ARTICLE IN PRESS

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Stratification and mixing in Lake Elsinore, California:An assessment of axial flow pumps for improving waterquality in a shallow eutrophic lake

Rebecca Lawson, Michael A. Anderson�

Department of Environmental Sciences, University of California, Riverside, CA 92521, USA

a r t i c l e i n f o

Article history:

Received 19 December 2006

Received in revised form

23 May 2007

Accepted 4 June 2007

Available online 12 June 2007

Keywords:

Aeration

Destratification

Stratification

Eutrophication

Stability

Mixing

nt matter & 2007 Elsevie.2007.06.004

thor. Tel.: +1 951 827 3757;michael.anderson@ucr.ed

a b s t r a c t

A 3-year study was conducted to quantify the effectiveness of a destratification system on

weakening thermal stratification and increasing dissolved oxygen (DO) levels in Lake

Elsinore, California. Biweekly measurements of temperature, DO, and other parameters

were made at 14 sites across the lake beginning in July 2003. A destratification system

consisting of 20 axial flow pumps fitted with 3 HP electric motors and 1.8 m diameter

impellers mounted 2 m below the water surface was installed in the spring of 2004 and

made fully operational in July 2004. An unusually wet winter of 2005 raised the summer

mean depth from 3.0 m in 2004 to 6.7 m in 2005. This study thus allowed us to quantify the

influence of axial flow pump operation on water column properties under shallow water

conditions (i.e., before and after axial flow pump installation), and also to compare the

effectiveness of the destratification system at two strongly different lake levels.

Transparencies increased substantially after the winter storms in 2005 and thermal

stability was shown to be strongly dependent upon lake level. Stratification and a large area

of anoxic sediments persisted despite pump operation in the summers of 2004 and 2005.

Acoustic Doppler current profiler (ADCP) measurements showed that mixing energy was

not being efficiently transmitted laterally into the water column.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Mixing in lakes occurs through a number of mechanisms,

including wind-induced turbulence, convective mixing, and

mixing due to inflows/outflows (Martin and McCutcheon,

1999; Imboden and Wuest, 1995). The response of a water

body to these mixing mechanisms depends on a number of

factors such as lake morphometric characteristics (Kling,

1988), and meteorological and climatic factors (Imberger,

1985; Imboden and Wuest, 1995). The extent and frequency of

mixing is a balance between the turbulent kinetic energy

(TKE) inputs to the lake relative to the resistance to mixing of

the water column. Resistance to mixing arises due to buoyant

forces in place as a result of thermal (or salinity-driven)

r Ltd. All rights reserved.

fax: +1 951 827 3993.u (M.A. Anderson).

stratification. The resistance to mixing, or stability, can be

defined as the amount of work required to overcome the

buoyant forces and mix the water column to an average

density without the addition or loss of heat (Idso, 1973).

When stratification is weak or inputs of TKE are high,

sufficient work is available to overcome the buoyant forces

due to stratification and mix the water column. During

periods of stronger stratification when natural mechanics

are not able to completely mix the water column, the lower

portion of the water column becomes isolated from the

atmosphere and gradients in the biological and chemical

properties can develop (Wetzel, 2001). Microbial respiration

commonly results in oxygen depletion in the lower portion of

the water column, especially in eutrophic waters; anoxia near

ARTICLE IN PRESS

Nomenclature

S stability (J/m2)

A0 surface area of the lake (m2)

Az area of the lake at depth z (m)

z depth (m)

z* depth of mean density (m)

r* volume average mean density (kg/m3)

rz density at depth z (kg/m3)

g acceleration due to gravity (m/s2)

rT density at any given temperature (kg/m3)

T temperature (1C)

D dissolved oxygen deficit (tons)

Cg concentration of O2 in the atmosphere (mg/L)

H temperature-dependent non-dimensional

Henry’s constant

Caq volume-weighted average dissolved oxygen in the

lake (mg/L)

V total lake volume (L)

TKEwind turbulent kinetic energy due to wind shear (W/

m2)

C�k dimensional wind efficiency coefficient

CD dimensional drag coeffieicent of the air–water

surface

ra density of air (kg/m3)

rw density of water (kg/m3)

U10 wind speed 10 m from the land surface (m/s)

Z height of the sensor from the land surface (m)

UZ wind speed at height Z (m/s)

TKEpumps turbulent kinetic energy from the axial flow

pumps (W/m2)

Qp flow rate at pumps (m3/s)

Up pump flow velocity (m/s)

Zmax maximum lake depth (m)

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 4 5 7 – 4 4 6 74458

the sediments, in turn, can lead to increased release of

nutrients from the sediments (Welch and Jacoby, 2001).

During a mixing event after a long period of stratification,

nutrients, sulfide, and other dissolved constituents are mixed

to the upper portion of the water column, where they can fuel

potential algae blooms, odors, and fish kills.

To prevent the occurrence of internal nutrient loading, fish

kills, and other problems in lakes, artificial destratification

systems are sometimes utilized (Quintero and Garton, 1973;

Vandermeulen, 1992; Cooke et al., 2005). There are currently

two techniques employed. The most common is a diffuser

system, which consists of the injection of air into the lower

water column and achieves mixing through the entrainment

of bottom water as the air bubbles rise and expand (Schladow

and Fisher, 1995). This technique also facilitates the dissolu-

tion of oxygen into the anoxic bottom waters, although direct

bubble-water exchange is relatively unimportant in shallow

lakes (Schladow, 1992). More significantly, the upward advec-

tive transport of the anoxic waters to the surface results in

mixing with higher DO surface waters and exchange across

the air–water interface. An alternative to upward mixing of

stratified waters via buoyant forces of air bubble plumes is the

downward mixing that can be achieved through axial flow

pumps (Cooke et al., 2005). In this system, impellers are

placed below the watersurface and used to drive warm, well

aerated surface water down toward the bottom of the lake,

where it can displace and mix with the cooler, oxygen-

depleted water (Punnett, 1991).

