The meromictic alpine Lake Cadagno: Orographical and biogeochemical description

Abstract

Lake Cadagno is a 21 m deep alpine meromictic lake situated at an altitude of 1921 m in the Piora valley in the southern part of central Switzerland. The bedrock of the valley containing dolomite and gypsum determines the chemistry of the water. The lake basin was created by glacial erosion and originally dammed by a glacial moraine. The water body is structured in 3 distinct layers, the oxic mixolimnion, the anoxic monimolimnion and a narrow chemocline in between. The water masses of the lake are stabilized by density differences of salt-rich water which is constantly supplied by subaquatic springs to the monimolimnion. In contrast the mixolimnion is fed by electrolyte-poor surface water. Sulfate, hydrogen carbonate, calcium and magnesium are the dominant ionic species. In the monimolimnion sulfide concentrations of more than 1 mM are found. The chemocline at a depth of 10 to 13 m is characterized by steep chemical and physical gradients. It contains dense populations of up to 10 5 cells/mL of phototrophic sulfur bacteria consisting of predominantly Chromatium okenii, C. minus and Amoebobacter purpureus. The lake has proven to be an excellent model system for studies of the role of planktonic bacteria which dominate the sulfur cycle.

Full text

The meromictic alpine Lake Cadagno:
Orographical and biogeochemical description
Claudio Del Don,
1
Kurt W. Hanselmann
1
, Raffaele Peduzzi
2
and Reinhard Bachofen
1,
*
1
Institute of Plant Biology, University of Zürich, Zollikerstr. 107, CH-8008 Zürich, Switzerland
2
Istituto Cantonale Batteriosierologico, via Ospedale 6/via Fogazzaro 3, CH-6900 Lugano,
Switzerland
Key words: Morphometry, geology, water chemistry, water density, diurnal and
seasonal changes, light regime.
ABSTRACT
Lake Cadagno is a 21 m deep alpine meromictic lake situated at an altitude of 1921 m in the Piora
valley in the southern part of central Switzerland. The bedrock of the valley containing dolomite
and gypsum determines the chemistry of the water. The lake basin was created by glacial erosion
and originally dammed by a glacial moraine. The water body is structured in 3 distinct layers, the
oxic mixolimnion, the anoxic monimolimnion and a narrow chemocline in between. The water
masses of the lake are stabilized by density differences of salt-rich water which is constantly sup-
plied by subaquatic springs to the monimolimnion. In contrast the mixolimnion is fed by electro-
lyte-poor surface water. Sulfate, hydrogen carbonate, calcium and magnesium are the dominant
ionic species. In the monimolimnion sulfide concentrations of more than 1 mM are found. The che-
mocline at a depth of 10 to 13 m is characterized by steep chemical and physical gradients. It con-
tains dense populations of up to 10
5
cells/mL of phototrophic sulfur bacteria consisting of predo-
minantly Chromatium okenii, C. minus and Amoebobacter purpureus. The lake has proven to be
an excellent model system for studies of the role of planktonic bacteria which dominate the sulfur
cycle.
Introduction
Lake Cadagno is a small meromictic lake, remote from industrial activities. The
specific water chemistry is defined by the geology of the catchment area. High
input of sulfate from gypsum coupled with a high productivity in the mixolim-
nion gives rise to a massive formation of sulfide in the anoxic monimolimnion.
In such lakes populations of planktonic phototrophic sulfur bacteria often de-
velop during summer stratification. These bacteria form visible blooms at the
interface between the oxic epilimnion and the anoxic hypolimnion, or on sedi-
Aquat.sci.63 (2001) 70–90
1015-1621/01/010070-21 $ 1.50+0.20/0
© Birkhäuser Verlag, Basel, 2001
Aquatic Sciences
* Corresponding author, e-mail: bachofen@botinst.unizh.ch

