Hydrobiologia 158: 253-265 (1988)J. M. Melack (ed.). Saline Lakes 253© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

Thermal stratification and the stability of meromixis in the Pretoria SaltPan, South Africa

P. J. Ashton & F. R. SchoemanNational Institute for Water Research, Council for Scientific and Industrial Research, PO. Box 395,Pretoria 0001, South Africa

Key words: Saline lake, meromictic, stratification, chemocline

Abstract

The Pretoria Salt Pan, South Africa, a small (0.076 km2), shallow (Zma = 2.85 m), hypersaline, maar lake,lies within a clearly-defined crater and is fed by a perennial, slightly saline (3 g 1-1) artesian spring. The lakehas two distinct solar-heated peaks in its temperature profile, each of these peaks located in a highly turbid(> 80 JTU) layer below a steep chemocline. The upper thermal peak, located at a depth of 10 cm, was transient,with a distinct diel pattern of diurnal heating and nocturnal cooling. The lower thermal peak, located belowa steep chemocline and centred at approximately 60 cm, was stable and showed a seasonal pattern of winterheating (maximum: 38.5 °C) and summer cooling (minimum: 27.4 °C). The unusual bathymetry of the lake,combined with the sheltering effect of the crater rim and steep salinity gradient between the surface (30- 80 g1- I) and bottom water (280-310 g 1-1) prevented windmixing of surface waters beyond a depth of approxi-mately 50 cm. During a 28 month study all water deeper than 55 cm remained anaerobic, and the lake appearedto be meromictic.

Introduction

Meromictic lakes, i.e. lakes that undergo partial orincomplete vertical mixing of the water column dur-ing an annual cycle, are comparatively scarce, butnevertheless are distributed throughout the worldand have been recorded from every continent(Hutchinson, 1957; Walker & Likens, 1975). Themajor characteristic of a meromictic lake is the pres-ence of a monimolimnion (Findenegg, 1935): a low-er, often more saline, anoxic layer that does not mixwith the upper or surface waters of the lake. The up-permost portion of the water column, the mixolim-nion (Hutchinson, 1957), may develop thermalstratification and circulates completely for at leasta portion of the year (Walker, 1974). The mixolim-nion and monimolimnion are separated by a transi-tional zone, the chemocline (Hutchinson, 1957).

Meromictic lakes possess an intrinsic resistance toholomixis which is brought about by complex inter-actions between the pattern of density stratificationand morphometric, climatic, hydrodynamic and bi-ological factors (Walker, 1974; Maclntyre & Melack,1982). The degree of resistance to complete mixingis, in effect, equivalent to the stability of the waterbody. Originally, Schmidt (1928) defined stability asthe minimum amount of work which the wind mustperform to transform a given stratified condition ina water column to a condition of uniform density,without loss of heat or solute. This concept of stabil-ity was elaborated by Hutchinson (1937) for the spe-cial case of chemically stratified meromictic lakes asthe minimum work required to mix a chemicallystratified lake, devoid of thermal stratification, touniform concentration. This definition pre-supposes that the thermal contribution to stability

254

is relatively small in these lakes. However, this is notgenerally so (Walker, 1974) and it is therefore usefulto distinguish between the contributions of the ther-mal and chemical components of total stability(Walker, 1974; MacIntyre & Melack, 1982).

Several meromictic lakes have been reported fromthe African continent (Beadle, 1966; Walker &Likens, 1975; MacIntyre & Melack, 1982) and, un-doubtedly, many more examples await descriptionand study. African meromictic lakes range in sizefrom the large, deep, freshwater Rift Valley lakes ofeast and central Africa to the small, saline craterlakes of eastern Africa. Recently, Ashton & Schoe-man (1983) reported preliminary limnological obser-vations on the Pretoria Salt Pan, the only salinecrater lake in southern Africa. This hypersalinemaar lake is one of the shallowest meromictic lakesso far reported (Walker & Likens, 1975; King & Tyler,1981). The position of the chemocline did not changeduring a 28-month study, indicating a high degree ofmeromictic stability. In this paper we describe theseasonal patterns of thermal and chemical stratifica-

tion and assess the factors that have contributed tothe development and maintenance of meromixis inthis lake.