The goal of destratification systems is to supplement

natural wind and convection-driven mixing, break thermal

stratification, and improve DO conditions across the lake.

While the principles of destratification are relatively simple,

the effectiveness of any destratification system depends upon

a number of factors, including the stability of the water

column, TKE inputs due to natural mixing processes, and

design-operation factors, such as pumping rate and size of

the system (Kirke and Gezawy, 1997; Vandermeulen, 1992).

The goals of this study were to quantify the natural mixing

processes and water column properties in a shallow eutrophic

lake located in Southern California, and evaluate the effects

of axial flow pump operation on temperature, DO concentra-

tion, water column stability, and related water column

properties.

2. Methods

2.1. Study site

Lake Elsinore is a relatively shallow polymictic lake located in

Riverside County, California (Fig. 1). The lake is chiefly used

for recreation, including water skiing, jet skiing, power

boating, and fishing. The lake also provides habitat for avian

and aquatic species. The lake is situated approximately 380 m

above MSL at the base of the 2000 km2 San Jacinto River

watershed, and is functionally a closed basin lake except

during extremely wet years. The region’s Mediterranean

semi-arid climate results in highly variable annual rainfall

(often o25 cm/year) that falls during the winter; summers

are hot and dry with annual evaporation rates that exceed

1.4 m/year (CIMIS, 2006). The lake is thus subject to sub-

stantial variations in lake level, with large declines during

droughts and rapid increases in lake levels during El Nino

events (Kirby et al., 2007). The region experienced a severe

drought in 1999–2004, when the lake level declined by almost

4 m and mean depth decreased from 5.3 to 3.3 m (unpublished

data). The lake surface area was also reduced during this

time, from about 13.1 to 10.5 km2. To slow the rate of lake level

decline, recycled water and local groundwater were added

from 2002 to 2004. Near record rainfall in the winter of 2005

dramatically increased the lake level, however, to a maximum

depth of approximately 11 m, a mean depth of 7.6 m, and a

surface area of about 14.7 km2.

ARTICLE IN PRESS

A1

A2

A3

A4

A5

B1

B2

B3

B4

C1

C2

C3

C4

D1

D2

0

km

0

1

2

3

4

5

NDepth, m

1.51.00.5

Fig. 1 – Lake Elsinore bathymetry at the start of the study in

the summer of 2003. Sampling locations represented by the

labeled diamonds; axial flow pump docking stations

indicated by closed squares.

WAT E R R E S E A R C H 41 (2007) 4457– 4467 4459

Lake Elsinore has historically experienced transient strati-

fication events during the spring and early summer that may

persist for periods as short as a few days to several weeks

depending upon lake level and meteorological conditions

(unpublished data). The lake is subject to relatively strong

daytime stratification during the summer months and high

rates of internal nutrient loading. Lake Elsinore is also prone

to intense blue–green algae blooms that can persist for much

of the year, as well as high oxygen demand, depletion of

dissolved oxygen (DO) near the sediments, and accumulation

of NH4-N and H2S near the sediments. Previous field

measurements indicate that transient thermal stratification

leads to rapid oxygen depletion within the water column,

declining from 6–8 to o1 mg/L within �1 week (unpublished

data). Such events have been linked to increased release and

accumulation of soluble-reactive phosphorus (SRP), NH3, and

H2S in the subsurface and in some instances to fish kills,

although no fish kills occurred in Lake Elsinore during the

time of the study.

To help reduce stratification and prevent the depletion of

DO and accumulation of nutrients and other constituents

near the sediments, a destratification system was installed in

Lake Elsinore in the summer of 2004. The destratification

system consisted of 20 axial flow pumps grouped together in

clusters of 4 pumps each placed at 5 docking stations

positioned at the perimeter of the high-speed zone (area of

no speed limit for boating activity) located in the center of the

lake (Fig. 1). The pumps were designed according to USACE

specification (Punnett, 1991) and consisted of a 3 HP motor

and 1.8 m impeller mounted 2 m below the water surface. The

goal of the system was to pump well-aerated surface water

downward, where it could mix with the cooler, oxygen-

depleted water, thereby weakening stratification and increas-

ing DO concentrations near the sediments.

2.2. Field sampling and measurements

Regular water column measurements were made at Lake

Elsinore from July 2003 to June 2006. Field measurements

were made at 14 sampling locations in 2003 and 2004, and an

additional sampling location (D2 in Fig. 1) was added in 2005

to account for the increased surface area and volume of the

lake after the winter storms (Fig. 1). Measurements of

temperature, DO, pH, and electrical conductivity were made

on a biweekly basis throughout the study and more fre-

quently in the summer of 2004, following axial flow pump

installation. Water column properties at the 14 (or 15)

sampling sites (Fig. 1) were recorded by making vertical casts

with a Hydrolab Datasonde 4a or Hydrolab Quanta. In 2003

and 2004, the casts were made in 0.5 m increments from

the surface of the water column down to the bottom,

within 0.1–0.3 m of the sediments. As the depth of the lake

increased in 2005, hydrolab casts were made every 0.5 m

down to a depth of 2 m, and then in 1 m increments

down to the bottom of the water column. The Hydrolab was

checked each morning prior to going to the field and

calibrated for pH, electrical conductivity, and DO as needed

using Fisher pH buffers, a 0.01 N KCl solution, and

water saturated with oxygen at a known concentration.

Temperature and pressure sensors were confirmed as work-

ing within factory specification during annual factory main-

tenance; surface depth (z ¼ 0 m) was set each sampling day at

the lake.

Transparencies were measured with a Secchi disk at the

corner sites, C1–C4 (Fig. 1), and often at some sites in the

middle of the lake as well. Water velocity measurements were

made on August 20, 2004 in a series of transects away

from the axial flow pumps using an RDI 600 kHz Work-

horse Sentinel acoustic Doppler current profiler (ADCP) in

bottom-track mode. Water mode 5 was used with 0.4 m depth

bins.

