The Merornixis of Sunfish Lake. Southern Ontario

Haulsn C. Durnrn aNn JonN C. FI . CenrRn

Departm,ent of Bi,ologyflniaers'ity of Woterloo, Waterloo, Ont.

Dururn, FI. C., l l+n J. C. H. Cenrnn. 1970. The rneromixis of Sunfish l.eke, southerrrOntario. J. Fish. Res. Bd. Canada 27: 847-856.

In the meromictic Sunfish Lake the large populations of phytoplankton that areobserved periodically were associated with circulation of the mixolimnion. Perhaps someorgarric dissirnilation took place in the lorver mixolimnion, recycling nutrients during springand fall circulation. Phytoplankton was practically absent frorn the epilimnion duringthe summer but a populationof Oscillatoria agordhii remairred in the metalimnion, whereit was responsible for a pronounced and persistent rletalimnetic oxygen maximum. Themonimolimnion contained about 5100 mg/liter of dissolved solids and the upperraixolinnion abor-rt 2200 rng/liter. Ventilation of the rnonimolimnion did not alwayscoir.rcide with circulation and subsurface springs may have been responsible. Circrrlationto the lake bottorn was observed only once in 18 months.

Received March 3, 1969

INTRODUCTION

IN a lrBnoulCrIC r,AKE the efficiency of the exchange betr,veen the nrixolimnionand the monimolimnion to a great extent controls the trophic level of theeuphotic zone. Thus in a permanently meromictic lake the monimolimnionacts as a nutrient trap, and, particularly if the importa,tion of nutrients fromtl.re r'vatershed is low and the lake is small, sheltered, and relatively deep,primary productivity wil l be low. Examples of this condition include Krot-tensee, Austria (Findenegg, 1965), and Green Lake, Nerv York (Jackson andl)ence, 1958). In meromictic lakes in r.vhich circulation may be complete givenfl ivourable meteorological conditions, some return of nutrients to tl.re euphoticzone is accomplished and the primary productivity is consequently higher.An example of this is Hall Lake, Washington (Edmondson, 1963; Edmondsonquoted in Frey, N,{S, 1967). Another consideration, often overlooked, is therecycling of nutrients through circulation in the mixolimnion itself. The effec-tiveness of this in supplying nutrients to tl 're euphotic zone l-ould in largelneasure depend on the intensity of organic dissimilation in the lou,er mixolim-nion, the degree of circulation acl.rieved, and its frequency (Findenegg, 1937;Kuznetsov, 1968). This effect apparentll ' t 'ould be more applicable to meromicticlakes that have occasional complete circulation. Tl.rere is, hou,ever, l i tt lepublished in{ormation on meromictic lakes of this type.

The present study rvas undertaken to explain the occasional appearanceof large algal blooms in the upper waters of Sunfish Lake, a small lake rvhoseph1'toplankton productivity is otherwise lou'. The lake is merornictic, full

Printed in Canada (J1395)

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848 JoURNAL FTSHERIES RESEARCH BOARD oF CANADA, voL.27, No. s, 1970

circulation being observed once during the 18 months of study but not sub-sequently. In the follorving description several aspects of the meromixis areexamined, in particular the influence of the periods of partial or full circulationon the gror'vth of the phytoplankton.

Sunfish Lake is situated at an altitude of 365 m about 8 krn to the westof Waterloo in southern Ontario. The lake l ies in a small depression surroundedby sandy kames over a Salina (upper Silurian) bedrock of shale, salt, andgypsum (P. F. Karror'v, Department of Earth Sciences, University of Waterloo,personal commnnication). Sunfish Lake and the nearby Spongy and Hofstetterlakes form a series of kettles on the outskirts of the Waterloo Hil ls. The verysmall rvatershed is mostly farmland or mixed deciduous r,voodland with somelremlock (Tsuga canadens,is) and cedar (Thuja occ,idental,is) in areas of poordrainage.

The morphometr ic data of Sunf ish Lake is as fo l lows: to ta l area,8.3ha; maximum length, 577 m; maximum width, 189 m; maximum depth, 20m; mean depth, 10.4 rn; vo lume development rat io 1.56; re lat ive dept l r ,

6 .157o ; max imum dep th (m) / ! a rea (ha ) ,6 .94 .The maximum depth is remarkably deep for the small area of the lake.

