis rate. The rate used here is the the lower end of the range (6.1 to 12.3 m 3 /km 2 per min) because deeper lakes generally require less air to mix than do shallow lakes (Pastorak, personal communication). According to Equation 19.6, the diffuser pipe length needed to destratify is inversely related to air-flow rate. Thus, pipe length would be 216 m, based on a 70 L/s air-flow rate and 182 m based on 120 L/s for destratification to occur in 5 days. For the example lake, Davis (1980) selected a high-density polyethylene pipe of diameter 50.8 mm, perforated with 1-mm diameter holes spaced at 0.3 m. An air pressure of 5.3 bar (5.5 kg/cm 2 ) at the compressor was calculated by summing the hydrostatic pressure represented by the water depth over the pipe, mean excess pressure above the hydrostatic pressure at the end of the pipe (related to pipe length), friction loss in the pipe (related to pipe diameter) and pressure drop from bends in the pipe. An air-flow rate of 108 L/s was recalculated for pipe length and pore size and number of holes with that compressor pressure (5.3 bar). That exceeded the calculated 70 L/s so the nominal pipe length of 250 m was considered adequate. A longer pipe length than the minimum calculated facilitates destratification with greater air distribution. These estimates can be obtained from nomographs in Davis (1980). While calculation of required free air-flow rate at the diffuser end and the initial estimate of minimum diffuser length to accommodate that rate are relatively straightforward, determining the required pressure at the compressor, and a more precise estimate of diffuser length incorporating all the pressure losses, is not straightforward and involves an iterative process (Meyer, 1991). Consistent with the above procedure, first obtain an initial estimate of diffuser length (Equation 19.6). Then, determine hydrostatic and internal pipe pressures to obtain a new estimate of free air flow from a single diffuser hole. From that air flow and knowing the diffuser hole-spacing and total air flow required, a new pipe length can be determined. With that pipe length, pressures can be recalculated and the process repeated until the optimum diffuser length is obtained. To simplify the process, Meyer (1991) incorporated the equations and charts from Davis (1980) into a spread- sheet, which allowed an iterative process of changing variables and formulas to arrive at an optimum diffuser length. Results using the spreadsheet procedure for a hypothetical reservoir are summarized as follows: • Surface area: 1,011,750 m 2 • Diffuser depth: 10 m • Volume above diffuser: 10,117,500 m 3 • Time to destratify 5 days: 432,000 s • Temp. range from 30°C @ surface to 21.8°C @ 25 m • Theoretical energy required (E) = stability (S) + solar input (R) (Equation 19.3): 1.9 × 10 8 J + 0.25 × 10 8 J = 2.15 × 10 8 J Air flow required (from Equation 19.4): L V D TQ D = + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ 373 1 10 4 1 10 4 3 3 . . ln . ⎠⎠ ⎟ ⎛ ⎝ ⎜ ⎜ ⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ 3 12/ L1625_C019.fm Page 481 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis Diffuser length, initial calculation (from Equation 19.6): = 89 m Selected: • Supply line: 500 m • Internal diameter supply line: 45 mm • Internal diameter diffuser: 35 mm Through iteration, an optimum diffuser length of 339 m and compressor pressure of 9.7 kg/cm 2 (135 psi) were determined. The iterative approach was used to estimate air-flow pressure and diffuser length for East Sidney Lake, New York, 85 ha, 15.7 m maximum depth and 4.9 m mean depth (Meyer et al., 1992). The respective values by using the Davis nomographs were 1.53 m 3 /min, 3.4 kg/cm 2 , and 107 m. Those using the iterative process were 2.19 m 3 /min, 3.9 kg/cm 2 , and 135 m. A destratifying time of 5 days was used with both procedures. To gain flexibility and control over the long and narrow reservoir, 244 m of total diffuser length was installed, with 8 separate 30-m lines spread through the reservoir. A 15 hp compressor was used to deliver 1.8 m 3 /min air flow at 3.6 kg/cm 2 pressure. The system operated satisfactorily during 1989–1990 to maintain destratified conditions (< 2°C difference surface to bottom) in the near field, but temperature difference was greater in the far field or whole lake, despite the extended lines. Also, bottom DO levels dropped below 3 mg/L. Use of a diffuser longer than calculated, i.e., “underloading” the diffuser, may have accounted for and restricted destratification capacity. However, the total air delivery per area to the reservoir, which was 2.1 m 3 /km 2 per min, relative to the Lorenzen and Fast criterion, was not discussed. That rate for East Sydney Lake was well below their median criterion and probably accounted for some of the less-than-expected water quality response, discussed later in this chapter. While a successful outcome for complete circulation depends on the