HYPOLIMNETIC OXYGEN DEPLETION MECHANISMS
IN LAKE ERIE
By
Chris Potos
U. S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
GREAT LAKES REGION
LAKE ERIE BASIN OFFICE
December 1969
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ABSTRACT
To the present, the mechanism of hypollmnetlc deoxygenatlon of tem-
perate lakes has been little understood. It fs the consensus of opinion
among llmnologlcal Investigators that a slow, progressive, sediment bio-
chemical oxygen uptake rate, exerted by microbiological flora In the de-
composition of sedlmented plankton and other degradable organic debris,
Is the mechanism responsible for depleting any hypollmnlon of oxygen dur-
ing stratification periods.
Success in the measurement of a positive depletion rate In the summer
of 1968 In the Lake Erie central basin and correlation of this rate with
existing sediment and hypollmnlon oxygen demand; Infers the probability
of still another operative factor - that of chemical oxygen demand sat-
isfaction. The total mechanism of the depletion, abetted by sediment re-
suspension due to wind Induced water turbulence, can be chemical and
microbiological In nature, both at one and the same time.
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INTRODUCTION
Topographically Lake Erie Is divided Into three subbaslns. The rocky
Island chain, north of Marblehead, separates the western basin from the
central, and a low wide sand and gravel ridge, north of Erie, Pennsylvania,
separates the central from the eastern basin.
The western basin Is relatively shallow, averaging 25 ft., with a max-
imum depth of 60 ft. In a small area In the Island passages. Because of Its
overall shallowness, a stable summer temperature-density stratification does
not form. However, Intermittent short-lived stratifications do occur during
especially calm weather, resulting In oxygen depletions near the bottom.
Ensuing winds quickly eliminate these temporary stratifications by efficiently
mixing the top and bottom water layers. Oxygen deficits may occur In the
western basin under Ice cover when mixing due to meteorological effects Is
eliminated and the entire basin becomes virtually stagnant except for limited
mixing due to tributary discharges and lake level Induced water movements.
FWPCA (1968) data have shown an oxygen deficit of 15 percent In the western
basin under Ice cover. Dissolved oxygen concentrations averaged 12.4 mg/l or
85 percent of saturation at 33°F.
The central basin averages 60 ft. with a maximum depth of about 80 ft. A
stable temperature density stratification does form beginning In June. In late
August, when maximum differences In strata temperature and density exist, the
thermocllne Is very thin, approximately 5 to 6 ft. and Is located 50 to 60 ft.
below the surface. The bottom cold water layer (hypolImnlon) which warms to
only 55-60°F, and which averages approximately 20 ft. In thickness, may lose
much of Its dissolved oxygen, while the warm water layer above the thermocllne
(eplIImnfon) remains near saturation. When stratification Is most stable the
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central basin hypo limn Ion volume Is about 17 percent of the total central
basin volume. The loss of dissolved oxygen Is progressive, although the
depletion rate Is variable and apparently dependent on meteorological condi-
tions as will presently be shown, with the greatest deficit occurring In late
August or early September. As the lake begins to cool In early September,
the thermocllne progressively deepens until the temperature of the eplllmnlon
approaches the hypollmnlon temperature about the first week In October. Den-
sity stratification ceases to exist and holomlxls takes place assuring uni-
form temperatures throughout the entire water mass.
The eastern basin averages 80 ft. with a maximum depth of 216 ft. The
temperature structure Is similar to the deeper Great Lakes. Summer stratif-
ication does occur, although because of the basin's greater depth and volume,
the hypollmnlon seldom becomes warmer than 40°F. The thermocllne Is thick,
on the order of 25 ft. when stratification Is most stable and Is located be-
tween approximately 50 to 75 ft. below the surface. The depth of the thermo-
cllne Increases as summer progresses and reaches 120 ft. or more by th e time
stratification disappears In November. During this period, eastern basin
hypollmnlon oxygen losses occur but seldom to less than 50 percent of sat-
uration. The large volume, low temperature, and relatively low algal pro-
ductivity are factors In minimizing oxygen depletion.
