Biogenic Meromixis and Stability in a Soft-Water Lake

Biogenic Meromixis and Stability in a Soft-Water LakeAuthor(s): David A. CulverSource: Limnology and Oceanography, Vol. 22, No. 4 (Jul., 1977), pp. 667-686Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2835250 .

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Biogenic meromixis and stability in a soft-water lake'

David A. Culver2'3 Department of Zoology, University of Washington, Seattle 98195

Abstract Hall Lake, Washington, was studied during a period of meromixis (1950-1953) and

during a period of holomixis (1968-1972). Comparisons of thermal cycles, light penetration, chemical distribution, and productivity indicate that the lake had a greater productivity while meromictic, even though the seasonal accumulation of dissolved materials in the deep waters was greater during the holomictic period. Calculations showed that the stability due to chemical stratification was actually greater during the holomictic period, supporting the idea that in biogenic meromictic lakes circulation may be controlled more by thermal phenomena than by chemical stratification. Turbid inflows during winter tended to destroy meromixis, but low transparencies due to algal blooms in summer tended to encourage it and may be the primary way by which biogenic meromictic lakes become permanently stratified.

In this paper I use data collected from Hall Lake, Washington, a soft-water lake that was studied during both a meromictic phase (1950-1953) and a holomictic phase (1968-1972), to examine the question of how a biogenic meromictic lake differs from a holomictic lake that undergoes seasonal oxygen depletion in the hypolimnion.

Hall Lake, Snohomish County, Washing- ton (47?49'N, 122?18.5'W), is a small (Ta- ble 1), soft-water lake located on the coastal plain east of Puget Sound, 19 km north of Seattle. It lies in a kettle created during the last advance of the Wisconsin glacial period, which would make the lake about 12,000-13,000 years old. A set of cores taken in 1969 with a Livingstone corer yielded a 14C age of 11,780 + 160 years B.P. for organic matter in contact with late-

' This work was supported by U.S. Atomic En- ergy Commission contract AT-(45-1) -2225-T23 to W. T. Edmondson and by a National Science Foundation predoctoral fellowship.

2I am grateful to W. T. Edmondson and G. C. Anderson for permission to use their extensive un- published data on Hall Lake for 1950-1953. C. S. Munch, K. F. Walker, and members of Dr. Ed- mondson's staff helped in numerous ways, particularly with fieldwork and chemical analyses between 1969 and 1971. This paper comprises part of a thesis submitted in partial fulfillment of the requirements for a Ph.D. degree at the University of Washington.

'Present address: Department of Zoology, 1735 Neil Avenue, Ohio State University, Columbus 43210.

glacial clays (Y-2653: M. Stuiver pers. comm.).

Hills (30 m high) provide wind protection from all directions except north, which makes the lake particularly prone to stratification, given its small fetch (220 m). At no time did I observe waves of greater than 10-cm amplitude. The lake drains via Hall Creek into Lake Ballinger, about 3 km southwest, which in turn drains into the north end of Lake Washington.

The environs of the lake are suburban, and numerous storm drains make the topographic watershed a poor indicator of the hydrologic watershed, which is about 120 ha. The area was homesteaded in the 1880s and a sawmill was established in 1905; logs floating on the lake contributed a significant amount of bark and other organic matter, which can still be seen as a discrete layer in sediment cores. Construction of a railroad extension to the lake in 1907 ac- celerated the pace of clearcutting around the lake. The last virgin stands were cut by 1917 (G. Hunter pers. comm.).

The first houses were built next to the lake in 1944. Progressive development of the watershed can be seen from aerial photographs taken in 1947, 1961, 1967, and 1974 (Fig. 1). Of particular importance is the disturbance created in 1963-1966 by construction of Interstate Highway 5 and widening in 1968-1969 of 212th St. SW next to the lake. A trunk sewer was also con-

LIMNOLOGY AND OCEANOGRAPHY 667 JULY 1977, V. 22 (4)



668 Culver

Table 1. Morphometric parameters, August 1968. Lake outline was obtained from aerial pboto 3 in Fig. 1.

Surface area (AO) 31.125 x 103 m2 (at mean water level) (3.1 ha)

Total Volume tV) 288.16 x 103 m3

Maximum depth (zmax) 16.2 m

Mean depth (V/Ao 9.26 m

Length 220 m

Width 165 m

Length of shorel ine 670 m

Shoreline development 1.08

Volume development (t/zmax) 0.572

Depth of center of gravity 6.96 m for isopycnic condition

structed in 1968, intercepting sewage that had previously been treated by individual septic tanks which in some cases drained into the lake.

Methods

For comparison of the two periods of the lake's history, it was necessary to establish that there had been no changes in morphometry, wind protection, basic water chemistry, climate, or biota. General de- scriptive data were collected for the lake, as was seasonal information on the vertical stratification of physical and chemical characteristics of the water column. A bathy- metric map was constructed from a series of handline measurements made in 1968 (Fig. 2) and an outline taken from the 1967 aerial photograph (Fig. 1). A poly- nomial fit to the cumulative volume curve (Table 2) was used to calculate the volume of a stratum bounded by any two depths (see Culver 1973). Cores were taken in 1968 to examine the influence of watershed disturbance on the sediments. X-rays were taken of the cores to delineate variation in organic and inorganic (primarily clay) input (see Culver 1975).

Temperature was measured with stan- dard bathythermographs (read to 0.50 C)

Table 2. Area (ha) and volume (m3 X 103) of specific strata at mean water level.

Depth Cm) Lake Area Volume of Cumulative strata volume

0 3.11 29.5 29.5

1 2.80 26.8 56.3

2 2.56 24.7 81.0

3 2.37 23.0 104.0

4 2.22 21.5 125.5

5 2.08 20.3 145.8

6 1.98 19.3 165.1

7 1.88 18.3 183.4

8 1.78 17.4 200.8

9 1.70 16.4 217.2

10 1.59 15.4 232.6

11 1.49 14.4 247.0

12 1.38 13.2 260.2

13 1.27 11.9 272.1

14 1.11 9.9 282.0

15 0.87 3.7 285.7

15.5 0.63 2.4 288.1

16 0.34

or with one of a series of three Whitney resistance thermometers (reproducible to 0.010, accurate to 0.10C). The small variations in calibration have nearly all been corrected, but in any event large blocks of data were collected with each instrument. Transparency was measured with a Secchi disk, and light transmission was studied with a relative irradiance meter with an opal glass diffuser and various combina- tions of colored glass filters.

Rainfall was recorded on the lake from 1970-1972 or inferred from records for the Everett, Washington, weather station; comparisons for periods when data were available for both showed good agreement. Water-level measurements in the 1950s were taken relative to a fixed point that no longer existed in 1968, but the two series of measurements were coordinated from



Biogenic meromixis 669

-~~~~~~~~~~~~~~~~~~~~~~~!

