Meromixis in Hypersaline Mono Lake, California. 3. Biogeochemical Response to Stratification and Overturn

Meromixis in Hypersaline Mono Lake, California. 3. Biogeochemical Response to Stratificationand OverturnAuthor(s): Laurence G. Miller, Robert Jellison, Ronald S. Oremland and Charles W. CulbertsonSource: Limnology and Oceanography, Vol. 38, No. 5 (Jul., 1993), pp. 1040-1051Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2838091 .

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Limnol. Oceanogr., 38(5), 1993, 1040-1051 ? 1993, by the American Society of Limnology and Oceanography, Inc.

Meromixis in hypersaline Mono Lake, California. 3. Biogeochemical response to stratification and overturn

Laurence G. Miller U.S. Geological Survey, Water Resources Division, Menlo Park, California 94025

Robert Jellison Department of Biological Sciences and Marine Science Institute, UCSB, Santa Barbara, California 93106

Ronald S. Oremland and Charles W. Culbertson U.S. Geological Survey, Water Resources Division, Menlo Park

Abstract Mono Lake is a terminal, saline lake that became ectogenically meromictic in 1982-1983 and remained

stratified until November 1988. During this period, the monimolimnion remained anoxic and nearly isothermal, while the upper mixolimnion was well oxygenated and exhibited a seasonal thermal regime. Dissolved sulfide and methane increased in the monimolimnion as a result of diffusive flux from the sediments. Winter mixing down to the chemocline distributed sulfide and methane throughout the mixolimnion. Lakewide inventories of dissolved sulfide and methane reflected the balance between increased concentrations and decreased monimolimnion volume over time. At overturn, the entire water column was isothermal and anoxic. Dissolved sulfide (380 x 106 mol) was oxidized in 1 week by molecular oxygen. Methane (12 x 106 mol) was removed more slowly by microbial oxidation and ventilation across the air-water interface.

Deep saline lakes are subject to periods of prolonged stratification (ectogenic meromixis) if water added by changes in the hydrologic budget is less dense than the water in the lake basin (Hutchinson 1957). Some examples are Soap Lake, Washington, Big Soda Lake, Ne- vada, and the Dead Sea, Israel, where meromixis has lasted for decades to centuries (Friedman and Redfield 1971; Kimmel et al. 1978; Stiller and Chung 1984). In lakes that approach steady state (e.g. Soap Lake, Big Soda Lake) the physical processes that control density also control the distribution of dissolved reactive solutes (e.g. reduced chemical species such as HS-, CH4, and NH3) and to some extent microbial and algal activity (Cloem et al. 1983a,b). It is unclear whether these same controls exist during nonsteady state periods.

Acknowledgments We thank E. Sterling, A. Maest, S. Pasilas, J. Thome,

E. King, C. Morris, J. Leisenring, and P. Blunden for field and laboratory assistance. Special thanks go to G. Dana and the staff at SNARL for logistical support. S. Rowghani and B. White of LADWP provided lake surface elevation readings for 1987-1989. We thank J. Carter for help with data analysis. The manuscript benefited from reviews by J. Cloern, K. Kuivila, J. Melack, and D. Hammond. We are grateful to the associate editor and reviewers for their thorough comments.

The breakdown of meromixis over a period ofyears seldom has been studied. Griesel (1935) examined evidence of mixed-layer deepening over a 20-yr period in the Hemmelsdorf See, but ended his observations before overturn. Steinhorn (1985) and others (Steinhorn et al. 1979) documented the successive deepening of the mixed layer of the Dead Sea during the late 1970s and overturn in 1979. Stiller and Chung (1984) used the Ra activity in Dead Sea surface water and deep water before turnover to esti- mate the time since the previous holomixis. Maclntyre and Melack (1982) described the gradual weakening of meromixis in a meromictic African rift valley lake.

The impact of destratification on the distribution of dissolved reduced chemical species and associated microbiological activity in de- volving meromictic lakes is poorly under- stood. In the first paper of this series on Mono Lake, California, Jellison and Melack (1993) described the changes in vertical mixing ac- companying the development (1982-1 983) and breakdown (1988) of meromixis. In the second paper Jellison et al. (1993) examined the distribution of nitrogen in Mono Lake over the same period. Here we report the chemical re- sponses of dissolved methane, sulfide, and oxygen to mixing in the lake.