The relative depth, rvhere the maximum depth is expressed as a percent oftlre mean diameter (Hutchinson, 1957), and the volume development arehigh, indicating the pronounced concavity of the lake basin. The main inletflows only intermittently and a direct passage of water from it to the nearbyoutlet, bypassing the lake, has been observed on several occasions. Thereare one or two other very small temporary inflow streams and the presenceof several subsurface springs has been reported by local residents. Whetheror not t l 'rese springs are rich in dissolved salts is not known. The littoral isvery narro\'v and mostly free of aquatic plants, but patches of Chara sp. occllr,t l 're most extensive being around the outlet. The lake bottom is covered bya black sulfide sapropel.

METTIODS

Systematic sampling was carried out weekly from February 7966 to July 1967 at a permanent

buoy near the deepest point in the lake (Fig. 1).

Frc. 1. Hydrographic map of Sunfish Lakeshowing the position of the sampling station

(f ) . Depths in meters.

l O O m \

Samples of water for chemical analysis and phytoplankton enumeration were collected

with a Kemerrer bottle at 1- or 2-m intervals from the surface to the bottom. At each sampling

interval the temperature and the percent saturation of oxygen at the observed ternperature were

measured with an Electronic Instruuents Limited Model 15A dissoived oxygen meter. A1l samples

rvere kept in polyethylene bottles and the further chemical analyses were generally performed

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DUI'HI I t r \ND CARTER: NIERON,I IXIS Ob- SUN|- ISI I L, \KI ' I '19

on the same day as collection. pH was measured in the laboratory with a Fisher Accumet pHmeter. Alkalinity (corrected for sulfide), nitrate nitrogen, and silicate (monthly) were measuredby the methods in Mackereth (1963). 'Iotal

phosphate phosphorus and sulfide were nreasuredby the methods of the American Public Health Association (1965). Sulfate was measured by theturbimetric method and iron by the 2,4,6-tripyridal-S-triazine method of the Hach ChemicalCompany (1965). In the comparison of chenrical concerltratiorrs above and below the chemocline(Table 1) all the analyses except total dissolved solids (TDS) 14rere performed by atomic absorptionanalysis. The

'IDS was estirriated by evaporation and rveighing.

'I'esrB 1. Sorne chemical concentrations (rng/liter) above ancl

below the chemocline in Sunfish Lake.

Depth

l m 1 8 m

TDS.TDS\ INFeZnCuN:rKNIgCa

2 1 2 022350 0 1TraceTraceTrace2 + . 0 2

' 2 . 8 )

1 2 . 1 7

5 1 6 050232 . 5 00 . 0 30 . 0 2'frace

3t .2 )2 . 8 )

1 4 . 0 31 0 5 . 0

June 6, 1968Dec. 10 , 1968Oct . 25 , 1968

Feb. 10, 1969

"TDS : tota l d issolved sol ids.

The populations of phytoplankton $-ere estimated by sedimentation of 100-rnl sampleswith Lugol's iodine followed by enumeration over an inverted microscope. Units are a single cell,a colony, or a filament 450 p long (i.e., the diameter of the field of the Whipple eyepiece used).

PHYSICAI, AND CHENIICAI- CHAR-q.CTERISTICS

TBupBnarunB

The isotherms of Sunfish Lake (Fig. 2) are indicative of meromixis.During February and N{arcb 1966 the 4 c isotherm was at a maximuln of14 m, the temperature increasing to 5 . 1 C near the bottom. During the winteroI 1966-67 the 4 C isotherm, init iatly at 13 rn in December, gradually roseto about 4 m by tl 're end of February suggesting some process of heating.During the summers of 1966 and 1967 the temperature near the bottom rvasIlever less than 5 .4 C and on several occasions there r,vas an inverse stratif icationexisting from about 15 n to the bottorn. Only during the circulation periodirr November-December 1966 rvas there sorre suggestion of circuiation tothe bottorn.

The thennal profi le for Noverriber 23, 1966 (Fig. 3), shows a maximumat about 16 m and the proli le for July 7, 1967, shorvs a slight but definiteternperature invcrsion frotn about 14 nr to the bottom.

