United States EPA-600/3-81-014
Environmental Protection February 1981
Agency
v>EPA Research and
Development
Evaluation of
Aeration/Circulation
As a Lake Restoration
Technique
Prepared for
Office of Water Regulations and
Standards
Criteria and Standards
Division
Prepared by
Environmental Research Laboratory
Office of Research and Development
Corvallis, OR 97330
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EPA-600/3-81-014
February 1981
EVALUATION OF AERATION/CIRCULATION •->( J^0"'"Llbrfl'y
& Protection
AS A LAKE RESTORATION TECHNIQUE
by
Robert A. Pastorok, Thomas C. Ginn
and Marc W. Lorenzen
Tetra Tech, Inc.
1900 116th Avenue, N.E.
Bellevue, Washington 98004
Project Officer
Spencer A. Peterson
Environmental Research Laboratory
Corvallis, Oregon 97330
Cento (SPM52) ,'
841 Cv.?sV.ut Street /
191Wi <_
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97330
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DISCLAIMER
This report has been reviewed by the Corvallis Environmental Research Laboratory,
U.S. Environmental Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and policies of
the U.S. Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
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ABSTRACT
Artificial circulation and hypolimnetic aeration are management
techniques for oxygenating eutrophic lakes subject to water quality
problems, algal blooms, and fishkills. Artificial circulation is achieved
by injecting diffused air into lower waters, by mechanical pumping of water
from one depth stratum to another, or by inducing turbulence at the surface
using large axial-flow pumps. Complete mixing leads to isochemical and
isothermal conditions with depth. In contrast, hypolimnetic aeration by air
or oxygen injection affects primarily bottom waters; in some instances, low
dissolved oxygen concentrations persist in the metal imnion. In general,
both restoration methods lower the concentrations of reduced compounds
(e.g., Fe, Mn, NH^, l^S) in lake waters, providing benefits for water supply
systems. Aeration may cause supersaturation of nitrogen gas, thereby
raising the potential danger of gas bubble disease in downstream fishes when
hypolimnetic waters are released from a reservoir. In some instances,
aeration/circulation has aggravated oxygen deficits and increased the
potential for massive fishkills. Usually, adverse impacts can be attributed
to faulty design of the aeration device, improper application of the
technique, or inadequate understanding of biological response mechanisms.
Whole lake mixing may reduce regeneration of nutrients from profundal
sediments, while often controlling blooms of blue-green algae. Models
predict that overall algal biomass will decrease in deeper lakes when light
limitation is induced by mixing. If destratification elevates epilimnetic
C02 levels and causes a sufficient drop in pH, dominance in the algal
community will likely shift from a nuisance blue-green species to a mixed
assemblage of green algae. This more edible resource combined with an
expansion of habitat and provisioning of a prey refuge in lower waters leads
to more abundant zooplankton and invasion of large-bodied daphnids. In most
cases, treatment enhances the abundance and species richness of benthic
macroinvertebrates in the profunal zone, causing a shift from predatory
chaoborids to a diverse assemblage of detritivores. Habitat expansion and
shifts in community structure of benthic macroinvertebrates potentially
elevate the abundance of fish food organisms. Although short-term
increases in fish growth and yield have been attributed to improvements of
food and habitat resources, documentation of long-term changes is lacking.
In southern regions, artificial circulation provides benefits for warm-water
fishes only.
Hypolimnetic aeration maintains the natural thermal structure of the
lake while oxygenating bottom waters. Thus, this technique is preferred for
management of water supply systems and cold-water fisheries. The potential
benefits of hypolimnetic treatment in controlling algal blooms are more
limited than those realized with whole lake mixing. While hypolimnetic
treatment usually lowers phosphate concentrations in bottom waters, the
long-term effects on internal loading of nutrients is unknown. Hypolimnetic
aeration or oxygenation appears to allow habitat expansion in zooplankton
and benthic macroinvertebrates as well as fishes.
111
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CONTENTS
Page
FIGURES vi
TABLES vi
GENERAL INTRODUCTION 1
ARTIFICIAL CIRCULATION 2
Mixing Devices and Applications 2
Effects of Artificial Circulation on Water Quality 5
Effects of Artificial Circulation on Phytoplankton 8
Effects of Artificial Circulation on Zooplankton and
Species Interaction in Open Water 17
Effects of Artificial Circulation on Benthic
Macroinvertebrates 22
Effects of Artificial Circulation on Fishes 26
HYPOLIMNETIC AERATION AND OXYGENATION 30
Introduction 30
Hypolimnetic Aeration Devices and Applications 30
Effects of Hypolimnetic Aeration/Oxygenation on
Water Quality 31
Effects of Hypolimnetic Aeration/Oxygenation on
Planktonic Microorganisms 34
Effects of Hypolimnetic Aeration on Benthic
Macroinvertebrates 35
Effects of Hypolimnetic Aeration/Oxygenation on Fish 36
SUMMARY 37
System Design and Application: Technical Problems 38
Benefits 38
Adverse Impacts 42
RECOMMENDATIONS 44
REFERENCES 48
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FIGURES
Number Page
1 Theoretical and observed peak biomass of algae in
Kezar Lake (adapted from Lorenzen and Mitchell 1975) 12
2 Beneficial effects of artificial circulation on phyto-
plankton (adapted from Shapiro 1979) 40
3 Some adverse impacts of artificial circulation and their
role in promoting blue-green algae blooms (adapted from
Shapiro 1979) 43
TABLES
Number Page
1 Selected Lakes and their Physical-Chemical Responses
to Artificial Circulation 3
2 Responses of Phytoplankton to Artificial Circulation 9
3 Epilimnetic pH Changes Associated with Artificial
Circulation 18
4 Responses of Zooplankton to Artificial Circulation 19
5 Responses of Benthic Macroinvertebrates to Artificial
Circulation 25
6 Selected Lakes and their Physical-Chemical Responses to
Hypolimnetic Aeration 32
vi
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GENERAL INTRODUCTION
Artificial aeration or circulation of lakes is commmonly used for
managing the ecological consequences of eutrophication. Unlike techniques
that prevent nutrient influx from the watershed (e.g. diversion of septic
effluents, treatment of inflowing waters, or modification of land use
practices) aeration/circulation affects nutrient cycling within the lake
only. Nevertheless, it can be used to enhance water quality by alleviating
a variety of problems arrising during thermal stratification and
deoxygenation of the hypolimnion. Aeration/circulation has proved
beneficial in management of domestic water supplies, downstream releases
from reservoirs, and industrial water systems. By inducing dramatic changes
in biological communities through influences on species abundance and
distribution, diversity, and trophic structure, the technique has potential
usefulness in control of algal blooms and improvement of fisheries. In some
instances, treatment has aggravated already existing problems or caused new
problems to arise, but these results can usually be attributed to faulty
design of the aeration device, improper application of the technique, or
inadequate understanding of the biological community and its response
mechanisms.
The broad range of aeration/circulation techniques has been divided
into two major groups: artificial circulation and hypolimnetic aeration.
Procedures which are designed to either mix the whole lake or provide
aeration without maintaining the normal thermal structure are classified as
artificial circulation techniques. Within this category, systems range from
high-energy mixing devices to low-energy aeration procedures; mechanical
pumps, rising air bubbles and jets of water can serve as mixing devices. In
most cases, mixing has been induced after the development of normal thermal
stratification; hence it is usually termed artificial destratification.
Destratification restores oxygen to deficient hypolimnetic waters whereas
artificial circulation before the onset of stratification can maintain
aerobic conditions near the lake bottom. Either technique leads to habitat
expansion for zooplankton, benthos and warm-water fish. However, complete
mixing may eliminate the cold-water habitat at mid-water or near the lake
bottom and cold-water fishes such as the salmonids may disappear from the
lake. Under certain circumstances, artificial circulation can eliminate
excessive algal growth or shift the community away from a uni-specific bloom
of a blue-green alga toward a mixed assemblage of more desirable green
algae.
Hypolimnetic aeration allows oxygenation of the bottom waters of a lake
without disrupting the normal pattern of thermal stratification. Both air
and oxygen in compressed form have been pumped into lakes. Hypolimnetic
aeration can effectively maintain aerobic conditions without loss of the
cool hypolimnetic water preferred for domestic and industrial uses and
required for the maintenance of cold-water fisheries.
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ARTIFICIAL CIRCULATION
MIXING DEVICES AND APPLICATIONS
The techniques used to circulate lakes can be broadly classified as
diffused air systems or mechanical mixing systems. The former category
includes all aeration devices exploiting the "air-lift" principle;
i.e. water is upwelled by a plume of rising air bubbles. Mechanical systems
employ standard diaphragm pumps, fan blades, or water jets to move water.
After reviewing design and field performance of a variety of
destratification techniques, Lorenzen and Fast (1977) concluded that
diffused air systems are less expensive and easier to operate than
mechanical mixing devices.
Compressed air is usually injected into the lake through a perforated
pipe or other simple diffuser (e.g. Fast 1968; Haynes 1973). Release of air
in deep water forms a trail of rising air bubbles which entrain water all
along their paths, creating turbulent mixing around a central zone of
upwelling (Kobus 1968; see figure A-l in Lorenzen and Fast 1977). Kobus
(1968) has shown that the amount of water flow induced by a rising bubble
plume is primarily a function of air release depth and air flow rate.
Therefore, artificial circulation is generally most effective if air is
injected at the maximum depth possible (Fast 1968J. Lorenzen and Fast
(1977) conclude that about 9.2 m3/min of air per 10° m2 of lake surface (=
30 SCFM per 106 ft*) will provide adequate water movement for surface
aeration, while maintaining minimal variation in temperature (< 2°C) and
algal concentrations throughout the water column. According to this scaling
rule, most aeration systems used in the past have been undersized (Table 1).
In a thermally stratified lake, mixing will normally be induced above
the air release depth only. Although an aerator located near the surface of
the lake may be unsuitable for destratifying a lake, it can be effective in
preventing the onset of stratification (e.g. Riddick 1957). In any case,
once a complete mix has been achieved, intermittent operation of the system
may be sufficient to maintain circulation depending on local meteorological
conditions, such as wind exposure and solar radiation.
Although most air compressors have been driven by electricity or
gasoline combustion, alternative energy sources are possible. For example,
Rieder (1977) has successfully coupled a wind power generating system with
an air compressor capable of destratifying small prairie lakes.
Mechanical mixing devices have been used less frequently than diffused
air systems, although they may be quite successful in certain circumstances
(e.g. Irwin et al. 1966; Toetz 1977a, b; Garton et al. 1978). A pumping
rate of about 10.9 n3/min was sufficient to destratify Stewart Hollow
Reservoir and Vesuvius Reservoir within 8 days (Table 1; Irwin et al. 1966).
However a pumping capacity of about 1.3 m3/min over a period of 10.1 days
did not give a complete mix of West Lost Lake (Hooper et al. 1953).
The axial flow pump designed by Quintero and Garton (1973) (also see
Garton and Punnett 1978; and Garton et al. 1978) uses a large fan blade to
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TABLE 1 SELECTED LAKES AND THEIR PHYSICAL-CHEMICAL RESPONSES TO ARTIFICIAL CIRCULATION
U)
Lak* Location
CllM'a rend Oroqen
•arvin Lnko Colorado
Sa*CiM 4 Lak* HlehlfM
Belt* L*ka lt*« Kentucky
jitivaralty Laka North Carolina
Miar Laka in* K*»a*Mra
Blag Ceoro* VI |JJJ Unltad RlnvdM
Indian Brook tea N*w York
rronpion Laka Muaylvanin
C.. tollow [»*«_„ Hl.con.ln
•...I**, te. j|»-» - Car-ny
Itarodworakla Laka to land
giia«n EllaiMtn n |JJJ united Kinadaai
Lake Mtorta Haw Mxieo
ralMMU Lake Kentucky
Tait *•• II Unitad UnodBB
l*7J
*••• a Lake OklabOH
I*7S
Teit ••• i Unltad IlngiltM
Haaa Air
41 10
• • IB J
11 * 1
2 B 02
a a
IB 7
10 00
1* 3
21
17 9
f 1
• 4 10 T
1 9 11
» 4 10 7
71 12 1
1.1 «
*J«|
• IB •
C«>>
0 B4f
0 110
2 9*1
2 OOB
4 1*1
1 400
41 tlO
1 211
2 40S
US
1 0*7
0 laO
0.1)1
0.1X1
Araa
1*
1 1
H
•0 *
M
142
7 1
IB B
314
T
120
2B 1
as a
40
JJ 1
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2 1
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ill
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4 91
1 04
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-------
TABLE I. (continued)
L*fc« Location Mrorinco Mai
Vaaiwiua "•• Ohio IrnU at «I 1*4* * 1
Vaoo*)on toadm BonataBon end Calin 1*7S 4 1
Corbott Lako British ColiMbla Juiitv and'ctlbriith l*1| l* '
•uciMMfi Lake Ontario Brown at al 1»">1 11
Lako ^airiievoon Unltod Klngooi Knooooit et •! 1**0 2* *
Arbuckle Lake }JJ) OklahoM Tooti I*l7a. bi m* 2* 1
Caviui ••• California Barnvtt 197S It
Hyru» Be. Ltah Drur> « .1 IOTS 21
«••> Lost Uha NieMo*n Hoap*r «t al 1«S1 11 0
El Capltan JJJJ California r«t 1MO »1
L«ko CalhOin HlniMMta Shapiro and rfanakuch 1*71 27 4
t.r..l. ... 0*l.h«« i«ch .t M l*» 37
rfAfflkormM KwltttrUnd HSUUi 1*47 "
b 6t • ttwporituro diffortntial bttwoon lurfac* and bottoa vator
e •••ponu p«rw*urii ID • aaccM depth. DO • dmolwd 0, nt • phospiMto.
TF • tOt«l phoaphorua MOj - nltr»t« tnt « ••ponitM
Pt • Iron ttn • MnoanoM
°*Piri '•' - ilpl •.».