Low dissolved oxygen was first observed In the central basin In 1929
(Fish I960). Carr, Beeton, and Allen (1963) corroborated the early observa-
tions and Indicated the oxygen-depleted area may be quite extensive. The
exact cause of the depletion was not clear, although a high sediment organic
content was suspected among other Interacting factors. FWPCA (1968) estim-
ated the central basin bottom water oxygen-deficient area (2 mg/l or lessjto
be approximately 2600 square miles or 25 percent of the entire lake area.
To the present, the mechanism of hypollmnetlc oxygen depletion has been
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little understood. Gorham (1958) suggested the decomposition of sedlmented
plankton as a principal cause. Gardner and Lee (1965) reported the hypo-
limnetic oxygen depletion of Lake Mendota to be the result of biochemical
oxygen demand of plankton and other organic debris accumulating In the sedi-
ments. In addition It was suspected, although no evidence was given, that
hypollmnetlc deoxygenatlon may have resulted through oxidation of reduced
Iron and sulfur species In the. bottom sediments.
It has been suspected for sometime by the author that sediment chemical
oxygen demand Is a contributing factor to oxygen depletion In Lake Erie and
It was so stated In the International Joint Commission Report on Pollution
of Lake Erie, Lake Ontario, and the St. Lawrence River (1969). Evidence to
complement these early suspicions will be given In the following discussion.
DISCUSSION
In the summer of 1968 FWPCA completed a cursory study on Lake Erie
central basin hypollmnetlc oxygen depletion rates. Analysis of water samples
from 20 locations on two north-south transects between Ashtabula, Ohio, and
Erieau, Ontario, and Cleveland, Ohio and Erleau, closely followed by routine
midlake sampling along the International boundary Indicated a large central
basin oxygen depletion In a short period. Positive depletion rates however,
were established for two stations only - the two transect stations that were
reoccupled as midlake stations (H-20-1, J-25-1) (See Fig. I). Station J-25-1
lost 6.1 mg/l of oxygen In 52 hours. This loss rate Is a true rate only If
the water masses were Identical at the time of both measurements. However
It Is possible that a low DO water mass moved Into the area at the time of the
second measurement. Assuming that the former case was operative, data from
station J-25-1 will be used to Infer mathematically the contribution of
chemical oxygen demand In the depletion of hypollmnetlc oxygen. The latter
case most likely is not probable since prevalent southerly winds would cause
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any significant hypollmnlon movement to be from north to south. DO measure-
ments at hypollmnlon stations to the north prior to the rapid depletion are
all greater than those of J-25-1.
Unpublished FWPCA k (velocity of reaction) rates of central basin muds
for 1968 average 0.05. The August 1968 k rate for J-25-1 sediments was 0.04,
determined on homogenized samples of the top four inches of mud as sampled
with a Peterson dredge. This rate Is less than one-fourth that for average
domestic sewage and Is indicative of a highly stable organic matter - one
composed In great part of low molecular weight, low energy compounds and much
water of hydratlon. At this rate (0.04) approximately 9 percent of the BOD
would be exerted on the first day at 20°C. However, during the period August
6-8, 1968, the hypollmnlon had only warmed to I4°C. Since the velocity re-
action rate Is dependent upon temperature as well as the amount of organic
matter present, the first day BOD exertion would be even less than 9 percent.
It is apparent that under the natural conditions occurring on August 6-8,
1968 the sediment and hypollmnlon BOD exerted In the 52 hour period under
consideration was not enough to account for the dramatic depletion described
above. Calculations completely shown In the appendix, place the total oxygen
depletion at 8 times that which can be accounted for by microbiological
uptake (BOD). It Is Indicated, the depletion most likely Includes a chemical
oxygen demand.
The calculated exerted BOD Is based on the amount of organic matter In-
cluded In a 1/4 In. (6.4 mm) layer at the surface of the sediments. It Is
assumed that all sediment associated organic matter was efficiently collected
(Peterson dredge) without any loss by dispersal Into the ambient water.