2.3. Calculations

The temperature profiles were averaged across the 14 (or 15)

sites and then used to calculate the total heat content of the

lake, and with the elevation–area relation for the lake, the

stability of the water column. Net heat flux was calculated as

the change of the total heat content over sampling events,

and then corrected for the time interval and lake surface area

to yield a net average heat flux (W/m2).

Thermal stability, as previously defined, is the amount of

work required to overcome buoyant forces due to stratifica-

tion and completely mix the water column without addition

or loss of heat (Wetzel, 2001). Schmidt stability, S (J/m2), was

calculated as (Idso, 1973)

S ¼g

A0

Z zm

z0

ðz� z�Þðrz � r�ÞAz dz; (1)

where A0 is the surface area of the lake (m2), Az is the lake

area at depth z (m), rz is the density (kg/m3) calculated from

the temperature at depth z, r* is the volume-weighted mean

density of the water column, z* is the depth where the mean

density occurs, dz is the depth interval and, g is the

acceleration due to gravity (m/s2).

ARTICLE IN PRESS

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 4 5 7 – 4 4 6 74460

The density of water at any given temperature (rT) was

calculated as (Martin and McCutcheon, 1999)

rT ¼ 1000 1�Tþ 288:9414

508929:2ðTþ 68:12963ÞðT� 3:9863Þ2

� �. (2)

Salinity effects on water density were ignored, since the

water column was uniform and generally low with respect to

electrical conductance on any given sampling date (data not

shown); thus, vertical density differences (Eq. (1)) arose only

due to temperature effects.

The DO profiles from the 14 (or 15) sampling sites were used

to derive the volume-weighted total DO and total water

column DO deficit. The total DO deficit, D (mg, and subse-

quently converted to metric tons) was calculated as

D ¼Cg

H� Caq

� �V; (3)

where Cg is the concentration of O2 in the atmosphere (mg/L),

H is the temperature-dependent non-dimensional Henry’s

constant, Caq is the volume-weighted DO concentration in the

lake (mg/L), and V is the total lake volume (L).

The DO profiles were also used to estimate the percentage

of anoxic (o1 mg/L DO) bottom sediments. For this calcula-

tion, it was assumed that the 14 (or 15) sites adequately

represent 70% of the lake, with the shallowest 30% of the lake

assumed to be well mixed by wave and boat-wake action and

is adequately aerated. This assumption is consistent with a

previous sediment survey that found that course textured

sediment was present in the shallowest 25% of the lake and

attributed to routine wave action, sediment re-suspension,

and focusing of fine organic sediment into deeper parts

of the lake (Anderson, 2001). The lack of organic material

would also limit the DO consumption by bacteria in this

shallow coarse-textured sediment (Trimmer et al., 2000). The

number of sites with DO levels o1 mg/L approximately

0.1–0.3 m above the sediments were divided by the total

number of sampling sites and multiplied by 70% to yield

the % area estimate.

TKE inputs into the lake due to wind shear were calculated

for comparison with water column stability and theoretical

mixing energy inputs due to axial flow pump operation. TKE

due to wind shear, TKEwind (W/m2), was calculated as

TKEwind ¼ C�krwCDra

rw

� �3=2

U3w; (4)

where Ck� is a dimensionless factor that accounts for the

efficiency of wind energy in mixing the surface layer, taken

here as 0.23 (Martin and McCutcheon, 1999), CD is a

dimensionless drag coefficient (1.3�10�3), ra is the density

of air (1.2 kg/m3), rw is the density of water (kg/m3), and Uw is

the wind speed 10 m from the land surface (m/s).

Wind speed and air temperature were taken from a CIMIS

weather station located at U.C. Riverside (CIMIS, 2006). The

weather station was located at N 33.97 and W 117.34 at an

elevation of 1020 ft above sea level, approximately 21 miles

from the study site. Wind speeds at this weather station were

generally comparable with those at a weather station

deployed on the east shore of the lake in 2001–2002, and thus

used in lieu of local data since local data were not available

for the study period. The height of the wind sensor from the

land surface was approximately 2 m, so the wind speed was

corrected to a height of 10 m using (Mackay and Yeun, 1983)

Uw ¼10:4

ln Zþ 8:1

� �UZ; (5)

where Uw is the wind speed at 10 m from the land surface

(m/s), Z is the height of the sensor from the land surface (m),

and UZ is the wind speed at height Z (m/s).

Theoretical TKE input into the water column due to the

operation of the axial flow pumps was estimated from rated

pump flows using an equation for energy input from

turbination (Imboden, 1980)

TKEpumps ¼QpðUpÞ

2rw

2A0, (6)

where TKEpumps is the TKE input due to operation of the axial

flow pumps (W/m2), Qp is the flow rate at the pumps (m3/s), Up

is the pump flow velocity (m/s), rw is the density of water

(kg/m3), and A0 is the surface area of the lake (m2). Theoretical

energy input based upon electrical horsepower rating of the

pumps was also estimated assuming 80% wire-to-water

efficiency.

3. Results and discussion

3.1. Lake level

The lake level varied dramatically over the course of this

study, with a maximum depth as low as 4.5 m in the summer

of 2004 and as high as 11 m in the winter of 2005 (as shown in

later contour plots, Figs. 3a and 4a). The lake level decreased

in the summertime about 0.5–1 m due to evaporative losses,

and increased in the winter due to runoff from precipitation.

Addition of recycled water and ground water slowed the

decline in lake level slightly in 2003 and 2004 relative to that

found in 2005. When the study began in 2003, the maximum

depth was approximately 5.7 m and had reduced to approxi-

mately 4.5 m by the time the axial flow pumps were installed

in July 2004. The maximum lake depth increased to approxi-

mately 10.5 m in July 2005 due to the increased runoff and

precipitation during the winter storms earlier that year.

Variations in lake level are important in this analysis

because the amount of work needed to mix the water column

generally increases with increasing lake depth (Kling, 1988).