Materials and methods

The study area - The Pretoria Salt Pan is locatedat 25°24'S and 280 05'E, approximately 50 kmnorth of the city of Pretoria in the Transvaal Prov-ince of South Africa. The lake is small (0.076 km2

in area) and lies at an altitude of 1045 m above sealevel (Wagner, 1922) within a clearly defined crater(Fig. 1). The crater rim is approximately circular inoutline, 1.1 km in diameter and varies in height be-tween 75 and 123 m above the high water level(Ashton & Schoeman, 1983). Wagner (1922)described the regional geology, and Ashton & Schoe-man (1983) have summarized the vegetation, soilsand climate of the crater and its surroundings as wellas morphometric features of the lake. The presentlake occupies an abandoned salt digging which was

Fig. 1. Aerial view of the Pretoria Salt Pan and the surrounding area. The artesian spring is located at the end of the promontory thatjuts into the lake from the north-western shore.

255

flooded in the early 1930's when one of the manyboreholes that were sunk struck a perennial sourceof water. This borehole now acts as an artesianspring and supplies, on average, 26000 m3 of slight-ly saline (3 g 1- l) water per year to the lake. This isapproximately 15% of the total water inflow(Ashton & Schoeman, 1983), the remainder beingderived from direct rainfall (27%) and runoff fromthe inner walls of the crater (58%). The lake is aclosed system and water is lost only via evaporation(approximately 175000 m3 per annum; Ashton &Schoeman, 1983).

The Pretoria Salt Pan is located within the high-veld summer rainfall zone (Schulze, 1965). Represen-tative climatic data for a five year period(1975-1980) from the Roodeplaat HorticulturalResearch Station 24 km south east of the PretoriaSalt Pan, one of 87 weather stations in this rainfallzone, are summarized in Fig. 2. Rainfall is highly er-

3

0

t, Ca

E.LE

20-

0-

I . .I I I I I I I I I I I I

EE

a

0.a0

0

.5.a

Fig. 2. Summarized annual climatic data for the RoodeplaatHorticultural Research Station. A: Average monthly values formaximum, mean and minimum air temperatures; B: Averagemonthly values for rainfall (solid histogram) and evaporation(open histogram). Vertical bars indicate range of evaporationvalues; numbers above each rainfall bar indicate average numberof days each month that rainfall was recorded. (Calculated fromunpublished data for the five-year period, 1975-1980).

ratic and seasonal giving rise to distinct wet and dryseasons; highest rainfalls are received as individualstorms during the austral summer months. Mini-mum rainfall and evaporation values are recordedduring the winter months, May to August.

A bathymetric map of the lake (Fig. 3) was drawnfrom a series of soundings taken with a weighted lineat 1 m intervals along 28 transects across the lake.From these measurements, hypsographic relation-ships between depth, area and volume were derivedby planimetry. Monthly variations in lake depth wererecorded at a fixed marker and changes in area andvolume were calculated from the hypsograph plots(Fig. 4).

Sampling and field measurements - Samples ofthe artesian spring and lake waters were collected atintervals of approximately 30 days for a period of 28months from February 1978 to May 1980. In the lake,samples were collected from 5 cm below the watersurface and then at 20 cm intervals to within 10 cmof the bottom sediments at the deepest point(Fig. 3). All lake samples were collected with a 3 litrecapacity, horizontally-mounted, opaque PVC VanDorn sampler. Water samples were stored in 2 litrecapacity polyethylene bottles on ice in the dark untilreturn to the laboratory. Sample processing andanalysis commenced within 24 hours of collection.

At the deepest point in the lake, water tempera-tures were measured at 5 cm intervals from the sur-face to the bottom with a Yellow Springs Model 57thermistor probe. pH was measured in the field witha portable Metrohm Model E-444 pH meterequipped with a glass electrode. Alkalinities werealso measured in the field by titration with 0.5 NHCI to a pH end point of 4.2. The extinction of pho-tosynthetically available radiation (400 to 700 nm)was measured with a Lambda Instruments ModelLI-185 quantum photometer. Readings were taken at5 cm intervals until total extinction was obtained.Measurements of wind speed within and outside thecrater were made on 21 of the 28 sampling visits us-ing a hand-held Apex Instruments Model AN-3anemometer and stopwatch. The South AfricanWeather Bureau provided unpublished data on airtemperatures, rainfall, evaporation and standard(10 m tower) wind speeds from the nearest weatherstation, the Roodeplaat Horticultural Research Sta-

L A. I

i

1

256

Fig. 3. Bathymetric map of the Pretoria Salt Pan. Contour intervals are in centimetres from the high water mark ( o ).

tion, located 24 km south east of the Pretoria SaltPan.