Hutchlnson (1957) believes that conditions In the top few millimeters of mud
are determined by diffusion of oxygen from the water on one hand, and reduc-
tion processes In the mud on the other. From Mortimer's (1941) data
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measuring sediment isopleths, the redox potential under Isothermal conditions
for ferric reduction (0.2v) on eutrophlc Esthwalte Water In England was found
on an average of 5 mm beneath the mud surface. The sediments below the 5 mm,
0.2 volt Isopleth were termed reducing In nature and devoid of molecular oxy-
gen as a result of the metabolic processes occurring In the sediments and the
curtailed dlffuslonal exchange resulting from the presence of the Interface
oxldatlve mlcrozone. Ruttner (1963) Is In agreement with these data. It Is
reasonable to conclude that the organic matter In the 6.4 mm layer used for
the calculations would be greater than the amount actually exerting a constant
non-weather Influencing demand on Lake Erie central basin hypollmnetlc dis-
solved oxygen.
Stirring and resuspenslon of the Interface sediments by wind-Induced
currents and/or Internal seiches with consequent exposure of high oxygen
demanding reduced chemicals beneath, Is strongly Indicated. The Immediate
demand of the exposed reduced chemicals, both organic and Inorganic, appar-
ently Is responsible for accelerating the depletion measured on August 6-8,
1968.
Examination of meteorological data reveals stronger winds on August 6,
1968, approximately four times the resultant wind speeds registered the week
prior, and three times the resultant wind speeds registered the week after.
The stronger winds were widespread throughout the central basin. The higher
resultant wind speeds registered for only one day on August 6, 1968 lend
credence to the suspicion that the uptake rate was greater than measured,
since rapid selective sedimentation most likely occurred following meteoro-
logical quiescence, quickly covering the exposed high oxygen demanding
sediments.
Current meters placed In the hypollmnlon periodically have recorded
hypolimn Ion movement of unusually high velocities (2 ft/sec.), much higher
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than bottom water velocities at other times of the year and capable of resus-
pendlng bottom sediments (FWPCA 1968). Fruh, Stewart, Lee, and Rohllch
(1966) credited the movement of wind-Induced currents In the hypolimnlon with
the capability of resuspendlng flocculent material from the surface of the
bottom sediments.
It Is the consensus of opinion among 11mnological Investigators that a
slow, progressive, sediment biochemical oxygen uptake rate, exerted by micro-
biological flora In the decomposition of sedlmented plankton and other degrad-
able organic debris, Is the mechanism responsible for depleting any hypo-
Mmnlon of oxygen during stratification periods.
If It Is assumed all the oxygen depletion during the study period to be
the result of biochemical oxygen demand, It would be necessary to resuspend
for 52 hours more than four Inches of sediment for exposure of the Included
volatile solids to the oxygen reserves of the hypollmnlon - a most unlikely
occurrence. These calculations are also shown In the appendix. In addition,
the above assumption Includes the proposition that the anaerobic bacteria
associated with the organic matter beneath the 0.2 volt Isopleth, would with-
out Interruption continue metabolism and reproduction when placed In an
aerobic environment, again a most unlikely occurrence. The transfer of
strict anaerobes to an aerobic environment would completely Inactivate them
due to Increased redox potentials or kill them through the mechanism of per-
oxide poisoning, (Salle 1961). The mlcroaerophlles and facultative anaerobes
would require some period of adjustment before metabolism could again be ef-
ficiently sustained, contradicting the rapid oxygen depletion encountered
during the study period.
Under meteorological and consequent hydro log lea I quiescence, the rate
of oxygen depletion most likely does proceed In a slow and progressive fashion.
With the exhaustion of the dissolved oxygen In the hypolimnlon, the
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microbiological populations then utilize chemically bound oxygen to satisfy
respi rational requirements.
As the proportion of reduced chemicals to oxidized chemicals In the
hypolimn I on during stagnation Increases, the oxidation-reduction potential,
the parameter which measures this proportion, begins to decrease from a max-
imum of 0.5 volt under Isothermal conditions. Nitrate Is reduced to nitrite
at 0.45-0.40 volt, nitrite to ammonia at 0.40-0.35 volt, ferric to ferrous
Iron at 0.30-0.20 volt and sulfate to sulflde at 0.10-0.06 volt (Ruttner
1963). With reduction and consequent solublMzatlon of the essentially ferric
hydroxide-phosphate oxldatlve mlcrozone at 0.3-0.2 volt, further leaching and
solublIIzatlon of other reduced chemicals beneath the former mlcrozone now
readily occurs, and to a great extent, leading to a highly reduced hypollm-
netlc environment. With autumnal turnover, the reduced solublIIzed chemicals
not utilized by algae are oxidized and resedImented, temporarily removing
them from llmnologlcal life cycles.