1040



Sulfide and methane in Mono Lake 1041

Mono Lake is in a tectonic depression east ofthe Sierra Nevada escarpment in the vicinity of Yosemite National Park, California (Jelli- son and Melack 1993). Water enters the lake by streams, springs, and direct precipitation (0.2 m yr-I over 150 kM2) and exits the lake by evaporation (1.1 m yr-', Vorster 1985). Since 1941, about 1.2 x 108 m3 yr -I of freshwater destined for the lake has been exported from the basin to Los Angeles; by early 1982 the lake was at its lowest historical level (1,942 m) and highest salinity (Blevins et al. unpubl. rep.). Two consecutive wet winters including the ENSO event of 1982-1983 resulted in the input of 4.3 x 108 m3 of freshwater to the lake surface and produced a 2.6-m rise in lake level (Oremland et al. 1987).

Methods Water-column and sediment samples were

collected at a principal station (No. 9) in the western basin of the lake near a permanently moored buoy (Jellison and Melack 1993, their figure 1). Fifteen water-column profiles, 10 gravity cores, and numerous surface sediment grabs were recovered from this site between July 1984 and August 1989. Samples from several other stations and transects were taken throughout the study, including a station oc- cupying a structural trench east of the main island in the lake (No. 4; water depth, 34 m) and a transect from station 9 to the western shoreline.

Water samples were recovered by Niskin bottle or by peristaltic pump (Oremland et al. 1987). Dissolved oxygen and temperature were measured with an in situ probe including a calibrated thermistor (precision, 0. 1C) and a membrane electrode that was insensitive to the presence of dissolved HS- (Cloern et al. 1 983a). Dissolved oxygen data were in good agreement with values determined by the Miller method (Walker et al. 1970) and with data collected by Jellison et al. (unpubl. rep.) with a membrane electrode without sulfide-insensitive electrolyte. Errors reported throughout this paper are 1 SD of the mean.

Water for analysis of dissolved constituents (ammonia, sulfide, chloride, and sulfate) was collected by overfilling a twice-rinsed 60-cm3 plastic syringe from the large end and quickly inserting the plunger without trapping any air bubbles. A nylon filter holder containing a 25-

mm-diameter, 0.4-,um Nuclepore filter was quickly inserted over the hub end followed by a 22-gauge needle. After rinsing the filter, the sample was expressed serially through the filter and needle to a 2-cm3 Vacutainer containing 0.25 cm3 of 15% zinc acetate (final Zn concn, 76 mM). Subsamples were stored as ZnS for up to 6 months before analysis for sulfide by colorimetry (Cline 1969) with standards prepared in 0.5 M NaCl or in Mono Lake surface water. Two standard curves, necessitated by differences in scale, were prepared for surface water and deep water, resulting in variable detection limits (surface water, 3-10 ,M HS-, deep water, 20-50 ,uM HS-).

Filtered subsamples for ammonia (NH3 + NH4+), chloride, and sulfate were next dis- pensed into polyethylene bottles, allowing no headspace. Subsamples were refrigerated and returned to the laboratory where ammonia was analyzed within several hours of collection (Solorzano 1969) with standards prepared in 0.5 M NaCl or in Mono Lake surface water. However, this surface water contained some ammonia (Jellison et al. 1993) and its use in preparing standards was discontinued after 1986. This blank was small compared with monimolimnion ammonia values and had lit- tle effect on later interpretations. Dual standard curves were prepared for ammonia in surface and deep water, resulting in detection limits of 2-5,uM NH3 in surface water and 10- 20 in deep water.

Sulfate and chloride samples were analyzed by ion chromatography (Oremland et al. 1987) after a 200-fold dilution. Chloride was also measured by coulometric titration with silver ions (Aminco titrator) and these values were averaged with the chromatographic results. The 1980 chloride data were determined by titration only. Accuracy was ?4% for sulfate and both chloride measurements, while precision was ? 4% for chromatographically determined chloride and ? 2% for sulfate and coulometric chloride measurements.

Dissolved methane was extracted at the time of sampling by a syringe-shaking and gas-phase equilibration (Rudd et al. 1974), stored at room temperature for several hours in serum bottles with rubber stoppers, and analyzed on site by gas chromatography with a flame ionization detector (Oremland et al. 1987; detection lim- it, 0.05,uM). Isopleths of dissolved sulfide and



1042 Miller et al.

methane were created by coarsely gridding all the available data (15 profiles over 5 yr). The plots were smoothed so that long-term trends are emphasized over transient phenomena.

Sediments were collected with either a piston (4-cm i.d.) or gravity (10-cm i.d.) corer that sampled the upper 1-2 m of the sediment column. Sediments were subsampled within several hours of collection by extruding in 2- or 5-cm-thick intervals. A freshly exposed interval was immediately sampled for methane (Oremland et al. 1987). Porosity was determined by weight loss on drying and calculated from the measured water content with the relationship of Bemer (197 1).