ment surfaces, at depths which are still reached by light. In permanently strati-
fied meromictic lakes (Walker and Linkens, 1975; Wetzel, 1983), blooms of pho-
totrophic sulfur bacteria are often present as a layer during the entire year. The
ecology of these organisms and their habitats have been recently reviewed
(Pedros-Alio and Guerrero, 1993; Van Gemerden and Mas, 1995).
Such a purple-red layer, formed by a bloom of Chromatium okenii, was first
observed 1913 in lake Ritom and in the nearby lake Cadagno (Düggeli, 1919,
1924), the site of the present observations. At this time various publications on
Lake Cadagno had dealt with water chemistry (Bourcart, 1906; Eder-Schweizer,
1924), plankton (Burckhard, 1910; Bachmann, 1924, 1928) and the fauna of the
lake bottom (Fuhrmann, 1897; Borner, 1927, 1928). During the past decade a
wealth of details of the biology of Lake Cadagno and the organisms present
have been reported by Wagener et al. (1990), Peduzzi et al. (1991), Joss et al.
(1994), Fischer et al. (1996), Birch et al. (1996), Schanz et al. (1998), Peduzzi et
al. (1998), Lehmann and Bachofen, (1999), Tonolla et al. (1999), Wiggli et al.
(1999), Bosshard et al. (2000a, b), Lüthy et al. (2000), Tonolla et al. (2000) and
Camacho et al. (this volume).
Lake Cadagno proved to be ideally suited for studies in microbial ecology
and for investigations on metabolic responses of aquatic bacteria, especially of
the phototrophs, to environmental signals such as light and environmental
redox conditions. On the other hand, the activities of the dense bacterial popu-
lation in a narrow layer causes strong gradients in the water chemistry, e.g. in
nutrient concentrations as well as in nutrient composition. The dynamics of the
water chemistry and the special conditions which follow from it, are conse-
quences of geological features, the lithology in the catchment area and the mor-
phology of the lake basin as well as the metabolism of the bacteria, mainly the
phototrophs and the sulfate reducers. As stated by Bourcart, (1906) “it serait
fort intéressant de faire des études approfondies sur ce lac si curieux”.
In this paper we enlarge the data on the bathymetric and orographic charac-
teristics, first presented by Garwood (1906) (Fig. 1), describe the geomorpho-
logic setting of the lake basin and the basic chemistry which are the core for the
richness of this microbial habitat. Detailed knowledge about the limnology of
the lake will help to better understand the unusual microbiology and form the
basis for future investigations on biological aspects of the various microbial
populations in the lake.
Materials and methods
Temperature, pH, conductivity, turbidity, oxygen concentration and light
These parameters have been measured in lake profiles simultaneously employ-
ing a multisensor unit (HPT c Züllig AG, Rheineck, Switzerland). Vertical
transmission and reflection of photosynthetically active radiation (SPAR 400-
750) have been determined with a cosine corrected Lambda sensor (LI-212 S,
Licor, Lincoln, Nebraska, USA). Detailed description of light measurements
are given by Fischer et al. (1996).
Lake Cadagno, a meromictic alpine lake 71

Sampling
The multisensor unit and the sampling devices were lowered from a stable wor-
king platform above the deepest point of the lake. Fine resolution sampling
was achieved with a syringe sampler consisting of 64 plastic syringes (50 mL)
mounted on an aluminum frame at distances of 5 or 10 cm. The syringes are ope-
ned simultaneously at the desired depth through pressure cylinders driven by
compressed air supplied by a cylinder which is attached to the sampler and
which can be controlled from the surface. Sample aliquots were divided im-
mediately after retrieval of the syringe sampler into preprepared vials at the
sampling site.
Chemical determinations
Carbonate alkalinity was titrated to an endpoint of pH 4.3 in small aliquots with
0.1 normal HCl (Titrisol Merck 9973). Other bases, e.g. HS
–
, NH
3
, HPO
4
2–
,
H
2
PO
4
–
, normally present in negligible concentrations (in the mixolimnion)
were determined by independent analytical procedures and the titrated alkali-
nity was corrected accordingly (necessary only for samples from the moni-
72
Del Don et al.
Figure 1. Map of the Cadagno region as illustrated by Garwood (1906)