Under conditions of meteorological and consequent hydrologlcal Insta-
bility, apparently microbiological deoxygenatlon of the hypolimn Ion Is aug-
mented by another oxygen depleting mechanism - that of chemical deoxygenatlon.
At these times the rate Is not slow and progressive as under quiescent condi-
tions. It Is Indicated that with meteorologically produced relatively rapid
hypollmnlon movement, the oxygen depletion rate can be, and Is In fact, very
rapid, depending upon the severity of the weather Instability. The total
mechanism of the depletion, abetted by sediment resuspenslon due to wind-
Induced water turbulence, Is chemical and microbiological In nature, both at
one and the same time.
SUMMARY
(I) Apparently the mechanism of hypollmnetlc oxygen depletion In Lake Erie
Is two-fold:
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(a) resplratlonal requirements of bacteria In the degradation
of organic matter mostly sedlmented plankton
(b) demand of ferrous Ion, sulfide Ion, and other reduced chemicals
when exposed to the overlying waters upon resuspenslon of Inter-
face sediments.
(2) Seiches and other wind and barometric pressure Induced bottom water move-
ments are capable of resuspendlng sediment materials to the overlying
waters.
(3) At the station studied, the sediment uptake rate was approximately eight
times that which can be accounted for by BOD, Indicating that the oxygen
depletion for the period under Investigation most likely Includes a chem-
ical oxygen demand.
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APPENDIX
PROOF OF COD RELATING TO LAKE ERIE
HYPOLIMNION OXYGEN DEPLETION
Between August 6-8, 1968 (52 hrs.) J-25-1 hypollmnlon station lost 6.1
mg/l of oxygen.
Assume area IOm x IOm with sampling point J-25-1 at Its center will have
similar conditions horizontally and that 1/4 In. (.00635m) layer of sediment
will be exposed to oxygen of overlying waters
0.00635m (100m2) = 0.635m3 of exposed sediment at 1005? solids
J-25-1 Solids Content = 20.65?
0.635 (.2065) = 0.1311m3
J-25-1 Volatile Solids (VS) = 33 mg/g » 3.3$
0.1311 (.033) = 0.00433m3 volume of exposed VS
Assume a specific gravity of I for VS
0.00433m3 x 9994l7g/m3 = 4327g of exposed VS ,
J-25-1 Sediment:
BOD =3.4 mg/g =3.4 mg/33 mg VS
"k" rate = 0.04 at 20°C
Ultimate BOD (L) = BOD 3.4 3.4 3.4
5_ = = =
-k.t. <-.04)(5) (-0.2) = 5.4 =
1-10 1-10 1-10 1-0.631 .369
9.2 mg/g = 9.2 mg/33 mg VS
But "k" rate Is temperature dependent
At I4°C - Temp, of hypoIImnIon Aug. 6-8
K = K (I.047)TI~T2
I 2
= (l.04)(l.047)(~6) = (.04) (.760) = .03 = k I4°C
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BOD exerted In 52 hrs. (2.08 days) (I4°C)
-k+)
Y = L (1-10
= 9.2 C|.|0(--03>(2-°8)]
* 9.2 d-IO"'062)
= 9.2 (1-0.865 = 9.2 (.135) = 1.25
=1.25 mg/33mg VS
I.25 s x x » 163.9g of 0_ necessary for sediment VS satisfaction
"33" 4327 ^
In 52 hrs. I4°C
J-25-1 Water
BOD "I.I mg/L
k rate - 0.05 at 20°C
Ultimate BOD (L) = BOD I.I
5 = -I.I
-k.t. (-.05)(5) .438
1-10 1-10
= 2.5 mg/a
at I4°C
K = K (|.047)T|"T2
I 2
• (.05)(l.047)"6 = (.05)(.760) = .04 = k at I4°C
BOD exerted In 52 hrs. (2.08 days)(!4°C)
Y « L (l-IO'k*f')
Y, - 2.5 (|-,0(--04)(2'08))
/..U8
= 2.5 (I-IO("'083))
= 2.5 (1-0.826) = 2.5 (.174) « 0.44 mg/k
Hypollmnlon Volume
5.182m " depth of hypollmnlon at J-25-1 on August 6-8
10m x 10 m x 5.182m • 518.2m3
518.2m3 x 999.3 U/m3 - 517,837 It
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0.44 mg/L x 517,837 L = 227.85g to satisfy water
BOD at 52 hrs. I4°C
163.9g 02 (Sediment VS satisfaction)
227.9g 02 (Water VS satisfaction)
39l.8g 02 (Total VS satisfaction)
But hypollmnlon at J-25-1 lost 6.1 mg/L of 0^ In 52 hrs.