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850 JoLrR\.- \L FrsHEr{rES RESn.\RCrr tso. \RD oF CAN-\DA, voL.2z, No. s, 1970

l l \ z o z z " \ z . L i x . ]

\ \ " ; : r 4

' ' * - l

r\ ll 'l-5

--?*r- .5.4 ,sg- .09; .un

, BO 't'L

'tu-t L

rzo f,." rsol rzd

, , "x; q i3a @

Sulf ide (ms/l i ter)Q 2 . O 4 O - ' g 2 0 4 0

20 40 60 20 40 60

5

E

E.c)o 1 5

7.2 t2 , t2 72zalt 70

M A M J J A S O N D J F M A M J J1 9 6 6 1 9 6 7

Sulfate (mgTliter)Temperature(C)

Oxygen (%saturation)

sto--60-- 700- sd-o---- sm----70oConductivity(micromho at 20 C)

Frc.3. Prol i les of temperature (C), conduc-tivity, dissolved oxygen, sullide and sulfate

on two dates in Sunfish Lake.

-v;

7F

5=0)ol,-*"'l'l

- ;{

Frc. 2. Isotherms (C) and isopleths of percerrtsaturation of dissolved oxygen and ol pH itSunf ish Lake, February 1966 to July 1967.Heavy bars are ice cover_ and stippled areas

are anaeroblc.

Drssor,vno OxycnN

The isopleths of dissolved oxygen (Fig. 2) clearly indicate prolongedperiods of anoxis belorv Il 12 m and the metalimnetic oxygen maximumduring the sumners. Althougli periods of circuiation rvould account for thepartial venl.i lation of the nroninrolininion in Novernber-Decernber 1966 andApril 1967 no such explanation can account for the oxygen present in July1966 or under the ice during the rvinter of 1966-67.

During the prolonged circulation of November-December 1966 the per-

centage saturation of dissolved oxygen in the mixolimnion fell to iess than80 and, except for one brief interval associated rvith a brief growth of pliyto-plankton imnrediatell, under the ice, remained less than 80cl unti l circulationthe foliou'ing spring. \ ' 'er)' l ikely this demonstrates the high biochemicaloxlrgen demand of the rronirnolirnnion and lorver rnixolir-nnion. On occasionsrvhen the dissolved oxygen nreter readings rvere checked against chemicalanalysis by the standard Winkler method the latter alr 'vays gave negativeresults rvhen tlrc' rneter registered tr:rce (less t.han .5/6 snturation) amounts.

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DUTI I IE AND C- \RTER: NI I IROI I IX IS OF SUNFISH LAKE 8 5 1

Irrobably, in the presence of H2S the oxygen electrode is nrore sensitive thanthe chemical rnethod.

rFI aNl Ar ,xarrxrry'rhere

\\ras a marked decreerse in pII rvith depth (F'ig.2). Tlie highestvalues, 8.0-8.8, \ l 'ere recorded nra in ly in the epi l imnion in the sunrmers ofboth 1966 and 1967. Frorn November 1966 to \ iay 196' l the pH of the sur facervaters fell to betr'veen 7 .4 and 7.8. There ivas l itt le evidence of seasonal changesin the pH ol the lon'er rnonimolimnion, the values being usualiy around neu-tralit-v.

Thc isopleths of alkalinity (F-ig. 4) indicate a marked increase with depth,particularly near the bottom. Tbe concentrations ranged from 75 ng/Iiter

c l o+o.q)

U

l 'rc. 4. Isopleths of :rlkalinity as rng/literof CaCO:, mg/liter of total phoJphatephosphorus, and filan-rents/millil ite; of Oscit-lator.ia agard.ltaT in Sunfish Lake, February1966 to July 1967. Heavy bars ere ice coverand stippled areas are 1000 filaments/

rnillil iter or more of O. agard.hii.

M A M1 9 6 6

F

zdo

Oscl lator ia agardhi i

, t l T s o r . r o . r F v e J

at the surface to over 350 mg/liter at the bottom. A decrease in alkalinityoccurred at all depths during the circulation of November-December 1966.Tl'rere \&-as a partial recovery toward the end of the period of observations.

Pgospuetn, NIrRAro, AND OrHEn OespRvlr roxs

There w-as a pronounced increase rvit l-r depth of total phosphate phosphorus(Fig. a) throughout tl 're study period. Concentrations varied from about0.01 nrg/l iter near the surface to over L mgfliter near the bottom. In general,

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852 JOUI{NAL F ISH} ' I { IES I I .ESEARCI I BOI \ I {D OlT CANADA, YOL.27, NO. 5 ' 1970

the stratif ication u,ith depth of total phosphate phosphorus in the mixolirnniorr

was most evident during thermal stratif ication, the isopleths clearly indicating

the periods of circulation. Organic dissimilation in the lorver mixolimnion

in the summer of 1966 is suggested by the general increase in total phosphate

phosphorus. The deep circulation of November-December 1966 is clearlv

reflected in the isopleths -v-et surprisingly tltere \\ras apparently l i tt le influence

on the concentrations belori- about 15 l-n.