To statistically evaluate the effects of the axial flow pumps

and natural wind forcing on the mixing processes and

aeration of Lake Elsinore, measurements taken during the

summers of 2003, 2004, and 2005 were averaged over the

entire summer for comparison. The summer is considered

here to be the months of July–September. The summer of

2003, before the installation of the axial flow pumps, consists

of 5 sampling dates; the summer of 2004, after the installation

and during operation of the axial flow pumps, consists of

8 sampling dates; and the summer of 2005, after the dramatic

increase in lake depth resulting from the winter storms,

consists of 6 sampling dates (Table 1). Axial flow pumps were

operational for the entire summer of 2005 as well. t-Tests

assuming unequal variances were performed comparing

each year with another (2003 vs. 2004, 2004 vs. 2005, and

ARTICLE IN PRESS

Table 1 – Summer averages (July–September) for 2003, 2004, and 2005

2003 (n ¼ 5) 2004 (n ¼ 8) 2005 (n ¼ 6)

Axial flow pump operation Not operational Operational Operational

Depth 3.2470.15a 2.9670.12b 6.7070.25c

Secchi depth (cm) 24.776.8a 18.771.4a 131.0717.3b

DT (1C) 0.5470.52a 0.3870.49a 1.1370.84a

Average temperature (1C) 26.670.76a 25.671.7a 27.071.4a

Average DO (mg/L) 2.4670.47a 3.5171.2b 4.6471.91b

DO deficit (tons) 166717a 115733b 2467158a,b

% Area anoxic sediments 45.2712.3a 33.1724.5a 50.7726.3a

Stability (J/m2) 21.576.0a,b 18.576.1a 45.8726.3b

Air temperature (1C)* 24.675.7a 23.376.0a 23.876.0a

Wind speed at 10 m (m/s)* 2.0071.40a 2.0971.36a 2.1471.36a

TKEwind (W/m2)* (1.0071.71)� 10�5,a (1.0471.80)� 10�5,a (1.0771.79)�10�5,a

Results from a t-test using unequal variances are indicated with superscripts.

Values followed by the same letter were not significantly different from each other (at pX0.05).

*24 h averages over the entire summer period.

J J A J O J J O J A

Date

300

200

100

0

Secch

i D

ep

th,

cm

O A

Fig. 2 – Average Lake Secchi depth over time. 0 cm denotes

the lake surface.

WAT E R R E S E A R C H 41 (2007) 4457– 4467 4461

2003 vs. 2005) and a test resulting in a p-value lower than the

critical value of p ¼ 0.05 considered the two sets of data

to be significantly different from one another. The results

from t-tests for the average depths yielded p-values lower

than the critical value, indicating statistically significant

differences in the average summer depths in 2003, 2004, and

2005 (3.2470.15, 2.9670.12, and 6.7070.25, respectively)

(Table 1).

3.2. Transparency

Light penetration and short-wave radiation inputs to surface

waters are dependent upon the transparency of water

(Wetzel, 2001); changes in transparency can thus alter the

heat budget and thermal properties of the water column. At

the start of the study (July 2003), Lake Elsinore was subject to

intense Oscillatoria blooms that yielded very low transpar-

encies (about 30 cm). Transparencies remained low in 2003

and 2004, around 20 cm (Fig. 2). The Secchi depth increased in

the winter of 2005 as the lake received more water from

precipitation and runoff and the lake level increased.

Transparencies continued to increase through the winter

and spring of 2005, reaching a high of 190 cm in early June

2005. Fluctuations were observed throughout the remainder

of the study and typically coincided with algae blooms. The

highest Secchi depth in 6 years was recorded in January 2006,

reaching 301 cm (Fig. 2).

A series of t-tests for the summer average Secchi depth for

2003, 2004, and 2005 yielded no significant difference between

the mean Secchi depths for the summers of 2003 and 2004

(Table 1). Not unexpectedly, the t-test results indicated that

the average transparency in 2005 (131.077.3 cm) was signifi-

cantly higher than the average transparencies of 2003 and

2004 (Table 1).

Although the massive amount of runoff brought large

quantities of nutrients to Lake Elsinore in the winter of

2005, the Secchi depth increased greatly. This may be due to

the dilution of fresh water to the lake, which accounted for a

decrease in electrical conductivity from 4.3 mS/cm in 2003 to

1.5 mS/cm in 2005. The reduction in salinity allowed for a shift

in the ecology of the lake. The algal community shifted away

from an Oscillatoria-dominated system to a more balanced

community with diatoms, green, and blue–green algae. The

zooplankton community also changed, where a healthy

population Daphnia that was essentially absent in 2003 and

2004 (Veiga Nascimento, 2004) developed in 2005 and appar-

ently more effectively grazed down the phytoplankton.

Although these are potential outcomes of a successful

destratification system (Cooke et al., 2005), the change

occurred in Lake Elsinore only after the winter runoff and

before axial flow pump operation began again in the spring of

2005.

The increase in the water column depth may have also

contributed to the greater clarity of the lake allowing for less

sediment re-suspension into the water column. At lower lake

levels, wind mixing can stir the bottom sediments and cause

increased turbidity in the water column, increase nutrient

release from the sediments, and increase algal productivity

ARTICLE IN PRESS

Dis

tan

ce

fro

m B

ott

om

, m

8121620242832

0

2

4

6

8

10

Temperature, °C

J J A J O J J O J A

Date

0

2

4

6

De

lta

T,

°C

O A

Temperature

Delta T

Fig. 3 – Thermal properties in Lake Elsinore (site A3, July

2003–June 2006): (a) temperature profiles and (b)

temperature difference between 2 m depth and bottom.

Horizontal bars indicate periods of axial flow pump

operation.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 4 5 7 – 4 4 6 74462

(Kann and Welch, 2005; Welch and Cooke, 2005). The increase

in lake level in 2005 may have thus lessened the importance

of these processes in the lake.