Density measurements and chemical analyses-In the laboratory, 250 ml aliquots of each watersample were transferred to glass measuring cylindersand placed in a Conviron constant temperature cabi-net at a temperature of 20°C + 0-1 °C for 6 hours.When the temperature of these water samples hadstabilized at 20 C, the density of each sample wasread using a series of Astell hydrometers (AstellLaboratory Services, London). These hydrometerswere certified accurate to 0.001 g cm - 3 . Periodicchecks of hydrometer accuracy were made usingstandard 50 ml capacity density bottles. Fifty mil-lilitre capacity density bottles were also used tomeasure the densities of water samples containing arange of solute concentrations at temperatures of 20,25, 30 and 35 °C. Measurements were made when thetemperature of the samples and density bottles hadstabilized at the appropriate temperature, usually af-ter 12 hours in a Conviron constant temperaturecabinet. The results were corrected for the effects ofthermal expansion and plotted graphically. Densi-

ties of water samples at intermediate temperaturesand solute concentrations were obtained by interpo-lation.

Prior to chemical analysis, all water samples werefiltered (Whatman GF/C) and, because of their highsolute content, diluted with deionized distilled waterto give final dilutions of 1/100 and 1/1000. Allchemical analyses for the major cations and anionswere carried out by the Water Quality Division of theNational Institute for Water Research using Techni-con AutoAnalyzers; the methods used have beendescribed in detail by Ashton & Schoeman (1983).Water analyses were within 5% of charge balance.Salinity was calculated as the sum of the major ionicconstituents (Hutchinson, 1957) and total dissolvedsolids (TDS) were determined gravimetrically afterevaporating a filtered (Whatman GF/C) sample todryness at 95 °C for 7 days.

Calculation of total heat content, stability andwind work - The total heat content of the lake wascalculated using the hypsographic relationships(Fig. 4) and thermal profiles according to Hutchin-son (1957). The stability equations introduced by

0 50 100 150 200I I I I I metres

257

(H.W.L.) 28

25

20

E

C0)E

u0

C1Q.0)

C10

SUMMER

% of area or volume at High Water Level (H.W.L.)

Fig. 4. Hypsographic plots of volume and area versus depth for the Pretoria Salt Pan. The seasonal changes in area and volume dueto evaporative water loss are also shown.

Schmidt (1928) and Hutchinson (1937, 1957) are wellknown. Following Walker (1974), we have equatedstability (sensu Schmidt & Hutchinson) with totalstability, and we have partitioned it into thermal sta-bility; that due to thermal stratification, and chemi-cal stability: that due to chemical stratification. Sta-bility values for the Pretoria Salt Pan (a closedsystem) were calculated using the equation of Idso(1973), which also provides an informative stability-depth plot. Chemical stability was computed assum-ing the temperature at each depth to be 20°C andthermal stability was computed as the difference be-tween total stability and chemical stability (MacIn-tyre & Melack, 1982). The values for stability can beconverted to units of work per unit area by includingthe gravitational constant, g. This permits direct

comparison with integrated values of windpower(e.g. Stauffer, 1980; MacIntyre & Melack, 1982;Ward, 1982).

Birge (1916) defined the work of the wind, WB, asthe amount of work per unit area required to pro-duce the observed temperature stratification patternby mixing the solar energy supplied at the surfacethroughout the lake. It was calculated using theequation of Idso (1973). Following MacIntyre & Me-lack (1982), we substituted piz, the initial density ateach depth in early morning before the onset of ther-mal stratification of the surface waters, for Pi. Thisallowed comparison of values for the work of thewind calculated at different times of the day. At eachdepth, the work of the wind was calculated accord-ing to MacIntyre & Melack (1982).

15

I

258

Results

Morphometry

At high water level, the lake has a maximum lengthof 350 m (transect A-A1; Fig. 3), a width of 275 mand is 0.076 km 2 in area. At this level, the maxi-mum depth of the lake is 2.85 m and the mean depthis 0.49 m. The development of volume and the rela-tive depth (sensu Hutchinson, 1957) are 0.17 and0.92%, respectively. The unusual bathymetry is dueto the topography of the abandoned salt diggings,where two access routes (visible as troughs on eitherside of the promontory) enter the flat-bottomeddeepest section.