Mean Air !•') I'M)
1 4 *J^ 1 SS4 42 *
IS t 11 0)
I 1* S 1 40* 21 2
« t U 0 41 • •
14 J* f 0 010 iO 7
' * I pwilp nw 1SI
It I "" 100 1100
It 4 IS 1 21 1 1*0
4 1 Dj^ 0 00* 1 4
* I 21 ) 11 9* 101 *
• 4 10 1 11 03 322
10 4 21 10 01 170 4
14 2 2f 101 1 41410
11 21 U 9 121
Jtarttlon Intanalty"
QA V
la1 /Bin) • ID*
aHial-flo» PUB?
12 > 12
4 SO I «4
0 10 0 47
24* 0 11
..1.1-fi. P-.
17 04 0 04
2 01 0 17
"-
00* 0 14
40* 02*
1 01- 0 14-
1 S4 0 20
S3 tO 0 OS
t 0 11
• ID* Ooforo Rfftn B 0
11 2
0 20 0
10 SI 1-4 0
J 17 0
4 10 I 0
11 *0
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i 42 « '
14* • 2-4
11 *
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1 17 J 0-14 0
1 11 *» .1
2 T« %ft ,]
1 4ft- .. .d
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1 IS
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ro
D o ro( T r PO j mit m
0
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a o ooo
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•
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•
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.
RlM*d to air ralHM daptti
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move water from the lake surface downward. A pump with a capacity of 102
nr/min completely destratified Ham's Lake after 3 days of operation in 1975
(Toetz 1977b). On the other hand, an array of 16 pumps with a total
capacity of 1,600 m3/min failed to mix Arbuckle Lake completely, although it
did lower the thermocline (Toetz 1979). Arbuckle Lake is more than twice as
deep as Ham's Lake (Table 1).
The Metropolitan Water Board of London induces mixing in water supply
reservoirs by discharging relatively warm river water from jet-type inlets
located near the bottom of each impoundment. A series of 9 jets, each with
a capacity of about 108 nr/min achieved adequate circulation in Queen
Elizabeth II Reservoir (Ridley et al. 1966).
EFFECTS OF ARTIFICIAL CIRCULATION ON WATER QUALITY
The onset of anaerobic conditions in the hypolimnion of a stratified
lake causes extensive chemical transformations in the surficial sediments,
which result in a transfer of nutrients to the overlying water (Mortimer
1941, 1942; Hutchinson 1957). The subsequent accumulation of Fe, C02, HgS,
NH4+ and other chemicals in the hypolimnion creates problems of quality
control for utilities supplying domestic and industrial waters (e.g. Teerink
and Martin 1969). Oxygenation of hypolimnetic waters raises the redox
potential near the lake bottom, greatly lowering concentrations of reduced
chemical species and eliminating taste and odor problems. Most aeration
systems have been installed with this goal in mind (Smith et al. 1975; Fast
1979a).
In a survey of 26 water suppliers conducted by the American Water Works
Association, 86 percent of the utility managers considered their artificial
destratification projects a success (AWWA 1971). However, 46 percent
reported that mixing created new water quality problems. Most of the
problems created by treatment involved elevation of turbidity levels or
algal blooms.
Chemical Parameters
In most cases, artificial destratification increases the concentration
of dissolved oxygen in bottom waters immediately (e.g. Hooper et al. 1953;
Lackey 1972; Haynes 1973); dissolved oxygen in the former epilimnion may
show a corresponding decrease due to a reduction in photosynthesis (Haynes
1973) or mixing of hypolimnetic waters with high BOD into the surface layer
(Ridley et al. 1966; Thomas 1966). Over a period of several weeks, the
oxygen content of the whole lake is increased (Table 1). The method of
destratification is probably irrelevant to the rate of oxygenation as long
as an adequate mix can be maintained. Since the direct transfer of oxygen
between rising air bubbles and water is unimportant except in very deep
lakes, the primary mode of aeration is through atmospheric exchange at the
lake's surface, even with diffused air systems (King 1970; Smith et
al. 1975).
Under some circumstances, oxygen depletion can not be prevented by
normal levels of artificial aeration (Table 1). For example, during mixing
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of Lake Roberts in July, a combination of reduced phytosynthetic activity in
cloudy weather and an unusually high BOD caused by decomposing Anabaena
scums created anoxic conditions throughout the lake leading to a massive
fish-kill (R.S. Kerr Research Center 1970; McNall 1971).
Oxygen levels influence redox reactions involving Fe, Mn and Al; in
turn, these elements and their complexes partly determine the availability
of nitrogen and phosphorus compounds through release processes occurring at
the surface of profundal sediments (Mortimer 1941, 1942; Hutchinson 1957).
Because of the importance of nutrient regeneration to algal production and
the interaction of chemical and biological components in controlling
nutrient levels, the effects of artificial circulation on phosphate and
nitrate concentrations will be discussed below in the section on plankton.
As hypolimnetic waters are brought to the lake's surface, excess gases
such as COo, HoS, and NH3 are released to the atmosphere (R.S. Kerr Research
Center 1970; Toetz et al. 1972; Haynes 1973). Along with oxygen and other
chemical species, these gases become isochemical with depth (Toetz et
al. 1972). Temporary rises of H2S and NHo may occur in surface waters
following mixing (R.S. Kerr Research Center 1970). Undoubtedly,
nitrification of NH^+ to N03~ is an important mechanism for elimination of
reduced ammonia compounds (Brezonik et al. 1969). After mixing, the
concentration of COo in the surface layer rises as hypolimnetic levels
decrease. The C02 content of the entire lake often falls slightly (Riddick
1957; Haynes 1973; Steichen et al. 1974), although temporary increases are
sometimes observed; e.g. during the 1966 mixing of Boltz Lake (Robinson et
al. 1969). Since changes in ambient C02 levels and related pH effects have
an important influence on species interactions in the phytopl ankton
community, these topics will be discussed in detail in a later section.
Some air injection systems may cause supersaturation of nitrogen gas
(No) relative to surface hydrostatic pressures (Fast 1979a, b). Dissolved
nitrogen concentrations of only 115 to 120 percent saturation can induce
substantial mortality among salmonids in rivers (Rucker 1972) and in
laboratory experiments (Blahm et al. 1976). Normally, the entire water
column is close to 100 percent saturation with respect to depth-specific
temperatures and pressures (Hutchinson 1957). Any rise in N2 above the
ambient concentrations is a potential problem when reservoir waters are
released downstream.
Fast (1979a, b) discusses the problem of N2 supersaturation during
artificial destratification of Casitas Reservoir in 1977. After 80 days of
aeration at 46 m depth, N2 levels in the zone of induced mixing (15 to 45 m)
were at 125 percent saturation relative to surface pressures. The waters
below 46 m had even higher N2 concentrations, up to 140 percent saturation
relative to the surface. Presumably, the aeration system did not greatly
influence N2 levels below the depth of air release, so such high
concentrations may be normal for this reservoir.
During spring circulation, N2 levels throughout the lake generally
equilibrate at 100 percent saturation with respect to surface temperature
and pressure. Any warming of the hypolimnion during summer will result in
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supersaturation relative to surface pressure and ambient temperature at
depth. But hydrostatic pressure probably maintains this "excess" gas in
solution throughout the metalimnion and hypolimnion. Even after
hypolimnetic aeration of Lake Waccabuc, No levels for most of the water
column were below the 100 percent saturation values adjusted for both
temperature and pressure (Fast et al. 1975a).
In any event, absolute concentrations of ^ will increase with depth in
stratified lakes, and the lower waters will be supersaturated relative to
the surface (Hutchinson 1957; Fast 1979a). If the body fluids of fish
equilibrate at N£ levels in deep water, and then the fish migrate to near
the surface, gas bubbles could form causing stress or mortality. To what
extent this occurs naturally, and whether aeration aggravates the problem
remains unknown. In Casitas Reservoir, fish-kills were avoided because
surface waters remained close to saturation and bottom waters were not
released downstream (Fast 1979b).
Physical Parameters
In almost every case, artificial circulation during summer promotes an
increase in the heat content of the lake, even when mixing is incomplete
(e.g. Toetz et al. 1972; Haynes 1973; Toetz 1977b, 1979; Kothandaraman et
al. 1979). Usually, the temperature of the upper waters decreases by a few
degrees, whereas deep waters are warmed by as much as 15 to 20 °C to
approximately the same temperature as the surface. Circulation during
winter actually reduces water temperatures overall because bottom waters are
no longer insulated from the cooler air by a surface layer of water or ice
(Lackey 1972; Drury et al. 1975).
Isothermy is difficult to establish because the destratification
process becomes less efficient as the lake comes closer to a complete mix
(Fast 1979a). Unfortunately, most destratif ication devices are low energy
systems, and a majority have been undersized with respect to the scaling
rule suggested above (Table 1; Lorenzen and Fast 1977). When more thermal
energy is absorbed at the lake's surface than the circulation device can
distribute, then microthermal stratification of 2 to 3°C provides algal
populations a surface refuge with high light levels (e.g. Fast 1973a; Drury
et al. 1975).
In small lakes, horizontal mixing is relatively complete, but in large
reservoirs, the destratification system will influence a limited area
(e.g. Leach et al. 1970; McCullough 1974). Of course, unaffected sections
of the lake may provide excellent experimental controls.
Artificial circulation has varied effects on water transparency,
depending on the intensity of mixing and the contribution of phytoplankton
to turbidity levels before treatment. In four of the lake case histories
examined, artificial mixing resulted in greater water transparency; and in
13 cases, transparency decreased or stayed the same (Table 1). When mixing
is induced during a surface bloom of blue-green algae, transparency will
increase immediately due to distribution of the algae throughout a greater
water volume (Haynes 1973). Thereafter, water clarity may be enhanced by
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destruction of the bloom through light limitation in deep lakes (Lorenzen
and Mitchell 1975) or through a change in some other environmental factor in
shallow lakes (Malueg et al. 1971). In contrast water clarity was reduced
in Section Four Lake when intense aeration resuspended inorganic sediments
and detritus from the lake bottom, nullifying the effect of a slight decline
in phytoplankton (Fast 1971 a). A rise in total seston after mixing
generally correlates with a decrease in transparency (Fast 1971 a; Drury et
al. 1975; Garton 1978; Garton et al. 1978). In West Lost Lake and Hyrum
Reservoir, algal blooms were responsible for the observed changes (Hooper et
al. 1953; Drury et al. 1975).
EFFECTS OF ARTIFICIAL CIRCULATION ON PHYTOPLANKTON
The effects of artificial circulation on phytoplankton populations are
extremely variable (see Toetz et al. 1972 for an earlier review), not only
because application of techniques and efficiency of mixing devices vary
among investigations, but also because alternative biological communities
exhibit different responses to the same kinds of perturbations. Moreover,
the desirability of a particular response depends on management goals. For
example, an increase in planktonic algae will be considered a nuisance if it
causes filter clogging and "taste and odor" problems in a water supply
system (Teerink and Martin 1969) yet the same "bloom" could have beneficial
effect by stimulating fish production (e.g. Johnson 1966; Oglesby 1977). An
understanding of the mechanisms underlying responses of specific biological
systems is essential to enhancement of our predictive power in future
applications of circulation techniques.
At one time, artificial circulation was regarded as a method for
reducing algal growth by one or several of the following mechanisms (Fast
1975):
1
"Preventing nutrient regeneration during anaerobic
conditions and thereby reducing internal loading"
2. "Increasing the mixed depth of the algae and thereby
reducing algal growth due to light limitation"
3. "Subjecting the algae to turbulence and rapid changes in
hydrostatic pressure as they are swept through a large
vertical distance of the water column".
It is now obvious that other effects of artificial circulation may
negate these influences and in some instances produce an opposite result,
i.e. increased algal biomass. In fact, of the 40 experiments in which
destratification was relatively complete, only 65 percent (=26 experiments)
led to any significant change in algal concentrations; of these, about 30
percent resulted in more algae than before destratification. Table 2
summarizes the responses of phytoplankton to artificial circulation for each
lake. When more than one experiment was conducted in a lake, the
predominant response is given unless the data are too variable to indicate
an overall trend; then, the responses for individual experiments are given.
Where mixing was complete, aeration caused a decrease in algal density or
8
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TABLE 2. RESPONSES OF PHYTOPLANKTON TO ARTIFICIAL CIRCULATION3
\o
Lake
Complete Mixing
Cline's Pond
I'jrvin Lake
Section 4 Lake
Boltz Lake
University Lake
Reference
Malueg et a). 1971
Lackey 1973a
Fast 1971a
Fast et al. 1973
Symons et al. 1967. 1970
Robinson et al. 1969
Weiss and Breedlove 1973
Algal
DensityC
-
-
-f
-
-
Algal Mean
Standing Chlorophyll -a.
Biomass3 Concentration
-
-
-
0
Green
Algae
0
-
-
-
•i
Blue-green Ratio
Algae Gr : Bl-gr
+
Oe o
•f
•f
Kezar Lake
King George VI
Indian Brook1'
Pranpton Lakeb
Cox Ilol1owb
Stewart Lake
U.K. Rcservoirb
Uahnbach Reservoir
Queen Elizabeth II
Lake Roberts
Falmouth Lake
Test Res. II
Buchanan Lake
Ham's Laker
Test Res. I
Turner et al. 1972
Haynes 1973
N.H.W.S.P.C.C.1971
Lorcnzen and Mitchell 1975
Ridley et al. 1966
Riddick 1957
McCullough 1974
Uirth and Dunst 1967
Wirlh et al. 1970
Barnes and Griswold 1975
Ridley 1970
Bernhardt 1967
HcNall 1971
R.S. Kerr Res. Cen. 1970
Syinons et al. 1967, 1970
Robinson et al. 1969
Knoppert et al. 1970
Brown et al. 1971
Steichen et al. 1974
Toetz 1977a, b
Carton 1978
Knoppert et al. 1970
g
+
«•
*
o
o+
0
0+
0
•f
0
0-
-------
TABLE 2. (continued)
Lake
Mirror Lake
4 Lakes'*
Starodworskle Lakef
Incomplete Mixing
Casltas Res.b
Hyrum Res.