Historically, the effect of destratification on water lake

transparency has varied from case to case (Lorenzen and Fast,

1977; Pastorok et al., 1981; Cooke et al., 2005). Aside from the

decrease in algal productivity associated with the reduction of

internal nutrient loading, transparency may increase as a

result of mechanical mixing in lakes, where surface algae

blooms are mixed further into the water column where light

may become a limiting factor (Cooke et al., 2005). On the other

hand, algae blooms may be fueled by hypolimnetic nutrients

mixed to the upper water column during destratification

causing transparencies to decrease such as in Lake Wilcox,

southern Ontario (Nurnberg and LaZerte, 2003). In some cases

there have been no changes in lake transparency due to

destratification (Toetz, 1979).

3.3. Temperature

Lake temperature profiles varied seasonally, with cool iso-

thermal conditions in the winter and warm thermally

stratified daytime conditions in the summer (Fig. 3a). Mini-

mum annual temperatures of approximately 10 1C were

generally found in December–January, while temperatures

exceeding 26–27 1C throughout the water column were

persistent through much of the summer. Temperature

profiles were averaged for all sites and the volume-weighted

average temperature and total heat content were calculated

for the lake. Statistical analysis revealed that there was no

significant difference in the average summer temperature

(Table 1). Complete destratification in lakes results in higher

average lake temperatures (i.e., such as in Section Four Lake

(Fast and Momot, 1973)).

Temperature profiles in July 2003 and 2004 showed that

warm surface water usually penetrated 0.5–1.5 m below the

surface, while the greater transparencies in 2005 (Table 1;

Fig. 2) resulted in deeper penetration of heat into the water

column, often 3–5 m below the surface (Fig. 3a). Consequently,

the depth of the thermocline was observed to be greater in

2005, thus resulting in a larger surface layer than observed in

previous years. The depth of the thermocline was considered

the mid-point of the depth interval at which the greatest

change in temperature occurred in the water column profile.

The average depths of the thermocline in the summers of

2003 and 2004 (0.2570 and 0.3470.20 cm, respectively) were

less than the average thermocline depth in 2005

(0.9670.84 cm). This may be attributed to greater light and

heat penetration due to an increase in the transparencies

observed after the winter storms of 2005 (Fig. 2). Kling (1988)

observed that the depth of the thermocline in 31 tropical

lakes in West Africa was strongly correlated with Secchi depth

(Kling, 1988). Fee et al. (1996) also found a strong influence of

lake transparency on the mixing depth in lakes of the

Canadian Shield.

While seasonal trends in water column temperatures are

apparent from the contour plot of profile data for the lake

(Fig. 3a), the large seasonal swings in temperature tend to

obscure the generally modest vertical gradients in tempera-

ture, where surface waters are often just a few degrees

warmer than bottom waters. To better highlight the extent

and duration of stratification present over the 3-year study,

the difference in the temperature (DT) between that found at

2 m depth and just above the sediments at site A3 (Fig. 1) was

determined (Fig. 3b). The 2 m depth was chosen based upon

our observations that the uppermost 2 m of the water column

experiences marked diurnal heating and cooling, while below

2 m or so the temperature is more influenced by longer-term

weather patterns, seasonal changes, and mixing, and is less

dependent upon the time of day of sampling (Fig. 3a).

Temperature differences between the 2 m and bottom

depths were generally close to 0 1C during the fall and winter,

confirming the notion of well-mixed isothermal conditions,

although fairly low DT values (e.g., o1 1C) were also typically

found in the summer of 2003 and 2004 (Fig. 3b). Higher DT

values were present in 2005, with values as high as 2.4 1C in

July and a summer mean value of 1.1370.84 1C (Table 1). By

comparison, the mean summer values for the summers of

2003 and 2004 averaged 0.5470.52 and 0.3870.49 1C, respec-

tively, although the large standard deviations and modest

ARTICLE IN PRESS

10

8

6

4

2

Dis

tan

ce fro

m b

ott

om

, m

0

0246810121416

Dissolved Oxygen, mg/L

J O J A J J J J A

Date

0

4

8

12

Dis

so

lved

Oxyg

en

, m

g/L

Theoretical DO

OAO

DO

Volume-weighted average DO

Fig. 4 – DO properties in Lake Elsinore (site A3, July

2003–June 2006): (a) DO profiles; (b) volume-weighted

average DO over time. Horizontal bars indicate periods of

axial flow pump operation.

WAT E R R E S E A R C H 41 (2007) 4457– 4467 4463

N sizes yielded no statistically significant difference between

the 3 years. The highest DT values in any given year were

found in the spring (March and April) and early summer

(June) and approached or exceeded 4 1C in 2005 and 2006

(Fig. 3b). The greater lake depth in 2005 and 2006 (Fig. 2)

increased the magnitude of DT and, more importantly, also

lengthened the duration of stratification, e.g., with DT

exceeding 1 1C every sampling event from March–July 2005

except one (Fig. 3b). The limited effect of axial flow pump

operation on stratification in Lake Elsinore can be compared

with similar systems installed at Arbuckle Lake and Ham’s

Lake (Toetz, 1977, 1979, 1981). The pumps effectively destra-

tified Ham’s Lake, where as much as a 10 1C decrease in DT

was observed between the reference year and the operating

year (Toetz, 1981). Arbuckle Lake was not successfully

destratified, but the thermocline was significantly deeper

and the lake mixed earlier during the operational year

compared with the control year (Toetz, 1979, 1981). The two

lakes differed in mean lake depth, however, with Arbuckle

Lake more than twice as deep as Ham’s Lake (9.5 and 2.9 m,

respectively). Ham’s Lake, which was successfully destrati-

fied, had a mean depth quite similar to Lake Elsinore in 2004,

although it had a greater maximum depth (10 m vs. approxi-

mately 5 m for Lake Elsinore).

3.4. Dissolved oxygen

DO depletion has been a continuing problem is shallow lake

management due to the high hypolimnetic and sediment

oxygen demand (HOD and SOD, respectively) that may result

from the high ratio of sediment area to water volume. SOD

has been found to account for a large percentage of the

overall HOD and can increase during artificial aeration (Beutel

et al., 2007). To alleviate DO problems in shallow lakes,

maintaining complete aeration is necessary.