6.1 mg/L x 517,837 L = 3,158.806 mg (3.l58.8g) lost In 52 hrs.
3J58.8 = approximately 8 times more oxygen depleted than can be accounted
391.8
for In BOD. Therefore depletion must Include chemical oxygen
demand.
At 20.7 percent solids and specific gravity 2.0
wt. J-25-1 Sediment = 388 Ibs/yd.3
= 230,637 g/m3
VS = 3.3 percent
230,637 (0.33) = 76,IIOg VS/m3
= 7,6M,OOOg VS/IOOm3
100m3 = 100m2 (area) x Im (depth)
3l58.8g 02 depleted In 52 hrs.
227.9g due to water BOD
2930.9g due to sediment BOD
BOD (52 hrs.) (I4°C) = 1.25 mg/33 mg VS
1.25 = 2930.9g x = 77,375.8g of VS to exert BOD of 2930.9g
33 x
77375.8
7,611,000 (Total VS In 100m3) x Im (depth) = 0.102m
0.102m x 39.37 In/m « 4.02 In.
0.102m x 1000 mm/m = 102 mm
II
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REFERENCES
Carr, J. F., Beeton, A.M., Allen, H., Factors associated with low dissolved
oxygen concentrations In Lake Erie, Proc. 6th Conf. on Great Lakes
Research p. 133 (1963).
Fish, C. J., Llmnologlcal survey of eastern and central Lake Erie 1928-1929,
USFWS, Spec. Scl. Rept. Fish. 334, 198 pp. (I960).
Fruh, E.G., Stewart, K.M., Lee, G.F., Rohllch, G.A., Measurement of eutroph-
I cat I on and trends, Jour. Water Poll. Contr. Fed. '38, 1237-1258 (1966).
FWPCA Lake Erie Environmental Summary 1963-1964, Dl, GLR (1968).
FWPCA Lake Erie Surveillance Data Summary 1967-1968, Dl, GLR (1968).
Gardner, W., Lee, G.F., Oxygenatlon of lake sediments, Int. Jour, of Air and
Water Pollution, pp 553-565 (1965).
Gorham, E. Observations on the formation and breakdown of the oxidized micro-
zone at the mud surface In lakes, Llmnol. Oceanog. 3 pp 290-298 (1958).
Hutchlnson, G.E., A Treatise on Limnology, Vol I, John Wiley i Sons Inc. New
York p. 720.
IJC Report on Pollution of Lake Erie, Lake Ontario, and the St. Lawrence
River, p. 126 (1969).
Mortimer, C.H., The Exchange of Dissolved Substances between Mud and Water In
Lakes, Jour, of Ecology p. 29-30 (1941).
Ruttner, F., Fundamentals of Limnology, 2nd Ed., University of Toronto Press,
Toronto, pp 206-207 (1963).
Salle, A.J., Fundamental Principals of Bacteriology 5th Ed. McGraw-Hill Book
Co. Inc., New York, pp. 329-330 (196.0.
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FIGURE LEGEND
MID-LAKE SAMPLING STATIONS, 1968 - DO STUDY
TRANSECT STATIONS
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FIGURE I
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