West Lost Lake
Pfafflkersee
Waco Res.b
Lake Haarsseveenb
Lake Catharine
El Capitanf
Arbuckle Lake
Lake Calhoun
Algal Mean
Algal Standing Chlorophyll- a
Reference Densityc Biomass^ Concentration
Smith et al. 1975
Knauer 1975
Irwin et al. 1966
LOSSOM et al. 1975
Barnett 1975
Orury et al. 1975
Hooper et al. 1953
Thomas 1966
Biederman and Fulton 1971
Knoppert et al. 1970
Kothandaraman et al. 1979
Fast 1973a
Toetz I977a. 1979
Shapiro and Pfannkuch 1973
09 09
0
.
* + +
+ +
•f
0
0
0
+7
0 0
t f
Green Blue-green Ratio
Algae Algae Gr : Bl-gr
05
+T
+
.
+
+
+
0
0
0
0
0
0 +
a * • Increase. - - decrease, 0 « no significant change
** Qualitative Information only
c cells or colonies per liter; weighted mean for water column unless noted
d weight per square meter of lake surface
e increase observed, but control year was unusual
' samples were taken near lake surface
9 increase observed, but it was correlated with large input of allochthonous nutrients
n Stewart Hollow Lake. Caldwell Lake, Pine Lake. Vesuvius Lake
-------
biomass in 13 of 23 lakes. In three lakes, the amount of phytoplankton
remained about the same, and in seven lakes It increased or the overall
response was unclear. Where mixing was incomplete, algal density generally
stayed the same or increased following treatment (Table 2). Although
artificial circulation usually has a negative influence on blue-green algae,
effect on green algae is ambiguous.
Changes in phytoplankton populations after circulation treatment are
discussed below under three primary modes of influence: physical, chemical,
and biological mechanisms.
Physical Mechanisms
In lakes where algal production is potentially limited by light,
several models predict a decrease in net photosynthesis and a reduction in
standing crop of algae as depth of the mixed layer increases (Murphy 1962;
Lorenzen and Mitchell 1975; Oskam 1978). Since destratification effectively
increases the depth of mixing, algae will then be spending a considerable
amount of time in dimly lit zones, perhaps below the compensation level in
deep lakes. Accordingly, Haynes (1973) observed a sharp decline in primary
production soon after aeration of Kezar Lake (although values thereafter
rose gradually to predestratification levels). At Lake Vaxjosjon (Sweden),
average primary production (gC m"^d~^' during summer of 1970 decreased by
about 30-40 percent relative to the control year (Bengtsson and Gel in 1975).
When aerators were operative for only two brief periods in summer 1971,
average production was only slightly lower than pretreatment levels.
Lorenzen and Mitchell (1975) have found good agreement between the
predictions of their model, relating maximum standing crop of algae to mixed
depth, and the results of experiments at Kezar Lake, New Hampshire (Figure
1). Although the model ignores the effects of mixing on algal losses by
sinking, grazing, and parasitism, it does appear to give a reasonable
estimate of maximum standing stock in a variety of circumstances. Perhaps
more importantly, it explains the apparently conflicting results obtained by
different studies of changes in algal abundance after mixing (Table 2). If
algae are limited by nutrients before circulation, a slight increase in
mixing depth could cause an elevation of standing crop (e.g. point A to
point B in Figure 1), a result opposite to that found in the light-limited
condition. Thus primary productivity and productivity per cell during
aeration of oligotrophic Section Four Lake were up to three times higher
than values during the control year (Fast 1971a). The particularly intense
aeration/circulation of this lake (Table 1) resuspended large quantities of
bottom detritus and probably made nutrients more available to algae. If
mixing shifts the controlling mechanism from nutrient limitation to light
limitation, a moderate increase in mixed depth will cause a substantial rise
of peak algal biomass or at best only a slight decline (A to C or B to C
respectively in Figure 1). However, for large increases in mixed depth, the
imposition of light limitation can cause substantial decreases in area!
algal biomass (A and B to D in Figure 1). It should be noted that when
water column biomass decreases with increased mixed depth, the concentration
of algae will decrease dramatically because less biomass is distributed in a
much larger water volume. Finally, because of differences in growth
11
-------
2 3
MIXED DEPTH, METERS
• THEORETICAL VALUES
• 1968.STRATIFIED
ffl 1969.DESTRATIFIED
D 1970.DESTRATIFIED
FIGURE 1 THEORETICAL AND OBSERVED PEAK BIOMASS OF ALGAE
IN KEZAR LAKE (ADAPTED FROM LORENZEN AND MITCHELL 1975)
-------
parameters among algal species, a major shift in species composition of
phytoplankton could generate a change in peak quantity of algae apart from
the effects of mixed depth.
Where algal abundance is relatively low, e.g. in oligotrophic lakes,
artificial destratification usually produces little change in cell
concentrations (Knoppert et al. 1970; Biederman and Fulton 1971; Toetz
1977a, b; but see Fast 1971a). In some instances, standing stock may have
increased due to change in mixing depth, whereas in others, the change was
small as a result of incomplete destratification. In any event, since the
slope of the ascending curve in Figure 1 equals the peak nutrient-limited
concentration of algae (Lorenzen and Mitchell 1975), the slope will be
smallest for oligotrophic lakes because these yield less algae per unit
volume than do entrophic lakes. Hence, any given change in mixed depth over
the range of nutrient-limited biomasses will result in only small
displacements of standing crop in oligotrophic lakes compared with potential
shifts in richer lakes.
A critical problem arises when surface waters heat rapidly and an
undersized circulation device is unable to achieve a complete mix
(i.e. isothermy). The resulting microstratification provides a
shallow-water "refuge" for some blue-green algae (e.g. Apham'zomenon
flos-aquae), yielding greater population densities after the aeration
treatment (e.g. Thomas 1966; Drury et al. 1975). The influence on standing
crop is unpredictable, depending upon the magnitude of the actual decrease
in mixed depth and the mode of limitation (cf. Figure 1). In El Capitan
Reservoir, incomplete mixing resulted in microstratification near the lake
surface and an increase in net primary productivity over pretreatment levels
(Fast 1973a). The use of a mechanical device like the Garton pump (Quintero
and Garton 1973; Garton and Punnett 1978; Garton et al. 1978), which induces
mixing by moving surface waters downward, might prevent microstratification
and associated problems.
When circulation treatment is effective, the increased depth of mixing
does lead to an expansion of the depth distribution of phytoplankton (Fast
1971a; Haynes 1973). Apart from long-term effects on standing crop, if a
roughly uniform profile of cell concentration versus depth is obtained soon
after mixing, population densities decline and water clarity is enhanced
(e.g. N.H.W.S.P.C.C. 1971; Haynes 1973).
On the other hand, artificial circulation sometimes reduces light
penetration by creating more turbid waters through resuspension of bottom
deposits (Hooper et al. 1953; Fast 1971 a). Water-jet inlet systems such as
the one used at Queen Elizabeth II Reservoir (United Kingdom)(Ridley et
al. 1966) especially aggravate this problem by reducing sedimentation of
debris.
The high turbulence also helps to retain algal cells in suspension,
reducing population losses due to sinking. Population decline in
Asterionella formosa has been related to sinking losses by Lehman and
Sandgren (1978TITnduced turbulence could account for increases in
Asterionella observed by Bernhardt (1967) and Fast et al. (1973) after
treatment.
13
-------
Artificial circulation effectively reduced the abundance of blue-green
algae in only 19 of the 30 experiments in which mixing was complete;
however, these 19 cases represent 76 percent of the instances where any
change was noted. Blue-green species often control their depth distribution
via buoyancy regulation to take advantage of specific optima in light,
temperature and/or nutrients (Fogg and Walsby 1971; Konopka et al. 1978).
Following artificial circulation, Bernhardt (1967) and Weiss and Breedlove
(1973) observed dispersion of metalimnetic populations and overall decline
of Oscillatoria rubescens and 0. tenuis respectively. Mixing may eliminate
the competitive advantage of Flue-green algae due to buoyancy regulation.
Mixing also potentially affects sensitive species by rupturing gas vacuoles
as cells are swept rapidly through zones of differing hydrostatic pressure.
Whatever the mechanism, Anabaena spp. are among the most sensitive forms
(Ridley 1970; Knoppert et al . 1970; Malueg et al. 1971; Steichen et
al. 1974; Barnett 1975).
Chemical Mechanisms
Occasionally, researchers interested in elevating fish yield have
successfully stimulated phytoplankton growth by mixing nutrient-rich bottom
waters into the trophogenic zone, which is generally poorer in dissolved
inorganics required by algae (Hooper et al. 1953; Hasler 1957; Schmitz and
Hasler 1958). In this case, an incomplete mix is desirable for deep lakes
where the maximum depth is greater than the mixed depth necessary to achieve
significant light limitation (cf. Figure 1). Johnson (1966) reported an
increase in production of phytoplankton and fishes after incomplete
destratification of Erdmann Lake; unfortunately, his experimental design was
confounded by application of rotenone before mixing. Also aeration
treatments have more direct effects on fish populations (see below).
More often, mixing techniques have been applied in attempts to reduce
algal blooms by curtailing regeneration of nutrients from profundal
sediments (Toetz et al. 1972; Dunst et alI. 1974; Fast 1979a). Direct
aeration of bottom waters combined with increased exchange across the
air-water interface leads to reoxygenation of the hypolimniom. Only five
studies reported a decrease in whole lake Og following artificial
circulation (Table 1); in these cases, resuspension of bottom detritus
probably increased BOD beyond the neutralization capacity of the oxygenation
technique. If the hypolimnij)n was previously anoxic oxygenation will
immediately reduce average P04= concentration in the deep water and in the
lake as a whole by precipitation of Fe+++ and Mn++ complexes (Fitzgerald
1970; Wirth et al. 1970; Haynes 1971; Weiss and Breedlove 1973; Toetz 1979),
although P04~ may occasionally increase in the upper waters (R.S. Kerr
Research Center 1970; Toetz 1979). When aeration is insufficient,
e.g. during destratification of Queen Elizabeth II Reservoir by water jets,
mixing may result in uniform P04= levels throughout the water column without
causing a decrease in the lower waters (Ridley et al. 1966).
An effective circulation will immediately reduce concentrations of NH4+
in the hypolimnion and throughout the lake, mainly by nitrification of NH4
14
-------
to N03", as the latter shows a corresponding rise (Brezonik et al. 1969;
Weiss and Breedlove 1973; Toetz 1979).
Formation of an oxidized microzone at the sediment-water interface
forms a barrier to the release of dissolved phosphate ions from the
decomposing sediments (Mortimer 1941, 1942); hence the term "bottom-sealing"
has sometimes been applied to circulation techniques (Shapiro 1979). Recent
work (Porcella et al. 1970; Kamp-Nielsen 1974, 1975) indicates that some
phosphorus still moves across the interface into the well-oxygenated water,
but aerobic muds might still act as a net "sink" for phosphorus (Mortimer
1971; Graetz et al. 1973).
Fast (1971a, 1975, 1979a) questions whether artificial circulation
reduces internal loading of phosphorus as has previously been assumed.
Although PO^- concentrations are indeed lowered by destratification, the
flux of nutrients from profundal sediments to the overlying water and
subsequent uptake by the plant community could actually increase. Under
aerobic conditions, the higher temperatures in the sediments after
destratification will stimulate decomposition and release of phosphorus to
overlying waters (Hargrave 1969, Kamp-Nielsen 1975). Simultaneously,
nutrient exchange across the mud-water interface is facilitated by increased
flow of water over the sediments and invasion of burrowing macroorgam'sms
which mix the sediments vertically (see below). Although tubificid worms
are unimportant in stimulating phosphorus release from sediments (Davis et
al. 1975; Gallepp et al. 1978), chironomid larvae do facilitate phosphorus
transfer from mud to water, possibly in a density-dependent manner (Porcella
et al. 1970; Gallepp et al. 1978). Lastly, it is unlikely that circulation
techniques can reduce internal loading of nutrients from sources other than
the profundal sediments; e.g. "leakage" from littoral macrophytes (Demarte
and Hartman 1974; Lehman and Sandgren 1978).
Even if artificial circulation does reduce phosphorus regeneration from
the sediments, significant changes in the biota will occur only if 1)
internal loading of nutrients is large relative to input from the watershed
and 2) algal growth is limited by phosphorus (Fast 1975). Although the
latter appears true in many instances (Likens 1972), Lane and Levins (1977)
caution against overreliance on the concept of a single limiting nutrient.
Also, lakes with nuisance algal blooms are usually eutrophic and, by
definition, experience high external loading. In Mirror Lake, Wisconsin,
blooms of Oscillatoria noted by Smith et al. (1975) during the fall 1972 and
spring 1973 mixing experiments were probably caused by allochthonous inputs
of phosphorus via storm sewers rather than by some direct effect of
circulation (Knauer 1975).
In any event, the effects of destratification on the concentrations of
dissolved inorganic nutrients in the upper waters of a lake are
unpredictable due to interactions with biota and organic factions (Toetz et
al. 1972; Fast 1975). For example, aeration of Lake Roberts during June did
not prevent a bloom of Anabaena. and total phosphate dropped throughout the
lake, probably because of uptake by the algae (R.S. Kerr Research Center
1970; McNall 1971). During July however, aeration was followed by a massive
die-off of Anabaena. perhaps due to a period of intense cloud cover and
15
-------
nutrient depletion. A rise in total phosphate accompanied the algal
population crash. Artificial circulation often elevates total phosphorus
levels by resuspension of organic detritus from the bottom or maintenance of
dead cells in suspension (Hooper et al. 1953; Fast 1971 a; Haynes 1973; Wirth
and Dunst 1967). A fraction of this detritus will be liable to
decomposition, and subsequent release of inorganic forms of phosphorus will
occur. Robinson et al. (1969) attributed an increase of organic nitrogen in
Boltz Lake and Falmouth Lake to release of celluar contents by dead algae as
they lysed or broke apart due to mixing. Finally, excretion of phosphorus
by zooplankton may be an important recycling mechanism within the epilimnion
(Devol 1979).