DO concentrations in Lake Elsinore varied seasonally, with

relatively high DO concentrations found throughout the water

column in the winter (Fig. 4a). Conversely, strong DO

gradients with depth were observed during the summer.

Anoxic conditions (DO concentration o1 mg/L) were often

found near the sediments, and DO super-saturation (DO

values higher than those based on Henry’s law) was

frequently observed at the surface resulting from high rates

of photosynthetic O2 production during algae blooms (Fig. 4a).

Anoxic conditions near the sediments were present even

when the lake level was quite low, indicating high rates of

oxygen consumption relative to re-supply from the atmo-

sphere, with axial flow pump operation not dramatically

altering these conditions (Fig. 4a). Slightly weaker vertical

gradients in DO concentrations were found in the summer of

2005 (Fig. 4a) that probably resulted from increased transpar-

encies (Fig. 2). The deepened surface layer may have allowed

increased primary productivity to greater depths as a result of

increased light penetration.

DO profiles for the sampling sites were used to calculate

volume-weighted average DO concentrations that varied

seasonally over the 3-year study (Fig. 4b). DO values ap-

proached theoretical concentrations in the winter and were

typically 2–5 mg/L lower than the theoretical concentrations

in the summer (Fig. 4b). The volume-weighted average DO

concentrations ranged from 2 to 4 mg/L in the summer of

2003, and then increased in the fall and reached theoretical

values in January 2004 with a high of 9.8 mg/L (Fig. 4b). The

average DO concentrations declined in the winter and spring

of 2004 back to summer values of 2–4 mg/L (about 4–6 mg/L

below theoretical values), despite efforts of the axial flow

pump operation (represented by the gray cross-hatched areas

in Fig. 4b). DO levels increased in late fall 2004, and after the

increase in lake level the volume-weighted average DO

concentrations remained high (between 7 and 10 mg/L) in

the winter. Concentrations fluctuated more strongly in the

spring and summer of 2005, often varying from 3 to 8 mg/L

between 2-week sampling intervals, but remained more

consistently under-saturated through the winter of 2006

(Fig. 4b).

A statistical analysis indicated that the volume-weighted

average DO concentration was lower in the summer of 2003

than the summers of 2004 and 2005 (Table 1). The t-test

indicated that there was no significant difference between the

summertime average DO concentrations in 2004 and 2005

(Table 1). Although the summer average DO values were

ARTICLE IN PRESS

0

20

40

60

80

% A

rea

of A

no

xic

Se

dim

en

ts

J O J O J O JA J A J A

Date

Fig. 5 – Percent area of anoxic sediments over time (July

2003–June 2006). Horizontal bars indicate periods of axial

flow pump operation.

0

40

80

120

Th

erm

al S

tab

ilit

y, J/m

2

J O J O J O JA J A J A

Date

Fig. 6 – Thermal stability over time (July 2003–June 2006).

Horizontal bars indicate periods of axial flow pump

operation.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 4 5 7 – 4 4 6 74464

higher in 2004 and 2005, optimal aeration would have resulted

in concentrations closer to theoretical values, around 8 mg/L,

and levels found during cooler isothermal conditions when

the water column was well aerated.

The DO profiles from the sampling sites were also used to

estimate the % area of anoxic sediments (taken as DO

concentrationso1.0 mg/L directly above the sediments

(Fig. 3a). At the start of the study in the summer of 2003,

27–59% of the sediments were observed to be anoxic; in the

fall the percentage decreased until finally reaching 0% in the

wintertime. The sediments remained oxic from December to

May with the exception of 1–2 weeks in April of 2004 and 2005

when rapid spring heating resulted in stratification (Fig. 3a

and b). In the summer of 2004, 25–70% of the sediments were

anoxic for the entire summer save for one mixing event in

August, where 0% anoxic sediments were observed (Fig. 5).

Axial flow pump operation did not appear to have any clear

effect on the % area of anoxic sediments in the summer of

2004. The % area of anoxic sediments declined until reaching

0% anoxic sediments in the winter of 2005. In the summer of

2005, the sediments were 47–70% anoxic for the entire

summer until a mixing event in late September, where 0%

of the sediments were observed to be anoxic. The lower

portion of the water column remained well aerated through-

out the fall of 2005 and winter of 2006, and anoxia was again

observed in the spring. t-Tests performed for the summertime

average % area of anoxic sediments indicated that there was

no significant difference between the summer average values

for 2003, 2004, or 2005 (Table 1).

The DO deficit, calculated as the observed DO subtracted

from the theoretical DO and expressed on a total-mass basis,

was highly variable over the course of the study, ranging from

531 metric tons in March 2005 to �130 metric tons in May

2006 (data not shown). DO super saturation was considered to

be the DO deficit below zero (i.e., a negative DO deficit),

indicating an elevated volume-weighted average DO concen-

tration and attributed to excess photosynthetic production of

O2. Results from the t-tests for the average summer DO deficit

indicated that the deficit in the summer of 2003 was

significantly higher than that in the summer of 2004

(Table 1). However, the average DO deficit in 2005 was not

significantly different than the summer averages of 2003 or

2004 (Table 1). This was attributed to the high variability of the

2005 data, although the reason for these large oscillations is

not entirely clear (Fig. 4b).

Successful destratification in lakes most commonly results

in an increase in lake-wide DO (Cooke et al., 2005). While the

volume-weighted average DO concentration in Lake Elsinore

was greater in 2004 and 2005 than in 2003, the % area of

anoxic sediments was not significantly different in any of the

3 years (Table 1). It is thus difficult to conclude that axial flow

pump operation had a significant effect on the overall DO

status of the lake. Moreover, the mean summer lake level was

lower in 2004 than in 2003, while mean air temperature and

wind speed were similar, so per unit volume, slightly higher

wind energy inputs would have been available to help mix the

water column. There also appeared to be greater DO super-

saturation in the surface waters (Fig. 4a), which may be

responsible for the increase in the volume-weighted average.