U nv

Fig. 1. Aerial views of Hall Lake: 1-August 1947; 2-August 1961; 3-April 1967; 4-August 1974. Abandoned right of way of interurban railroad cuts across photographs diagonally north of the lake; 212th St. SW curves along south side. Widening of 212th St. is obvious between views 3 and 4. Interstate highway 5 (lower right in 3 and 4) was built between 1963 and 1966.

occasional measurements made at the outlet culvert. A continuous water-level re- corder was used in 1972.

Water was collected for chemical analy- sis with 50-cm Kemmerer (1950s) or 75-cm Van Dorn water bottles. Oxygen, iron, and alkalinity samples were taken first from the sampler in glass-stoppered reagent bottles; the remainder was kept for other measurements. This resulted in a small difference in the actual depth sampled for the various ions, but these differences have been corrected in the figures. These corrections

may seem trivial, but Hall Lake is often so sharply stratified that ignoring stratification within the sampler could result, for exam- ple, in a difference of 6.5 mg liter1 for iron in a sample from "15 m" (9 December 1972). The Kemmerer used in the 1950s did not sample the lake microstratification as effectively as did the free-flushing Van Dorn used more recently.

Although I tried to maintain uniformity of technique for chemical analyses throughout this study, there have been some changes (Culver 1973). In the recent work,



670 Culver

C U T L E T R 5~~~~~~~~~~~~~~~NE

Fig. 2. Morphometric map of lake. Depths in meters; contours drawn for mean water level.

conductivity was measured on samples re- turned to the laboratory, whereas most of Edmondson's conductivity measurements were made in situ (Edmondson 1956). Some oxidation of ferrous iron undoubtedly occurred in the recent samples, yielding lower values for conductivity since ferric iron would precipitate.

Oxygen was measured by the unmodified Winkler method until 1971, when a modification involving use of phosphoric rather than sulfuric acid (Montgomery et al. 1964) was adopted to avoid interference due to ferric iron. The largest deviation due to ferric interference observed in my tests was 0.3 mg 02 liter-' in the presence of 1.2 mg total iron of which about half was ferric.

The acidity titration was performed occasionally in the 1950s and frequently in 1972. No inflection occurred in the potentiometric titration, but Stumm and Morgan (1970) gave a way of calculating end point, which generated a value of pH 8.4 for water collected 1 July 1972. Iron may have interfered, as the deep water (anoxic) samples turned rusty at about pH 8, the approximate isoelectric point of Fe (III) (O'Melia 1972).

Our determination of dissolved inorganic carbon by acid titration was subject to some interference by ammonia, manganese hydroxides, iron hydroxides, silicate, and possibly dissolved organic matter. Oxida- tion of ferrous iron during titration was prevented by rapid acidification to about pH 5, with slower approach to the end point at 4.60 (Singer and Stumm 1968).

The methyl orange end point (pH 4.3) used in the 1950s was too low according to direct potentiometric titration or theoreti- cal calculation by the method of Stumm and Morgan (1970). In any event, the error is small (ca. 1%).

Reactive phosphate and total phosphorus were divided into dissolved and particulate fractions by centrifugation in the 1950s and by filtration (0.45-,um filters) in recent studies. The two methods are not truly comparable, especially considering the greater chance of aeration of anoxic samples during centrifugation, which would encourage precipitation of ferric phosphate. Seston determinations were made from the centrifuged samples (1950s) or by weigh- ing the tared filters after filtration and dry- ing (Eaton et al. 1969).

Stability calculations were performed for the two periods, since they provided the opportunity to include thermal, chemical, and sestonic contributions to the stratification patterns. Although problems with static stability calculations limit their use- fulness, the deficiencies should be the same for meromictic and holomictic periods of Hall Lake. A comparison of the recent year when the lake failed to circulate (1969-1970) with both periods gives further information on the contribution of density to meromixis.

In calculating meromictic stability, Hutchinson (1937) ignored thermal stability, since most mixing in temperate zone lakes will occur in winter when thermal stability is minimal. In Hall Lake, this is certainly the case for the destruction of stratification, but as the onset of thermal stratification is important for terminating winter mixing, the thermal density component is of interest. Seston can also have an important effect on the density of the water and can be added to the effects of thermally and chemically induced density.

I have estimated density values for Hall Lake by assuming a linear change in density with concentration of dissolved salts. The calculated densities are based on chemical concentration measurements and are thus subject to error from those substances that were not specifically measured.




Chemical stability results from a differential concentration of dissolved substances, so unless a specific ion is significantly abundant and stratified, it can safely be ignored. Materials accumulatinrg in the deep water of Hall Lake include free C02, bicarbonate ion, ferrous iron, ferric iron, ammonia, silicate, phosphate, and organic carbon.

Data used to calculate stability were the concentrations of oxygen, total dissolved phosphorus, bicarbonate ion, free C02, nitrite, nitrate, total dissolved nitrogen, dissolved iron, and total seston. Phosphorus was expressed as phosphate, total dissolved nitrogen (Kjeldahl) as ammonia, and all others as the ion listed. Concentrations were converted to grams per cubic centi- meter.

Densities due to thermal conditions were calculated by first estimating the average temperature for the stratum and then ob- taining the density from the table provided by Hutchinson (1957), interpolating lin- early as required. Variation of the heat capacity of water with temperature was ignored. The inferred densities due to temperature, dissolved salts and gases, and particulate matter were added for each depth to give a total density estimate for each date that full chemical samples were taken.

Following Walker (1974) these densities were then used to calculate stability of stratification for the lake for both meromictic and holomictic phases. As I wished to examine static stabilities, I used the closed basin model for the calculation though the basin is not in fact closed. By repeating the calculation using only temperature data, temperature data plus chemical data, and finally temperature plus chemical data plus seston, I could assess the relative contributions (positive or negative) of the three factors.

Results

Examination of X-rays of cores indicated no significant changes in the morphometry of the lake since 1953, although the sedimentation rate has been high. Sedimenta- tion since deposition of the bark layer (1914) has been about 14 cm, due primar-

ily to road construction. Clay laminations, which although extremely fine are precisely repeated at various points in the lake, indicate that major slumping and important morphometric modifications have not occurred during the period of observation.

I found no satisfactory way to assess possible changes in wind stress since 1950. As removal of trees in the watershed (see Fig. 1) has not included those closest to the lake, wind protection has probably not changed much since 1950. No on-the-lake measurements of wind velocity are available.

The climate of the Hall Lake region has not changed since 1950. Most rainfall in the watershed (ca. 75%) occurs from Oc- tober through February. The annual rainfall has not changed since 1950, but the annual pattern of water-level fluctuation is significantly different (Fig. 3). The summer fluctuations of recent years are not entirely due to greater sampling frequency. The watershed is delivering more of the summer rains to the lake now than it did when less of the watershed was paved and when water retention by soil and evapo- transpiration were more important. The water-level fluctuations necessitated correc- tion of the sampled depths for temperature and chemical measurements.