Biological Mechanisms
An effective destratification often causes a dramatic shift in species
composition of the phytoplankton community, from dominance by one or a few
species of blue-greens to predominately an assemblage of green algae (Table
2). Fortunately, green algae remain dispersed in the water without forming
nuisance "scums" on the surface as many blue-greens do. Moreover,
zooplankton readily graze on green algal species, whereas they reject the
inedible and sometimes toxic blue-greens or grow poorly on them (Arnold
1971; Porter 1973; Webster and Peters 1978). On the other hand, some
gelatinous greens actually profit from passage through the gut of a Daphm'a
(Porter 1975).
King (1970) suggested that blue-green algae dominate the plankton of
enriched lakes because of their efficiency in taking up C02 at the low
ambient concentrations and high pH of these waters. Presumably, their
ability to fix nitrogen, regulate vertical position, and jvoid being eaten
by grazers contribute to the competitive advantage of blue-greens over other
algae.
Shapiro (1973; and et al. 1975, 1977) has induced the blue-green to
green shift in experimental enclosures by adding C02 or HC1, both of which
lower the pH of the water. Moreover, addition of-NOj" and P0^= facilitates
the shift. Since the blue-greens decline precipitously before the greens
begin growing rapidly, Shapiro et al. (1975, 1977) suggest that the shift is
mediated by the action of cyanophages, viruses specific to blue-green algae
(Shilo 1971; Lindmark in Shapiro 1979), rather than by a direct competitive
replacement. At high pH, cyanophages are inhibited, but when pH is lowered,
they are capable of lysing blue-greens. Indeed, the release of large
quantities of P04S and NH3 to the water after the sudden decline of
blue-greens in the enclosures suggests that lysis is occurring.
Destratification essentially mimics Shapiro's experimental treatments
by adding C02 and nutrients to the surface waters through: 1) mixing of
hypolimnetic C02 and nutrients into the surface layer, 2) recarbonation of
waters by atmospheric exchange, and 3) decreasing the ratio of primary
production to respiration through deepening of the mixed layer. In
experimental enclosures, a change in algal species composition occurs only
at pH values less than 8.5, and the results are unpredictable between pH 7.5
and 8.5 (Shapiro et al. 1975, 1977).
16
-------
Whether or not artificial circulation results in a shift from
blue-green algae to green algae apparently depends on the effect of mixing
upon pH in the upper waters. Although the results are not clear-cut, lakes
where the ratio of green algae to blue-green algae increased following
circulation generally showed a decrease in pH, whereas pH stayed the same or
increased in mixing experiments that failed to produce the shift to greens
or even stimulated growth of blue-green algae (Table 3). Where pH remained
high after treatment, perhaps addition of COo through circulation couldn't
satisfy algal demands for C02 due to stimulation of photosynthesis by
recycling of hypolimnetic nutrients (Shapiro et al. 1975).
In Kezar Lake during 1969, mixing caused a temporary rise in pH, but
after 20 days of aeration, the pH dropped from 9.0 to 7.1, and at least a
small increase in the ratio of greens to blue-greens ensued
(N.H.W.S.P.C.C. 1971). Destratification by pumping hypolimnetic water to
the surface maintained relatively low pH in the epilimm'a of four Ohio lakes
and prevented the usual fall blooms of blue-green algae (Irwin et al. 1966).
Although mixing caused a temporary decrease of epilimnetic pH in Ham's
Lake (1973 experiment) and Starodworski Lake (Poland), the pH remained above
7.3 in both cases, failing to produce a shift from blue-green algae to green
algae (Tables 2 and 3). In Hyrum Reservoir, where aeration caused
microstratification and a reduction io mixed depth, pH of the surface waters
rose sharply to 9.2 during a bloom of Aphanizomenon (Drury et al. 1975).
Partial mixing in Arbuckle Lake, El Capitan Reservoir and Lake Catherine
generated little change in pH and no apparent shifts in algal species
composition.
EFFECTS OF ARTIFICIAL CIRCULATION ON ZOOPLANKTON AND SPECIES INTERACTIONS IN
OPEN WATER
Artificial circulation generally leads to an increase in the abundance
of zooplankton and an expansion of their vertical distribution (Table 4) .
Several studies reported no effects of mixing on the zooplankton but this
result is probably due to inadequate sampling design (Eufaula Reservoir,
Bowles 1972), incomplete mixing (Hyrum Reservoir, Drury et al. 1975;
Arbuckle Lake, McClintock 1976, Toetz 1977b), or lack of control data (Ham's
Lake, Arbuckle Lake, McClintock 1976). Because the data are limited,
conclusions expressed below about the effects of artificial circulation on
individual zooplankton species must remain tentative.
Depth Distribution
During normal summer stratification, the absence of oxygen in the
hypolimnion restricts zooplankton to the upper waters of lakes (Fast 1971b;
Heberger and Reynolds 1975; Brynildson and Serns 1977). The onset of low
oxygen and high carbon dioxide conditions in the hypolimnion produces an
upward displacement of microcrustacea on a seasonal basis (Langford 1938;
Heberger and Reynolds 1975). Some zooplankters avoid the hazardous chemical
conditions associated with summer stratification by entering a resting
stage. For example, Cyclops bi cuspi datus thomasi in Lake Erie is
17
-------
TABLE 3. EPILIMNETIC pH CHANGES ASSOCIATED WITH ARTIFICIAL CIRCULATION
Lake
Group la
Cline's Pond
University Lake
Kezar Lake
Stewart Hollow
Cladwell Lake
Pine Lake
Vesuvius Lake
Buchanan Lake
Group Hb
Parvin Lake
Test Res. I & II
Starodworski Lake
Lake Calhoun
Ham's Lake
Arbuckle Lake
Lake Catharine
Hyrum Res.
El Capitan Res.
Direction
Reference of Change
Malueg et al. 1971
Weiss and Breedlove 1973
N.H.W.S.P.C.C. 1971
Haynes 1973
N.H.W.S.P.C.C. 1971
Haynes 1973
Irwin et al. 1966
Irwin et al. 1966
Irwin et al. 1966
Irwin et al. 1S66
Irwin et al. 1966
Brown et al. 1971
Lackey 1972
Knoppert et al. 1970
Lossow et al. 1975
Shapiro and Ffannkuch 1973
Steichen et al. 1974
Toetz 1977b
Toetz 19775
Toetz 1979
Kothandaranan et al. 1979
Drury et al. 1975
Fast 1563
-
-
1968 -
1969 +
-
0
0
.
-
0
0?
.
0
1973 -
1975 0
1975 -
1977 0
0
±
0
pH Values
Before
6.2-9.6c
7.6<»
9.4
6.6
6.B
6.B
7.3
6.9-7.2
6.8-7.3
7.1
6.6-7.2d
?
9.0-9.4d
8.0-8.Sd
8.5
>8
7.71<1
•».7.5d
>8<«
7.8-8.99
7.3-8.6
8.0-3.5
7.5
>8
7.39
*7.5
>8
7.2-9.2
7.7-8.3
a Group I » Lakes in which the ratio of green algae to blue-green algae increased after
treatment
b Group II ' Lakes in which the ratio of green algae to blue-green algae decreased or
stayed the sane after treatment
c Control section
d Control year, suirarer values
18
-------
TABLE 4. RESPONSES OF ZOOPLANKTON TO ARTIFICIAL CIRCULATION1
Lake
Buchanan Lake
Lake Roberts
Lake Calhoun6
Stewart Lake
Indian Brook Reservoir
Mirror Lake
Parvln Lake
El Capltan Reservoir
Starodworskl Lakec
Eufaula Reservoir**16
Main's Lakec
Arbuckle Lakec'e
Hyrum Reservoir*1 «e
Reference
Brown et al. 1971
McNall 1971
Shapiro and Pfannkuch 1973
Barnes and Grlswold 1975
Riddlck 1957
Brynildson and Serns 1977
Lackey 1973b
Fast 1971b
Lossow et al. 1975
Bowles 1972
HcCllntock 1976
McC Unlock 1976
Drury et al. 1975
Abundance
•»•
+
*
1974 -
1975 +
+
1973 +
1974 +
-
+
+
0
0
0
0
Depth
Distribution
+
+
+
+
4
+
•f
0
0
0
0
0
Ratio
Copepods: Cladocerans
-
t
4-
0
0
0
0
a + • Increase, - = decrease, 0 • no significant change
Weighted mean density or standing stock
c Zooplankton distributed to bottom before mix
Inadequate sampling design or lost samples
e Incomplete mix
-------
essentially epibenthic during July; but when oxygen is depleted near the
bottom in August, the copepodites encyst and enter a diapause stage
(Heberger and Reynolds 1975). This disappearance of JC. b. thomasi from the
water column is not observed in the other Great Lakes where oxygen remains
abundant. Moreover, the relative abundance of £. J3. thomasi has increased
in Lake Erie over the years since 1929-1930 as oxygen depletion has become a
more serious problem (Bradshaw 1964). Aeration should select against
species like£. b_. thomasi that are tolerant of low oxygen concentrations;
unfortunately, too little data exist to test this prediction.
In general, artificial destratification allows zooplankton to
redistribute themselves throughout the water column (Table 4), although this
may be a passive process for weak swimmers like rotifers and Holopedium
(McNaught 1978). Before aeration of El Capitan Reservoir, the zooplankton
occupied the upper 10 m, but after 17 days of mixing, 85 percent were found
below 10 m (Fast 1971b). At the air release station, however, the upwelling
water concentrated zooplankton near the surface where they were fed upon by
shad. Lackey (1973b) found that the depth distributions of Cladocera and
rotifers were generally unaffected by artificial circulation, but Diaptomus
spp. tended to occur in deeper water during the treatment year.
Even in lakes where zooplankton occupy the entire water column before
treatment, circulation usually shifts the vertical profile of the population
toward lower depths. Although zooplankton were distributed throughout all
depths in Ham's Lake before destratification, only 6 percent of the total
community was found at 6 m and below (McClintock 1976). During August when
mixing was strongest, 42 percent were found at 6 m or below. Lossow et
al. (1975) observed a concentration of Daphnia hyalina near the bottom of
Starodworski Lake (Poland) following destratification.
Some species with unique distributions, e.g. the cold-water form
Daphnia longiremis, are eliminated from lakes at the time of oxygen
depletion (Heberger and Reynolds 1975). Artificial aeration should allow
such species to persist throughout the summer, unless treatment elevates the
water temperature above their upper tolerance limit.
Zooplankton Abundance
In one of the most complete studies to date, Brynildson and Serns
(1977) documented a four-fold increase in Daphnia spp. after mixing of
Mirror Lake in September, 1974. In an earlier experiment £• pulicaria
doubled but JD. galeata mendotae decreased by two-thirds, leaving the average
density of daphnids about the same as before mixing. During the 1974
experiment, ])• pulicaria declined by one-half and was replaced by smaller
species, J3. £. mendotae "and especially ]). retrocurva. Total aeration during
spring 1973 stimulated early onset of a normal population pulse in
D. pulicaria, presumably by supplying oxygen to ephippial eggs lying in mud
Below 5 m.Although the density of small cladocerans including Bosmina
longirostris and Diaphanosoma leuchtenbergianum showed no significant change
after circulation, calanoid and cyclopoid copepods increased during both
experiments.
20
-------
During aeration of Starodworski Lake, the relatively large Daphnia
hyalina appeared for the first time and became especially abundant in lower
water (Lossow et al. 1975). Bosmina longirostris declined to particularly
low densities during summer of the treatment year. In contrast, the larger
B. coregoni, which is characteristic of lakes with well oxygenated
"Rypolimnia, increased greatly. Predation pressure on Bosmina spp. was
probably less during aeration since Chaoborus larvae declined; Chaoborus
spp. are significant predators on small zooplankton, including Bosmina"
(cf. Pastorok, in press).
Shapiro et al . (1975) found that the abundance of Daphnia
spp. increased 5 to 8 times during artificial circulation of Lake Calhoun
compared with the previous control year. Moreover, the large-bodied
J). pulex invaded the lake and became reasonably common after treatment.
Other zooplankters, including cyclopoids, Pi aptomus, Bosmina and
Diaphanasoma showed population increases, although none were as dramatic as
the changes in Daphnia spp. Although Lackey (1973b) reported a significant
decline in the population of J3. schodleri and Cladocera in general during
treatment of Parvin Lake, the control year may have been unusual due to
absence of the late summer bloom of Aphanizomenom flos-aquae (cf. Lackey
1973a).
Aeration should favor those species which normally cannot tolerate low
oxygen concentrations. Heisey and Porter (1977) showed that filtering and
respiration rates of £• Jl« mendotae decline linearly with decreasing oxygen
over a wide range of concentrations. £. magna shows a greater tolerance of
low 02 and its rates are independent of ambient oxygen above 3 mg $2 Per
liter. JD. £. mendotae populations should exhibit a stronger response than
]3. magna following artificial circulation; unfortunately, no studies
involved j). magna, and the evidence for ]3. £. mendotae is equivocal
(e.g. Brynildson and Serns 1977). In general, copepods may be less tolerant
of low oxygen conditions than cladocerans are (Heberger and Reynolds 1975;
Gannon and Stemberger), although individual species vary widely in their
requirements (Heisey and Porter 1977). An increase in the ratio.of copepods
to cladocerans following artificial circulation could be interpreted as a
shift of the community away from species characteristic of eutrophic lakes
toward species indicative of oligotrophic lakes (Gannon and Stemberger
1978), but the limited data show variable responses in zooplankton
composition (Table 3). Perhaps this ambiguity can be traced to Brooks'
(1969) assertion that species composition of zoopl anktonic herbivores is
mainly controlled by competition for resources among species and especially
by the character and intensity of fish predation on the zooplankton (see
below).