molimnion). Ammonia was determined in filtered samples colorimetrically
after the methods given by the EDI (1983a). Cations of alkali, earth-alkali
and transition metals were determined by ICP-AES at the Federal Institute
for Forestry, Snow and Landscape, Birmensdorf (Switzerland). Soluble reac-
tive phosphorous was quantified after filtration of the water through glass
fiber filters (Whatman GF/F) by the molybdenum method (EDI, 1983b).
Sulfide was determined colorimetrically according to the methylene blue me-
thod described by Gilboa-Garber (1971). Sulfate was measured by ion exchange
chromatography (precolumn Wescan 269-003, separating column Wescan 269-
001) with a conductivity detector (Wescan 213A). Separation was achieved at a
flow rate of 1.2 ml min
–1
with p-hydroxybenzoate (4 mM, pH 8.5) as eluant.
Elemental sulfur was combusted in an oxygen atmosphere and determined
as sulfate by titration with barium perchlorate (0.02 M) containing Thorin as the
endpoint indicator.
Calculations
Water exchange area, sediment-water exchange area and lake volume were cal-
culated using the appropriate formulas for truncated cones based on a morpho-
metric map with contour lines spaced 1 m apart. The density of the water (
r
eff
)
was calculated from the temperature, the conductance and the chemical and
particle composition of the water according to
r
eff
=
r
max
· h
(T)
· h
(
k
)
· h
(m)
where
r
max
is the maximal density of pure water at 3.98°C, h
(T)
the function to
correct the density for temperature deviation (Weast et al., 1986), h
(
k
)
the func-
tion to correct the density for dissolved materials according to Pytkowicz
(1979), and h
(µ)
is the function to correct the density for the different suspended
particles. Ca
2+
, Mg
2+
, Na
+
, K
+
and HCO
3
–
and SO
4
2–
accounted for more than
99% of the dissolved compounds in the mixolimnion. For the monimolimnion
sulfide, phosphate, iron, manganese and ammonium were included in the cal-
culation (Millero et al., 1977). The influence of the conductivity on
r
eff
can be
expressed by the function (Weast et al., 1986)
h
(
k
)
= 7.7981 · 10
–7
·
k
25
+ 0.9999728 (g/cm
3
)
where
k
25
is the conductivity in µS cm
–1
normalized to 25°C. The conversion fac-
tor 7.7981 · 10
–7
has been derived applying partial molal volume calculations for
the chemical entities analyzed.
For the density contribution of the organisms in the bacterial layer a mean
density of 1.1 g/cm
–3
was used for bacterial biomass (Guerrero et al., 1984; Loch,
1989). Based on the turbidity measurements with the HPT-multisensor the fol-
lowing empirical relationship was derived
h
(µ)
= 1 – 2.9392 · 10
–14
+ 1.0564 · 10
–13
µFU – 2.4238 · 10
–16
µ
2
FU
µFU is the reading of the turbidity in formazan units.
Lake Cadagno, a meromictic alpine lake 73