For comparison, Toetz (1981) found that, although axial flow

pumps installed in Arbuckle Lake did not effectively mix the

entire water column, the volume of anoxic water decreased,

thus indicating partial success in destratification in the lake.

3.5. Thermal stability

Schmidt stability calculations were made using the average

temperature profiles measured between the hours of 10:00

am and 2:00 pm on the days of sampling; thus, the values

obtained represent average daytime stabilities. Stabilities

were generally observed to be low in the winter months,

typically o10 J/m2 and often o5 J/m2, while summer daytime

stabilities were observed to be much higher (Fig. 6). Daytime

stabilities in the summer of 2003, prior to the installation of

the axial flow pumps, ranged from about 20 to 30 J/m2 (Fig. 6).

Similar values were observed in the summer of 2004 following

the installation and operation of the axial flow pumps. Mean

stabilities for these two summer periods were very similar

ARTICLE IN PRESS

WAT E R R E S E A R C H 41 (2007) 4457– 4467 4465

(21.576.0 and 18.576.1 J/m2, respectively) and not significantly

different from each other (Table 1). Much higher daytime

stabilities were present in the summer of 2005, however, with

values exceeding 60 J/m2 and approaching 100 J/m2 in July and

early August 2005 (Fig. 6). Stabilities declined later in the summer

to yield a mean (July–September) value of 45.8726.3 J/m2, a value

statistically significantly higher than that for the summer of 2004,

although not significantly different from 2003 at p ¼ 0.05

(Table 1). Rapid warming in the spring (Fig. 3a) resulted in

increased stabilities in March and April in all 3 years, with

sharply higher values also found in May and June in 2005 and

2006 (Fig. 6). These periods of higher stabilities coincide with

comparatively large DT values (Fig. 3b), declines in average DO

concentrations (Fig. 4b), large areas of anoxia (Fig. 5), and greater

DO deficits (Table 1).

Although the mixing depth was potentially greater, the

thermal stabilities in the summer of 2005 were frequently

much greater than in previous years of the study (Fig. 6). This

can be attributed to the increase in maximum depth. The

winter storms of 2005 increased the maximum depth of Lake

Elsinore from about 5 to 410 m (Fig. 3a). This had a

tremendous effect on lake stability, where values in the

summer of 2005 were observed to be as much as 4 times

higher than those observed in previous summers. It has been

observed in temperate as well as tropical lakes that greater

maximum depth usually corresponds to greater values in

stability (Johnson et al., 1978; Kling, 1988). Kling (1988) showed

that thermal stability was correlated with maximum depth

from his survey of tropical lakes in West Africa, conforming to

a relationship of the form (Kling, 1988)

log S ¼ 0:416þ 1:48� ðlog zmaxÞ; (7)

where S is thermal stability (J/m2) and zmax is the maximum

depth (m). When applied to maximum depths of 5 and 10 m

(approximate range of maximum depths found in Lake

Elsinore during this study), the equation yields predicted

stabilities of 28 and 79 J/m2, respectively. A predicted value of

28 J/m2 is quite similar to those found at Lake Elsinore in the

summers of 2003 and 2004, while a predicted thermal stability

of 79 J/m2 is similar to values found in the summer of 2005

(Fig. 6). Thus, the empirical relationship developed by Kling

(1988) for lakes in West Africa (without supplemental TKE

inputs from destratification systems) quite reasonably repro-

duced observed daytime stabilities in Lake Elsinore, irrespec-

tive of axial flow pump operation. This provides additional

indirect evidence that the axial flow pumps have not

substantially altered the stability of the water column relative

to natural wind-forcing and convective mixing processes.

3.6. TKE and meteorological conditions

Meteorological factors play a dominant role in lake mixing

processes and thermal stratification. It is important, then, to

compare meteorological conditions for the 3 study periods

(summers of 2003, 2004, and 2005). Hourly air temperature

and wind speed data were averaged for each of the 3

summers. Hourly wind speed values corrected to 10 m height

were used to calculate the TKE due to wind shear (TKEwind),

with the calculated TKEwind values then averaged for each of

the summers.

There were no significant differences for the average air

temperature, wind speed, or TKEwind for any of the years of

the study (Table 1). The large standard deviations in the

summer average wind speeds, and thus TKEwind (Table 1),

arise from the pronounced diurnal variation in wind

speeds that are close to 0 m/s in the early morning hours

and peak in the afternoon, at speeds up to 7–10 m/s

(CIMIS, 2006). When maximum wind speeds occur during

the same period as maximum heating, effects of TKEwind on

water column stability can be damped substantially due to

the high resistance to mixing resulting from increased

buoyant forces set up by surface heating (MacIntyre and

Melack, 1995).

TKE provided by wind shear, combined with that from

convective mixing during periods of cooling, is usually

sufficient in the fall and winter to overcome the buoyant

forces caused by differences in temperature and vertically

mix the lake. However, these buoyant forces are stronger in

the summertime, as the surface heat flux is much greater,

and the energy provided by wind and convection may not

overcome the strong density gradient. The aeration system

installed in Lake Elsinore in the summer of 2004 was intended

to provide enough TKE to the lake, in combination with the

TKE provided by wind and convection, to overcome these

buoyant forces and mix the lake. The theoretical amount of

TKE provided by the axial flow pumps was calculated from

Eq. (6) using the rated flow rates (1.89 m3/s/pump) and

assuming a velocity of 0.72 m/s (from flow rate and cross-

sectional area of the turbine). Inserting these values and the

surface area of the lake (approximately 10 km2 in 2004 and

14 km2 in 2005) into Eq. (6), the TKE inputs due to the axial

flow pump operation in 2004 and 2005 were 9.8�10�4 and

7.0�10�4 W/m2, values that are 65–98� greater than the TKE

input due to natural wind mixing (Table 1). Based on these

values, axial flow pumps operation of o1 day would have

been sufficient to overcome the average stability in 2003. That

the average daytime stability in 2004 was reduced only 3 J/m2

from the mean 2003 value implies that, despite approxi-

mately 212 months of operation, the axial flow pumps were

very inefficiently delivering mixing energy to the water

column of the lake. Assuming a reduction of 3 J/m2 over

212 months of operation (6.5�106 s), one estimates that the

actual TKE input may be as low as 5�10�7 W/m2 or o0.1% of

its theoretical capacity.