Dry density was determined by measuring the volume of a known weight of sediment (n = 13). Pore-water density was assumed constant with depth at 1.10 g cm-3. Methane determinations that were made on a per weight basis were corrected to pore-water concentrations by using the measured porosity to cal- culate wet density or by assuming a porosity of 0.90. Total organic C analyses were made on carbonate-free (HCI leached and fumed) samples with a LECO carbon analyzer and are reported as a percent of the original dry weight.

Pore water was separated from sediments by either centrifugation (8,000 x g for 15 min) followed by filtration (0.4-,um Nuclepore) or by gas-pressure (N2) squeezing through 0.4-,um Nuclepore filters. Pore-water squeezers were made of solid PVC and used rubber gaskets and a latex rubber diaphragm to apply pressure to the surface of the sediment interval. Pore waters were expressed through a series of 9-cm- diameter filters directly into N2-flushed plastic syringes. Subsamples for dissolved sulfide, ammonia, sulfate, and chloride were analyzed with the same methods described for water- column samples (see above). There was no dif- ference in the sulfate concentrations of sediments that were slurried with zinc acetate (10%) before centrifugation and those that were un- treated, suggesting that oxidation of interstitial sulfide to sulfate was of minor importance during core processing.

Pore-water concentration profiles of reduced species in the upper 10 cm of all sediments were modeled after Klump and Martens (1981) to determine concentration gradients at the sediment-water interface. Exponential equa- tions were fit by least-squares minimization to

all sulfide data from 10 cm to the sediment surface (bottom-water values) with

C,= (C. - Co)(-e-z) + C. (1)

where Cz, CO, and C0 are the concentrations at depth z, infinity, and the sediment-water interface. C, and a were calculated; C0 was fixed at the bottom-water concentration. The gradients were calculated at z = 0, and the errors were estimated from the range in both a and COO required to bracket the data (?30% for HS-). A linear equation was fit to all methane data in the upper 10 cm because an exponential fit was not realistic (would not con- verge). The flux of reduced species out of the sediment was calculated from the best-fit concentration gradient with Fick's first law of diffusion:

Flux =-D d (2)

where b is porosity at the interface, D is the sediment diffusion coefficient at 5?C corrected for viscosity (ML = 1.2 x flwater) and tortuosity (02 = 1.24), Cis concentration, and z is positive downward (DCH4, Isenogle 1985, DHS-, Li and Gregory 1974; tortuosity correction, Ullman and Aller 1982).

Whole-lake inventories of sulfide and methane were obtained by integrating the volume- weighted concentrations for each date that a water-column profile was obtained. The volume below 26-m depth (Pelagos unpubl. rep.) was multiplied by the deepest sample concentration to determine the uninvestigated inventory below 26 m. This calculation results in underestimated inventories if concentrations increased significantly below 26 m. Ammonia inventories estimated by this method of calculation were <10% (15?56 x 106 mol, n = 15) higher than inventories measured by Jel- lison et al. (1993). The validity of the calculated inventory also assumes that station 9 was representative of the lake (i.e. that there were no significant horizontal concentration gradients). No horizontal methane concentration gradients were found in a transect between station 9 and the western shore (samples collected below the oxycline at 17-m depth in 200-m intervals over 1 km, [CH4] = 8.4?0.4 ,uM). Also, dissolved sulfide and CH4 profiles were nearly identical at stations 9 and 4 in August




Jul Oct Oct Apr Oct Feb Nov Mar 1980 1984 1985 1987 1987 1988 1988 1989 (604) (521) (526) (496) (522) (528) (549) (564)

30

1 0 E~~~~~~~~~~

18 100M m W in Error: Fc(28 (577) (557) (543) (555) (4)(568)

30 68 52 (621)

1980 1982 1984 1986 1988 1990 Fig. 1. Vertical profiles of dissolved chloride concentration in the water column vs. depth below surface. Concen-

tration scale and error bar are constant for all profiles. Top and bottom concentrations (mM) are given in parentheses. Meromixis developed in spring 1983; overturn occurred in November 1988. Depth of the pycnocline-*.

1988. Whole-lake inventories of dissolved oxygen in October and November 1988 were determined from the water-column data obtained by Jellison et al. (unpubl. rep.).