The sill at the outlet of the lake was at a level of -0.11 m. Water levels greater than this resulted in surface outflow which currently occurs sporadically throughout the year. In the 1950s surface outflow occurred only in the cooler months, so loss of nutrients via the outlet was negligible during periods of peak algal productivity.

Air temperatures in the Puget Sound area are ameliorated by the presence of the ocean and periods of ice cover on Hall Lake are brief, usually lasting 2-3 weeks in January-February. The greatest accu- mulations of ice occurred in January of 1950 and 1969. Depending on weather patterns, ice may not form at all, as in 1953 and 1970. Differences in thermal climate between the meromictic 1950s and the holomictic recent years were small (Fig. 4) so did not contribute to the differences in observed stratification patterns.



672 Culver

.2 - .2

o .4

198I 19591 1197015972 1963 Jan Jul Jan Jul Jn ulJan Jul Jan Jul

-j- LU

_j .2~~~~~~~~~~~~~~~~~. -j-

19Q 19569 1970297 1963 Jn JlJan Jul Jan Jul Jan Jul Jan Jul

Fig. 3. Water-level changes (m) relative to fixed point. Note annual drop in summers of 1950- 1953 relative to recent years, when a greater proportion of pavement in watershed delivered more summer rain to lake.

A comparison of the annual cycles in the concentrations and distributions of major chemical species for the holomictic and meromictic periods showed no striking differences, other than an increase in bicarbonate and iron concentrations (Table 3). This increase, the increased surface inflow, and the decrease of sewage input are the primary differences that must be taken into account in comparing the meromictic and holomictic phases of Hall Lake. Other differences between the two periods were probably due to one of the above, changes in circulation patterns, or both.

The transparency of Hall Lake waters has increased markedly since the 1950s (Fig. 5). This may be reasonably explained by the differences in phytoplankton stand- ing crops for the two periods. Anderson (1954) found total phytoplankton abundances for the top 4 m of 32 and 56 cm3 m-2 in late summer 1951 and 1952. Munch

(1972) found peak abundances of 6, 21, 14, and 2 cm3 m-2 in the euphotic zone for the summers of 1968, 1969, 1970, and 1971.

Relative irradiance measurements in the summers of 1952 and 1968 (Fig. 6) further emphasized the effect of the large algal blooms of the 1950s. Note that the light reaching 0.5 m (as a percentage of the surface light intensity) was much less in 1952 than in 1968 and that the loss was greatest in the shorter wavelengths, as is typical of scattering by small particulate matter. Light levels at 1 m on 9 July 1952 were similar to those at 3 m on 23 July 1968; Secchi transparencies on these dates were 1.50 and 3.35 m. Seston weight for the closest date (7 July 1952) was 19.2 mg liter-' at 2 m. This seston was 61% organic (by loss on ignition) and contained chlorophyll a equivalent to 36.3 ug liter-'. Seston at 2 m on 27 June 1968 was 8.4 mg liter-', and the chlorophyll a concentration was 7.1




pg liter-1. Measurements of loss on ignition are not available for 1968. The lower transparencies in 1969 resulted from silt input during road construction in winter 1968- 1969; in addition to the effect of the silt itself, associated nutrients stimulated the largest algal blooms observed in the recent period. Road construction affected both Secchi transparency (1963 and 1969) and winter seston totals for the lake (Fig. 7).

The lower transparency of the lake water in the 1950s is reflected in the relatively shallow penetration of the 60 isotherm (Fig. 8). The annual heat income ?ba was corre- spondingly greater in the 1969-1973 period (Table 4) as a thicker layer of the lake was heated by direct radiation, and back radiation and evaporative heat loss were pro- portionately decreased (surface temperatures were similar during the two periods) (Fig. 8).

Calculation of annual heat incomes (?ba) is dependent on available temperature data on the date of maximum and minimum heat contents, which can rarely be accom- plished with certainty. Heat incomes for 1963 and 1968 are probably too low, since the earliest temperature profiles available for those years were from 5 February and 5 April respectively, whereas the minimum was in January in those years adequately sampled. The differences in the annual heat incomes between the 1950s and the recent period must be due to the observed differences in transparency, as there is no evidence that wind or insolation patterns have changed. Increased flushing in recent years may have contributed to the lower winter minimum heat content but is unlikely to have increased summer maximum heat content.

The minimum heat content in winter used to be higher than it now is, with hypolimnion temperatures staying near 50C in the 1950s. Only an 8 mg liter-' difference in dissolved solids is necessary to prevent displacement of 50C water by 40C water, and the density differential between the mixolimnion and monimolimnion in Hall Lake (1950) was four times that. During

20

67- 680 *

690 0 0 67

0 --66 150

0

5 6 5 FJO

10~~~~~~~ 50 051~ ~~~~*05

63 68.

0 - 0 6692 050

050~~~~~~~~~~~~ 05JIFIM IA M 50 NID

Fig. 4. Monthly average temperatures at Ev- erett, Washington (representative of Puget Sound area ). Weather station's "long term average".. plotted as solid line; data for individual years as open ( 1950-1953 ) and closed circles ( 1963, 1967- 1972). Maximum and minimum for each month during these years labeled as to year. Obviously mean monthly temperatures were not consistently different between the two periods.

recent years, the difference has been even greater ( e.g. 130 mg liter'1 in 1972) .

The increase in temperature toward the bottom evident in 1950 and 1951 (Fig. 8) occurred also in other years but is less evident due to the intervals chosen for the isotherms. The depth of minimum tempera,ture was commonly near 12 m, with increases to the bottom as great as 0.5? but typically 0.1? or less. Regression of total heat content for the 12-16.2-m stratum from 10 April-18 August (i.e. from date of initiation of the thermocline to time of maximum heat content) yielded estimates of 2.1856 x 1010 and 1.8482 x 1010 gcal accumulated in the stratum per year for 1952 and 1972. Dividing these values by the area of mud below the 12-m contour (1.39418 x 108 CM2) gives heat flux estimates of 15.7 and 13.3 gecal cm-2 yr-1 for 19-02 and 1972. Use of the area of the 12-m contour instead of the area of the mud be-



674 Culver

Table 3. Representative chemical analyses from meromictic and holomictic periods. All concentrations in mg liter-1 as the form listed in parentheses.