The electrical power consumed by the pumps can also be

used to provide an alternative estimate of theoretical mixing

energy input to the lake; assuming a 25% wire-to-water

transfer efficiency and a lake area of approximately 10 km2,

one estimates a total energy input of 1.1�10�3 W/m2 (20

pumps�3 HP/pump�746 W/HP�1 J/s/W�0.8/107 m2¼ 1.1�

10�3 W/m2), a value in good agreement with that estimated

from Eq. (6). Irrespective of the method of calculation, it is

apparent that very little energy is being efficiently trans-

mitted into the water column by the axial flow pumps.

3.7. Velocity measurements

A series of transects away from one of the axial flow pumps

was made with an ADCP on August 20, 2004. An example of

one of the transects is plotted in Fig. 7 (the others were very

ARTICLE IN PRESS

5 10 15 20 25 30 35

Distance from Pump, m

-5

-4

-3

-2

-1

De

pth

Be

low

Su

rfa

ce

, m

051015202530354045

Fig. 7 – Horizontal velocities near axial flow pump measured

using an ADCP.

WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 4 4 5 7 – 4 4 6 74466

similar). The measurements began at the docking station

(x ¼ 0) and ended in this particular transect 38 m away (Fig. 7).

An area of high horizontal velocities (20–45 cm/s) was found

just near the docking station extending laterally out to about

5–8 m in the lower portion of the water column (Fig. 7).

Vertical velocities (not shown) decreased quickly with in-

creasing distance from the axial flow pumps, from values of

�30 cm/s (i.e., downward) immediately adjacent to the

docking station to +10 cm/s (i.e., upward) approximately 7 m

from the edge of the pumps. Downward velocities of

approximately �10 cm/s were again observed about 10 m

away from the edge of the pumps. These measured vertical

velocities are lower than the value used in the TKE calculation

(72 cm/s), and result from our inability to measure velocities

directly below the axial flow pumps. Horizontal velocities of

about 10–20 cm/s extended 18–20 m away from the pump,

while low velocities (typically near 2 cm/s) were observed

thereafter extending to the end of the transect and similar to

velocities measured hundreds of meters from the pumps.

The high-energy zone in Fig. 7 effectively maps the

magnitude and loci of energy flow to the water column

provided by the axial flow pumps. Water is being pumped

downward at high velocities, where an apparent scouring of

the bottom sediments has occurred to a depth of about 40 cm

(represented by the lower black line in the Fig. 7). The

measured flow velocities indicate that a localized circulation

cell developed around the axial flow pump with little far-field

lateral spreading. Measurements made at other axial flow

pumps in 2004 and at higher lake levels in 2005 yielded very

similar results.

While comparatively few studies have been published on

the impacts of axial flow pump destratification systems on

lakes, Punnett (1991) did note that pumping at too great a rate

for mixed water to spread through the lake can result in

pumped water being recirculated in localized cells around the

pumps (e.g., as seen as Beech Fork Lake; Punnett, 1991).

Velocity measurements indicate that such circulation cells

are present in Lake Elsinore (Fig. 7). Assuming an active

mixing cell that extends 24 m from each docking station and

an additional 3 m to account for the distance from the center

of the docking station (total radius of 27 m), one estimates

that the 4 pumps at each docking station mix a circular area

of 2290 m2. In total, the axial flow pumps are estimated to

have actively mixed 11,451 m2 or about 0.1% of the total lake

surface area at that time.

The ineffectiveness of the axial flow pumps in weakening

stratification and improving DO conditions in Lake Elsinore is

thus thought to result from a number of factors, including its

shallow depth, basin morphology, pump configuration, and

lake trophic state. Punnett (1991) previously recognized basin

size, shape, and placement of the axial flow pumps to have

major influences on system efficiency. Unlike most reservoirs

and many natural lakes, Lake Elsinore lacks a well-defined

‘‘deep hole’’ and rather has only small horizontal gradients in

depth. The comparatively flat bottom topography and shallow

depth (Fig. 1) thus limits the development of a strong lateral

gradient in temperature and density that would help move, by

gravitational flow, cooler, more dense water toward the axial

flow pumps. That is, lateral mixing is induced both by the

local advective currents set up by direct pump action as well

as by indirect gravity-driven flows toward the pumps that

would result from the presence of locally warmer, less dense

water at the sediments. Excessive turbulence production due

to the configuration of the pumps into arrays of 4, combined

with the shallow lake depth, further inhibited effective

mixing. Finally, the intense daytime heating in the region

during the summer limits the effectiveness of wind mixing

(MacIntyre and Melack, 1995), while the high oxygen emand

in the subsurface make it difficult to substantially improve

DO concentrations in Lake Elsinore. This contrasts, e.g.,

Ham’s Lake, where axial flow pumps were able to destratify

the water column in approximately 1 week (Toetz, 1977).

4. Conclusions

1.

Installation and operation of 20 axial flow pumps, in arrays

of 5 rafts with 4 pumps each, had little effect on

stratification and DO levels in the lake.

2.

Lake depth and water quality changes resulting from the

winter storms in 2005 had a more substantive impact on

water column properties than axial flow pump operation.

3.

Excessive turbulence and local circulation near the axial

flow pumps, combined with its shallow depth and flat

bottom topography, are responsible for the very low

efficiency in net mixing energy transfer to the water

column in Lake Elsinore.

4.

Results of this study indicate that care must be taken in

the design and operation of axial flow pumps for improv-

ing water quality in shallow lakes.

Acknowledgments

This work was supported by Lake Elsinore and San Jacinto

Watersheds Authority. Thanks to Ed Betty and Jacob Wake-

field-Schmuck for their vital assistance in the field.

ARTICLE IN PRESS

WAT E R R E S E A R C H 41 (2007) 4457– 4467 4467

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