size and length of diffuser pipes, results indicate that for best results in improvement of water quality, as well as achieving destratification, adherence to the Lorenzen and Fast criterion is also advisable. Mechanical mixing devices have been used less frequently than compressed air (Table 19.1). Two types of pumps have been developed for destratifying reservoirs: (1) axial-flow pumps with a large propeller (6 to 15 ft diameter) that generates a low velocity jet (Punnet, 1991), and (2) direct drive mixer with a small propeller (1 to 2-ft diameter) that generates a high velocity jet (Stefan and Gu, 1991; Price, 1988, 1989). Design of a pumping system to destratify a lake or reservoir depends on the desired time to destratify (or rates of circulation) and depth of hydraulic jet penetration. Time to destratify in turn depends on the degree of stratification Q= 0.196 J sln 1+ 10 m 10.4 ()(. )215 10 432 10 8 3 × × ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟ = 144 5.L/s L = ×+ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ × 373 10 1175 10 1 10 432 6 . (. ) ( m m 10.4 3 110 144 51 1 10 4 3 s) s) 3 (./ln . + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎜ D ⎜⎜ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ L1625_C019.fm Page 482 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis or resistance to mixing. The number of pumps needed to achieve a given depth of penetration and time to mixing can be calculated (Holland, 1984; Gu and Stephan, 1988; Stefan and Gu, 1991). Destratification was complete (Δt < 3°C) for 4 of the 10 cases for pumps and jets cited in Table 19.1. Mixing devices powered by solar and wind energy are available commercially, but published results of effectiveness were unavailable for inclusion here. 19.3 THEORETICAL EFFECTS OF CIRCULATION 19.3.1 D ISSOLVED OXYGEN (DO) The principal, and probably the most reliable, effect of circulation is to raise the dissolved oxygen (DO) content throughout the lake over time. If the lake is destratified, the DO content in what was the hypolimnion will increase, and that in the epilimnion will decrease, at least at first. This can occur from simple dilution. Additional reasons why the surface water DO may decrease are the transfer of oxygen-demanding substances toward the surface and a decrease in photosynthesis in the photic zone due to increased mixing depths (Haynes, 1973; Ridley et al., 1966; Thomas, 1966). DO will continue to increase as circulation is maintained, largely because water undersaturated with oxygen is brought into contact with the air. While the vertical transport of water is achieved by entraining water through releasing compressed air at some depth, little oxygen increase is achieved through direct diffusion from bubbles (King, 1970; Smith et al. 1975). 19.3.2 NUTRIENTS Internal loading of P theoretically can be decreased through increased circulation. This would occur in situations where the dominant mechanism of P release was from iron-bound P in anoxic hypolimnetic sediments. By aerating the sediment-water interface of lakes where iron is controlling P solubility, P should be adsorbed from solution by ferric-hydroxy complexes (Mortimer, 1941, 1971; Stumm and Leckie, 1971; Chapters 8, 18, 20). Thus P would be prevented from migrating from high concentrations in sediment interstitial water to the overlying water. Calcium may control P solubility in hardwater lakes, rather than iron, or the iron/phosphorus ratio may be too low to control P release (Jensen et al., 1992), in which case the release rate could be due largely to a function of aerobic decomposition of organic matter (Kamp-Nielsen, 1975). In that event, internal P loading may actually increase as temperature at the sediment-water interface is raised in the circulation process. Also, some sediments with a low Fe:P ratio have a high organic and water content and are very flocculent, and may have a high loosely bound P fraction (Boström, 1984). In that latter situation as well, internal loading could actually increase from such sediments following circulation. P exchange rates are dependent upon circulation at the sediment-water interface and that process could be enhanced by mixing (Lee, 1970). Degree of wind mixing had a dominant effect on summer internal loading of P in shallow Moses Lake, Washington (Jones and Welch, 1990). Internal loading of P may be high in unstratified, shallow, eutrophic lakes in which the sediment- water interface is usually oxic (Jacoby et al., 1982; Kamp-Nielsen, 1975; Søndergaard et al., 1999). Therefore, reduced internal P loading probably cannot be expected to result from artificial circu- lation. Internal loading and whole-lake TP may decrease in shallow stratified lakes following circulation (Ashby et al., 1991), but the concentration available for growth in the photic zone may increase, as has been observed (Brosnan and Cooke, 1987; Osgood and Stiegler 1990). Thus, depth is an important