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The meromictic alpine Lake Cadagno: Orographical and biogeochemical description


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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.
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The meromictic alpine Lake Cadagno:
Orographical and biogeochemical description
Claudio Del Don,
Kurt W. Hanselmann
, Raffaele Peduzzi
and Reinhard Bachofen
Institute of Plant Biology, University of Zürich, Zollikerstr. 107, CH-8008 Zürich, Switzerland
Istituto Cantonale Batteriosierologico, via Ospedale 6/via Fogazzaro 3, CH-6900 Lugano,
Key words: Morphometry, geology, water chemistry, water density, diurnal and
seasonal changes, light regime.
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
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
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
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Aquatic Sciences
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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
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
, 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-
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
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.
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 (
was calculated from the temperature, the conductance and the chemical and
particle composition of the water according to
· h
· h
· h
is the maximal density of pure water at 3.98°C, h
the function to
correct the density for temperature deviation (Weast et al., 1986), h
the func-
tion to correct the density for dissolved materials according to Pytkowicz
(1979), and h
is the function to correct the density for the different suspended
particles. Ca
, Mg
, Na
, K
and HCO
and SO
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
can be
expressed by the function (Weast et al., 1986)
= 7.7981 · 10
+ 0.9999728 (g/cm
is the conductivity in µS cm
normalized to 25°C. The conversion fac-
tor 7.7981 · 10
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
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
= 1 – 2.9392 · 10
+ 1.0564 · 10
µFU – 2.4238 · 10
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
had to be included, however, for the pycnocline.
Local water stability was calculated according to Imboden and Wüest
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,
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-
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.
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
Summer Winter
Altitude of water surface [m] 1921 1918
Maximum depth Z
[m] 21 18
Mean depth Z [m] 9.27 7.87
Median depth Z
[m] 8.5 7.8
Relative depth Z
[%] 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
[m] 2109 1924
Shore line development l
[m] 1164 1168
Surface area A
] 261043 215839
Total volume V
] 2419850 1699600
Benthic contact area M
] 356300 271050
Area error E [–] 0.001050 0.000462
Information value of map I [–] 0.998721
Depth values are calculated in meters above the sediment surface. For the definition of the param-
eters see Appendix.
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
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
[m] [m] [m
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
z ® (a: summer, b: winter). Summer values were taken on August 18,
1987, winter values on March 31, 1985
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
z ® (a: summer, b: winter). Sampling dates as in Fig. 1
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
and 1.00060 g/cm
for summer and winter situations, respectively.
The permanently anoxic monimolimnion has a volume of 201800 m
, the
mixolimnion is more than ten times larger (2318000 m
). 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
. The exchange area between the oxic and anoxic water
bodies varies between 81000 m
in 13 m depth towards the end of the summer,
and 113000 m
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
. The sediment area through which hydrogen sulfide
and methane exchange in the profundal benthos measures 114800 m
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
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
to about 800 µS/cm
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
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
Lake Cadagno, a meromictic alpine lake
Del Don et al.
Table 3. The ionic composition of subaquatic spring water
Depth of springs Conductivity Conductivity Density
Calcium Magnesium S Cations Sulfate S Carbonate
S Anions
[m] [µS/cm] 20°C [µS/cm] 20°C [g/cm
] [mM] [mM] [meq/L]
[mM] [mM] [meq/L]
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
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
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
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
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
Values from 3 different campaigns during August and September 1990.
From Uhde (1992), campaigns 1991.
Campaigns 1989.
S carbonate = [HCO
] + [CO
meq = milliequivalents = c
· z
= concentration of ion i in millimole/l
absolute value of charge of ion i.
Density calculated according to Wüest (1987) for 20°C.
– = Not determined.
fall overturn (OctoberNovember) the mixolimnion is uniformly mixed and
saturated with more than 8 mg/L
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
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
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
and 0.1 h
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
and Mg
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
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)
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
mL giving the water a purple color due to the presence of carotenoids, mainly
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)
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
S in mg/L (SH
S = [H
S] + [HS
] + [S
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
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
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
to 10
cells mL
. Such large numbers would give a density increase
of 7.7 · 10
to 7.7 · 10
. 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.
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.
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Del Don et al.