The deviation from 1 is small, in spite of the seemingly high turbidity in the
transition layer. For the calculation of the water density changes the correction
by h
(µ)
had to be included, however, for the pycnocline.
Local water stability was calculated according to Imboden and Wüest
(1995).
Cell volumes of the layer of phototrophic bacteria were calculated from
microscopical measurements taking cells as spheres at each end or as cylinders
with 2 half spheres depending on their growth state. Isopleths were calculated
using Systat 5.2.1 (Statistical Production & Service Solutions, Inc., Evanston,
USA).
Results
Geographic location and morphology
Lake Cadagno (Lago di Cadagno), situated at 1921 m above sea level, belongs
to a group of 9 small lakes in the Piora depression in the central Alps of Swit-
zerland. The Piora valley spans over 8.5 km and is surrounded by a chain of
mountains which determine the watershed. The lake serves as one of the reser-
voirs for the hydroelectric power plant in Piotta. Part of the volume is drained
every winter which leads to an annual water level fluctuation of 3 m.
The lake basin was formed during the last glacial period, an estimated 8000
years ago as derived from pollen analyses in peat cores of a nearby swamp
(Stapfer, 1991). The west and southwest sides of the basin are filled in by glacial
deposits, and moraines build a natural dam. Avalanches, small land slides and
accumulations of dolomite “sand” have contributed to the clastic deposits in the
lake as determined from seismic profiles along several transects (Kriege, 1918;
Dal Vesco et al., 1964).
The north slope of the Piora valley contains metamorphic crystalline rocks,
the south is built by the so-called Lukmanier layer, composed of crystalline
rock as well. Below the glacial deposits the bottom of the valley consists of a
karstic system of Rauwacke and Dolomite which also contain some gypsum.
These rocks allowed the carving of a karstic hydrological system from which a
portion of the water entering lake Cadagno originates. Water penetrates
through the coarse material of the moraines and through the karstic dolomite
and reappears as underwater springs in the southern and western part of
the lake.
The water from the north is mainly in contact with silicate rocks; it is low in
salt content and determines the water composition of the mixolimnion. Many
small springs discharge into swamps which overflow into the lake. Water also
enters the lake in deeper zones through bottom springs of low or high conduc-
tance.
The slope line at the south-west side is flat and the water level low. In con-
trast, on the north side the mountains rise steeply and the shore drops accord-
ingly (see also Fig. 1). The main parameters describing the lake morphology are
summarized in Table 1.
74
Del Don et al.

A bathymetric map has been constructed based on 210 depth measurements
across 16 north-south transects at distances of 50 m resulting in a 1:1000 depth
contour map with 1 m depth resolution and a calculated optimal determination
information value of 0.9987 (Hakanson, 1981). The lake parameters calculated
from the map are given in Table 2. From the hypsographic equations which can
be calculated from the data given in Table 2 we determined areas and volumes
of specific layers.
Seasonal weather fluctuations and lake dynamics
The large and often rapid changes in atmospheric temperature, wind, radiation
and precipitation patterns in the alps govern the physical stability of the water
masses and the biological activities in this lake. Temperature extremes in daily and
weekly weather patterns during summer and fall are between + 20°C during the
day and near 0°C at night. The water surface freezes in December and the lake
becomes covered with ice- and snow-layers of up to 2 m thickness for 6 months.
Lake compartmentalization and transition zones
The lake can vertically be partitioned into distinct compartments which are
defined by the depth profiles of temperature, oxygen concentration and density.
Temperature dependent density stratification creates a well defined hypolimnion
below about 11 m depth during summer. The size of the epilimnion varies diur-
Lake Cadagno, a meromictic alpine lake 75
Table 1. Summary of the parameters which describe lake Cadagno
PARAMETER
a
Summer Winter
b
Altitude of water surface [m] 1921 1918
Maximum depth Z
max
[m] 21 18
Mean depth Z [m] 9.27 7.87
Median depth Z
50
[m] 8.5 7.8
Relative depth Z
r
[%] 3.64 4.43
Maximum effective length Le [m] 842 769
(Azimut 45‰)
Maximum effective width Be [m] 423 380
(Azimut 61.6‰)
Shore line length l
max
[m] 2109 1924
Shore line development l
d
[m] 1164 1168
Surface area A
Zmax
[m
2
] 261043 215839
Total volume V
Zmax
[m
3
] 2419850 1699600
Benthic contact area M
Zmax
[m
2
] 356300 271050
Area error E [–] 0.001050 0.000462
Information value of map I [–] 0.998721 –
a
Depth values are calculated in meters above the sediment surface. For the definition of the param-
eters see Appendix.
b
The values for the winter situation are based on a water surface level of 1918 m above sea level.