Thermal features

An isotherm plot for the deepest portion of the lakeduring the 28 month study period is shown in Fig. 5.Diel patterns of thermal stratification and destratifi-cation in the surface waters (0-40 cm) during theaustral summer months (November to February)complicated construction of the isotherm plot andhave been omitted from Fig. 5 to improve clarity.They are described in detail later. During the winter

285

250

4)c

EOo

.0o

a

200

150

100

50

O

months, surface waters were usually isothermal.Within the chemocline, water temperatures rose toa maximum of 38.6°C in spring and decreased to aminimum of 27.4°C in autumn. The temperaturesof the bottom waters immediately above the sedi-ments varied by less than 0.3 degrees Celsius duringthe year. The maximum temperature zone shown inFig. 5 was located between 2.2 and 2.4 m above thesediments and remained remarkably constantthroughout the study. This zone was positionedwithin the deeper portion of the lake, immediatelybelow the junction between the deeper section andthe shallower peripheral regions (Fig. 6). This maxi-mum temperature zone gives rise to a persistentmesothermal temperature profile in the deepest por-tion of the lake (Fig. 7).

The interplay of temperature and salinity to createthe observed density profiles in the Pretoria Salt Panis shown in Fig. 7. Reduced surface temperaturesduring winter (July), combined with increased salin-ities due to evaporation, reduce the density gradientbetween mixolimnion and monimolimnion. Sum-mer inputs of rainfall and runoff decrease the salini-ty and density of the mixolimnion, steepening thedensity gradient.

Typical summer and winter light energy profilesare shown in Fig. 8. Light extinction was extremely

-- 2-- 2

FIM I A I I .1 .I I NI 1 F MIA MI I J I A IS IOI N I D I J I FI M A I M

285

250

200

150

100

50

0

1978 1 1979 i 1980Fig. 5. Isotherm plot for the Pretoria Salt Pan from February 1978 to May 1980. Summer (November to February) isotherm lines forthe surface waters (0-40 cm) have been omitted for clarity.

- .~

24

262

2- \24

22

26 ~ ~ ~ 2

24

I . .'.' ' I ' - I . I- I- .. . .- .- . -.- .-

259

SUMMER E--292WINTER ' 272-

< MM TMPERATUE ZONE _....- - - - 200,

,, i\ ZOO ,,

\o uu

I- I === = 10 8o

o0

Fig. 6. Portion of bathymetric profile along transect A-Al

(Fig. 2) to show location of maximum temperature zone.

p (g cm- 3 )

1.040 1.080 1.120 1.160a ,_, __ __ . . .

rapid during summer as a result of high turbiditiesand phytoplankton densities, and total light extinc-tion occurred within 40 cm. In winter, lower turbidi-ties and phytoplankton densities permitted light topenetrate to approximately 50 cm before total ex-tinction occurred. However, the lower water levelsduring winter allowed some light to penetrate themaximum temperature zone. Secchi disc transparen-cies varied from 7 cm in summer to 19 cm in winter.

The surface waters of the lake (0-40 cm) were

1.200I I I ~.... . I I T ('C

Fig. 7. Profiles of density (p), salinity (S) and temperature (T) in the Pretoria Salt Pan for one annual cycle.

260

Irrodionce, PAR (E.m - 2 . s )

ETC

Ef)(nU,

Go

a

240

ZONE OF MAXIMUMTEMPERATURE

220 -

Fig. 8. Light energy profiles for late winter (August) and summer(January). Horizontal bars indicate water level on each samplingdate.

usually isothermal in the early morning and intensethermal stratification developed in the upper 15 cmduring calm summer days (Fig. 9). Water tempera-tures at a depth of 5 - 10 cm reached maximal valuesof 35-37 °C by 16h00 and then cooled during thelate afternoon and night, becoming isothermal oncemore by early morning. In summer, diurnal periodsof wind mixing in the surface waters occasionallygave rise to isothermal water temperatures.

Profiles of water temperature, total dissolvedsolids (TDS) and total stability for August 1979 andFebruary 1980 are compared in Fig. 10. The temper-ature profiles indicate that the temperature of thebottom water layers remained virtually constant,while large changes in temperature occurred in themixolimnion and in the upper monimolimnetic

L.

