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
S 1
• 0
1 7
*a
OA
C-V-in
2 1
1 21
ill
0 40
1 Bl

4 91
4 91
1 04
I tl
0 17

1 »«
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1 10
1 01

2 01
o ts


ration tnnnnlt *
cyv
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10 2
2 9
20
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I 00
1 It
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«... ,„
2 17
1 B4
0 10
0 B4
UiAl-tlOU fUBB
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aaiel'flow PMB>
•aUl-fle* PMBI
a>lol-riM p^

V*
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11 10
100
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IB 7
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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
* ii e
i 42 « '
14* • 2-4
11 *
0 10 17 *t
1 17 J 0-14 0
1 11 *» .1
2 T« %ft ,]
1 4ft- .. .d
2 Ot " *
0 Oft 10 7
1 IS





I*k. Boopo.^
ro
D o ro( T r PO j mit m
0
• *
*
.

a o ooo
-
• 0 0
•
*
• 0 O 000
•
0
.





 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-

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 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

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                     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)

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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

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     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

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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

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     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

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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

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             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

-------
 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

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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

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     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

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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

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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)

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     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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                                REFERENCES
Ambuhl,  H.   1967.  Discussion of impoundment destratification by mechanical
pumping.  (W.  H.  Irwin, J.  M.  Symons, and G.  G.  Robeck).  J.  Sanit. Eng.
Div., Amer.  Soc.  Civil Eng.  93:141-143.'

American Water Works  Association.   1971.   Artificial  destratification  in
reservoirs.   Committee Report 63:597-604.

Andersson, G., H. Berggren,  G.  Cronberg, and  C.  Gel in.  1978.   Effects  of
planktivorous and benthivorous fish on organisms and water  chemistry  in
eutrophic lakes.  Hydrobiologia 59:9-15.

Arnold,  D. E.   1971.  Ingestion, assimilation,  survival, and reproduction  by
Daphnia  pulex fed  seven species of  blue-green algae.   Limnol.  Oceanogr.
16:906-920.

Barnes,  M. D.  and  B.  L. Griswold.   1975.   Effects  of artificial  nutrient
circulation  on lake productivity and fish growth.  Speciality Conference  on
Lake Reaeration Research, Amer. Soc.  Civil Eng., Gatlinburg, Tennessee.

Barnett,  R.  H.  1975.  Case  study of reaeration of  Casitas  Reservoir.
Speciality Conference on Lake  Reaeration Research,  Amer. Soc.  Civil Eng.,
Gatlinburg,  Tennessee.

Bartell, S.  M., and J. F. Kitchell.   1978.   Seasonal impact of planktivory
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Bengtsson,  L., and  H.  Berggren.   1972.   The  bottom  fauna in an oil
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Bengtsson, L., H. Berggren, 0.  Meyer, and B. Verner.  1972.  Restaurering  av
sjoar med kulturbetingat  hypolimniskt syrgasdeficit.   Limnologiska
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Bengtsson,  L., and C. Gel in.   1975.   Artificial  aeration  and  suction
dredging methods  for controlling water  quality.  Proc. Symp. on Effects  of
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Bernhardt, H.   1967.  Aeration of Wahnbach Reservoir without  changing the
temperature  profile.  J. Amer.  Water  Works Assoc. 9:943-964.

Bernhardt, H.   1974.  Ten years experience  of reservoir aeration.   Seventh
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Biederman, W. J., and E.  E.  Fulton.  1971.   Destratification  using air.
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Blahm,  T.  H., et al.  1976.   Gas supersaturation  research, National Marine
Fisheries  Service Prescott  Facility - 1971 to 1974.  Pages 11-19 jji D. H.
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Admin,   (as  quoted by Fast  1979).

Bowles, L. G.  1972.  A description of the spatial and tenporal  variations
in species composition and distribution  of pelagic net zooplankton in the
central  pool of Eufaula Reservoir, Oklahoma, with comment on forced  aeration
destratification experimentation.  Trans.  Kansas  Acad. Sci. 75:156-173.

Bradshaw, A. S.   1964.  The crustacean zooplankton  picture:  Lake Erie
1939-49-59; Cayuga 1910-51-61.  Verh.  Internat.  Verein.   Limnol.
15:700-708.

Brezonik,  P., J. Delfino, and G.  F.  Lee.  1969.  Chemistry of N and Mn  in
Cox Hollow Lake, Wisconsin,  following destratification.  J.  Sanit. Eng.
Div., Amer.  Soc. Civil Eng. 95:929-940.

Brooks, J. L.  1969.  Eutrophication and changes  in the composition of the
zooplankton.  Pages 236-255 J_n National Academy  of Sciences, Proc.  Symp.  on
Eutrophication: Causes. Consequences. Correctives.  Washington,  D.C.

Brown,  D. J.,  T.  G.  Brydges,  W. Ellerington,  J. J.  Evans,  M. F.  P.
Michalski, G. G. Hitchin,  M. D. Palmer, and D. M.  Veal.  1971.  Progress
report  on  the destratification of  Buchanan Lake.  Ont. Water Res.  Comm., AID
for Lakes  Program (Artificially  Induced Destratification).

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