Amoults oi l i trste nitrogen varied inconsisteutly with depth and time

frorn a t race to 0.25 mg/ l i ter . None I 'as detected belorv 12 mby the rnethod

trsed. Concentrations of dissolved sil icate varied from about 5 to 10 mgfliter.

T}rere \vas more than tr-ice the concentration of TDS above the chemo-

cline as tirere rras belorv ('fable 1). The most abundant element analysed

was calciun-r. Of the other elements analysed only potassium \\ias more con-

centrated above tl 'ran belolr, the chemocline.on November 23, 1966, the chen'rocline rvas evidently betu,een 13 and

15 m (Fig. 3) u,ith the lake circuiating to about 15 m. The position of the

chemocline rvas not nearly so evident on July 7 , 1967, although a temperature

rlinimum was at 14 rn.Amounts of sulfide near the botton.r \\ ere usually :rroLtnd 40 mg/liter and

sul fate betu,eetr 60 and 72 nrg/ l i ter (F ig.3) . On November 23, 1966, t race

anrounts of sulhde rvere {ound at the surface. Sulhde rvas generall} 'detectabie

at 9 or 10 m during thermal stratif ication o{ the mixolinlnion.

Secchi disc transparency \\ras generally between 4 and 5 m during the

sunrlner months but during the periods of circulation it decreased to 2 2! n.

PHYTOPLANKTON

Over t[e entire period of observation, by far the most abundant phyto-

plankter was a species ol Osc,iLlatoria, tentatively identified as O. agard'hii

Gomont but in some respects resembling the variety O. agard,hi'i 'isothrix

Skuja. When observations began in February 1966, a fairly large population

existed under the ice and r,vas particularly dense betrveen 2 and 4 rn iFig.4).During circulation of the mixolimnion in April 1966, t l ie population declined

to less than 250 fi laments per mill i l i ter. Coincident u,ith the onset of thermal

stratif ication in X,{ay the population increased rapidly and by t}re end of \4ay

rvas in excess of 1000 li laments per mill i i i ter betu'een 2 and 8 m. In June, July,a.nd, August the population r,vas practically confi,ned to the metalimnion and

densest betrveen 6 and 8 m. In Septep-rber and early October, t l-re center of

the bloom descended further to betureen 8 and 10 m. With the onset of autumn

circulation, the population became scattered fairly uniformly through the

mixolimnion. Other phytoplankton lvere very sparse in the epil inrnion during

bot[ sumnrers and consisted chiefly of lorv nutnbers of several species of Chro-

ococcsles. During the trinter of 1966 67, there was a srnall increase in Oscil-

lator' ia immecliateiy under the ice to about 250 fi laments per mill i l i ter but

a large population, such :rs u'1s {ound in February 1966, did not develop.

A1so, u1like the previous -vear, a large population did not develop inrmediately

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DUTHIE AND CARTER: MEROI , I IX IS OF.SUNFISH LAKE 853

after thermal stratif ication in May 1967. The increase was gradual and notunti l the 3rd week in June was there a population in excess of 1000 fi lamentsper mi l l i l i ter between 5 . 5 and 7.5 m.

Only four other species of phytoplankton formed persistent populationsof appreciable abundance. These were Asterionella forrnosa Hass., Stephano-discus hantzsch'ii Grun., Cryptomonas erose E., and Cryptomonas curaata E.(Fig. 5). All four species had periods of growth before and during the spring

Frc. 5. Variations in popuiationsof four species of phytoplanktonin Sunfish Lake, February 7966to July 7967. Heavy bars are ice

cover.

overturn in both years and A. formosa and C. erosa also had rninor bloomsin early winter 1966-67. The spring increase in 1966 was greater in each in-stance than that in 1967, particularly so for A. formosa.