3- E9 c)

. 0

.4

0ctu

22

2

3

25 30 3

waters, the zone of maximum temperature. The TDSprofiles indicate no significant depression of thechemocline and only slight changes in TDS in themonimolimnion. The relatively large change in TDSwithin the mixolimnion are due to the inflow of lesssaline runoff and rainfall in summer. The profiles oftotal stability also indicate no depression of thechemocline. The increased stability values for thechemocline and upper monimolimnetic waters areagain due to the presence of less saline waters andlower water temperatures in the mixolimnion. In themonimolimnion, the relatively constant TDS gra-dient suggested that the water in this zone had notmixed.

The total heat content of the lake was lowest dur-ing autumn (April - May) each year and rose rapidlyduring winter to a spring (September) maximum(Fig. 11). This was followed by a short period of rap-id heat loss during early summer, a more gradual lossof heat during mid-summer and a final rapid loss ofheat during early autumn.

Values of total (chemical plus thermal) stabilityalso followed a distinct annual pattern (Fig. 11). Thelowest values were recorded during winter(June -August) and the highest values were found inlate summer (February) each year. The generally lowvalues for total stability (range = 60470 to 75750ergs cm-2) reflect the shallowness of the lake. Thevalues for chemical stability alone were greater thanthose for total stability, indicating that the thermalcomponent decreased the total stability of the water

Temperature (C)5 25 30

Fig. 9. Diel record showing diurnal heating and nocturnal cooling of surface waters during summer (29 December 1979) at the PretoriaSalt Pan.

0

5

10

15

20

25

30I

._._ .. .... . I

I I

- - - - - - · ·

1

Temperature (C)

EU

va

4o0o0

0

TDS (gt -) S (g-cm cmI1 cm 2)

Fig. 10. Water temperature, total dissolved solids (TDS) and total stability (S) profiles for August 1979 (solid lines) and February 1980(dotted lines) in the Pretoria Salt Pan.

" 1500IE

o 1400

c 13000o

a' 1200

o

F- 1100

80

N

E0E

70 0

6)60

r 'm''Ma s ''N 'J' F 'MIAIMI J JAIS'OIND I J FIMIAI' M1978 1979 1980

Fig. 11. The variation in total heat content above 0 °C and total (chemical plus thermal) stability in the Pretoria Salt Pan from February1978 to May 1980.

column. This is highlighted in Table 1 which summa-rizes the seasonal variations in heat content and sta-bility values over two annual cycles. The negativevalues for thermal stability are a result of the solar-heated mesothermal profile. Since the mesothermalprofile persisted throughout the study, early morn-

ing thermal stability values were usually negative.Diurnal changes in the values of total heat con-

tent, thermal stability and work of the wind at fourtimes of the year are summarized in Table 2. Thesevalues are representative of the diurnal rangesrecorded at the Pretoria Salt Pan. On each occasion,

261

__ - - - - - - - -II. . .. .- ..- .- - . . .

262

Table 1. Maximum and minimum values for heat content (HC:cal cm- 2), total stability (S: ergs cm- 2 ), chemical stability (S,:

ergs cm- 2) and thermal stability (S.: ergs cm- 2) during 1978 and1979 at the Pretoria Salt Pan. (Thermal stability values calculatedas the difference between total and chemical stability values).

1978 1979

minimum maximum minimum maximum

HC 1104.8 1464.7 1114.8 1492.0S 60470 73280 61860 74480

Sc 61980 73670 63190 74970

St - 1510 -390 - 1330 -490

with the change in heat content since early morning

WB = 4.5 AHC-15.2 (r2 = 0.99).

On 21 sampling visits, wind speeds were measuredin the crater at a height of 2 m above water level andat the lowest point of the crater rim, 75 m abovewater level. On these occasions, wind speeds at thecrater rim varied between 0.4 and 8.1 m sec- 1 and,in all cases, wind speeds within the crater were be-tween 8 and 15% of the values recorded at the rim.

Discussion

Table 2. Examples of diurnal changes in heat content (HC: calcm-2 ), thermal stability (St: ergs cm- 2), and work of the wind(WB: ergs cm-2) at the Pretoria Salt Pan.