Eddy diffusivities at the pycnocline were averaged over the period of heat input to the top of the monimolimnion (Jellison and Melack 1993). Eddy flux was obtained by multiplying the average diffusivity by the sulfide and methane concentration gradients at the pycnocline. Loss due to entrainment was estimated from changes in the depth of the pycnocline (corrected for changes in lake level as the pycnocline elevation):

Entrainment = [%AV] [inventory] (3) [area][time]

where %AV is the fractional decrease in monimolimnion volume caused by the deepening of the pycnocline. In this manner, entrainment simply removes dissolved reduced compounds stored above the final pycnocline ele-

vation. We assumed that the mixolimnion was a sink for dissolved reduced species, and the amount of reduced species stored in the mixolimnion was small compared to the amount stored in the monimolimnion (whole-lake inventory = monimolimnion inventory).

Results Water column-In 1980, before meromixis,

the water column was isohaline with a chloride concentration near 620 mM. Chloride concentrations ranged from 500 mM at 16 m to 628 mM at 26 m in October 1984 (Fig. 1). The effect of enhanced runoff in 1982 and 1983 caused an 18.6% dilution of the surface water and establishment of a well-developed chemocline coincident with the main pycnocline. Chemocline depths based on conductivity gradients (Jellison and Melack 1993) are shown in Table 1 and Fig. 1 for selected sampling dates. Dissolved sulfate ranged from 101 mM in surface water in 1984 to nearly 130 mM in

Table 1. Hypsographic parameters.

Surface Pycnocline Pycnocline elevation* deptht elevation* Pycnocline Monimolimton - - ~~~~~~areat: volumet:

(m) (kM2) (km3)

18 Oct 84 1,944.5 14.0 1,930.4 100.35 1.191 8 Oct 85 1,944.0 16.0 1,928.0 92.05 0.960 4 Nov 86 1,944.4 19.0 1,925.4 82.07 0.734 13 Oct 87 1,944.1 19.0 1,925.1 80.79 0.709 16 Aug 88 1,943.7 24.0 1,919.7 56.56 0.335 22 Nov 88 1,943.5 28.0 1,915.5 32.82 0.147 * Above sea level; data supplied by LADWP. f Below surface; based on conductivity (Jellison and Melack 1993); during 1988 based on temperature (Jellison et al. unpubl. rep.). : Data from Pelagos (unpubl. rep.).



1044 Miller et aL

TEMPERATURE (OC) 0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20

0

4 Mix

8 ~~Mix

12 z - Mix

16 Chmi a ~~~~~~~~~~~Chin 20 . ..... .... . . . . .

24 Mon Mon ? 0 40 80 120 160 200 0 40 80 120 160 200 0 40 80 120 160 200

DISSOLVED OXYGEN (gM) Fig. 2. Vertical profiles of temperature (C1) and dissolved oxygen (0) in the water column during meromixis (A-

January 1987; B-April 1987) and after overturn (C-November 1988). Measurement error (? 1 SD) is <1 symbol size. Shown also are the mixing regimes of the lake: mix- mixolimnion; chm-chemocline region defined by chloride and sulfate profiles; mon-monimolimnion.

bottom water in 1986. Ratios of sulfate to chloride were constant (0.22?0.01) throughout the water column and over the study period.

During meromixis, temperature and dissolved oxygen varied seasonally in the mixolimnion but not in the monimolimnion (Fig. 2A,B). Surface water temperature ranged seasonally from 1.5 to 22.6?C. A seasonal thermocline developed annually in early spring and stabilized in summer and early fall at 10-20-m depth. In winter (Fig. 2A) the lake developed inverse thermal stratification. Bottom water remained between 3.7 and 4.6?C at all times during the year until overturn in November 1988 (Fig. 2C). Immediately after overturn, the water column was nearly isothermal at 8.7 ?0.1?C (n = 27).

Dissolved oxygen in the surface water ranged from 114 ,uM in August 1986 to 182 in Feb- ruary 1988. A well-developed oxycline existed at about the depth of the thermocline during stratification (Fig. 2A,B). After overturn in November 1988, the entire water column became anoxic for several months (Fig. 2C, G. Dana pers. comm.).

Dissolved sulfide increased in the monimolimnion from 130 ,uM in 1984 to a maximum of 1,820 by February 1988 before decreasing to 890 ,uM in August 1988 (Fig. 3A). A small amount (< 10 ,M) of dissolved sulfide

was present throughout the water column between overturn in November 1988 and March 1989. Sulfide was otherwise absent from the mixolimnion (Fig. 4A).

Dissolved methane increased in the monimolimnion from 25.5 ,uM in 1984 to a maximum of 57.0 in February 1988 before decreasing to 38.3,uM in August 1988 (Fig. 3B). Following overturn in November 1988, methane was uniformly distributed at 3.4?0.2,uM (n = 8, Fig. 4B). Four months later, in March 1989, methane was still 3.1?0.2 ,M (n = 8) throughout the water column. The maximum concentration in the mixolimnion before No- vember 1988 was 1.3 ,M (January 1987).