The growth of zooplankton populations following destratification could
be caused by several factors. First, mixing often resuspends organic
detritus from the bottom sediments, thus creating additional food resources
for filter-feeders, especially Cladocera (Saunders 1972). This effect might
be nullified by resuspension of mineral particles which interfere with
filtering mechanisms and decrease growth of herbivores. In any event,
resuspended detritus would be refractory material, poorly assimilable by
zooplankters. Secondly, the shift from blue-green algae to green algae
21
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after some mixing treatments (Table 2) furnishes a more edible resource to
herbivorous zooplankton. Finally, by bringing oxygen to the bottom waters
of a lake, circulation extends the habitat for both zooplankton and
plantivorous fishes, distributing them throughout a greater volume (Table 4
and see below for information on fish distribution). Furthermore, the dimly
lit bottom waters can serve as a refuge for zooplankton, protecting them
from planktivorous fishes which depend on relatively high light levels for
efficient feeding (Zaret and Suffern 1976). The reduction in encounter rate
between fish and their prey lessons predation pressure on the zooplankton,
allowing population growth and invasion of large-bodied forms, especially
Daphnia (Shapiro et al. 1975; Shapiro 1979; cf. Hrbacek et al. 1961;
Andersson et al. 1978; and DeBernardi and Guissani 1978).
In turn, large herbivores such as Daphnia pulicaria are more effective
grazers of algae than are small zooplankton (Haney 1973; Hrbacek et
al. 1978). They also release less phosphorus per unit body weight than the
smaller forms do (Bartell and Kitchell 1978). During the year following
removal of fish from the Poltruba backwater, Hrbacek et al. (1961) observed
a decline in algal populations; although total zooplankton numbers were
lower than the previous year, large species (e.g. Daphnia hyalina) became
dominant in the community. Andersson et al. (1978) found that dense
populations of fish in experimental enclosures resulted in low numbers of
planktonic cladocerans, high concentrations of chlorophyll, and blooms of
blue-green algae. In enclosures without fish, large cladocerans prospered
and grazed the phytoplankton down to low levels.
EFFECTS OF ARTIFICIAL CIRCULATION ON BENTHIC MACROINVERTEBRATES
Aeration/circulation of stratified eutrophic lakes has a potential for
significantly affecting the benthic faunal communities. During
stratification, the profundal benthos of eutrophic reservoirs may be limited
due to anoxic conditions and reduced profundal sediments. In such
situations, the profundal benthos may be either non-existant or limited to a
few highly adapted taxa such as Chironomus spp., Chaoborus spp. or
oligochaetes.
Several recent studies have examined the responses of benthic
macroinvertebrate fauna to lake circulation. Most of the case studies
involved circulation by diffused air techniques; however, an axial flow pump
was used at Ham's Lake.
Destratification of Ham's Lake, using an axial flow pump resulted in
measureable changes in the density and structure of macrobenthic communities
(Wilhm and McClintock 1978). Three biotic parameters - organism density,
diversity and species number - displayed significant correlations with
dissolved oxygen concentrations in benthic samples collected at both
stratified and destratified sites during 1976. At the stratified sites the
dissolved oxygen was less than 0.2 mg/1 at the 5-m depth. Benthic samples
collected at those locations were dominated by Chaoborus (>90 percent of
organisms) and had correspondingly low diversities and species number (<4).
Species numbers at the 5-m destratified site varied between 8 and 16, with
higher diversities and lower percentages of Chaoborus than the stratified
site.
22
-------
Aeration of Lake Starodworskie (Poland) resulted in marked changes in
the benthic faunal assemblages. Prior to aeration the benthic macrofauna
other than Chaoborus (mainly Chironomidae and Oligochaeta) were generally
confined to depths less than 10 m (Sikorowa 1978). Alternatively, Chaoborus
were relatively common (1,000-5,000 per m2) at depths greater than 10 m.
After aeration the total densities of profundal macrofauna increased
considerably, ranging from 2.5 to 18.0 times the pre-aeration densities,
depending upon depth. Littoral densities also increased to about 5 times
pre-aeration values. However, species composition changed considerably. By
the second year of aeration, Chaoborus was only a very minor component of
the benthos, displaying a 99 percent reduction of the population. The
benthic habitat was occupied primarily by Chironomidae, Oligochaeta,
Hydracarinae and Heleidae.
In Lake Catherine installation of a venturi-type aeration device
resulted in a partial destratification (Kothandaraman et al. 1979). Based
on a limited number of benthic samples it appears that the benthic organism
density and number of taxa were elevated near the aerator when compared with
a nearby stratified lake or a stratified station in Lake Catherine.
Prior to destratification the profundal zone of El Capitan Reservoir
was devoid of benthic macroinvertebrate fauna (Inland Fisheries Branch
1970). The existing benthos was comprised of chironomids, oligochaetes,
clams and nematodes which occurred only at depths less than 10 m during the
prolonged summer stratification. During two summers of destratification by
aeration, the benthic communities colonized the deeper areas of the
reservoir. During the first summer of destratification, chironomids
occurred to depths of 27 m, although at lower densities than in the littoral
zone.
Aeration of University Lake resulted in elevation of the density and
taxa number of chironomids (Weiss and Breedlove 1973). The greatest
increases over the control year values occurred in profundal areas near the
air diffusers. It is not clear whether chironomid assemblages over the
entire lake bottom were affected, or if the effects of aeration were
confined to profundal areas. The study results were confounded by different
nutrient impacts during the control (low runoff) and aeration (high runoff)
years. Weiss and Breedlove (1973) also noted the occurrence of high
Chaoborus densities near the air diffuser during the aeration year; however,
there were no quantitative comparisons between control and treatment years.
Destratification of a mesotrophic montane reservoir (Parvin Lake)
resulted in no significant changes in the abundances of Asellus (Isopoda),
Chaoborus or Lumbriculus (Oligochaeta) (Lackey 1973c). Abundances of the
amphipod Hyalella increased at the shallow-water station (2 m) during
destratification; however, no changes in abundances occurred at the deeper
stations. A generalized decline in chironomid abundance was detected at all
areas sampled. The decline was most pronounced in the profundal zone (10 m)
where average chironomid densities following destratification were only
about 2.5 percent of pretreatment values.
23
-------
A decline in chironomid abundances was also observed at oligotrophic
Section Four Lake following aeration (Fast 1971a). Midges emerged from
somewhat deeper water during aeration, although in general the depth
distribution of benthic fauna was unaffected.
For eutrophic reservoirs the documented responses of benthic
communities to lake aeration/circulation have been relatively consistent;
i.e. increases in number of taxa, diversity and biomass, expecially in the
profundal areas (Table 5) . The only cases of a generalized decline or no
change in organism densities were associated with two lakes receiving low
nutrient inputs, Parvin Lake and Section Four Lake. Although the
hypolimnion of Parvin Lake was normally anoxic during late summer while the
deeper areas of Section Four Lake remained oxic, the mechanisms producing
declines in chironomid densities may have been similar. Both lakes normally
had dominant chironomid assemblages in deep water prior to aeration. For
example, although the Parvin Lake hypolimnion was anoxic during late summer,
the mean density of organisms in July-August was 325/mz. The decline in
overall densities may have resulted from increased midge emergence due to
the warmer bottom temperatures during lake circulation. In both lakes, the
other insect larvae and invertebrates such as Asellus and Hyalella, which
were abundant in littoral areas, did not invade the hypolimnion following
aeration. Therefore, overall profundal biomass declined.
Four of the five lakes in which Chaoborus formed a significant
component of the profundal benthos displayed similar responses, i.e. a
general decline in Chaoborus densities following aeration. The exception
was Parvin Lake in which Chaoborus densities did not change significantly
during aeration. Two of the aerated lakes displayed pronounced changes in
Chaoborus during aeration. In Cox Hollow Lake there was an overall decline
in Chaoborus associated with replacement of £. punctipennis by £. albatus
(Wirth et al. 1970). Prior to aeration Chaoborus was the only profundal
macroinvertebrate in Stewart Lake, but following treatment the larvae were
almost completely absent, having been replaced by oligochaetes and
chironomids (Barnes and Griswold 1975).
£. punctipennis is a commonly encountered chaoborid in stratified
eutrophic lakes. This species appears to be highly adapted to co-occurrence
with fish by undergoing vertical diel migrations into anoxic bottom strata.
Field studies have indicated that the migratory C. punctipennis occurs in
lakes with fish while the non-migratory £. americanus is excluded from fish
lakes (von Ende 1979). Moreover, introduction of fish predators into lakes
has resulted in the virtual elimination of £. americanus and pronounced
reductions in the densities of £. trivittatus. a deeper dwelling species
(Northcote et al. 1978).
Therefore, the larvae of certain Chaoborus spp. appear to avoid fish
predators by occupying dark anoxic waters during the day. Chaoborus then
migrate to surface strata at night where they feed and recharge with oxygen
under conditions of reduced fish predation.
Oxygenation of deeper waters may also affect the vertical migratory
behavior of Chaoborus. In a series of laboratory experiments, LaRow (1970)
24
-------
TABLE 5. RESPONSES OF BENTHIC MACROINVERTEBRATES
TO ARTIFICIAL CIRCULATION
Lake
Reference
Organism
density
No. of Species
(or diversity)
NJ
in
Ham's Lake
Starodworskie Lake
Lake Catherine
Parvin Lake
El Capitan Reservoir
Cox Hollow Lake
University Lake
Stewart Lake
Section Four Lake
Wilhm and McClintoch 1978
Sikorowa 1978
Kothandaraman et al. 1979
Lackey 1973c
Inland Fisheries Branch 1970
Wirth et al. 1970
Weiss and Breedlove 1973
Barnes and Griswold 1975
Fast 1971a
varied
. Chironomids only
Chironomids -, others 0
-------
demonstrated that under high oxygen concentrations most Chaoborus larvae
(88.2 percent) did not migrate to the surface layers, but remained in the
bottom stratum. Under low oxygen conditions a high percentage migrated to
the surface stratum after sunset.
The documented distributional characteristics of Chaoborus are
consistent with the observed declines in Chaoborus densities during lake
aeration. Aeration of bottom strata would remove the anoxic refugia of
Chaoborus. thus exposing the species to intense fish predation. Since third
and fourth instar Chaoborus are relatively large organisms (6 to 15 mm) they
are a preferred food item for zooplanktivorous fish (Northcote et al. 1978;
von Ende 1979).
In lakes showing declines in Chaoborus densities during aeration, the
profundal areas were occupied by increased densities of other fauna such as
oligochaets, chironomids and other insect larvae. Such detritivores
responded to the generally rich deposits of organic material by establishing
relatively high standing crops. In some cases there was a noticeable
decline in the amount of leaf litter in the bottom sediments following
extended periods of aeration.
In summary, aeration may not only increase the standing crop of benthic
fauna, but may modify the trophic structure of the community by a reduction
in zooplankton predators (i.e. Chaoborus) and an increase in benthic
detritivores. The increased production of profundal benthos would provide a
potential increase in the availability of fish food organisms. Organisms
such as chironomid larvae are frequently important food items for a variety
of fish species. The high utilization of benthic fauna and the influence of
fish predation on prey population densities is indicated in field studies
such as Andersson et al. (1978).
EFFECTS OF ARTIFICIAL CIRCULATION ON FISHES
In stratified lakes, fishes may be prevented from utilizing the total
potential habitat due to low oxygen levels in the hypolimnion. This may be
especially critical for cold-water species such as salmonids which may be
compressed into a narrow layer of available metal imnetic habitat by warm
water above and anoxic conditions below.
The responses of fishes to lake aeration/circulation techniques have
not been intensively studied. However, several case studies are available
which have examined depth distribution, survival and growth rates following
lake aeration.
Prior to destratification of Mirror Lake, trout and yellow perch were
confined to the epilimnion and metalimnion (Brynildson and Serns 1977).
During the spring and late summer the maximum depth occurrence of the two
fish species was about 5 m and 7 m, respectively, corresponding to a
dissolved oxygen level of about 3 to 4 mg/1. After destratification fish
were distributed throughout the water column to the maximum depth of 13 m.
Trout were essentially evenly distributed while yellow perch occurred from 4
to 13 m.
26
-------
Total aeration of Mirror Lake in the fall and hypolimnetic aeration in
winter also decreased the trout winter-kill as evidenced by the significant
angler catch of "carry-over" stocked trout (Brynildson and Serns 1977).
After partial destratification of Lake Arbuckle in late summer, gizzard
shad, freshwater drum, white crappie and black bullhead all displayed
increased depth distribution when compared with pre-circulation conditions
(Gebhart and Summerfelt 1976). In 1975 the total available fish habitat (as
defined by the 2 mg/1 DO isopleth) was increased from 53 percent of lake
volume in August to 99 percent of total volume in September following
treatment. It is interesting to note that although freshwater drum and
white crappie were essentially confined to the epilimnion and metalimnion
during mid-summer, considerable numbers of gizzard shad and black bullhead
were collected from the anoxic hypolimnion to a depth of 20 m. Apparently
these two species, both of which feed on bottom detritus, were migrating
into the hypolimnion for short periods to feed.
Increases in available fish habitat at Lake Arbuckle were generaly
associated with faster growth rates in fishes (Gebhart and Clady 1977).
This relationship was especially evident in bottom-feed ing species such as
gizzard shad and channel catfish. The authors indicated that the maximum
increases in fish growth rates occur when available habitat is expanded
early in the growing season (i.e. May).
In a study of Section Four Lake, Fast (1971a) observed no positive or
negative effects on rainbow trout due to aeration. Although the depth
distribution was modified in that trout occurred primarily near the bottom
during aeration, there appeared to be no other effects of the warmer,
isothermal conditions.
A survey of anglers on University Lake indicated that the catch per
unit effort (primarily bluegill) increased during aeration (Weiss and
Breedlove 1973). Anglers were observed to concentrate near the bubble zone
which seemed to act as a fish attractant.
The angler harvest of bluegills at Cox Hollow Lake was also increased
during a multi-year aeration project (Wirth et al. 1970). The increased
habitat and production of benthic fauna did not result in concommitant
increases in the growth of the stunted bluegill population, however.