Date Time HC St WB

20 Aug. 1978 07h50 1357 -935 013h00 1389 -632 10815h45 1401 -494 157

23 Nov. 1978 07h20 1307 - 357 012h50 1383 431 33416h10 1421 875 491

17 Feb. 1979 07h00 1262 427 013h10 1343 1195 36316h00 1370 1458 471

21 May 1979 07h30 1129 - 879 012h25 1149 -707 8816h00 1157 -633 128

thermal stability and the values for work of the windwere lowest in the early morning and increased withthe increase in heat content, reaching a maximum inthe late afternoon. Ninety-nine percent of the vari-ance in the change of thermal stability since earlymorning was explained by a regression on change inheat content

AS t = 10.52 AHC-39.1 (r2 = 0.99)

where ASt is the change in thermal stability andAHC is the change in heat content. Values for workof the wind (WB) were also very closely correlated

The chemical stability of the Pretoria Salt Pan(6.0-7.6 x 104 ergs cm- 2 ) is lower than that ofmost other meromictic lakes, the exception beingLake Sonachi, Kenya (see Table 6 in Maclntyre &Melack, 1982). The development and maintenanceof meromixis in the Pretoria Salt Pan is regulated byseveral factors. These include: lake basin and cratermorphometry, a constant inflow of less saline springwater at the lake surface, the diurnal periodicity ofthe winds and thermal stratification and seasonalchanges in rainfall.

Walker & Likens (1975) suggested that both catch-ment topography and lake basin morphometry con-tribute to the development and maintenance ofmeromixis in a lake. This concept was expanded forcrater lakes by Melack (1978), who suggested that theratio D/H, where D is the maximum diameter (fetch)of the lake and H is the minimum height of the craterrim above it, provided an appropriate index of theexposure of a crater lake to wind mixing; the smallerthe ratio, the more protected is the lake. The low in-dex of exposure (4.67 during summer when the lakewas full), indicated that the crater rim provided alarge measure of protection from the prevailingwinds. This effect was enhanced during winter, whenevaporative water losses reduced the fetch and sur-face area of the lake and the exposure index fell toapproximately 3.3 during August and September.These values for the exposure index are amongst thelowest that have been recorded (Melack, 1978;MacIntyre & Melack, 1982). In contrast to thosecrater lakes with low exposure indices listed by Me-

263

lack (1978) and MacIntyre & Melack (1982), thePretoria Salt Pan exhibits intense chemical stratifi-cation. Though the relative depth of the PretoriaSalt Pan (0.92%) is one of the lowest on record andreflects unusual bathymetry, it is unlikely that un-derwater morphometry alone was sufficient to pre-vent the lake from mixing completely. Nevertheless,the location of the maximum temperature zonewithin the deeper, central portion of the lake (Fig. 6)suggested that lake morphometry may have assistedin preventing complete mixing.

Wind data for a five-year period (1975-1980)from the Roodeplaat Agricultural Research Stationare shown in Fig. 12. Averaged over a 24-hour peri-

I1, 200o':

E_ 150c

3 100

'o 50

0)

-1

c0'

_ O

o

. 40

0 20

0a

0c

- MAY, JUNE, JULY--- = OCTOBER, NOVEMBER,

DECEMBER

29

23

1410 12

4 '_ 5-- .L-_-qI I I I I

05 10 45 20 25 30 35 4.0

Average wind speed (m sec- )

4'5

Fig. 12. Histograms of average daily wind run each month (upperdiagram) and relative frequency of wind speeds during summerand winter (lower diagram) at the Roodeplaat HorticulturalResearch Station, Transvaal. Vertical bars in upper diagram indi-cate range of values, while horizontal bars indicate the summerand winter months examined in lower diagram. In the lower dia-gram, numbers refer to number of days in each three-month peri-od. (Calculated from unpublished data for the five-year period,1975-1980).

od, daily wind speeds over the surrounding country-side are low and rarely exceed 2.3 m sec - . Howev-er, average wind speeds (i) have a definite periodicityduring a diel cycle and follow a pattern similar tothat recorded for Lake Sonachi, Kenya (MacIntyre& Melack, 1982). Wind speeds are low at night(f < I m sec - ) and begin to increase gradually fromapproximately 08h00. From mid-morning to mid-afternoon (10h30 to 16h00), wind speeds are general-ly between 2 and 4 m sec t- , with the highest valuesusually recorded in mid afternoon. Wind speeds de-crease during the late afternoon and early evening.Intermittently during the day, wind speeds increaseand may average between 5 and 8 m sec - for peri-ods of up to one hour. On occasion, wind speeds mayincrease suddenly and gusts of up to 18 m sec- 'have been recorded. These events are rare, however,and usually of less than one hour's duration. Withinthe crater, average wind speeds are usually very low(A < 1 m sec- ) for most of the day. Air flow withinthe crater is gusty with several changes in direction.The highest wind speed measured in the crater (1.5 msec-t, 2 m above water level) was recorded duringmid-afternoon in November 1979 at a time whenwind speeds on the crater rim averaged 12 m sec- Ifor a period of two hours. The energy provided bythis wind mixed the upper 50 cm of the lake toisothermy at 16h00, the only occasion that this fea-ture was observed during the study. Wind speedsmeasured over the surface of the Pretoria Salt Panare very much lower than those reported by MacIn-tyre & Melack (1982) for Lake Sonachi, Kenya, andemphasize the high degree of protection afforded bythe crater.