Ammonia increased in the monimolimnion from 139,uM in 1984 to 600 in October 1987 before decreasing to 313 ,M in August 1988. Following overturn in November 1988, ammonia ranged from 33.2 ,uM at 16 m to 58.6 at 25 m, and averaged 41?5 ,uM (Fig. 4C, n = 8). By March 1989 the mean ammonia concentration had increased to 54?3 ,uM with a slight trend increasing with depth. Ammonia concentration was elevated in the surface mixed layer in August 1989 (mean, 45?5 ,uM, n = 5) while values as great as 125 ,uM were observed in the monimolimnion. The maximum concentration of dissolved ammonia in the mixolimnion before overturn (November 1988) was 22.0 ,uM (April 1987).




SULFIDE (gM)

10

15

.0 ,00 ::3~400 20 -

25 -

~~ 25 ( 8/86 ~~~~~~~1/87 78 28 8/88 3/89

10

320 0. _ o5 --

25 /

30 1O/84 5/85 10/85 8/86 11/87 7/87; 2/88; 8/88; 3/89

1556

1984 1985 1986 1987 1988 1989 1990 YEAR

Fig. 3. Distribution of dissolved sulfide and dissolved methane in the water column. Figures are drawn from grids of over 200 data points, based on 15 vertical profiles.

v A NOV 1988 1 7 9 105 OAU9 1989 1 1 8 1 1 10

K 20 ; 20

30 L I I I I I I I I I I I I 30 0 200 400 600 800 1000 0 10 20 30 40 50 60 0 50 100 150 200 250 300

SULFIDE (,UM) METHANE (,UM) AMMONIA (,UM) Fig. 4. Vertical profiles of dissolved sulfide, dissolved methane, and dissolved ammonia in the water column.



1046 Miller et al.

DISSOLVED SULFATE (mM) 0 20 40 60 80 100 120 140

20

E?40 0

z60-

80-

100

120 I I 2 3 4 5 6 7

ORGANIC CARBON (weight %) Fig. 5. Vertical profiles of sediment organic carbon (0)

in one core and interstitial dissolved sulfate (1, A) in two cores collected at station 9 in August 1986.

Sediment and pore water-Organic C ranged from 3% near the sediment surface and at 20- cm depth to nearly 7% at 62-cm depth (Fig. 5). Dissolved sulfate concentrations decreased from near-bottom water values (130 mM) at the sediment-water interface to 37 mM near 120-cm depth. Dry density was nearly constant throughout the sediment column at 2.3 ?0.1 g cm-3 (n = 13) and was independent of depth (data not shown).

Porosity varied between 0.80 and 0.92 in all sediments with the highest values at the sediment-water interface. Three determinations between 0.70 and 0.80 were associated with gray volcanic ash layers in the sediments at 60-70 cm.

Dissolved sulfide increased with depth in the sediments and ranged from 983 uM at 2.5 cm to 18,500 at 109-cm depth (Fig. 6A). Replicate cores were collected and analyzed for sulfide in August 1988. Domagalski et al. (1989) reported similar profiles of dissolved sulfide and total organic C in Mono Lake sediments.

Methane ranged from near-bottom water values (55.7 uM at 2.5 cm) to supersaturation at 1 atm (> 1,200 uM) by 1--m depth in some sediment cores. Methane generally increased with depth through the upper 50 cm. Values > 2,200,uM measured in cores collected in Au- gust 1986 and October 1987 were associated with visible bubbles.

Sediment fluxes and water-column inven-

Table 2. Pore-water concentration gradients, diffiusiv- ities, and calculated fluxes from the sediment to the monimolimnion. Flux is positive upward.

OC/Oz* D x 10t Fluxt (Umol cm-4) (cm2 s-') (mmol m-2 d-')

HS- 0.92, n = 5 0.76 6.3 (0.43-1.58) (?0.05) (2.8-12)

CH4 0.07, n = 12 0.95 0.6 (0.024.16) (?0.05) (0.2-1.5)

* Best-fit gradient at z = 0; range in parentheses; n-number of profiles. t Molecular diffusion coefficient corrected for viscosity at 5?C (0.8 centipoise)

and tortuosity (02 = 1.24); uncertainty in parentheses. Flux = -tD(aC/Oz), where t = 0.90; range in parentheses is due to com- pound uncertainty in the gradient and diffusivity.

tories-The calculated diffusive flux across the sediment-water interface ranged from 0.6 mmol m-2 d-1 for dissolved methane to 6.3 for sulfide (Table 2). Uncertainty in the tracer diffusion coefficient (0.05 x 10-5 cm2 S-1 Li and Gregory 1974) was included in the calculated range of flux.