... The lake's distinct geochemistry and microbial communities have been well-characterized. Waters below the chemocline are fed by groundwater input from a karstic system composed of dolomite and gypsum, and are therefore rich in Ca 2+ , Mg 2+ , SO 4 2− and HCO 3 − relative to overlying waters (Del Don et al., 2001, Figure 1). In anoxic deep waters, dissolved concentrations reach 10 2 μmol kg −1 sulfide, ∼3.5 mmol kg −1 sulfate, and ∼1 μmol kg −1 Fe (e.g., Ellwood et al., 2019, see supplement). ...
... High densities of anoxygenic photoautotrophic green and purple sulfur bacteria are found at the chemocline in summer (Tonolla et al., 2003). These bacteria exert control on geochemical gradients in this zone (e.g., S, Fe;Del Don et al., 2001;Berg et al., 2016) and can form a 0.3-1.2 m thick mixed layer through bioconvection during summer (Sepúlveda Steiner et al., 2019, 2021Sommer et al., 2017). ...
... Observed deep water [Cr] enrichments likely reflect the integrated accumulation of Cr released from sinking particles and near-surface sediments as well as the potential contribution from groundwater (Del Don et al., 2001), with concentrations further modified by turbulent mixing. As there is no modification of particulate δ 53 Cr with depth, released Cr must be of similar isotope composition as the particulates (δ 53 Cr ≈ 0 ‰). ...
Full-text available
Chromium stable isotope composition (δ⁵³Cr) is a promising tracer for redox conditions throughout Earth’s history; however, the geochemical controls of δ⁵³Cr have not been assessed in modern redox-stratified basins. We present new Cr concentration and δ⁵³Cr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δ⁵³Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ⁵³Cr from oxidative continental weathering.
... Water sample collection and physicochemical profiling. This study was conducted in Lake Cadagno (21 m deep), a high alpine meromictic lake situated at 1,921 m above sea level in the Southern Alps of Switzerland (73). Due to the extensive number of samples collected and experiments performed for process measurements, sampling was done over two subsequent days. ...
Full-text available
As a window into the past, this study offers insights into the potential role that microbial guilds may have played in the production and recycling of organic matter in ancient Proterozoic ocean chemoclines. The new observations described here suggest that chloroplasts of eukaryotic algae were persistent in the low-oxygen upper chemocline along with the purple and green sulfur bacteria known to dominate the lower half of the chemocline.
... Deeper lakes in particular seem to be vulnerable due to their tendency to mix incompletely (Danis et al., 2004). H these are not the only factors, as the chemical composition of the sediment and underlying bedrock determines the nature and quantity of compounds that can be released under anoxic conditions (Boehrer & Schultze, 2008;Del Don et al., 2001). Additionally, Matzinger et al. (2007) suggest that meromixis is stimulated under a high rate of warming, which puts lakes at risk that lie in parts of the world that are experiencing rapid warming. ...
Full-text available
• Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning. • We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic. • Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter. • The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition. • A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate. • Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.
... This small alpine lake has a maximum depth of 21 m and a surface area of 0.26 km 2 . Deep, ion-rich, subaquatic inflows (Del Don et al. 2001) lead to permanent stratification and qualify Lake Cadagno as a crenogenically meromictic lake (Boehrer and Schultze 2008). The stratified water column exhibits a chemocline separating the oxygen-rich upper layer (top $10 m) from the anoxic and sulfide-rich deep-water (deepest $10 m). ...
Full-text available
In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria‐populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well‐established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally‐unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48‐h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high‐resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.
... In regions with steep reliefs, erosional incision can also expose SO 4 2--rich layers that are subsequently washed out by rain or flowing water. For example, some meromictic alpine lakes have a saline hypolimnion fed by a sulphate-bearing layer in the bedrock that has been "tapped" by erosion (e.g., Lake Cadagno; Del Don et al., 2001). Other natural SO 4 2sources include volcanic brines, hydrothermal fluids, natural saltwater intrusions ("saltwater wedges") in coastal areas, and hypersaline subglacial Antarctic environments (Blume et al., 2016;Lyons et al., 2019). ...