Aeration of Stewart Lake resulted in a decline (^30 percent of
pre-aeration levels) in the total bluegill population by the second summer
of aeration (Barnes and Griswold 1975). Fish mortalities during the initial
aeration period may have resulted from lowered dissolved oxygen; however,
the overall effect was beneficial to the stunted sunfish and catfish
populations. By the second summer of aeration, there was evidence of
improved growth rates and condition factors for both fish species.
Aeration of Casitas Reservoir in southern California has allowed the
establishment of a year-round trout fishery (Barnett 1975). Although trout
are stocked only during cooler ambient temperatures (<20°C), the aeration
27
-------
provides habitat adequate for trout survival during the summer (T < 22°C, DO
> 5 mg/1). As a result of the multi-year survival, a trophy trout fishery
has developed when fish become concentrated in the summer.
Nighttime aeration of Puddingstone Reservoir did not result, however,
in the formation of conditions suitable for year-round trout survival (Fast
and St. Amant 1971). The lake oxygen concentrations were increased to >6
mg/1 throughout the water column but temperature was also elevated to
25.5°C, resulting in conditions unsuitable for trout survival.
At another southern California reservoir (El Capitan) warmwater fish
species such as channel catfish and threadfin shad expanded their depth
distribution considerably following aeration (Fast 1968). Walleye also
increased their depth distribution, although not to the same extent as the
other species.
Increased depth distribution of fishes following aeration/circulation
was also observed at Lake Calhoun by Shapiro and Pfannkuch (1973). Echo
soundings during aeration revealed that yellow perch, bluegill and crappie
occupied most of the formerly uninhabited hypolimnion.
Lake aeration/circulation has also been shown to be beneficial as a
salmonid management technique by providing rearing habitat for anadromous
species and preventing over-winter mortality in ice-covered lakes. Johnson
(1966) demonstrated a more than 3X increase in coho salmon smolt migration
and survival during a 3-year period of aeration of Erdman Lake. The higher
smolt production was attributed to increased available habitat and primary
production, but the effect of aeration is unclear because rotenone was
applied to the lake just before aeration.
Halsey (1968) has demonstrated that incomplete autumnal turnover and
oxygenation may be a cause of winter fish mortalities in ice-covered lakes.
Winter mortalities were prevented at Corbett Lake (British Columbia) by a
short period of aeration just prior to ice formation. The aeration resulted
in sufficient oxygen under the ice, while under natural conditions the low
autumnal oxygen concentrations were rapidly depleted during ice cover.
The importance of proper selection of aeration depth and time in
preventing winter trout mortalities is exemplified by the studies of Halsey
and Macdonald (1971). Fall aeration of Yellow Lake for a period of four
days prior to ice formation resulted in overall oxygen concentrations of <3
mg/1, at which time aeration was terminated. The reduced oxygen
concentration after aeration resulted in the death of an estimated 5,000
trout.
The most commonly measured parameter in evaluating the effects of
aeration/circulation on fish populations is depth distribution. In all
cases where depth distribution has been evaluated, fish have been observed
to expand their vertical distribution downward in response to lake
destratification.
28
-------
It is generally assumed that the results of expanded habitat following
destratification are beneficial to fish populations because of increased
food supply and alleviation of crowding into eplimnetic strata during the
summer. Of the surveyed studies, those at Lake Arbuckle were probably the
most comprehensive in examining fish response. The results at that lake
(Gebhart and Clady 1977) did indicate an apparent increased growth of
bottom-feed ing fishes; however, the results were not apparent in all species
and displayed some variance according to year of study. Increased growth
was also observed at Stewart Lake (Barnes and Griswold 1975), although the
apparent cause was the selective elimination of much of the previously
stunted bluegill populations. Therefore, although the stimulation of fish
growth and production by aeration/circulation appears to be a conceivable
benefit of the technique, it has not been evaluated in most of the past or
current projects. It should also be emphasized that fish would generally
display a more delayed response to lake aeration than lower trophic levels.
Most of the review studies were of limited duration and fish populations may
not have reached equilibrium with the modified lake environment. Moreover,
some of the lakes contained already stressed populations (e.g. overcrowded
and stunted centrarchids) which would be slow to respond to habitat
improvements.
An increase in the angler catch rate of game fish was noted in several
cases. Increase in fish harvest rate could be due to two quite different
responses in the fish populations. 'The short-term increase in fish catch
immediately after aeration may be due to an attraction of fish to the
aeration point due to rheotactic response or increased food availability.
Anglers generally respond quickly to fish concentrations, with resultant
increases in catch rate.
A longer-term benefit to fish catch may be due to increased fish
survival during critical periods. This would be especially true for
salmonids occurring in eutrophic lakes with ice cover during the winter. In
such lakes with severe "winter-kill" problems, the management approach is
usually to stock the lake following ice breakup in the spring. With poor
multi-year survival the occurrence of large game fish is limited. By
aerating the lake during fall and/or winter, mortality rates are reduced,
and the potential for fall trout stocking with over-winter survival of large
fish exists.
Several of the successful lake circulation projects from a fishery
standpoint were located in cooler climatic areas of North America. In
warmer areas, the potential benefits for cool-water fisheries (e.g. trout)
are more limited. Circulation of the lake in summer will increase the heat
budget and may result in adverse water temperatures for maintenance of
salmonid fisheries, such as occurred in Puddingstone Reservoir. Localized
aeration resulting in only partial destratification could allow for some
cooler areas with sufficient DO for trout survival (e.g. Casitas Reservoir);
with only partial circulation, however, the potential for other benefits
such as water quality changes would be considerably less.
Adverse impacts of aeration/circulation may be associated with lake
mixing during stratified conditions when hypolimnetic oxygen depletion has
29
-------
already occurred. Following destratification the oxygen demand associated
with sediments and reduced compounds can lower dissolved oxygen in the
entire lake to levels which are not adequate to support fish life. Although
a dissolved oxygen concentration of at least 5 mg/1 would generally be
required to maintain good game fish populations (U.S. EPA 1976), fish can
survive for short periods at considerably lower oxygen levels. However, if
dissolved oxygen concentrations drop below 2 to 3 mg/1 considerable
mortalities of desirable game species would be expected.
HYPOLIMNETIC AERATION AND OXYGENATION
INTRODUCTION
Hypolimnetic aeration and oxygenation are recently developed methods
for adding dissolved oxygen to the bottom waters of a lake without
disrupting the normal pattern of thermal stratification. Because these
techniques improve water quality without greatly altering the lake's heat
budget, they represent a viable alternative to artificial circulation
whenever restoration plans specify maintenance of a cold water resource.
Since hypolimnetic systems preserve the original pathways for atmospheric
exchange, oxygen transfer is mainly confined to the interface between
injected bubbles and hypolimnetic water, and oxygenation is slower than with
artificial circulation.
The major goals of most programs of hypolimnetic aeration have been to
improve water quality and provide new habitat or expand existing habitat for
cold-water fishes. Unlike artificial circulation which modifies algal
distributions, hyolimnetic aeration has only indirect effects on most
phytoplankton populations.
HYPOLIMNETIC AERATION DEVICES AND APPLICATIONS
Fast and Lorenzen (1977) reviewed 21 designs for hypolimnetic aerators
and proposed dividing them into the following categories: 1) mechanical
agitation, e.g. aeration of withdrawn water onshore by discharge into a
splash basin before returning it to the hypolimnion, 2) pure oxygen
injection, 3) air injection systems, including a) full air-lift design,
which lifts bottom water to the surface in a vertical tube and then returns
it to the hypolimnion, b) partial air lift design, which aerates
hypolimnetic water without transport to the surface, and c) downflow air
injection system, which mechanically pumps water upward and injects air as
it is returned to the hypolimnion.
The first reports of hypolimnetic aeration described the mechanical
agitation system used at Lake Bret, Switzerland (Mercier and Perret 1949;
Mercier and Gay 1954; Mercier 1955). Although the system was relatively
inefficient in terms of oxygen dissolution for a given energy input, it
successfully elevated oxygen content of the hypolimnion and improved water
quality by reducing concentrations of iron and carbon dioxide.
Partial and full air lift systems circulate water within vertical tubes
by injecting compressed air at the bottom. As rising air bubbles "lift"
30
-------
hypolimnetic water, oxygenation occurs and waste gases along with some COo,
^S, and NH-j are vented to the atmosphere. Although the partial air lift
systems have greater effluent Oo concentrations, they aerate less water
volume and the total 02 dissolved^ is less than with full air lift designs.
Most of the energy used to compress the air is "lost" in waste discharge
since the air does not expand greatly while it is in contact with the water.
The LIMNO system, a partial air lift design, is the most widely used aerator
at the present time.
Fast et al. (1976) and Lorenzen and Fast (1977) compared costs of pure
Q£ injection by side stream pump (SSP), a partial air lift design (LIMNO),
and two full air lift systems. Although the SSP system had a relatively low
capital cost, the full air lift design used by Fast (1971a) is the least
expensive design considering overall costs of construction, installation and
operation; and it is almost twice as efficient as the other systems in terms
of energy consumed to dissolve a given amount of oxygen.
The physical characteristics of some lakes and their hypolimnetic
aerators are summarized in Table 6. Fast et al. (1976) and Lorenzen and
Fast (1977) present methods for estimating the size of a hypolimnetic
aerator needed for any site. The steps involved in this process are: 1)
estimation of hypolimnetic volume, 2) estimation of oxygen depletion rates
in the hypolimnion, 3) estimation of required oxygen input capacity and
expected aeration period each year, and 4) selection of an appropriate
aeration/oxygenation system based on the foregoing information. In general,
the aeration system should be oversized to allow for unpredictable
variations in (^ consumption, hypolimnetic volume, equipment shutdown, or
other factors. In addition, intermittent operation of an oversized system
may be less costly and more efficient than continuous operation of a smaller
system.
EFFECTS OF HYPOLIMNETIC AERATION/OXYGENATION ON WATER QUALITY
The physical and chemical changes associated with hypolimnetic
aeration/oxygenation are summarized in Table 6.
Unlike artificial circulation, hypolimnetic aeration generally
maintains cold temperatures in the hypolimnion while increasing dissolved
Og. Successful operation of hypolimnetic aerators usually elevates
temperatures of the bottom waters by less than 4°C throughout the summer
(Table 6). In the Hemlock Lake experiment, the temperature of the
hypolimnion rose more than 2°C per week, and eventually, the lake
destratified early (Fast 1971a). However, this can be attributed to a
defect in design of the system, since water leaked through the walls of the
vertical tower.
In almost all aeration experiments, 02 concentration in the hypolimnion
increased, from 0 mg/1 to as much as 7 mg/1 (Lake Waccabuc, Jarlasjon,
Larson Lake and Wahnbach Reservoir) or more (Hemlock Lake, Ottoville
Quarry). The rise in average 02 concentration for the whole lake reflects
elevation of hypolimnetic values (Table 6). In Mirror Lake and Spruce Run
Reservoir, the aerators were undersized, and hypolimnetic 02 concentrations
31
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TABLE 6. SELECTED LAKES AND THEIR PHYSICAL-CHEMICAL RESPONSE TO HYPOLIMNETIC AERATION
Aeration Intensity Hypolimnlon Response9
Depth (.) ~i_B Ar»
Lake
Lake Uaccabuc
Mirror Lake ,'7j
Larson Lake
Jarlasjon
CO Spruce Run Res. !?,?
N> n"*
Hemlock Lake
Ottoville Quarry
Wahnbach Res
* Response parameters
Location
New York
Wisconsin
Wisconsin
Sweden
New Jersey
Michigan
Ohio
W Germany
Reference
Fast et al I975a
Garrell et al 1977
Smith et al 1975
Smith et al. 1975
Bengtsson and Gel in 1975
Whipple et dl 1975
Fast 197la
Fast et al )97Sb
Uverholtz et al 1975
Bernhardt 1967, 1974
DO * dissolved 0?. P04 « phosphate. TP
Max
13
13 1
11 9
24
U.I
18 6
18
43
• total
NU3 = nitidtL', NII4 - aunioii i urn , Ic • irun, Mn =
Mean Air (mJ) (ha)
13 4 053 53 6
7 6 12.8 0 400 53
40 119 0.1UB 4.U
9 3 24 78 84
12 2
18 6 18
18 0.063 0 73
19 2 41 63 214 5
phosphorus.
IlldlUJdllCSl-
"air
7 93
0 45
0 1b
22 H
O.lb
2 8
0 11
9
T Fe
CO DO P04 TP N03 NH4 Nn
0 * 6 * -
... 0-000
*3 0 * » *
o +
+1° *--+.-
0 * 0 0 - -
0 * 0 0
«?°/wkb *
• 3" *
*4« *--*-.
Uuection of change in hypol inmetic coiiLcntratioii » - increase, - - dec reuse, 0 - no significant change
11 Eventual destralification due to water leakage through walls of aerator tower
-------
never rose above 1.7 mg/1 and 3 mg/1, respectively. Anoxic zones developed
within the metalimnion during treatment of Lake Waccabuc (Garrell et
al. 1977), Jarlasjon (Bengtsson and Gel in 1975) and Larson Lake (Smith et
al. 1975). If the aerator is undersized, anoxic zones will probably occur
above and below the water outfall, as in the 1973 experiment at Spruce Run
Reservoir (Whipple et al. 1975). In some cases, a metal ii.netic minimum of
dissolved 62 serves as a barrier to fish movements (Bengtsson et al. 1972),
whereas in others, it does not (Whipple et al. 1975; Serns 1976)).
Like artificial circulation, hypolimnetic aeration can effectively
reduce concentrations of Fe, Mn, HoS, NH4+, C02, and other chemicals
associated with anoxic conditi ons iTabl e 6). In general, a rise in
hypolimnetic NOj" concentration accompanies the decrease in Nfy"*", suggesting
increased nitrification in the oxygen rich waters (Bengtsson and Gilen 1975;
Garrell et al. 1977).