The calculated total lakewide inventory of dissolved sulfide ranged from 107 x 106 mol in October 1984 to 1,100 x 106molinOctober 1987 (Fig. 7A). After complete overturn in No- vember 1988, the sulfide inventory was 14 x 106 mol and decreased to 6 x 106 mol by March 1989. The inventory of dissolved sulfide increased to 60 x 106 mol by August 1989.

The total lakewide inventory of dissolved methane was 28 x 106 mol in July 1984 and increased to 45 x 106 mol in January 1987 (Fig. 7B). After overturn, the methane inventory was 9.8 x 106 mol and decreased to 2.6 x 106 mol by August 1989. Eddy flux ranged from 0 to 21 mmol mr-2 d- 1 for HS- and from 0.2 to 1.4 for CH4 (Table 3).

Discussion Stratification and mixing in the water col-

umn-The influx of freshwater to the surface of the lake in 1982 and 1983 had a large effect on the density structure of the water column (Jellison and Melack 1993). Chloride profiles (Fig. 1) show that dilution of the saline lake water was restricted to the upper 16 m by 1984. From 1984 through April 1987 the chemocline deepened and remained well defined between 14- and 19-m depth. Jellison and Melack (1988) reported deepening of the mixolimnion during this period. Temperature initially exerted less influence on the density of the water than salinity (0.5 vs. 1.5 x 10-2 g cm-3 in 1984, Jellison and Melack 1993), yet it was closely




0

20

40

60

80

0 Aug 1986 0 Aug 1986 ,+ 100 Z Nov 1986 E *LI Nov 1986

E A Aug 1988 A Aug 1988

' 120 I I I I I I I I 0 10,000 20,000 0 1000 2000 3000

00

2 .m9V May 1986 V A *O~O Aug 1986

E LI9 Nov 1986 4 ~~~~~~~~~~~~~~~~~~Fl Apr 1987

0 *0 ?~~~~~~~~~~~JulI1987 EA * * *0~~~~~~~~~ct 1987

A Feb 1988 6 4.AAug 1988

V Mayl1986 8 0 Aug 1986

El Nov 1986 AA Aug 1988

10 . 0 2000 4000 6000 8000 0 500 1000 1500 DISSOLVED SULFIDE (pM) DISSOLVED METHANE (gM)

Fig. 6. Vertical profiles of dissolved sulfide and dissolved methane in sediment interstitial water at station 9. Panels A and C represent entire core profiles. Panels B and D represent detail of the upper 10 cm of sediment and show all the data. Solid lines-best fit to the data above 10 cm using the models explained in the text. Dotted lines-model fits adjusted to the range of data. Values at 0 cm are bottom water concentrations. Replicate cores were collected in August and November 1986 and August 1988.

coupled with the distribution of dissolved nu- trients and gases. In summer the thermocline and oxycline were nearly coincident above the chemocline (Fig. 2B). Salinity increases in the mixolimnion led to lower density differences

between the mixolimnion and the monimolimnion (Jellison and Melack 1993). By 1988 the water column was isohaline and thermal control of the density structure resumed.

Dissolved sulfide and methane increased an-



1048 Miller et al.

1,500 1 1 I1 I 1,500 j) 22 Nov 88

E 1,000

E500 -J

0 60 I

22 Nov 88

E % 40

z z 20

1984 1985 1986 1987 1988 1989 1990 YEAR

Fig. 7. Total (lakewide) inventories of dissolved sulfide and dissolved methane vs. time. Date of overturn is shown.

nually in the anoxic portion of the mixolimnion until winter mixing redistributed them throughout the mixolimnion where they were subject to chemical, physical, and biological removal (Oremland et al. 1987; Cloem et al. 1983a). Decreasing water-column inventories resulted from winter mixing events that deepened the mixed layer, entrained water from the monimolimnion, and sometimes produced lower bottom-water concentrations. For in- stance, decreases in monimolimnion sulfide, methane, and ammonia concentrations at station 9 and concurrent ammonia decreases at two other sites in the lake during winter 1987- 1988 (Jellison et al. 1993) may have resulted

from lakewide mixing with waters of the mixolimnion.