27 October 1952 9 December 1972

0 m 4 m 8 m 14 m 15.5m 0 m 4 m 8 m 12 m 14 m 15 m

Silica (SiO2) 8.9* 14 14 9.2t 9.5 10.5 11.1 12.8

Total Iron (Fe) 0.01 0.08 0.22 2.38 4.34 0.62 0.43 0.61 0.43 13.40 20.94

Partic. Iron (Fe) 0.01 0.03 0.03 0.49 1.26 0.16 0.16 0.18 0.14 0.17 0.31

Calcium (Ca) 7.5* 9.2 9.2 4.73t 4.10 6.00 6.93

Magnesium (Mg) 2.7* 4.1 3.6 6.041 6.19 5.88 6.70

Sodium (Na) 4.8* 4.6 4.3 9.08P 8.67 9.42 9.78

Potassium (K) 1.9* 2.4 2.4 2.10t 1.98 2.15 2.45

Bicarbonate (CO2) 18.3 23.8 25.0 53.1 53.7 40.0 39.9 40.3 40.3 78.1 98.8

Free CO2 (CO2) 1.76? 9.68 9.68 13.6 17.16 6.97 9.17 9.25 9.85 29.31 46.41

Sulfate (SO4) 13* 7.8 3.7 - - - - - -

Chloride (CI) 5* 4 4 - - - - - -

Nitrate (N) 0.042 0.045 0.036 0.065 0.014 0.20 0.20 0.20 0.21 0.11 0.23

Nitrite (N) 0.007 0.007 0.009 0.006 0.007 0.007 0.007 0.008 0.007 0.003 0.004

Ammonia (N) 0.0 0.0 0.0 2.547 3.324 0.049 0.049 0.055 0.047 0.556 0.934

Total 0.619 0.460 0.643 3.072 3.669 0.581 0.590 0.550 0.565- 2.345 3.442 Trivalent N (N)

Phosphate (P) 0.002 0.001 0.002 0.184 0.223 0.006 0.006 0.006 0.006 0.152 0.432

Phosphorus (P) 0.029 0.049 0.181 - 0.249 0.020 0.019 0.018 0.020 0.163 0.430

Oxygen (0) 6.52 4.87 0.10 0.00 0.00 5.26 5.18 5.14 5.10 0.00 0.00

pH 7.10 6.90 6.60 6.65 6.64 7.00 7.02 7.08 7.04 7.07 7.05

Conductivity 76.5 72.5 77.7 104.2 106.5 133.7 131.8 131.9 133.7 181.7 210.2 (jamhos cm-1 180C)

Color 55* 90 130

* Analyzed by C. S. Howard. t 27 June 1952. : 13 August 1968. ? Analyzed by M. R. Cheek.

low it does not significantly change the estimates. The hypolimnetic heating can easily be explained by geothermal heating, which Hutchinson (1957) gave as 37 gcal cm-2 yr-1. Given the uncertainty of gener- ating a heat content from a single profile in the center of the lake and the small temperature changes involved, it is unlikely

that the 15% difference in accumulation rates for the two years is significant.

Oxygen concentration traditionally has been used to determine the depth to which mixing occurs in autumn, although oxygen is far from a conservative element. The common practice is to define as meromictic those years in which the lake maintained




1950 51 52 53 1961 62 63 6 9 70 71 72 (12) (18) (26) (7) (1) (1) (9) (31) (58) (72) (53) (37)

20

v5.

6- s-MEROMICTIC--- ---MERO--- ------ HOLO-eMEROeaHOLOMICTIGC

I! t74 Freeway construction 212 th St SW modification

begins & sewer construction

Fig. 5. Secchi transparency. Vertical bars indicate range of measurements for each year; mean value shown by height of horizontal bar and seasonal distribution of measurements in that year by its length. Only single observations available for 1961 and 1962 ( 0 ); number of observations shown in parentheses.

an oxygen concentration of zero in the deepest water throughout the year (Fig. 9), although thickness of the monimolimnion may have varied greatly throughout the year. Given the possible positive er- rors from ferric interference before Septem- ber 1971, it is preferable to adopt the 0.5 mg liter-' isopleth as indicating the approximate position of the zero isopleth in the water colunm. The monimolimnion was never a discrete layer. Instead, for both periods there was a general increase of substances (except oxygen and nitrate) toward the bottom: this increase often ceased short

100

80-

0.5m 1968 60- I m 1968

40- 0.5m 1952

20 -I m 1952

3m 1968

400 500 600 700 WAVELENGTH (nm)

Fig. 6. Relative irradiance as function of wavelength and depth for 9 July 1952 (0) and 23 July 1968 ( 0 ).

Table 4. Heat income and autumnal heat loss. Heat of fusion of ice was ignored. Annual heat income is difference between maximum and minimum divided by surface area of the lake. Autum- nal heat loss is similarly calculated from summer maximum and minimum of the following winter and is a measure of autumnal circulation intensity. For purposes of comparison, a minimum heat content of 1.153 X 10o gcal would result from an average temperature of 40C for the whole lake. Dates are coded as year, month, and day (e.g. 500323 =23 March 1950).

Min Max Annual Autumnal Date heat Date heat heat heat of content of content income loss

min 101 gcal max 1012gcal (gcal cm-2)

500323 1.4274 500828 2.6817 4,030 4,802

510208 1.1871 510822 2.6221 4,610 4,655

520102 1.1733 520818 2.7186 4,965 4,574

521231 1.2948 530727 2.5033 3,883

630205 1.2723 630725 1.9264 2,102

680405* 1.8528 680804 3.3389 4,775 7,678

690118 .9491 690731 3.0946 6,893 5,385

700109 1.4185 700814 3.6240 7,086 7,738

710119 1.2153 710808 3.6862 7,939 8,309

720128 1.100 720905 3.4489 7,547 7,760

730106 1.0335 - -

*Earliest date available.

of the bottom in the meromictic period, indicating diffusion from sediments over a long enough period to produce some sort of steady state, as Weimer and Lee (1973) found for methane in Lake Mary. The monimolimnion is difficult to define in both of these lakes.

Erosion of the monimolimnion in Hall Lake was similar to that seen by Kjensmo (1967) in Skjennungen, except that in his lake ice cover began early and effectively ended circulation by mid-November. With- out the protection of ice cover, except for brief periods, Hall Lake continued to gain oxygen until thermal stratification began in April, although the charging of the deep-



676 Ctulver

10 1950 1951 1952 1953 1 1963

8 8 0

6 6 j

4 ) 4 4

2 . 2

n ~~~~~~~~~~0 0Jul Jan Jul Jan Jul Jan Jan Jul

10 I968/\;l 1969 1970 1971 1972

2 LA- " .

Jul Jan Jul Jan Jul Jan Jul Jan Jul Fig. 7. Total seston. Total in lake, dashed line; totals in 0-4-, 4-8-, 8-12-, and 12-16-n strata,

solid lines. Huge amounts of seston in early 1963 and 1969 due to highway construction.

est strata with oxygen was most noticeable between November and January (Fig. 10).