D ISCUSSION

Of known meromictic lakes in North America, Sunfish Lake is chemicallymost similar to Green and Round lakes, Nerv York (Likens; MS, 1967). How-ever, Green Lake has high transparency, paucity of phytoplankton, andlow productivity (Jackson and Dence, 1958) and in these respects is quiteunlike Sunfish Lake. Green Lake is also deeper (50 r-n), and complete circulationhas never been reported. Takahashi et al. (1968) concluded that the causeof permanent stratifrcation of Green Lake is crenogenic, l i tt le contributionto the meromixis coming fror-n biological activity within the lake. In SunfishLake, since the presence of saline subsurface springs has not been demon-strated, and considering the biological activity rvithin the lake, meromixisis probably largely biogenic, its maintenance assisted by the morphometry

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854 JoTTRNAL FISHERIES RESE. \RCI I BoARD oF 'C-A.NADA, voL.27 , No. s , 1970

of the lake basin and its sheltered situation. It is highly unlikely that mero-

mixis can be attributed entirely to morphometric features as Northcote and

Halsey (1969) postulated for Yellow Lake, B.C. The nature and antiquity

of meromixis in Sunfish Lake is currently under investigation by microfossil

and chemical analysis of the sediments.The concentrations of sulfate and sulfide in Sunfish Lake (Fig. 3) are,

hor,vever, too great to be entirely biogenic (cf. Hutchinson, 1957). In GreenLake the maximum amount of sulfide is about 56 mg/liter (Turano and Rand,

MS, 1967) compared with 40 mg/liter in Sunfish Lake. On the other hand

the sulfate content of Green Lake is about 1500 mg/liter compared with

only 70 mg/liter in Sunfish Lake. The origin of the sulfate in Green Lake

has been ascribed to leaching from the surrounding geological formations

and possibly the sulfate in Sunfish Lake is derived by leaching from the bedrock,

which is knor,vn to contain gypsum, or from agricultural runoff.The decrease in pH with depth (Fig. 2) is probably caused by increasing

amounts of carbon dioxide, t,hich has a much greater influence on pH than

the increasing alkalinity (Fig. 4). In Green Lake, Turano and Rand (MS,

1967) point out that the pH is more affected by the ratio of carbon dioxide

concentration in the monimolimnion to that in the mixolimnion than the

corresponding ratio of alkalinity concentrations.The concentration of phosphate in the monimolimnion of Sunfish Lake

is very large and its origin is uncertain. There are no entirely unpollutedwaters in the vicinity u'ith which comparisons may be made so the contributionfrom farming and domestic activit ies cannot be evaluated. The concentrationsof total phosphate in the mixolimnion are dependent partially upon circulation

and eddy diffusion across the chemocline and partially upon tropholyticprocesses in the mixolimnion itself.

The very low amount of soluble iron in Sunfish Lake water (Table 1)

is unusual for an inland meromictic lake (cf. Hutchinson, 1957), althoughferrous iron is completely absent in the monimolimnion of Lake Mary, Wis-

consin, a biogenically meromictic lake (Hutchinson, 1957). In both casesiron is presumably deposited in the sediments as FeS. Conversely, the con-

centration of TDS (Table L) is higher tl-ran in either Lake Mary or Stewart'sDark Lake and comparable to Round and Green lakes, New York (Likens,

MS. 1967). The increase of total dissolved solids in the mixolimnion and the

decrease in the monimolimnion between June 5 and December 5, 1968' is

probably the result of partial autumn circulation into the monimolimnion.

The occurrence o{ oxygen in the monimolimnion did not always correspondto periods of deepest circulation. The oxygen may may have entered the

lake via subsurface springs, but the irregular nature of the partial venti lation

is unexplained. Turano and Rand (MS, 1967) found a concentration of about

I mg/liter in the monimolimnion of Green Lake. The low concentrationsof oxygen in Sunfish Lake rvere sometimes associated rvith H2S. Sorokin(1966) described a similar occurrence in meromictic Lake Belovod and discussed

the conditions under u'hich it mav occur.