criterion in determining the candidacy of shallow lakes for complete circulation from not only phytoplankton production related to available light, but also internal P loading. Unless oxic conditions will substantially reduce P internal loading, maintaining stratified conditions may be preferable for limiting P availability in the photic zone. L1625_C019.fm Page 483 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis Other potential changes in chemical content resulting from complete circulation are the con- version of ammonium to nitrate and the complexation and sedimentation of trace metals such as manganese and iron. Ammonium decrease can largely be attributed to increased nitrification, which requires aerobic conditions (Brezonik et al., 1969; Toetz, 1979). This effect will be greater the longer that duration and completeness of hypolimnetic deoxygenation proceeded prior to circula- tion. The decrease in trace metals like manganese and iron should also be greater in lakes with larger oxygen deficits prior to aeration increases. Because these metals diffuse from the sediment in their reduced, soluble forms, aeration will promote their oxidation and subsequent complexation and precipitation. This can be an important benefit in lakes used for drinking water supplies. 19.3.3 PHYSICAL CONTROL OF PHYTOPLANKTON BIOMASS Circulation can reduce phytoplankton biomass through light limitation, brought about by providing a greater depth of mixing of plankton cells in the water column so that the total light received during their brief period in the photic zone is insufficient for net photosynthesis (photosynthesis in excess of respiration) and thus any growth or increase in cell mass. This is known as the “critical depth” concept, first formulated to predict the timing of the spring diatom bloom in the ocean (Sverdrup, 1953). By knowing light at the surface, compensation depth, and the extinction coefficient, the critical depth can be calculated as the point above which net production is possible; when that calculated depth exceeds the mixed-layer depth, a bloom can occur. This model is dependent upon some relationship between light intensity and gross photosynthesis, assuming a constant rate of respiration. The same concept applies in lakes (Talling, 1971). The combination of low surface light intensity and deep mixing prevented net photosynthesis during winter in relatively deeper lakes (> 30 m) of the English Lake District, but not in the shallower lakes (10 m). Growth rate during the spring phytoplankton maximum was directly related to light intensity in a long-term data series (Neale et al., 1991). Normally, lakes are shallow enough to allow some net photosynthesis even in winter, but decreasing mixing depth, as stratification develops and surface light intensity increases in the spring, usually accounts for the large increase in net photosynthesis and the spring diatom bloom in deeper lakes. Light can limit maximum phytoplankton biomass even in shallow eutrophic lakes (Sheffer, 1998). A 35-year data base from Lake Võrtsjär (270 km 2 , mean depth 2.8 m), Estonia, showed that the water level change produced a 2.5 times difference in mean depth resulting in biomass levels significantly lower in high water level years (Nõges and Nõges, 1999; Nõges et al., 2003). Thus, artificial circulation may produce light-limiting benefits in shallow, eutrophic lakes with normally high particulate matter concentrations and light extinction. The concept of physical control of phytoplankton growth was extended to the effects of artificial circulation in eutrophic lakes (Lorenzen and Mitchell, 1975; Murphy, 1962; Oskam, 1978). Forsberg and Shapiro (1980) and Shapiro et al. (1982) integrated the effects of nutrients with those of physical factors. By increasing the depth of mixing, a lake potentially can be returned to a winter condition where light is limiting, assuming maximum depth and light attenuation are sufficient. Increasing mixing depth would not be great enough in most cases to prevent net biomass production completely, which is not expected. This effect of mixing depth is clearly shown in results from Kezar Lake (Figure 19.3; Lorenzen and Mitchell, 1975). Increased mixing depth though complete circulation is expected to substantially reduce algal biomass due to light limitation alone. However, nutrients may initially be limiting in the epilimnion, so that a slight increase in mixed depth may entrain water with higher nutrient content from below and biomass may increase (point A to point B in Figure 19.3). At some point light will limit and productivity and biomass will decrease (point C to point D). Note that biomass is plotted as mass per area (g/m 2 ), which was expected to decrease by only 38% for a mixing-depth increase of 2 to 6 m. Biomass concentration (g/m 3 ), however, was expected to decrease by 80%, which would also include the effect of water column dilution. This model predicted only the potential productivity without nutrient limitation and included no losses L1625_C019.fm Page 484 Sunday, December 18, 2005 11:29 PM Copyright © 2005 by Taylor & Francis [...]