Chemical mass balance-Table 2 shows the calculated sediment fluxes of sulfide and methane to the monimolimnion for the best-fit concentration gradients at station 9. These esti- mates will be too low if ebullition of methane bubbles from the sediments occurs (Oremland et al. 1987), biogeochemical processes acting at the sediment-water interface are important, or pore-water advection due to groundwater flow is important. If we assume no sulfide or methane was present at the onset of meromixis (spring 1983) and no process other than input from the sediments was adding or removing dissolved solutes from the water column, then 340 x 106 mol of sulfide and 34 x 106 mol of methane should have accumulated by October 1984. We measured 110 and 32 x 106 mol of sulfide and methane, respectively. The measured methane inventory was within the range predicted by the sediment flux. However, the sulfide deficit (predicted - observed) implied either that the assumption of no net sulfide change in the water column was incorrect or that sulfide flux from the sediment was over- estimated. Because the caveats mentioned above led to underestimates of the flux, we felt it was more likely that processes that removed sulfide were responsible for the lower measured inventory. Removal processes include, but are not restricted to, biogeochemical re- actions in the mixolimnion.

The methane flux to the mixolimnion was corroborated by comparing calculated CH4 losses (Table 3) by eddy diffusion (0.2-1.4 mmol m-2 d-1) and entrainment (0-0.7 mmol m-2 d-1) with the flux of methane across the air-lake interface measured by static cham- bers. For the period October 1985-October 1987, the measured flux ranged from 0.01 to

Table 3. Annual whole-lake storage, eddy diffusive flux across the pycnocline, and loss by entrainment during winter mixing for HS- and CH4 (mmol m-2 d-').

Storage* Eddy fluxt Entrainmentt

HS- CH4 HS- CH4 HS- CH4

Oct 84-Oct 85 4.0 -0.1 0 0.6 0.9 0.3 Oct 85-Nov 86 8.6 0.2 13 1.1 3.2 0.4 Nov 86-Oct 87 19 -0.2 0.5 0.2 0 0 Oct 87-Aug 88? -24 -0.3 21 1.4 23 0.7 * (A inventory)/[(area of pycnocline)(time between dates)]. t (Eddy diffusivity)(concentration gradient at pycnocline) for the period of thermal stratification, normalized to time between dates. t [(% change in monimolimnion volume)(inventory)]/[(area of pycnocline) (time between dates)]. ? August 1988 was the last sampling date before overturn.




Table 4. Initial and final lakewide inventory, sediment flux, and loss by entrainment during descent of the thermocline and during overturn for HS- and CH4 (106 mol).

Inventory* Entrainmentt Sediment flux Inventoryt

HS- CH4 HS- CH4 HS- CH4 HS- CH4

16 Aug-15 Nov 88 490 26 160 9.5 56 5.3 390 22 15 Nov-22 Nov 88 390 22 380 12 2.7 0.3 14 9.8 * Measured inventory 16 August 1988; calculation of inventory for 15 November 1988 given in text. t Integrated loss between 13 and 23 m for 16 August-I 5 November 1988, calculated for 15-22 November 1988 as entrainment = initial inventory + sediment

flux-final inventory. : Calculated inventory 15 November 1988; measured inventory 22 November 1988.

4.8 mmol m-2 d-l (n = 15), with a median value of 0.3 mmol m-2 d-l (Miller and Or- emland 1988). Aerobic methane oxidation in the mixolimnion was inferred from a 20-40oo shift in 613C of residual CH4 in the mixolimnion (Oremland et al. 1987).

Overturn and postmeromixis-Vertical concentration gradients of major dissolved solutes were small in the lake throughout most of 1988. We used the water-column inventories of sulfide and methane in August and a detailed his- tory of the thermocline elevation from August to November (Jellison et al. unpubl. rep.) to constrain the amount of dissolved HS- and CH4 present in the water column at the time of overturn (15-22 November 1988, Table 4). The thermocline (pycnocline) descended from 13 m on 18 August to 23 m on 15 November, entraining the inventory of HS- and CH4 between 13 and 23 m. Sediment flux was assumed constant during this period and eddy diffusion was not included because of the rapid change in pycnocline elevation. The amount of dissolved HS- and CH4 below the pycnocline was then calculated as the initial inventory minus entrainment plus sediment flux, based on the area of the pycnocline in August (24 m). These calculations indicated that dissolved sulfide decreased by 20% and dissolved methane by 16% between 16 August and 15 November 1988.

The calculated inventory for 15 November was then used as the initial inventory for the overturn event, which occurred between 15 and 22 November 1988. The final inventory on 22 November was measured, so that the loss from the hypolimnion could be calculated as the loss due to entrainment by means of the mass balance relationship (above) after ac- counting for the ongoing sediment flux. Over- turn entrained 380 x 106 mol of HS- and 12 x 106 mol CH4 in 1 week (Table 4).