Nevertheless, in the 1950s, the deepest strata remained anoxic and the deep sediments remained anoxic. The one recent meromictic episode was during winter 1969-1970, after watershed disturbance the previous winter. Oxygen depletion began more rapidly during summer 1970, particularly in the 12-16-m stratum (Figs. 9, 10), though the algal abundance was not nearly so high as it had been the previous summer (Munch 1972). The net amount of oxygen added to the lake before thermal stratification in April 1970 was also lower. This cumulative effect of incomplete winter circulation is obviously important for maintaining meromixis, since there were similar

patterns throughout the 1950s. Neverthe- less, t-he lake remained holomictic after 1970, indicating that more than reduced sediments is involved in maintaining meromixis.

The greater total phosphorus concentration of the surface waters in the meromictic era (Table 3) is further evidence of high algal biomass during that period. While reactive phosphate concentrations were comparable for the two periods, except when large amounts of silt were being washed into the lake (cf. Figs. 7 and 11) during road construction, total phosphorus was consistently higher in the 1950s. This was probably due to input from septic tanks around the lake before sewer construction in 1968-1969. It is likely that the change




1950 1951 1952 1953 1963 0~~~~~~~~~~~~~~~~~

6u u a u JJn Jul Jjan Ju8a u

C1 0 [ J2 .0 4 0

4.- 4 * .

4 4

12-

'dJan Jul Jan Jul Jan Jul Jan Jul Jan Jul

* Q98 196 7 7 7

0~~~~~~

78 4~~~~~~~~~~~~ 8I

4 '- 4

4~~~~

16 Ja-n Jul Jan Jul Jan Jul Jan Jul Jan' Jul 1968 1969 1970 1911972

Fig. 8. Isotherms. Contour interval, 2?C. Dots along bottom of each panel indicate dates of temperature profiles used in constructing diagram.

from meromixis to holomixis was partly due to decreased loading of phosphorus to the epilimnion after 1969.

Calculation of the persistency (sensu Hutchinson) for the meromictic and holomictic periods (Fig. 12) resulted in values consistently higher during recent years. At all times of year, except winter, the thermal component is the only one of any signifi- cance, as may be expected. The thermal calculations provide an additional indica- tion of smaller heat income in the 1950s due to lower transparency at that time. Note that 1969 had the lowest recent summer persistency maximum and that the date of maximum heat content for 1972 was obviously not sampled. The thermal persistency returns to zero for both the meromictic and holomictic periods during December

or January, implying that there is no thermal resistance to mixing (or even negative resistance as indicated by a positive persistency) at the time of minimal heat content. Values of density, persistency, and temperatures for 10 winters are included in Table 5. In many cases complete data are unavailable for chemical and sestonic calculations.

Another way to compare the resistance to mixing of the lake during the two periods is to examine the resistance remaining on dates when the lake has cooled to the same surface temperature for both periods, such as 80 on 8 November 1951 and 13 Novem- ber 1971 (Fig. 13). The progression of circulation for both years is shown in the distribution of stability with depth curves in Fig. 14. The integrals of the curves in



678 Culver

1950 1951 1 952 1953 1 1963 O0 ...... .. . . . * ........ . . . .... . . .. . *. I . .-............... -.

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

1 6

Jan Ju I 10

4 i4 .5

16Jan Jul Jan Jul Jan Jul Jan Jul Jan Jul

12 198 8 899 128 0 19o 1 8 1972

4~~~~~~~~~~~~~~ E 10

4L 4 5

12 4) 0 0

16 4 2.5~~~~~~~~~~~~~.


oxygen profiles used in constructing diagram. Zero isopleth has been smoothed slightly.

Fig. 14 are the values plotted in Fig. 12 and listed in Table 5 for the case where sestonic, thermal, and chemical contributions to density are considered.

While the lake had mixed to about 8 m in 1971 and only to 5 m in 1951 when the surface temperatures were 80, the resistance to mixing remaining was essentially the same. Since the remaining stratification in 1951 was 3 m closer to the surface where wind stress occurs, the lake should have been more likely to circulate fully in 1951 than in 1971, although this obviously did not happen. The contribution of chemical stratification to the total stabilities (or persistency) on the above dates was 1.65 and 2.29 g cm cm-2 for 1951 and 1971, or about 10% of the total. Unfortunately, bicarbonate ion, dissolved iron, and free-CO2

concentrations were not measured from 1969 to mid-1971, so that the winters of 1969-1970 and 1970-1971 are represented by stability calculations for temperature plus seston plus only part of the chemical stratification. In the fourth curve on each year (right triangles in Fig. 14) the initial effect of the establishment of thermal stratification can be seen, although it is shal- lower in 1952 than in 1972.

Seston can occasionally contribute significantly to the stability calculation, as was the case in late 1968 and early 1969. (Com- pare the last two columns of Table 5 for 14 December 1968, 9 January 1969, and 2 Feb- ruary 1969, then examine a year such as 1971-1972 when very little silt came in during winter.) The effect of silt on persistency can even be seen in Fig. 12 for De-




25 1 1950 1951 1952 1953 1963

20-

vTOT0 -TOTAL 0-4 M

5-


25 1968 1969 1970 ' 1971 '7< 1972

20TOTAL I /

N AI

5 4-8 M 8-12 M 12 1GM

Jan Jul Jan Jul Jan Jul Jan Jul Jan Jul

Fig. 10. Total oxygen. Total in lake, dashed line; individual strata, solid lines.

cember 1968 and January 1969 where the persistency curve including seston does not follow the other two, but indicates a higher value for persistency. As Frey (1955) pre- dicted, this finely divided clay does indeed contribute to the stability calculation, but the lake was freely circulating at this time since the mud was reaching the bottom waters of the lake as a density current, carry- ing significant oxygen with it. All other deviations of the persistency curves from one another on Fig. 12 were due to chemical stratification, not seston. There was, however, a heavy seston load in winter 1963 (Fig. 7; Table 5).

Although the biota do not contribute much gravimetrically to the stratification of a biogenic meromictic lake (up to 70% of the seston by dry weight), they are important, as has been discussed earlier. Pri- mary productivity in the trophogenic zone does provide the materials that, upon de- composition in the tropholytic zone, con-

sume oxygen, but my data indicate that its role in producing a light-absorptive layer at or near the surface of the lake is more important. Unfortunately there are few direct measurements of primary productivity in Hall Lake (Table 6). If one assumes that the 1952 experiments fully sampled the last half of the year and that the production of the first half of the year was equivalent, the annual production was 200 g C m-2 yr-1. Since the above assumptions and the 72-h incubation periods used in 1952 all tend to underestimate primary productivity, the true value may well have been 50 or 100% higher than this figure. The single 14C experiment in 1972 was run for 2.5 h. The algal blooms in the 1950s, especially those in the metalimnion, are reflected in chlorophyll a abundance (Fig. 15).