nally at this high alpine location due to large temperature changes at the water
surface between day and night. Mixing by convective turbulence can reach as
deep as 5 m. In summer, the thermocline is situated between 7 and 8 m (Fig. 2a),
while during winter we observe a temperature inversion (Fig. 2b) with a ther-
mocline between 1 and 2 m below the ice. During the winter months, the
monimolimnion and the sediment act as heat sources. Geothermal heat from
the underwater springs or metabolic heat produced by sedimentary microbial
activity might explain the higher temperature near the bottom during the win-
ter months (Fig. 2b).
The density profile in summer shows two zones with water masses which are
potentially more stable than those above and below (Fig. 3). Local stability is
expressed by the Brunt-Väisälä-Frequency (N) (Imboden and Wüest, 1995).
Higher frequencies indicate more stable water masses. In summer, the upper
pycnocline (pycnocline 1) situated between 6 m and 7 m depth is due to temper-
ature dependent density stratification while the second one between 8 and 9 m
above the sediment (pycnocline 2) originates from the higher salt content in the
monimolimnion. During fall, destratification moves the pycnocline 2 to about
6.5 m above the sediment. A rather constant density is characteristic for the
homothermal monimolimnion during the summer period. Of special interest is
the zone between 11 and 12 m showing almost constant temperature and thus
minimal stability (Fig. 3, insert). The irregularity in the profiles suggests that
76
Del Don et al.
Table 2. Dimensions describing the morphology of lake Cadagno
Depth above deepest Circumference of Water exchange
point, Z contour lines area, A
Z
[m] [m] [m
2
]
21 2110 261000
20 2070 247900
19 2040 234000
18 1920 215800
17 1890 195200
16 1800 175400
15 1720 159400
14 1660 145800
13 1600 134300
12 1540 123400
11 1480 113300
10 1410 103000
9 1340 92100
8 1250 81200
7 1170 70500
6 1050 58100
5 870 45600
4 750 35400
3 650 27500
2 550 20500
1 450 13000
0 110 800

Lake Cadagno, a meromictic alpine lake 77
Figure 2. Depth profiles of temperature ■■
.
and the temperature gradient
d
T/
d
z ® (a: summer, b: winter). Summer values were taken on August 18,
1987, winter values on March 31, 1985

78
Del Don et al.
Figure 3. Depth profiles of the water density ■■
.
and the Brunt-Vaisälä-Frequency (N) ® as indicators for the stability of the water masses (a: sum-
mer, b: winter). Sampling dates as in Fig. 1. Insert: Magnification of zone of bacterial layer

Lake Cadagno, a meromictic alpine lake 79
Figure 4. Depth profiles of oxygen concentration ■■
.
and the oxygen gradient
d
[O
2
]
d
z ® (a: summer, b: winter). Sampling dates as in Fig. 1

80
mixing by convection, probably induced by the active movement of the large
motile bacteria, Chromatium okenii, destabilizes the density gradient. Maximal
densities observed in the monimolimnion are 1.00025 g/cm
–3
and 1.00060 g/cm
–3
for summer and winter situations, respectively.
The permanently anoxic monimolimnion has a volume of 201800 m
3
, the
mixolimnion is more than ten times larger (2318000 m
3
). The redox-transition
zone located between 10 m and 12 m depth is a few m in size and contains the
greatest part of the active population of the phototrophic sulfur bacteria, it mea-
sures up to 100000 m
3
. The exchange area between the oxic and anoxic water
bodies varies between 81000 m
2
in 13 m depth towards the end of the summer,
and 113000 m
2
measured 10 m below the surface just after the ice has melted. If
the bacteria contained in a 1 m thick layer at 10 m depth would all sink to 13 m
their packing density would thus increase by a factor of 1.4.
The litoriprofundal which extends between 8 m depth and 12.5 m depth and
which is partially covered with bacterial mats comprises a sediment-water
exchange area of 474000 m
2
. The sediment area through which hydrogen sulfide
and methane exchange in the profundal benthos measures 114800 m
2
equiva-
lent to 17.4% of the total sediment surface. Since 30% of the total lake volume
are drained in late fall, 70% of the littoral remains uncovered for six winter
months.
Typical for a meromictic lake, the monimolimnion remains constantly ano-
xic. During summer the midpoint of the oxygen gradient (the oxycline) is locat-
ed between 11 and 12 m above the sediment (Fig. 4). It is lowered to 7 m above
the sediment during the fall overturn (Fig. 5). During the period of ice cover
oxygen consumption in the mixolimnion lifts the oxycline again to about 15 m
above sediment. The habitats in which fish can survive are thus further reduced
by this upward movement of the anoxic-oxic transition zone.
Stratification in the water column
Lake Cadagno is clearly a meromictic lake. Complete mixing of the water body
is prevented by the salt dependent density increase in the monimolimnion
which stabilizes the water masses even under homothermal conditions. The pro-
files of temperature, conductivity and of the calculated density, typical for
summer and winter situations (Figs. 2 and 3), illustrate the dependence of the
pycnocline on temperature and dissolved solutes during the annual cycle. The
separation into monimolimnion and mixolimnion, whose upper limit is establish-
ed after the fall overturn, is maintained during the winter season below the
massive ice cover. The increase in salt concentration in the deep water is seen in
the conductance increases during winter from 400 µS/cm
–1
to about 800 µS/cm
–1
near the sediment-water interface. This leads to an upward movement of the
pycnocline in spring.
Del Don et al.