Chemical oxidation with molecular oxygen appears to be the major sink for dissolved and colloidal sulfide. The complete oxidation of 1 mol of sulfide requires 2 mol of oxygen. If the amount of oxygen stored in the water column on 25 October 1988 (190 x 106 mol) reacted with 380 x 106 mol of sulfide then a stoichiometry (02/HS -) of 0.5 resulted if sulfide were the only sink for oxygen. J0rgensen et al. (1979) found a stoichiometry of 1.0 at the sulfide- oxygen interface in Solar Lake and suggested that sulfide is oxidized beyond elemental sulfur but not to sulfate. Complete oxidation of 380 x 106 mol of sulfide to sulfate by means of unknown oxidants would have resulted in a net increase of only 0.2 mM So4-2 -a change (in percentage terms) too small to be verified. Evasion of H2S during overturn was probably not a significant sink because H2S was <0.2% of total reduced inorganic sulfur at the pH (9.8) of Mono Lake.

Oxidation of reduced or partially oxidized sulfur compounds on a slower time scale than the initial oxidation of sulfide may have pre- vented dissolved oxygen from increasing during December and January (J0rgensen 1990). Dissolved oxygen in the lake remained very low for a period of several months after overturn (G. Dana pers. comm. and this study). Thus, invasion of oxygen would have been rapid because of the high gradient across the air-water interface; however, no oxygen was present in the water column through January 1989. By March 1989, 260 x 106 mol 02 were dissolved in the water column.

Methane was primarily lost by ventilation across the air-lake interface. Of the 26 x 106 mol of CH4 dissolved lakewide in August 1988, 9.5 x 106 mol were entrained and then lost between August and November. If the CH4 remaining in the lake on 15 November 1988 was then mixed conservatively throughout the



1050 Miller et al.

water column, a concentration of 7.6 AM would have resulted. Only 3.4 AM was measured immediately after overturn. Methane oxidation in the anaerobic water column may have removed up to 2.5 x 106 mol of CH4 in the week before sampling (calculated with an anaerobic oxidation rate of 0.64 mmol m-2 d-l and a lake area of 166.8 kMi2, N. Iversen pers. comm.). By contrast, Rudd and Hamilton (1978) measured CH4 oxidation rates as high as 32 mmol m-2 d-I during fall overturn of eutrophic freshwater Lake 227 in the Canadian shield.

In Mono Lake, 9.4 x 106 mol of CH4 was presumed lost by evasion to the atmosphere in 1 week (calculated as 11.9 x 106 mol entrained less 2.5 x 106 mol oxidized anaero- bically). A methane evasion rate, calculated as the loss divided by the lake area, was 8.1 mmol m-2 d-l or 26 times the median measured methane flux during the period of stratification (0.3 mmol m-2 d-1, Miller and Oremland 1988). The evasion flux calculated from mass balance is slightly greater than that (0.25-6.4 mol m-2 d- 1) calculated with

Flux = KL x CW (4)

where KL is the piston velocity (3.5 cm h-1) and Cw the surface water concentration (Wan- ninkhof 1986; Miller and Oremland 1988). By comparison, 2.34 x 104 mol of CH4 was reported lost by evasion during fall overturn in Lake 227 (Rudd and Hamilton 1978) which resulted in a higher evasion rate of 76 mmol m-2 d-I because that lake was smaller. We conclude that methane evasion at overturn of Mono Lake is comparable to the episodic methane flux from freshwater or saline lakes that are mono- or dimictic.

The water column was well mixed during winter 1988-1989 (Fig. 2C). The sulfide inventory was low at the onset of thermal stratification, however CH4 was elevated. Thermal stratification in spring preceded a period of elevated dissolved reduced species in the bottom water (Fig. 4). From March to August 1989 sulfide inventories increased at 3.4 mmol m-2 d- 1. Methane inventories decreased from March to August 1989 as oxidation and evasion continued to remove excess methane.

The 1982-1983 ENSO event is considered to be the strongest of the past century (Ras- musson and Wallace 1983), but many other El Nifio events of strong and moderate intensity

have occurred (Quinn et al. 1987). The link between regional climate and ENSO remains uncertain however, and not all events are associated with increased precipitation. Rare as the large events may be, the addition of 108 m3 freshwater could initiate another multiyear period of meromixis in Mono Lake, particu- larly if salinity is maintained at or above its present level. If and when ectogenic meromixis occurs, the effects on the biogeochemical cycling of sulfur and carbon are likely to be sub- stantial.

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Submitted: 6 February 1991 Accepted: 10 January 1993 Revised: 18 February 1993



Documents

Meromixis in Hypersaline Mono Lake, California. 3. Biogeochemical Response to Stratification and Overturn