The effect of the wind on the lake surface isreduced by the diurnal pattern of stratification in thesurface waters. Thermal stability, the work neededto return a thermally stratified lake to isothermy, isclosely correlated with the total heat content whichis maximum in the late afternoon each day. Thoughdiurnal increases in thermal stability were low, theywere still sufficient to resist mixing in the surfacewaters. Nocturnal sensible heat losses returned thesurface waters to isothermy and stable mesothermaltemperature profiles were recorded after dark on sev-en occasions. These provide further evidence for thestability of stratification and shallowness of mixing.

264

Deeper mixing of the surface waters could occur ifconvective heat losses were augmented by wind mix-ing. However, wind stresses between 20h00 and08h00 are usually low (<0.1 dynes cm-2) and areinsufficient to extend the mixing depth. The time re-quired to change the lake from a stratified to ahomogeneous condition as a result of wind mixingwas calculated according to Ward (1982) as the quo-tient of the potential energy of stratification and thepower from the wind available for mixing the watercolumn (Table 3). Sustained wind speeds greaterthan 1 m sec-I are rare within the crater, and it istherefore highly unlikely that meromixis will be de-stroyed by wind mixing alone.

The values for the power of the wind presented inTable 3 are those for the summer months when thesurface area of the lake is greatest. During winter, thesurface area of the lake is often 30-35% smallerthan in summer. Correspondingly less wind powerwould be available for mixing the lake and the timesrequired to affect complete mixing would be longer.The lowest stability values at the Pretoria Salt Panare recorded during winter when the prevailing windspeeds are clearly insufficient (Fig. 12; Table 3) tocompletely mix the water column. Increased stabili-ty values in early summer (Fig. 11) are sufficient tooffset the seasonal increases in wind speeds (Fig. 12).

In contrast to many other meromictic lakes (Walk-er, 1974; King & Tyler, 1981; MacIntyre & Melack,1982; Steinhorn, 1985) the Pretoria Salt Pan gainsmost heat during winter and the total heat contentof the lake is maximal during the austral spring(Fig. 11). At the same time, the warm lens within thechemocline reaches its maximum temperature in thespring and then cools during summer (Fig. 5). This

Table 3. The duration and speed of lake surface winds required

to completely mix the Pretoria Salt Pan.

Wind velocity Power available Time(m sec- ) for mixing required

(Watts) (days)

12 283 0.346 35.4 2.713 4.4 21.841 0.2 480.50

is brought about by the deeper winter heating per-mitted by lower mixolimnetic turbidity (Fig. 8) andheat is stored in the chemocline. In summer, however,higher mixolimnetic turbidities ensure shallow heatpenetration and consequent storage closer to the air-water interface where it is easily lost by nocturnalconduction (Fig. 9). Greater wind-mixing of themixolimnion would transport summer heat deeperinto the lake and possibly prevent the above two fea-tures from occurring.

The Pretoria Salt Pan is most stable during sum-mer (Fig. 11) because the surface waters arefreshened by rainfall and runoff (Fig. 2) and thetemperature difference between the mixolimnionand the chemocline is least at this time (Fig. 7). Dur-ing winter, when the Pretoria Salt Pan is least stable(Fig. 11), the density gradient could become even lessstable if an unusually cold year occurred or if evapo-ration losses were excessive. Under these conditions,the lake might not undergo full turnover, but thechemocline could deepen. However, the range of cli-matic fluctuations that characterize the wintermonths (Fig. 2) suggest that this is unlikely to occur.

Based on the range of wind speeds recorded in thecrater and their power available for mixing, it is likelythat the Pretoria Salt Pan became meromictic quitesoon after filling for the first time (during the early1930's). The continuous inflow of less saline (3 g1-1) spring water contributes some 78 t of salts tothe lake each year and the salinity of the lake wouldhave risen rapidly after the initial filling phase as aresult of evaporative concentration. More impor-tantly, the continued spring inflow and the inflowsof summer rainfall and runoff combine to maintainthe difference in densities between mixolimnion andmonimolimnion and regulate the stability of thewater column.