Air injection can cause supersaturation of nitrogen gas (No) in the
'hypolimnion relative to surface hydrostatic pressures (also, see
above--ARTIFICIAL CIRCULATION). During hypolimnetic aeration of Lake
Waccabuc, N2 concentrations in bottom waters, increased from near saturation
to 150 percent saturation after 80 days of system operation (Fast et
al. 1975a). Even higher levels of No are possible with longer periods of
aeration or greater release depths (Fast 1979a, b). At 150 percent
saturation, N2 would have caused a severe die-off of fishes if the
hypolimnetic waters had been released to a stream. Within Lake Waccabuc,
however, fish may have adjusted their behavior to avoid any potential
problems (Fast et al. 1975a).
Aeration effectively reduces hypolimnetic phosphate concentrations
(Bernhardt 1974; Smith et al. 1975; Bengtsson _and Gilen 1975). In the long
term, phosphorus flux from the sediments may be diminished because
hypolimnetic aeration creates an oxidized microlayer at the mud-water
interface without stimulating decomposition by raising sediment temperatures
(e.g. Bengtsson and Gilen 1975).
The overall effectiveness of hypolimnetic aeration in regulating
nutrient availability to aquatic plants depends upon the importance of
phosphorus regeneration from the sediments relative to influx from the
watershed. During the first year of aeration at Lake Waccabuc, phosphorus
content of the hypolimnion decreased by about 30 percent; in contrast,
phosphorus concentrations increased greatly during the second year of
treatment (Garrell et al. 1977). High precipitation during spring and
summer of the second year probably facilitated transport of nutrients in
septic effluents to the lake. Consequently, external loading swamped the
effect of aeration on internal nutrient dynamics (Garrell et al. 1977).
Unlike artificial 'destratification, hypolimnetic aeration appears to
have little effect on pH of the epil imnion (Smith et al. 1975), and no
changes in water transparency have been noted. This is not surprising
considering the general lack of influence on depth of mixing, sediment
resuspension, and epilimnetic algal densities. In some cases, hypolimnetic
aeration may affect algal abundance indirectly by modifying species
33
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composition and abundance of zooplankton (Fast 1979a; also, see below), but
this response requires further documentation.
EFFECTS OF HYPOLIMNETIC AERATION/OXYGENATION ON PLANKTONIC MICROORGANISMS
Because hypolimnetic aeration leaves the normal vertical profiles of
most algae intact, any influence on algal densities would probably be
mediated by chang in nutrient cycling and shifts in species composition and
abundance of zooplankton, benthic fauna, and fish (Fast 1979a). As
mentioned above, hypolimnetic aeration may decrease internal loading of
phosphorus but the ultimate effect on algae depends on the relative rates of
internal versus external loading. In the event that internal loading is
important, the greatest effect of aeration on algal standing crop should be
observed after natural destratification at a time when hypolimnetic
nutrients accumulated in the absence of treatment would normally be
recycled. In the long term, control of phosphorus release from the
sediments could reduce primary production in some lakes, but much aeration
research remains to be done in this area.
Since hypolimnetic aeration has little or no influence on epilimnetic
concentrations of CO? and pH, changes in species composition comparable to
the dramatic shift from blue-green algae to green algae after whole lake
mixing have not been observed.
On the other hand, hypolimnetic aeration can cause profound changes in
the zooplankton community. After hypolimnetic aeration, changes in food
resources for the zooplankton may be less important than with artificial
circulation, but responses in the predation regimes are similar. By
providing new habitat for both fish and zooplankton, aeration essentially
dilutes the populations and reduces the intensity of predator-prey
interactions(cf. Fast 1979a; Shapiro 1979). In addition, the dimly lit
hypolimnion can serve as a daytime refuge for the large bodied zooplankton
that would otherwise be selectively eaten by visually hunting fishes. Thus,
a relatively large species, Daphnia pulexi invaded Hemlock Lake during
hypolimnetic aeration and exhibited a significant population growth of about
88 times its initial density (Fast 1971a). Apparently, JD. pulex was
previously excluded from the lake by intense planktivory in the epilimnipn,
and the smaller herbivores dominated the zooplankton community. Following
treatment, the small bodied cladocerans Bosmina and Diaphanasoma increased
to a lesser extent than Daphnia, and the population of the copepod Diaptomus
remained essentially the same. A combination of competitive pressure from
J). pulex and predation by Chaoborus larvae may have prevented more dramatic
responses in these relatively small herbivores. Fast (1971a) reported a
significant increase in the abundance of Chaoborus spp. following aeration.
These insect larvae, which "prefer" small to medium sized prey, remove a
considerable portion of the zooplankton standing crop each day (Pastorok, in
press).
Without aeration, all zooplankton in Hemlock Lake were limited to
shallow water (<11 m) above the zone where Op was depleted (Fast 1971 a, b).
After treatment, J3. pulex probably exhibited a typical diel migration
pattern, remaining in bottom waters during the day to avoid predation and
34
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moving up at night to exploit the phytoplankton crop in the epilimnion.
Fast (1979a) suggests that the intense grazing activity of ID- pulex may have
limited phytoplankton populations; but it is difficult to assess the role of
nutrient depletion and regeneration in causing the exaggerated oscillation
of algal density during the treatment year.
Confer et al. (1974) and Fast et al. (1975b) observed slight increases
in hypolimnetic zooplankton densities during aeration/oxygenation of several
lakes, but the effect of aeration on zooplankton and algal abundance in Lake
Waccabuc was insignificant (Confer et al. 1974). Considering the technical
problem of water leakage from the aerator tower during the Hemlock Lake
experiment (Fast 1971a), the foregoing conclusions about responses of
zooplankton to hypolimnetic aeration must be regarded as tentative.
Although Linder and Mercier (1954) found little change in total
zooplankton abundance following hypolimnetic aeration of Lake Bret, the
copepods, including Cyclops strenuus, £. 1euckarti, and Diaptomus gracilis,
were more common during August and September of the treatment year. After
becoming rare during cultural eutrophication of the lake, Diaphanasoma
brachyurum and the rotifer Notholca longispina increased following
treatment; Linder and Mercier (1954) considered these species indicative of
a return to oligotrophic conditions. The crustaceans as a whole were
distributed closer to the surface of the lake following treatment, possibly
because light extinction with depth was greater at that time.
When algae are concentrated in relatively deep water, hypolimnetic
aeration can affect the population directly. In Mirror Lake for example,
circulation currents produced by a hypolimnetic aerator distributed an
Oscillatoria rubescens population throughout the bottom waters; previously,
the dense population (1.9 mg/1 biomass and 14 mg/1 in 1972 and 1973,
respectively) occupied a narrow zone at the interface between the
metalimnion and the hypolimnion (Smith et al. 1975). Subsequent releases of
phosphorus and nitrogen by the algae affected the concentrations of these
nutrients in the hypolimnion, and decomposition of dead cells offset any
decrease in BOD due to artificial oxygenation (Smith et al. 1975).
EFFECTS OF HYPOLIMNETIC AERATION ON BENTHIC MACROINVERTEBRATES
Only three studies have examined the effects of hypolimnetic aeration
on benthic macroinvertebrates.
Although hypolimnetic aeration of Hemlock Lake eventually caused
destratification, the water column remained stratified for 10 weeks during
treatment, and the changes in benthic fauna illustrate the expected
outcomes. During the treatment year, total numbers of benthic organisms
almost doubled while biomass decreased slightly compared to the previous
control year (Fast 1971a). Chironomids accounted for a large portion of the
change, increasing their numbers by 65 percent and their biomass by 52
percent. Chapborus spp. increased their numbers by 250 percent, although
biomass fell by 22 percent probably because of a predation induced shift
toward a smaller species. The numbers of oligochaetes were also elevated by
treatment, but mayflies and odonates showed no response.
35
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As a result of aeration, the depth distribution of chironomids and
Chaoborus spp. shifted toward the profundal zone, especially during late
summer. Predation by trout increased in bottom waters as their own depth
distribution expanded (see below), and selective removal of large Chaoborus
would explain the shift from £. flavicans to C. punctipennis. a smaller
species.
In Lake Jarlasjon (Sweden), the benthic fauna did not recolonize the
bottom after aeration in spite of improved oxygen levels (Bengtsson and
Berggren 1972). However, the bottom of this lake was heavily contaminated
by oil.
The relative abundance of benthic species varied greatly between the
control arm and an experimental arm of Spruce Run Reservoir; but no effect
of aeration could be substantiated because of the difference in morphometric
features of the two areas (Whipple et al. 1975).
EFFECTS OF HYPOLIMNETIC AERATION/OXYGENATION ON FISH
Eutrophic lakes usually do not support cold-water fisheries on a
sustainable yield basis. During the summer months, epilimnetic temperatures
are too high for prolonged survival, and lower waters are generally devoid
of oxygen. Although fish do use habitats low in dissolved oxygen, the lower
limit for acceptable survival and growth is about 5 mg/1 (U.S. EPA 1976).
In certain circumstances, some trout may survive in limited refuges of cold
oxygenated water, e.g. near incoming streams or cold springs.
Artificial circulation can extend the habitat of warmwater fishes and
their food organisms (e.g. Brynildson and Serns 1977), but by raising water
temperature it may eliminate any existing habitat for cold-water species.
Hypolimnetic aeration maintains cold, oxygenated water capable of supporting
salmonid populations. Although few studies have sampled fish populations
during hypolimnetic aeration, the data generally show good survival of
stocked trout and expansion of existing habitat for natural populations of
cold-water fishes (e.g. Fast 1971 a, 1973b; Overholtz et al. 1977; Garrell et
al. 1978).
During summer of a control year at Hemlock Lake, rainbow trout were
limited to the upper 10 m of the lake by anaerobic conditions in the
hypolimnion (Fast 1973b). Using a hypolimnetic aerator, Fast raised the
dissolved oxygen levels in the bottom waters from 0 mg/1 to over 9 mg/1
(saturation). As mentioned above, aeration gradually destroyed thermal
stratification, but the lake did remain thermally stratified for 10 weeks
during treatment. Immediately before aeration, trout occupied depths
between the surface and 6 m; but after only 20 days of treatment, the fish
were distributed throughout the water column. During the control year, the
trout fed almost exclusively on Chaoborus; after aeration, Daphnia pulex and
Chaoborus were the most important prey, respectively. Both of these food
species undergo extensive vertical migrations, accounting for the
wide-ranging distribution of trout. In addition, gradual lowering of water
temperatures in the epilimnion by "leakage" across walls of the aerator
tower made a greater portion of the lake available to trout.
36
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Hypolimnetic oxygenation of a small quarry (0.73 ha) by Overholtz et
al. (1977) resulted in the creation of suitable trout habitat in the
hypolimnion during the summer. After oxygenation most trout occurred in the
hypolimnion at temperatures less than 20°C and at depths below 4 m. During
the same period gizzard shad preferred depths less than 5 m (>12°C),
although oxygen concentrations were adequate for survival throughout the
water column. During the summer of 1975 hypolimnetic oxygen concentration
reached 16 to 20 mg/1 (at 8 to 12°C) without apparent adverse effects on
trout survival.
Hypolimnetic aeration of Lake Waccabuc resulted in summer utilization
of the hypolimnion by rainbow trout, a condition prevented previously by
anoxic conditions (Garrell et al. 1978). During a 24-mo period the stocked
trout displayed good growth. Stomach content analysis indicated that the
fish were feeding primarily on Chaoborus and Chironomidae in the
hypolimnion. A few fish stomachs contained organisms representative of the
epilimnion, indicating a migration through the low-oxygen metalimnion.
Indeed, sonar traces done by Fast et al. (1975a) confirmed that trout were
distributed throughout the lake, some having moved into shallower water
after being stocked directly into the oxygenated hypolimnion.
In southern lakes, hypolimnetic aeration may create a "two-story"
fishery with warm-water species in shallow water and cold-water species in
deep water, each species being limited to a portion of the lake by their
respective thermal tolerences (Fast 1975, 1979a). In more northern areas,
epilimnetic temperatures during summer would be suitable for trout survival,
and cold-water fish might be found throughout the lake (e.g. Northcote et
al. 1964). During hypolimnetic aeration, a metalimnetic deficit of
dissolved oxygen will not necessarily prevent fish movements between upper
and lower waters (Serns 1976; Garrell et al. 1978).
SUMMARY
Artificial circulation and hypolimnetic aeration are cost-effective
restoration techniques which can solve a variety of problems arising from
anoxia in the hypolimnion of a eutrophic lake. The major benefits derived
from aeration/circulation are enhancement of water quality for consumptive
uses, control of algal blooms, and improvement of recreational fisheries.
Although the biological effects of aeration/circulation are notoriously
unpredictable in general, some specific benefits are realized quite
consistently, e.g. improvement of habitat for fishes. Moreover, the risk
of adverse impacts can be minimized by proper application of techniques and
further refinements in the design of aeration/circulation systems.
Before making a commitment to a particular management strategy and
system design, it is important to evaluate site-specific interactions
between biological and chemical components of the lake system,
i.e. mechanisms controlling algal populations, BOD levels, and oxygen
depletion rates in the hypolimnion. The possible detrimental effects of a
properly executed aeration/circulation program are summarized below under
adverse impacts. Any undesirable outcomes that can probably be avoided by
37
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refinement of technique are listed as technical problems associated with
system design and appl-ication.
SYSTEM DESIGN AND APPLICATION: TECHNICAL PROBLEMS
A. Artificial Circulation
1. Placement of air release. If the air diffuser is located too
far above the lake bottom, an anaerobic zone will persist below
the air release depth.
2. Undersizing the system. When the system capacity is undersized
with respect to the lake volume and area, an incomplete mix will
result. In the case of a Garton pump or similar mechanical
device located at the lake surface, the thermocline may be
lowered, but an anoxic zone would persist near the lake bottom.
If an air diffuser system is undersized, microstratification at
the lake surface will encourage algal blooms. With any system,
horizontal mixing will be limited in very large lakes when only
one device is used.
3. Oversizing the system. If artificial mixing is too vigorous,
sediments may be stirred and resuspended in the water column.
4. Oxygen depletion. When a lake is destratified too quickly after
a long period of anoxia, mixing of hypolimnetic waters and
bottom muds high in BOD into the surface layers may cause 02
depletion throughout the lake and a fish-kill.