Light penetration
Light intensity determines colonization of pelagic and benthic habitats by pho-
totrophic organisms. Planktonic, oxigenic eucaryotes are preferentially found at
depths down to 8 m. Furthermore, the development of a dense population of
phytoflagellates has often been observed especially in the microoxic region of
the metalimnion at a depth of 10 to 11 m. Mass developments of phototrophic
purple sulfur bacteria occur at depths between 11.5 and 13.5 m just below the
oxycline where the light intensity is still a few percent of the surface radiation
(Fig. 6). The light climate typical for summer situations has been described in
more detail by Fischer et al. (1996).
The transition zone of the litoriprofundal is covered with compact bacterial
mats composed of cyanobacteria, Beggiatoa spp. and large areas of purple pig-
mented sulfur bacteria. In winter, below a thick layer of snow and ice, only a
fraction of a percent of the surface radiation is measured at the water-ice inter-
face. Turbidity indicating microorganisms is less pronounced at depths between
8 and 12 m as indicated by increased light penetration.
Dynamics of the lake chemistry
Over the seasons, the chemical as well as the biological parameters change in
the lake concomitantly with the physical parameters temperature and density.
Large variations in the chemical composition are typical for the mixolimnion
during the annual cycle. As is seen from the oxygen concentration during the
81
Figure 5. Oxygen concentration isopleths during an annual cycle. From November to the end of
May the lake surface was 3 m lower than during the summer. Numbers are in mg O
2
/L
Lake Cadagno, a meromictic alpine lake

82
Del Don et al.
Table 3. The ionic composition of subaquatic spring water
a
Depth of springs Conductivity Conductivity Density
f
Calcium Magnesium S Cations Sulfate S Carbonate
d
S Anions
[m] [µS/cm] 20°C [µS/cm] 20°C [g/cm
–3
] [mM] [mM] [meq/L]
e
[mM] [mM] [meq/L]
e
measured calculated
12.5 1509 1.08570 4.94 3.48 16.84 7.40 0.71 16.22
12.5 1376 0.89928 4.55 3.27 15.64 6.50 0.73 14.47
9 331 0.18770 0.96 0.85 3.53 1.15 0.60 3.50
9 1485 1513 0.63996 4.47 3.46 15.86 7.55 1.44 17.98
9
b
1322 1310 0.91363 4.03 2.91 13.88 6.17 1.57 15.48
8.5 498 0.26802 1.95 1.03 4.96 1.66 0.79 4.71
8
c
1402 0.94705 4.55 3.27 15.64 6.50 1.47 > 14.47
8 1130 1313 0.91856 4.15 2.76 13.82 6.27 1.40 15.34
8
b
1097 1087 0.75502 3.19 2.36 11.10 5.00 1.48 12.80
5.5 559 0.31153 1.50 1.10 5.20 1.60 1.60 6.40
5.0
c
447 476 0.31123 1.45 1.03 4.96 1.60 1.58 > 4.70
2.5 230 332 0.20022 0.92 0.79 3.43 1.22 0.63 3.71
a
Values from 3 different campaigns during August and September 1990.
b
From Uhde (1992), campaigns 1991.
c
Campaigns 1989.
d
S carbonate = [HCO
3
–
] + [CO
3
2–
].
e
meq = milliequivalents = c
i
· z
i
 = concentration of ion i in millimole/l
.
absolute value of charge of ion i.
f
Density calculated according to Wüest (1987) for 20°C.
– = Not determined.