The relatively slight changes in TDS concentra-tions in bottom monimolimnetic waters during thestudy suggest that much of the incoming salt loadmay be lost to the sediments by precipitation. Thisimplies that the chemical constituents in themonimolimnion may be at or near saturation levelsand are unlikely to undergo marked changes underthe present conditions. Data collected on a subse-quent visit to the lake on 11 August 1984, four yearsafter the completion of this study, support the

265

hypothesis. On this occasion, monimolimnetic con-centrations of the major cations and anions differedby less than 3o from the values recorded in May1980. However, conclusive proof of the validity ofthe hypothesis must await the collection and analysisof suitable sediment cores.

Acknowledgements

Thanks are due to C. P. Albertyn of the South Afri-can Weather Bureau for the provision of climaticdata, J. J. Erasmus of the Soutpan Agricultural Ex-perimental Farm for providing facilities at thePretoria Salt Pan and G. R. Batchelor for assistancewith the collection of field data. We are indebted toS. MacIntyre and J. M. Melack for their construc-tive criticism of the manuscript. This paper is sub-mitted for publication with the permission of theNational Institute for Water Research.

References

Ashton, P. J. & F. R. Schoeman, 1983. Limnological studies onthe Pretoria Salt Plan, a hypersaline maar lake. 1. Morphomet-ric, physical and chemical features. Hydrobiologia 99: 61-73.

Beadle, L. C., 1966. Prolonged stratification and deoxygenationin tropical lakes. I. Crater Lake Nkugute, Uganda, comparedwith lakes Bunyoni and Edward. Limnol. Oceanogr. 11:152-163.

Birge, E. A., 1916. The work of the wind in warming a lake. Trans.Wis. Acad. Sci. 18: 341-391.

Findenegg, I., 1935. Limnologische Untersuchungen im KartnerSeengebiete. Ein Beitrag zur kenntnis des Stoffhaushaltes InAlpenseen. Int. Revue ges. Hydrobiol. 32: 369-423.

Hutchinson, G. E., 1937. A contribution to the limnology of aridregions. Trans Connecticut Acad. Arts Sci. 33: 47-132.

Hutchinson, G. E., 1957. A Treatise on Limnology. 1. Geography,Physics and Chemistry. John Wiley, New York. 1015 pp.

Idso, S. B., 1973. On the concept of lake stability. Limnol.Oceanogr. 18: 681-683.

King, R. D. & P. A. Tyler, 1981. Meromictic lakes of South-westTasmania. Aust. J. mar. Freshwat. Res. 32: 741-756.

MacIntyre, S. & J. M. Melack, 1982. Meromixis in an equatorialAfrican soda lake. Limnol. Oceanogr. 27: 595-609.

Melack, J., 1978. Morphometric, physical and chemical featuresof the volcanic crater lakes of western Uganda. Arch. Hydrobi-ol. 84: 430-453.

Schmidt, W., 1928. Uber Temperatur und Stabilitatsverhiltnissevon Seen. Geogr. Ann. 10: 145-177.

Schulze, B. R., 1965. Climate of South Africa. Part 8, GeneralSurvey. Government Printer, Pretoria. 330 pp.

Stauffer, R. E., 1980. Windpower time series above a temperatelake. Limnol. Oceanogr. 25: 513 -528.

Steinhorn, I., 1985. The disappearance of the long term meromic-tic stratification of the Dead Sea. Limnol. Oceanogr. 30:451- 472.

Wagner, P. A., 1922. The Pretoria Salt Pan - A Soda Caldera.Geol. Surv. Memoir 20. Govt Printer, Pretoria, 136 pp.

Walker, K. F., 1974. The stability of meromictic lakes in centralWashington. Limnol. Oceanogr. 19: 209-222.

Walker, K. F. & G. E. Likens, 1975. Meromixis and a reconsideredtypology of lake circulation patterns. Verh. int. Ver. Limnol.19: 447-458.

Ward, P. R. B., 1982. Physical limnology. In Thornton, J. A. (ed.)Lake McIlwaine: the Eutrophication and Recovery of a Tropi-cal African Man-made Lake. Monographiae Biologicae Vol.49: 23- 34. Dr W. Junk, The Hague.