B. Hypolimnetic Aeration
1. Undersized aeration capacity. By underestimating the oxygen
consumption rate in the hypolimnion or by overestimating the
rate of oxygen dissolution by the system, the aerator may
provide insufficient oxygen.
2. Unintentional thermal destratification. Side stream pumping of
pure 02 may mix the lake or cause significant warming of the
hypolimnion if the discharge velocity is high. Water leakage
through the vertical 'er of a full air lift system will cause
similar problems.
Assuming an effective application of techniques, i.e. sufficient
oxygenation by hypolimnetic aeration or complete lake mixing in the case of
artificial circulation, aeration/circulation will produce some or all of the
following benefits and adverse impacts.
BENEFITS
A. Improvement of Water Quality
38
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1. Both artificial circulation and hypolimentic aeration can
provide adequate aeration, although circulation does so more
rapidly. Either technique minimizes taste, odor and corrosion
problems by oxygenating bottom waters, raising their pH and
lowering concentrations of reduced compounds. Hypolimnetic
aeration maintains a cold water resource as well.
2. Artificial circulation generally reduces the temperature of the
surface water and lowers evaporation rates.
3. As long as sediment stirring is avoided, enhancement of water
clarity can be expected when aeration/circulation distributes
algae throughout all depths and controls blooms.
B. Control of Nuisance Algae
Figure 2 summarizes the mechanisms which may contribute to the
beneficial effects of artificial circulation on phytoplankton
populations.
1. A shift from blue-green algae to green algae will probably
follow artificial circulation when pH declines to 7.5 or below
resulting in "activation" of cyanophages. pH changes as
hypolimnetic CC^ is mixed into the surface waters and as algal
uptake of CC^ falls due to a reduction in light availability.
2. The abundance of blue-green algae may also decrease due to
disruption of vertical profiles and the potential effects of
variations in hydrostatic pressure.
3. The increases of mixed depth and suspended silt will probably
induce light limitation of peak algal biomass, especially in
deep lakes. However, the prediction of a reduction in algal
crop depends on maintenance of a uniform vertical distribution
or nearly so; moreover, if algae are limited by nutrients rather
than light before treatment, a moderate increase in mixed depth
may actually cause greater algal growth. In any event, a given
change in mixed depth will usually produce a larger change of
algal biomass in eutrophic lakes than in oligotrophic lakes.
4. Artificial circulation stimulates sediment decomposition,
resulting in mineralization of organic fractions and
consolidation of the sediments. In the long-term, treatment
probably reduces internal loading of nutrients by oxygenating
hypolimnetic waters and surficial profundal sediments, creating
a sink for phosphorus compounds. However, the importance of
mixing, sediment composition and decomposition rates in
determining nutrient exchange across the mud-water interface
demands further investigation.
5. At present, there is no evidence that hypolimnetic aeration will
control algal blooms.
39
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CIRCULATION
NUTRIENT
INACTIVATION
CYANOPHAGE
ACTIVITY
BLUE-GREEN TO
GREEN ALQAE SHIFT
HABITAT
FISH — ZOOPLANKTON
PREDATION
ON ZOOPLANKTON
©
ZOOPLANKTON
GRAZING
9 INCREASE IN RESPONSE PARAMETER
0 DECREASE IN RESPONSE PARAMETER
FIGURE 2 BENEFICIAL EFFECTS OF ARTIFICIAL CIRCULATION
ON PHYTOPLANKTON (ADAPTED FROM SHAPIRO 1979)
-------
6. Artificial circulation effectively increases the grazing
pressure on phytoplankton by shifting the community toward more
edible forms and by elevating the abundance of large
zooplankton.
7. The relative importance of light, nutrientj and grazing in
controlling algal biomass will undoubtedly vary among sites;
this accounts for some variation in the responses of different
communities to treatment.
C. Effects on Benthic Macroinvertebrates
1. Aeration/circulation may produce changes in benthic organisms
without corresponding shifts in planktonic biomass and
production, e.g. Ham's Lake experiment (1976).
2. The distribution and abundance of benthic macroinvertebrates
increases following aeration/circulation. Changes in abundance
may be greater after hypolimnetic aeration than they are with
circulation because the latter elevates water temperatures,
causing rapid turnover of populations and earlier emergence of
benthic insects.
3. Aeration/circulation induces a shift in trophic structure of the
macroinvertebrate community, with infaunal detritivores (mainly
chironomids, oligochaetes) replacing predatory insects
(Chaoborus) which exploit zooplankton prey.
D. Improvement of Fisheries
1. Aeration/circulation prevents winter-kill and summer-kill of
fishes by alleviating anoxic conditions and eliminating toxic
gases.
2. Artificial circulation expands habitat for warmwater fishes. In
northern lakes where surface temperatures remain below 22°C
throughout the summer, mixing should create or expand habitat
for cold-water fishes.
3. Hypolimnetic aeration creates habitat for cold-water fishes and
fosters a two-story fishery.
4. By enhancing their habitat and food supply, aeration/circulation
has great potential for improving growth of fishes,
environmental carrying capacity, and overall yield. However,
little evidence exists for these long-term benefits. In
addition, an increase in recreational yield may result simply
from a change in catch per unit effort due to concentration of
fishes near the aeration device.
41
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5. Accrual of maximum fisheries benefits will be achieved by
treatment before the development of full stratification.
ADVERSE IMPACTS
A. Water Quality
1. Destratification facilitates a temporary recycling of nutrients
by mixing hypolimnetic waters into the trophogenic zone.
2. Artificial circulation may raise suspended silt levels by
slowing rates of sedimentation and possibly increasing sediment
resuspension. Often, water transparency decreases due to silt
load and temporary algal blooms.
3. Hypolimnetic aeration has no known adverse impacts on water
quality.
B. Nuisance Algae
Figure 3 summarizes the potential mechanisms producing undesirable
changes in phytoplankton communities after artificial
circulation/destratification.
1. The recycling of hypolimnetic nutrients and elevation of total
posphorus by artificial destratification may stimulate a
temporary algal bloom.
2. An immediate dilution of algae following destratification
effectively lowers zooplankton filtering rates and the intensity
of grazing on phytopl ankton. This may cause short-term
increases in algal biomass before zooplankton populations grow
to post-treatment levels.
3. Decline of algal sinking rates following artificial
destratification will favor heavy algae without buoyancy
adaptations.
4. A temporary rise in algal biomass following destratification may
favor blue-green algae by depleting CC^ and keeping pH levels
high. In turn, the intensity of zooplankton grazing is
effectively reduced.
5. The assemblage of blue-green algae and the alternate green algal
association represent alternative stable states of the
community. Maintenance of blue-green algae following some
destratification experiments probably results from initial
stimulation of algal growth by nutrient recycling and failure to
lower surface pH values.
42
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CIRCULATION
DETRITUS
UJ
0
WATER
TRANSPARENCY
TOTAL
PHOSPHORUS
0
ALGAE
SINKING
ALGAL
DISTRIBUTION
RECYCLE
HYPOLIMNETIC
NUTRIENTS
ALGAL
ABUNDANCE
ZOOPLANKTON
GRAZING
EPILIMNETIC
CO,
8 INCREASE IN RESPON SE PARAMETER
& DECREASE IN RESPONSE PARAMETER
J
e
CYANOPHAGE
ACTIVITY
GREEN TO BlUE-
QREEN ALQAE SHIFT
FIGURE 3 SOME ADVERSE IMPACTS OF ARTIFICIAL CIRCULATION
AND THEIR ROLE IN PROMOTING BLUE-GREEN ALGAE
BLOOMS (ADAPTED FROM SHAPIRO 1979)
-------
C. Macrophytes
1. In lakes with shallow littoral shelves, macrophytes may invade
or expand to nuisance levels if water transparency improves
following artificial circulation.
D. Fisheries
1. Aeration/circulation may raise ^ gas concentrations to levels
capable of inducing gas bubble disease in fish.
2. Artificial circulation may eliminate habitat for coldwater
fishes in southern lakes where metal imnetic populations existed
before treatment.
3. Regardless of precautionary measures, artificial
destratification involves some risk of extensive oxygen
depletion and fish-kills.
RECOMMENDATIONS
A. System Design and Application
1. Aeration/circulation is recommended as an inexpensive, efficient
restoration technique, potentially useful for treating the
symptoms of eutrophication when alternative management schemes,
such as the control of nutrient influx, are deemed too costly or
technically unfeasible.
2. Release of compressed air or a mechanical pump will achieve
adequate mixing in shallow and moderately deep lakes. However,
a combination of a surface pump and a bottom aerator will
probably give the best results in very deep lakes, especially
where intense surface heating could cause thermal
microstratification and associated algal blooms if only air
diffussion is used.
3. The full-air lift design is recommended for hypol imnetic
aeration because it is the least costly system to construct,
install, and operate. In terms of oxygen dissolved per
kilowatt-hour, it is almost twice as efficient as other systems.
However, each system has unique properties that might be
considered relevant for a prospective aeration site. Injection
of pure oxygen should be considered as an alternative to
aeration when a potential for No supersaturation exists and
downstream releases are inevitable.
4. Air diffusers should be located near the lake bottom to avoid
development of anoxia in the deepest portion of the lake basin.
Diffusers should be oriented such that released air does not
stir the surficial sediments directly.
5. Approximately 9.2 m3/min of air per 106 m2 of lake surface (= 30
SCFM per 106 ft2) is recommended to attain good mixing. Unless
44
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necessary to achieve an adequate mix, more intense aeration
should be avoided because of problems resulting from
resuspension of bottom sediments.
Hypolimnetic aeration rates should be determined by the method
outlined in Lorenzen and Fast (1977).
6. Unless an elevation of algal productivity is desired, artificial
circulation techniques should be applied before full development
of thermal stratification to avoid post-treatment recycling of
nutrients accumulated in the hypolimnion.
7. When the hypolimnion is already anoxic, aeration might be
started slowly and gradully intensified to force nutrient
precipitation and oxygenation of the bottom layer and avoid
mixing high BOD waters into the surface stratum. This might
avert possible oxygen depletion throughout the lake and a
subsequent fish-kill.
B. Improvement of Water Quality
1. When a cold water supply is needed, and control of algal blooms
is not critical, hypol imnetic aeration is recommended. On the
other hand, artificial circulation is preferred whenever
limitation of algal biomass is desirable, oxygenation of the
metalimnion is required, or loss of cool water is acceptable.
Although untested, the combination of a hypolimnetic aerator
with a mid-water mixing device might be used to lower the
themocline and oxygenate the entire lake while maintaining cold
bottom water.
2. Either aeration/circulation method is recommended for use by
water supply managers seeking to alleviate "taste and odor"
problems resulting from high concentrations of Fe, Mn, H2S and
other chemicals which accumulate in the anoxic hypolimnion.
3. When water transparency is a primary amenity, artificial
circulation should be applied cautiously to avoid resuspension
of bottom sediments (see recommendation A-3 above) and algal
blooms (see section C below).
C. Control of Nuisance Algae
Although aeration/circulation techniques cannot be considered a
"cure-all" for algal problems, the following recommendations should
increase the likelihood of bloom control while reducing the risk of
undesirable results.
45
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1. Hypo!imnetic aeration should be considered as a method for bloom
control only in cases where internal loading of nutrients is
high relative to external loading, and blooms occur following
natural destratification in autumn or during early spring as a
result of prior recycling of hypolimnetic nutrients.
2. Mixing techniques are recommended when the nuisance species is
known to be sensitive to disruption of its vertical profile and
variable hydrostatic pressures. Usually these are buoyant
blue-green algae, such as Anabaena spp. and Oscillatoria spp.,
with depth-specific light and nutrient requirements.
3. When a lasting reduction of algal standing crop is desired, an
evaluation of limiting mechanisms should preceed treatment
(cf. Lorenzen and Mitchell 1975; Lorenzen and Fast 1977).
Mixing techniques should be applied only in lakes where algal
biomass is limited by low light levels or could be limited by
reduced light availability resulting from an increase in mixed
depth. Although a temporary reduction in algal biomass and a
shift in species composition may follow mixing of a shallow
lake, artificial circulation will probably not control total
algal growth in shallow lakes.
D. Enhancement of Fisheries
1. Artificial circulation is appropriate for northern lakes where
surface waters would remain below 22°C during summer allowing
distribution of both cold-water and warm-water fishes throughout
the lake.
2. Hypol imnetic aeration is recommended for southern lakes where
high water temperatures in the epilimnion and metalimnion along
with anoxic conditions in the hypolimnion otherwise preclude
establishment of a cold-water fisheries.
3. Hypol imnetic aeration is recommended when improvement of
fisheries is the only consideration, e.g. when control of algal
blooms is unnecessary.
E. Future Research
1. Observational methods and experimental designs could be greatly
improved. Ideally, at least two years of pretreatment data are
required for proper evaluation of the effects of any
perturbation on biological communities in lakes. Within-lake
controls such as large enclosures or unaffected stations are
also desirable. Chemical observations should focus on the flux
of nutrients between various compartments of the system,
especially sediment-water exchange, in addition to standing
quantity within each compartment.
46
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2. A team research approach is desirable in assessing the impact of
aeration/circulation on lake ecology.
3. Integration of mathematical models predicting peak algal biomass
(e.g. Lorenzen and Mitchell 1975) with conceptual models
explaining shifts in alqal species composition (Shapiro et
al. 1975; Shapiro 1979) could form a basis for a priori
hypotheses about community responses amenable to experimental
testing. A systems analysis approach to lake ecosystems could
provide a holistic view necessary to understand the complex
response mechanisms operating during aeration/circulation
treatment.
4. Long-term responses of lake systems to treatment need to be
examined. Organisms with long generation times and slow
turnover rates (e.g. fishes) may require up to five yesrs or
more to reach equilibrium growth and carrying capacity.
5. A general area requiring additional research concerns how
trophic structure and species composition of communities
determines responses to aeration/circulation.
6. The possibility that nitrogen sup^rbaturaMon resulting from
aeration could induce gas-bubble disedse in fish needs to be
investigated further.
47
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