Differences in algal abundance and productivity between the 1950s and 1968-1972 might reasonably be explained by changes



680 Culver

16 - 32 16 1950 1951 1952 1953 1963 TOTAL -

12 - 24 -

Kg8 _ j Kg 16 TOTAL

4 8 8-12 M 12-IG M

4-8M~ 4 -8 M o0 -4 M. ~ 0Q4,

Jan Jul Jan Jul Jan Jul Jan Jan Jul

16 16 r 1968 ', 1969 1970 1971 1972

11 11' 1

12-

Kg 8 _ -

Jan Jul vlan Ju~~~l Ja Jul Ja ul Jn u

Fi. O1.Toal 7eciepopae oesaecag o 93 n ihcreainbtensr 112-1GM stu M

8-12 M 4-8 M

O -4 MJu Jan Jl Jan Jul Jan J ul Jan Jul Jan Jul

Fig. I11. Total reactive phosphate. Note scale change for 1963, and high correlation between surface phosphate concentration and particulate matter coming in with runoff in early spring (Fig 7). Incomplete circulation in 1969-1970 is reflected in rapid release of large amounts of phosphate into 12-16-rn stratum in 1970.

in nutrient loading to the lake. No other marked changes in the biota have been no- ticed between the meromictic and holomictic periods, except for a switch from Daph- nia rosea to Daphnia pulex in the plankton, which will be discussed elsewhere. The lake still stratifies each summer, deep waters are anoxic for long periods, and the chemistry of the water is relatively unchanged. The total circulation in recent winters has not allowed establishment of benthic or- ganisms much below 6 m nor caused tre- mendous changes in planktonic species composition.

Discussion The heat incomes recorded for the mero-

mictic and holomictic periods in Table 4 may seem low, but Hall Lake is small and

protected and the hypolimnion seldom warmer than 50C. Kjensmo (1967) re- ported values of summer heat incomes (? ) for two meromictic lakes, Skjennungen (5,627 gcal cm-2) and Store Aaklungen (6,028 geal cm-2) and two similar dimictic lakes, Store Tryvann (4,763 geal cm-2) and Bjordammen (3,726 geal cm-2), so the values for Hall Lake are not unreasonable.

The consistently lower heat incomes during meromictic years, including winter 1969-1970, suggest that in biogenic mero- nictic lakes heat accumulation and loss are somehow related to meromixis. During meromictic years, the lake gained less heat in summer and retained more heat in winter than in holomictic years. As I pointed out previously (Culver 1975), this suggests that meromixis in biogenic meromictic lakes is not maintained solely by the magnitude




Table 5. Total meromictic stability (persistency), Schmidt thermal stability, and density difference during periods of maximum circulation, 1950-1973. Not all contributors to chemical stability were measured each time. Substances measured, in order of importance, were bicarbonate ion, free- C02, iron, 02, nitrogen, and phosphorus.

Dissolved Schmidt Meromictic stability matter thermal ( cm cm-2)

Temp (?C) Total density (q cc-1) (mg Q ) stability Thermal T + C Date Surf. Btm. Surface Bottom Surf. Btm. g cm cmL Thermal + Chem + Seston

500323* 6.22 4.72 0.99998897 1.00005588 28.1 60.1 -5.69 -5.25 -9.35

501214* 5.50 4.56 1.00000792 1.00006722 16.0 69.9 -2.68 -2.75 -7.05 510130* 3.89 4.44 1.00002205 1.00006750 22.1 69.2 +0.14 +0.20 -3.54 510227* 4.72 4.72 1.00002668 1.00007577 31.0 80.0 -0.10 +0.01 -4.74

511108t 8.22 4.44 0.99990236 1.00005641 34.8 54.9 -22.54 -21.72 -23.38 -23.09 511211t 5.00 4.56 1.00002851 1.00006278 32.3 62.6 -1.06 -1.06 -3.64 -3.43

520207t 5.57 4.75 1.00001844 1.00005465 33.5 54.5 -1.64 -1.51 -3.29 -3.42 520226t 5.13 4.75 1.00002529 1.00005897 31.2 59.8 -0.45 -0.41 -2.51 -2.38

520313 ? 6.25 4.74 0.99999537 1.00005304 29.8 54.6 -4.47 -4.23 -6.30 -5.89

521209 t 4.82 4.91 1.00003068 1.00006256 31.4 63.5 +0.10 +0.13 -3.39 -3.48

630205-t 4.75 4.20 1.00002263 1.00007287 -0.54 -0.51 -6.74

630219? 6.85 4.10 1.00000659 1.00011411 61.6 68.2 -4.79 -4.10 -5.20 -10.45

630321?11 8.40 4.30 0.99991351 1.00010199 55.5 75.8 -11.36 -9.56 -11.81 -14.54

681214tll 6.12 5.93 1.00003633 1.00009141 53.1 55.5 +0.93 +0.97 -0.68 -4.63 690109t11 3.36 4.03 1.00007351 1.00011527 51.5 53.3 -0.43 -0.41 -0.65 -6.39

690202t 1.34 3.82 0.99997081 1.00005877 -2.76 -2.48 -2.04 -6.35

69122214:11 6.19 5.57 0.99997882 0.99999710 -1.90 -2.00 -1.92

700109** 4.94 5.56 1.00000175 0.99999823 +0.27 +0.43 -0.41

701223-4 4.35 5.22 1.00000823 1.000000lI +0.57 +0.67 +0.43

710119t1l 4.58 3.93 1.00000460 1.00000394 -0.32 -0.28 +0.20

711113 8.35 4.92 0.99991986 1.00008128 62.8 85.4 -22.92 -22.74 -25.03 -24.95 711227? 2.55 4.95 1.00005264 1.00006403 67.3 67.7 -0.45 -0.20 -0.21 -0.33 720128? 1.35 4.45 1.00001493 1.00007166 69.7 70.0 -1.19 -0.97 -1.06 -1.17

721029 10.15 4.30 0.99978360 1.00019951 66.5 197.2 -48.92 -48.12 -57.97 -57.90

721209 4.75 5.61 1.00006685 1.00019054 69.9 206.4 +0.82 +1.07 -7.50 -7.66

730106 2.95 4.05 1.00006400 1.00012584 68.3 103.9 -0.73 -0.67 -2.92 -4.33

* 02 and HCO3 only.

t No free C02 or Fe.

+ No HC03.

? No Fe.

|| No total nitrogen.

** 02 and seston only.

of the chemical concentration gradient (although there must be one if the monimolimnion temperature is >40C) but also by the intensity of the downwelling currents. Therefore, the greater heat content in win-

ter during the 1950s is due more to a decreased influence of surface waters than to accumulated dissolved substances in the monimolimnion. The decreased transparency mentioned above was responsible for



682 Culver

1961 1962 1963 -250 :&_ 195 19 51 1952 1953

-200 -

.,-150

E 0. -100

-50-

0 - . . ___

Jan Jul Jan Jul Jan Jul Jan Jul

-400 fo 9 1968 1969 970 9 71 1972

-350-

-300

- -250-

0 -200- E

c-150-

-100

-50-

0 J an Jul Jan Jul Jan Jul Jan Jul Jan Jul

Fig. 12. Meromictic persistency. Each panel is three superimposed graphs of persistency: first, a plot of persistency due to thermal stratification alone, then thermal plus chemical persistency, and finally thermal plus chemical plus sestonic inputs to persistency. Dots in top panel are isolated values for 1961, 1962, and 1963.

the lower maximum total heat content in the 1950s, and, together with the decreased heat loss in winter, created the decreased annual heat gains recorded in Table 4.