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DIJTHIE . {ND C- \RTER: MEROMIXIS OF SUNFISH LAKE

The metalimnetic oxygen maximum (Fig. 2) generally extended from3 or 4 m to the upper part of the Osc,illator,ia bloom, at about 7 m. In thesummer ol 1967 the uppermost l imit of the oxygen maximum was shallorv,averaging 3-6 m. In September 1966 the lower edge of the Osc,illator,ia bloomwas in water of less than 10/6 oxygen saturation and in the presence of atleast 1 mg/liter of hydrogen sulfide. In all important respects the behaviourof. O. agardhii in Sunfish Lake is similar to that described in Myers Lake,Indiana (Eberly, 1959), and NIclish Lake, Indiana (Eberly, 1964a), bothof which have metali*-t i. ollgen maxima. Eberly Q96aa) uses the ratiomaximum depth (m)/"(area (ha) and suggests that it has to exceed 3 beforea metalimnetic oxygen maximum can develop. In Sunfish Lake this ratiois 6.94. Eberly (1964b) reviews the occurrence of metalimnetic oxygen maximain lakes of the world and their morphometric characteristics. Ninety percentof the lakes had a volume development ratio of greater than 1.00; in SunfishLake the ratio is 1.56, indicating the pronounced concavity of the lake basin.Another ratio used by Eberly is relative depth. In Sunfish Lake it is 5.15,compared u.' i th 4.16 for Mclish Lake and only 2.94 for Myers Lake (westbasin). Included in Eberly's review are at least four lakes possessing metalim-netic oxygen maxima that are also meromictic. Their relative depth ratiosvary from 1.53 to 10.21 and inversely with their area. Thus Sunfish [-akeundoubtedly is related to the class of lakes found in Wisconsin and Indianathat show metalimnetic oxygen maxima or meromixis or both. They are allof glacial origin and have a significantly higher relative depth ratio than lakesin the same area without metalimnetic oxygen maxima or meromixis. Similarlythe small size and high relative depth of Sunfish Lake make it cornparableto a group of Japanese lakes with metalimnetic oxygen maxima (Eberly,1964b).

The periods of growth of major species of phytoplankton other thanO. agardhii in Sunfish Lake (Fig. 5) were associated with circulation of themixolimnion. In every case the population declined upon, or shortly after,thermal stratification. In spite of the deep circulation and considerable ex-change across the chemocline in November-December 1966 the populationsin December 1966 and in the following spring were not so great as in the springof 1966. However, the nature of the autumn 1965 circulation was not studied.

It is improbable that phosphate is l imiting at any time, though nitratemight be. The isopleths ir 'dicate l itt le association between the amounts ofphosphate and the populations of. O. agard.hii. In the epil imnion in summerthe concentration of total phosphate pl-rosphorus never fell much below 0.01mg/liter, yet the total population of phytoplankton from lune to Septemberin the upper 3 or 4 m rarely totalled more than 10 cells/ml at any time. RecentlyRigler (1968) postulated that the concentration of inorganic phosphate inlake water is much lower than indicated by the molybdenum blue test. Ifthis is so, lack of phosphate may perhaps l imit growth in Sunfish Lake. Itis equally possible that iron or some other substance is l imiting. The mero-mixis of Sunfish Lake undoubtedlv hinders the return of nutrients from the

8.55

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856 JOURNAL FISHERIES RESEARCH BOARD OF CANADA, VOL.27, NO. 5 , 1970

monimolimnion but this alone cannot explain the low productivity of phyto-plankton in the epil imnion during the summer. The same features of morphorn-etry that contribute to the meromixis of Sunfrsh Lake must also help main-tain a very stable epil imnion, allor,ving l itt le eddy diffusion across the thermo-cline and thus preventing any return of nutrients except during the springand fall circulations.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Dr J. Sparling, Department of Botany,

University of Toronto, in carrying out most of the atomic absorption analyses. The research was

supported by a grant to H. C. Duthie from the National Research Council, Ottawa, Canada.

REFERENCES

AunnrclN PuerIc Hnelrrr Assocr.q.rron. 1965. Standard methods for the examination ofwater and waste water. 12th ed. Amer. Public Health Ass. Inc., New York, N.Y.

EeBnr-v, W. R. 1959. The metalimnetic oxygen maximum in Myers Lake. Invest. IndianaLakes Streams 5: 1-46.

1964a. Primary production in the metalimnion of Mclish Lake (Northern Indiana),an extreme plus-heterograde lake. Verh. Int. Ver. Theor. Angew. Limnol. 15: 394-401.

1964b. Further studies on the metalimnetic oxygen maximum, with special referenceto its occurrence throughout the world. Invest. Indiana Lakes Streams 6:103-139.

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Hecn CnrutcAr, CoMpANy. 1965. Water and sewage analysis procedures no. 9. Ames,Iowa. 108 p.

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TunaNo, V. S. , exo M. C. ReNn. MS, 1967. Some chemical observat ions on Fayet tevi l leGreen Lake, New York, p. 151-187. In Dante| F. Jackson [ed.] Some aspects of mero-mixis. Symp. Meromictic Lakes. Dep. Civil Eng., Syracuse Univ., Syracuse, New York.

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