fall overturn (October–November) the mixolimnion is uniformly mixed and
saturated with more than 8 mg/L
–1
of oxygen as deep as 14 m (Fig. 5). After
freezing, when photosynthetic oxygen production is almost abolished, the vari-
ous chemical and biological oxygen consuming activities persist and the oxic/an-
oxic interface moves upward. The 1 mg L
–1
boundary reaches a depth of about
8 m below the ice cover in late spring. During the melting period the mixo-
limnion fills up with oxygen- and nutrient-rich water and oxygenic photosynthe-
sis starts immediately (Bertoni et al., 1998; Schanz and Stalder, 1998). The bound-
ary between oxic and anoxic conditions drops to a depth below 10 m (end of
June).
During summer when light penetrates into the zones which lack oxygen but
are rich in hydrogen sulfide (Figs. 4, 5 and 8), a dense population of phototrophic
sulfur bacteria develops. Their activity and position in the water column is regu-
lated by the intensity and quality of the radiant flux which reaches the depth of
the oxycline and by the availability of reduced sulfur species as electron donors
(Fig. 8). Due to the presence of sulfate reducing bacteria in the layer and the
ability of the phototrophic sulfur bacteria to utilize hydrogen sulfide in the light
and to produce it in the darkness (Lüthy et al., 2000; Tonolla et al., 2000), the
sulfidocline shows diurnal fluctuations between 11 and 13 m. Furthermore,
physical oscillations with frequencies of 0.2 h
–1
and 0.1 h
–1
have been observed
depending on the depth of the measurement (Egli et al., 1998). During the sum-
mer season, the oxygen/hydrogen sulfide interface drops towards deeper zones
due to the activity of the phototrophic bacteria (Figs. 5 and 8), from 11 m in
early summer to 13.5 m depth in fall. The circulation in the mixolimnion in fall
pushes the oxycline even deeper before the surface freezes in winter.
Besides sulfate and carbonate ions, the cations Ca
2+
and Mg
2+
are released
from the gypsum containing dolomitic rock into the monimolimnion (Fig. 7).
Since carbonate is the main buffer in the lake water, the pH is stabilized at a
value of about 7.0 (Fig. 9). The pH in the mixolimnion, however, increases
during the summer to values as high as 8.5 to 9.0 due to oxygenic photosyn-
thesis. In contrast, during snow-melt (end of May) the pH of the surface water
often drops to 5.3 due to the acidity of the lake ice and in the snow cover during
the winter.
In summary, gradients in chemical parameters are more pronounced in sum-
mer and large concentration differences in many constituents of the water be-
tween mixo- and monimolimnion give rise to steep gradients in the redox tran-
sition zone. In contrast, in winter, concentrations of many compounds increase
in the deeper layers while primary productivity in the upper zones is low. This
leads to less pronounced gradients at the transition zone.
Biological dynamics of the lake during the annual cycle
In situ turbidity is a good measure for the distribution of bacterial biomass. Tur-
bidity is low in the mixolimnion indicating lower bacterial biomass in the oxic
production layer and no inorganic scattering material brought in by runoff
water. Microscopy and pigment analyses of the turbidity below 11 m in summer
Lake Cadagno, a meromictic alpine lake 83

84
Del Don et al.
Figure 6. Spectral distribution of the sunlight at the surface and at various depths in summer (a,
24. 8. 1985) and in winter (b, 30. 3. 1985) and penetration of radiation as deep as the redox transi-
tion zone (c)
wavelength