The differences in thermal cycles in the lake between the meromictic and holomictic periods were more likely the cause than the effect of the meromictic condition. Similar effects would be expected for any lake of similar morphometry that received

an increase in nutrient income. Hall Lake was decidedly humic both during the 1950s and in recent years, although few quanti- tative data are available. Changes in transparency due to dissolved matter could of course generate much the same results as an algal bloom, and may be quite important, as in Stewart's Dark Lake in which 99% of all light is absorbed in the top meter (Hasler and Likens 1963). Humic




0 0 --.

m27 Dec71 2 _ 1ol

| . ~~8 Nov 51/

4 - 2Jan52i !,?'! E 8 I 0

-r 8 - ,!/ *1iv71 Li 10 - A

12 - OA

14 - A,1

16 - 1 1 I l I I 2 3 4 5 6 7 8 9

TEMPERATURE (?C)

Fig. 13. Vertical distribution of temperature during late fall and midwinter 1951-1952 and 1971. Note how much deeper water has been warmed in 1971.

material did not cause the meromixis in 1969-1970 in Hall Lake, since there is no evidence that more humic material entered the lake in 1969 than in other years when the lake circulated completely.

The rapid fluctuation of the water level precluded accurate comparison of flushing times for the two periods, but it is evident that flushing is higher now than it was in the 1950s. As the inflow is often silt-laden and can easily penetrate the small density change at the chemocline, the increased inflow certainly contributes to the increased frequency of circulation. That it is not so important as the thermal effects discussed above is demonstrated by failure to circulate completely in 1969-1970, although rainfall that winter (September-April) was no less than in 1968-1969 or 1971-1972. Rainfall was 25% higher in winter 1970- 1971, which may have contributed to destruction of the meromixis that had been established in 1969-1970. Winter rainfall in the 1950s was comparable to that in recent years with the exception of 1952-1953, which was quite dry.

The total amount of iron and phosphorus suspended or dissolved in the water column

Table 6. Primary production, 1952 and 1972. Oxygen experiments of 1952 were recalculated as mg C m2 d-' assuming a PQ of 1.0.

25 16 28 15 28 Jun Sep Oct Dec Feb 52 52 52 52 72

mg 02m-2 d-1 2,290 570 140 33 -

mg C mr2 d-1 1,720 425 105 25 29

Compensation 4 m 3 m 2 m 3 m 6 m depth

decreased during autumnal circulation in both the 1950s and the holomictic period. Ferric hydroxide flocs formed as ferrous iron was oxidized to ferric with the progressive erosion of the chemocline. Ammo- nia and other substances may be removed via sedimentation after adsorption to ferric hydroxide flocs, and the presence of clays in the inflow water can increase the effi- ciency of this scrubbing activity. The high concentrations of iron, phosphorus, and ammonia in the deepest water in late fall may thus be a result of sedimentation phenomena as well as of the release of these substances from the sediments.

When the sediments are not oxidized during autumnal circulation, as was always the case in the 1950s and also for winter 1969-1970, phosphate release is facilitated, as can be seen by the large amount of phosphate released in the 12-16-m stratum be- ginning in April 1970 (Fig. 11). Much of this phosphate presumably came in with the mud during 1969 (Fig. 7), as the seston load in the lake was quite small in 1970. The relatively high phosphate accumulation in 1972 began much later in the season (Fig. 11).

The work required to destroy the late fall stratification by wind stress can be less important than the introduction of oxygen into the hypolimnion, which can oxidize ferrous iron and effectively remove it by sedimentation. Low redox potentials in the deepest strata can reduce some of the ferric floc to ferrous iron and effectively concen- trate the entire chemical contents of the hypolimnion in a narrow zone near the bot-



684 Culver

0 ' o l -231 gcmcm2 A -2

E 6 0 1 /l3Mar52 o\ 0 , .\2-0 gcm c

1 -3.4 -- -3 0. 1. . A201 -125

0 ~~~~~~~~~~A-122

2o pe n -. 2 j~~ o~ 8 Nov 51

~ 6 ~/13Mar52 0 u~~~~~~~~~~ ,.

0 ~27 Feb7~ ~ 5o7 n- I A

10 A1 \ ,IIDec 5I il-27 Dec 71 I AI

12 - A ~: 1 A

\A 0 ~~~0 0 b

14 2 Jan 52\A \' A f ,~~~~~ /A&,l-28 Jan 72

0. 2 ,12. 0. -2 2K 3. gocm cm cm gocm cm cm

Fig. 14. Distribution of stability throughout autumnal mixing in 1951 and 1971. Symbols same as in Fig. 13. Numbers shown in upper right of each corner show integrated values for stablity like those plotted in Fig. 12.

tom, thus increasing the resistance of this layer to further admixture of surface waters. This probably results in a net horizontal displacement of iron and phosphorus of surface waters toward the deepest zone as has been found in Ursee (Tessenow 1973).

The greater productivity of Hall Lake in the 1950s than in the holomictic period is counter to previous views of the influence of a chemocline on the productivity of a meromictic lake. Frey (1968) and Findenegg (1965) claimed that meromixis should decrease productivity for a given nutrient supply since the monimolimnion acts as a nutrient trap. This is true only if return of nutrients from the deep hypolimnion is a significant proportion of the nutrient supply to the euphotic zone, since the difference between a productive and nonproductive lake is a function of the balance between nutrient supply to the euphotic zone and sedimentation, regardless where the nutrients come from.

Although major periods of total circulation do not occur in a meromictic lake, the high concentration of important algal nutrients in the monimolimnion may easily make up for the lack of a complete circulation, and the nutrients are available in a soluble form, free to diffuse into the mixolimnion. Furthermore, the sediments of the monimolimnion are reduced at all times so that iron and phosphorus can diffuse out of the sediments year round. Perhaps the association between low productivity and meromixis, where it occurs, is due to a correlation between topographic wind protection and small watersheds, which would tend to generate a smaller nutrient input.