... Analysis and Simulation in Ecology, Vol III Academic Press, New York pp 475–588 Chriswell, B and M Zaw 199 1 Lake destratification and speciation of iron and manganese Environ Monit Assess 19: 433–447 Cooke, G.D., E.B Welch, S.A Peterson and P.R Newroth 199 3 Restoration and Management of Lakes and Reservoirs, 2nd ed Lewis Publishers and CRC Press, Boca Raton, FL Davis, J.M 198 0 Destratification of reservoirs. .. response parameter FIGURE 19. 7 Potential adverse effects of artificial circulation, including the promotion of blue-green algal blooms (From Pastorak, R.A et al 198 1 Evaluation of Aeration/Circulation as a Lake Restoration Technique 600/ 3-8 1-0 14 USEPA; Shapiro, J 197 9 In: Lake Restoration USEPA-440/ 5-7 9-0 01 pp 161–167; with modification.) Copyright © 2005 by Taylor & Francis L1625_C 019. fm Page 495 Sunday,... parameter FIGURE 19. 6 Potential beneficial effects of artificial circulation on phytoplankton (Modified from Pastorak, R.A et al 198 1 Evaluation of Aeration/Circulation as a Lake Restoration Technique USEPA-600/ 3-8 1-0 14; Shapiro, J 197 9 In: Lake Restoration USEPA-440/ 5-7 9-0 01.) Copyright © 2005 by Taylor & Francis L1625_C 019. fm Page 491 Sunday, December 18, 2005 11:29 PM Circulation L1625_C 019. fm Page 492... Forsberg, B.R and J Shapiro 198 0 Predicting the algal response to destratification In: Restoration of Lakes and Inland Waters USEPA 440/ 5-8 1-0 10 pp 134–139 Gächter, R 198 7 Lake restoration Why oxygenation and artificial mixing can not substitute for a decrease in the external phosphorus loading Schweiz Z Hydrol 49: 170–185 Garton, J.E., R.G Strecker and R.C Summerfelt 197 8 Performance of an axial-flow pump... 197 8 Light and zooplankton as algae regulating factors in eutrophic Biesbosch reservoirs Verh Int Verein Limnol 20: 1612–1618 Pastorak, R.A., T.C Ginn and M.W Lorenzen 198 1 Evaluation of Aeration/Circulation as a Lake Restoration Technique USEPA-600/ 3-8 1-0 14 Pastorak, R.A., M.W Lorenzen and T.C Ginn 198 2 Environmental Aspects of Artificial Aeration and Oxygenation of Reservoirs: A Review of Theory,... 11:29 PM Kitchell, J.A and J.F Kitchell 198 0 Size-selective predation, light transmission, and oxygen stratification Evidence from the recent sediments of manipulated lakes Limnol Oceanogr 25: 389–402 Knoechel, R and J Kalff 197 5 Algal sedimentation: The cause of a diatom-blue-green succession Verh Int Verein Limnol 19: 745–754 Knoppert, P.L., J.J Rook, T Hofker and G Oskan 197 0 Destratification experiments... 161–167 Shapiro, J 198 4 Blue green dominance in lakes: The role and management significance of pH and CO2 Int Rev ges Hydrobiol 69: 765–780 Shapiro, J 199 0 Current beliefs regarding dominance by blue greens: The cases for the importance of CO2 and pH Verh Int Verein Limnol 24: 38–54 Shapiro, J 199 7 The role of carbon dioxide in the initiation and maintenance of blue-green dominance in lakes Freshwater... Smeltzer and G Zoto 198 2 Experiments and Experiences in Biomanipulation — Studies of Biological Ways to Reduce Algal Abundance and Eliminate Blue Greens USEPA-600/ 3-8 2–096 Sheffer, M 199 8 Ecology of Shallow Lakes Chapman and Hall, London Sirenko, L.A., N.V Avil’tseva and V.M Chernousova 197 2 Effect of artificial aeration on pond water on the algal flora J Hydrobiol 8: 52–58 Smith, S.A., D.R Knauer and T.L... result of complete circulation These involve changes in (1) CO2 and pH, (2) distribution of buoyant cells, and (3) grazing by zooplankton, all of which could be results from increased circulation Blue-green algae-dominated cultures shifted to dominance by green algae in response to decreased pH and associated increases in free CO2 concentration (King, 197 0, 197 2; Shapiro, 197 3, 198 4, 199 0; Shapiro and. .. Lehman, J.T and C.D Sandgren 197 8 Documenting a seasonal change from phosphorus to nitrogen limitation in a small temperate lake, and its impact on the population dynamics of Asterionella Verh Int Verein Limnol 20: 375–380 Lorenzen, M.W and A.W Fast 197 7 A Guide to Aeration/Circulation Techniques for Lake Management Ecol Res Ser USEPA-600/ 3-7 7-0 04 Lorenzen, M.W and R Mitchell 197 5 An evaluation of artificial . algicides and herbicides). 19. 2 DEVICES AND AIR QUANTITIES Introduction of compressed air through a diffuser or perforated pipe located at depth employs the air-lift method of circulating lakes and reservoirs, . be calculated (Holland, 198 4; Gu and Stephan, 198 8; Stefan and Gu, 199 1). Destratification was complete (Δt < 3°C) for 4 of the 10 cases for pumps and jets cited in Table 19. 1. Mixing devices. sediment-water interface of lakes where iron is controlling P solubility, P should be adsorbed from solution by ferric-hydroxy complexes (Mortimer, 194 1, 197 1; Stumm and Leckie, 197 1; Chapters