Lake Cadagno, a meromictic alpine lake 85
indicate the presence of blooms of phototrophic sulfur bacteria, dominated by
the large Chromatium okenii and colonies of Amoebobacter purpureus (Tonolla
et al., 1999; Bosshard et al., 2000a and b). They reach densities of up to 10
6
per
mL giving the water a purple color due to the presence of carotenoids, mainly
okenone.
Bacteriochlorophyll a is the major chlorophyll found in the turbidity layer,
but also chlorophyll a is present at the upper edge of the layer. Some of the Chl
a may be associated with sedimenting algal biomass from the mixolimnion, but
also populations of photosynthetically active phytoflagellates have been ob-
served at these microoxic depths (Camacho et al., this volume). Many changes
in the chemistry of the lake water are controlled by the metabolism of the bac-
terial populations. The bacteria maintain the strong gradients and act as a che-
mical nutrient filter between mixo- and monimolimnion. The pH of the water at
different depths (Fig. 9) is a good indicator for the kind of metabolic processes
which dominate (Hanselmann, 1986).
Figure 6 (continued)

86
Del Don et al.
Figure 7. Concentration isopleths of (a) the sum of the carbonates (mval/L), (b) Calcium (mg/L)
and (c) Magnesium (mg/L) during the annual cycle

Lake Cadagno, a meromictic alpine lake 87
Figure 8. Sulfide concentration isopleths during an annual cycle. Compare with Fig. 5. Numbers
are SH
2
S in mg/L (SH
2
S = [H
2
S] + [HS
–
] + [S
2–
]
Figure 9. Variation of pH in the bacterial layer during the annual cycle. Insert: pH drop in the sur-
face water during melting of the ice

Discussion
The data presented demonstrate that the meromictic Lake Cadagno is an excel-
lent model to study biological processes occurring at chemical redox transition
zones and of their consequences on water chemistry.
The anoxic monimolimnion is characterized by high concentrations of
hydrogen sulfide since the sulfate contained in the subaquatic spring water pre-
sents a readily available electron acceptor for anaerobic respiration. Through
sulfate-reducing and fermenting activities sedimenting biomass is degraded and
partially oxidized in the hypolimnion and in the upper layers of the sediment
thereby producing carbon dioxide and hydrogen sulfide which are released into
the overlaying water.
The year-round stratification is stabilized through the higher density of the
water fed to the monimolimnion by underwater springs, this prevents fall circu-
lation from reaching depths below 14 m. It would hardly be sufficient, however,
to prevent full circulation in winter in the absence of a protective ice cover. In
general, the density decreases continuously from the lake bottom to the sur-
face showing two depths with more pronounced changes. They correspond to
the two pycnocline indicated in Fig. 3. This is best seen in the changes in local
stability expressed as N
2
.
The dense microbial populations may themselves create density changes in
the water column. A significant drop in stability is observed especially in sum-
mer in the bacterial layer between 11 and 12 m depth. Under typical summer
conditions (Figs. 2a, 3a, and 4a) the total bacterial counts reach concentrations
of up to 10
6
to 10
7
cells mL
–1
. Such large numbers would give a density increase
of 7.7 · 10
–5
to 7.7 · 10
–4
g/cm
–3
. Thus, the large number of suspended particles
contribute to the density increase as much as the sum of the dissolved salts. The
bacterial particles are responsible for a layer of nearly constant density and high
instability within the overall density gradient (Fig. 3, inserts). In this layer of
high bacterial density, sedimenting dead biomass becomes accessible for degra-
dation by fermenting or anaerobically respiring organisms. On the other hand,
the same bacterial layer acts also as a chemical filter for compounds produced
within it and those diffusing from the monimolimnion towards the surface. This
role of the bacterial layer is especially important for the removal of the toxic sul-
fide but also for many other nutrients which are liberated in the sediments and
in the monimolimnion.
ACKNOWLEDGEMENTS
We thank the Swiss National Science Foundation for generous financial support (grant 3.276-0.85),
Prof. Dr. H. Matthias, Institut for Geodesy, ETHZ, for the determination of the lake morphology,
PD Dr. F. Schanz for the use of the instruments for light measurement, H. Maag, for scuba diving,
Dr. M. Tonolla for the use of the data in Fig. 6, and Dr. A. Wüest for the computer program to cal-
culate the instability of the water masses.
88
Del Don et al.

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