Findenegg (1965) contrasted two holomictic lakes, Worthersee and Klopeiner See, which both become strongly stratified thermally each summer. After the initiation of thermal stratification, the phytoplankton abundance in Worthersee de- creases steadily toward autumn, in contrast to Klopeiner See in which the phytoplankton increase. He explained the difference




0 20 20 10 10 10 10 4

108 1 4 0 20 30~~~20

~ y

E 10 ' o () t

12 ~ ~ ~ 10

16

Jan Jul Jan Jul Jan Jul Jan Jul 1950 1951 1952 1953

Fig. 15. Isopleths of chlorophyll a concentration. Contour interval 10 Ag liter"'. Dots above panel indicate dates of chlorophyll measurements on which graph is based.

by the fact that summer nutrient loading to the euphotic zone is greater in Klopeiner See since it is a smaller lake than Worther- see. When ve consider the difference in density gradient across the thermocline and the chemocline of a biogenic meromictic lake (1,760 vs. 42 mg liter-' for Hall Lake on 18 August 1952), it is surprising that Findenegg then explained the low productivity of meromictic lakes such as Krotten- see by loss of nutrients through the chemocline. Production in a meromictic lake must be affected by the nutrient loading to the euphotic zone. In Hall Lake, the volume of the euphotic zone was smaller in the 1950s than it is now, intensifying the effect of surface inflow from septic tanks.

The greater chemical stratification in recent years than in meromictic years is surprising, since the greater productivity of the 1950s should generate a greater accumulation of dissolved materials per unit time in the hypolimnion.

The chemical and sestonic components of persistency never accounted for more than 5 or 6 g cm cm-2 of the persistency, or as little as 1% of the summer total. Weimer and Lee (1973) found an even lower value for winter chemical stability in Lake Mary

(ca. 1 g cm cm-2); they apparently did not include dissolved gases (e.g. CO2 and methane) in their calculation, although they measured them. During the periods of minimum thermal stratification, Hall Lake in recent years had greater meromictic stability than in the 1950s, which in turn had higher stability values than Lake Mary, which is still meromictic. The persistency or integrated meromictic stability of biogenic meromictic lakes is thus a poor indicator of whether a lake will circulate. Only the relative magnitude of the maximum stability obtained in summer seems to be related to circulation the following winter, and the higher the stability in summer the greater the likelihood of circulation the following winter.

Chemical stratification is critical to maintaining meromixis during periods of iso- thermy, but Hall Lake showed a greater increase in dissolved matter (particularly dissolved inorganic carbon) with depth during holomictic years than it did during meromictic years. This explains why static stability calculations based on thermal and chemical contributions to density are in- adequate to explain whether a lake will circulate in a given year, except insofar as



686 Culver

the proportion of the lake heated above 40C figures prominently in the summer thermal stability of the lake and in the likelihood of overturn the following winter. Large inputs of silt tend to cause major circulation, in contrast to the suggestions of Frey (1955), but may generate biogenic meromixis if they bring significant amounts of nutrients with them (e.g. phosphate and iron). Circulation is primarily a result of thermal phenomena affecting the magnitude of downwelling currents in a basin with significant wind protection. The importance of the biota in biogenic meromixis is primarily a result of its influence on transparency and transparency's effect on thermal stratification. Since all lakes are subject to the same interaction, the tendency for Hall Lake to become meromictic with adequate nutrient input has implica- tions for the circulation patterns and chemical stratification of other eutrophic lakes that have a strong tendency toward stag- nation.

References ANDERSON, G. C. 1954. A limnological study of

the seasonal variations of phytoplankton populations. Ph.D. thesis, Univ. Wash., Seat- tle. 268 p.

CULVER, D. A. 1973. Biogenic meromixis in a soft-water lake. Ph.D. thesis, Univ. Wash., Seattle. 215 p.

. 1975. Physical, chemical, and biological factors in the initiation and destruction of biogenic meromixis in a soft-water lake. Int. Ver. Theor. Angew. Limnol. Verh. 19: 776- 783.

EATON, J. S., G. E. LIKENS, AND F. H. BORMANN. 1969. Use of membrane filters in gravi- metric analyses of particulate matter in natu- ral waters. Water Resour. Res. 5: 1151- 1156.

EDMONDSON, W. T. 1956. Measurement of conductivity of lake water in situ. Ecology 37: 201-204.

FINDENEGG, I. 1965. Relationship between stand- ing crop and primary productivity, p. 271- 289. In C. R. Goldman [ed.], Primary productivity in aquatic environments. Univ. Calif.

FREY, D. G. 1955. Langsee: A history of

meromixis. Mem. Ist. Ital. Idrobiol. 8(suppl.): 105-119.

. 1968. Biological characteristics of meromictic lakes, p. 63-95. In D. F. Jack- son [ed.], Some aspects of meromixis. Trans. Symp. Meromictic Lakes (Fayetteville, N.Y. Green Lakes State Park, 23-24 April 1965).

HASLER, A. D., AND G. E. LIKENS. 1963. Bio- logical and physical transport of radionu- clides in stratified lakes, p. 463-470. In V. Schultz and A. W. Klement, Jr. [eds.], Radio- ecology. Natl. Symp. (1st) Radioecol. NTIS.

HUTCHINSON, G. E. 1937. A contribution to the limnology of arid regions. Primarily founded on observations made in the Lahontan Basin. Trans. Conn. Acad. Arts Sci. 33: 47-132.

. 1957. A treatise on limnology, v. 1. Wiley.

KJENSMO, J. 1967. The development and some main features of "iron-meromictic" soft water lakes. Arch. Hydrobiol. Suppl. 32, p. 138- 312.

MONTGOMERY, H. A., N. S. THOM, AND A. COCK- BURN. 1964. Determination of dissolved oxygen by the Winkler method and the solu- bility of oxygen in pure water and sea water. J. Appl. Chem. 14: 280-296.

MUNCH, C. S. 1972. An ecological study of the planktonic chrysophytes of Hall Lake, Wash- ington. Ph.D. thesis, Univ. Wash., Seattle. 228 p.

O'MELIA, C. R. 1972. Coagulation and floccu- lation, p. 61-109. In W. J. Weber, Jr. [ed.], Physicochemical processes for water quality control. Wiley-Interscience.

SINGER, P. C., AND W. STUIMM. 1968. Kinetics of the oxidation of ferrous iron, p. 12-34. In Coal mine drainage research. Proc. Symp. (2nd), Mellon Inst.

STUMM, W., AND J. J. MORGAN. 1970. Aquatic chemistry. Wiley-Interscience.

TESSENOW, U. 1973. Solution, diffusion and sorption in the upper layer of the lake sediments. 2. Recent accumulation of ferrous phosphate (vivianite) in the sediment of a meromictic bog lake (Ursee, Hochschwarz- wald) by postsedimentary displacement. Arch. Hydrobiol. Suppl. 42, p. 143-189.

WALKER, K. F. 1974. The stability of meromictic lakes in central Washington. Limnol. Oceanogr. 19: 209-222.

WEIMER, W. C., AND G. F. LEE. 1973. Some considerations of the chemical limnology of meromictic Lake Mary. Limnol. Oceanogr. 18: 414-425.

Submitted: 2 December 1975 Accepted: 2 December 1976



Documents

Biogenic Meromixis and Stability in a Soft-Water Lake