Sulphate (SO42-) concentrations in freshwaters have increased globally over the last decades even though a strong reduction in atmospheric sulphur (S) deposition has occurred across large parts of North America and Europe. However, the extent and effects of increased SO42- concentrations in freshwater and terrestrial ecosystems remain poorly understood regarding many aspects of ecosystem structure and functioning. Here, we review the sources of SO42- pollution, environmental impacts on freshwater ecosystems and bioremediation opportunites and we identify key knowledge gaps and future research needs. Natural sources of dissolved SO42- in freshwater ecosystems include mineral weathering, volcanic activity, decomposition and combustion of organic matter, oxidation of sulphides, and sea spray aerosols. Acid mine drainage, fertiliser leaching from agricultural soils, wetland drainage, agricultural and industrial wastewater runoff as well as sea level changes are the main direct and indirect sources of the anthropogenic SO42- input to waterbodies. Increasing SO42- concentrations in freshwater systems influence the biogeochemical processes of carbon, nitrogen and phosphorus. Similarly, iron availability can be critical in determining the adverse effects of SO42- on environmental receptors. The literature reviewed clearly demonstrates that SO42- pollution may have toxic effects on aquatic plants and animal organisms, including, among others, fishes, invertebrates and amphibians, and it may also have negative implications for human health. Bioremediation systems provide opportunities to mitigate the impacts of SO42-, but removal efficiencies range widely from 0% to 70% across treatment systems such as constructed wetlands, permeable reactive barriers and bioreactors. We conclude that examination of increased SO42- concentrations and fluxes at different spatial scales is urgently needed as the ongoing global perturbation of the S cycle is likely to be accelerated by climate change and human development activities. The adverse effects of this on freshwater organisms worldwide may prove detrimental to the future well-being of humans and ecosystems. Field-scale research to estimate the ecotoxicological effects of elevated SO42- concentrations is recommended as is widespread implementation of large-scale wetland restoration and bioremediation systems to reduce SO42- loads on freshwater ecosystems.
... Uncertainties are large because knowledge about "hydraulic gradients and transmission coefficients along the world's coasts" (Church, 1996) is limited. Groundwater discharge has also been observed in lakes but quantification studies are still scarce (e.g., Colomer et al., 1998;Del Don et al., 2001;Bonhomme et al., 2011;Lewandowski et al., 2015;Rosenberry et al., 2015). ...
Full-text available
Pockmarks are circular depressions on the floor of oceans and lakes and constitute potential hot spots of gas ebullition and/or groundwater discharge. Marine pockmarks are well-studied, whereas lacustrine pockmarks are virtually unexplored. In Lake Neuchatel (Switzerland) four giant pockmarks of 80 to 150 m in diameter are located along the northern shore and adjacent to the karst system of the Jura Mountains. Two pockmarks have a ~60 m-deep chimney filled with mud; two are funnel-shaped 12 and 29 m deep holes. We present evidence for the presence of groundwater in the pockmark chimneys and active lacustrine groundwater discharge (LGD) at both pockmark types. Temperature, electrical conductivity and calcium concentrations of the pore water in the chimneys show values typical for karst water (maximal sampling and profiling depth: 41 m into the pockmark) and contrast strongly with the properties of the lake water. TOC and TIC indicate that the chimney mud is homogenized and composed of liquefied sediments from the entire deglacial to Holocene lacustrine sediment succession. Mini mud volcanoes apparent on the suspension surface imaged with a remotely operated vehicle (ROV) localize the groundwater exit points and confirm LGD. LGD is further corroborated by electrical conductivity anomalies detected above the lutoclines and within a funnel-shaped pockmark during the ROV survey. We conclude that the giant pockmarks in Lake Neuchatel represent a type of subaquatic springs that connect the water body of the lake with the karst system. A next essential research step will be to quantify LGD via the pockmarks in order to assess their lake-wide relevance. This study underlines the existing need for research on the connectivity of lakes and oceans with groundwater systems for completing our understanding of the hydrological cycle.
Full-text available
Accumulation of methane in oxic waters of lakes and the ocean has been widely reported. Despite the importance for the greenhouse gas budget, mechanistic controls of such “methane paradox” remain elusive. Here, we use a combination of CH4 concentration and isotopic (δ13CCH4, δDH2O and δ18OH2O) measurements, plankton incubations and microbial community assessments to demonstrate the existence of the methane paradox in oxygenated waters of a meromictic lake (Lake Cadagno, Switzerland). Following mass dynamics using water isotopes, we exclude the possibility that the accumulation of CH4 at the thermocline results solely from lateral transport. Interannual variability in the magnitude of the methane paradox (between 0.5 and 5 μmol L−1) is associated to stratification patterns, changes in zooplankton biomass and planktonic detritus accumulation along density gradients, as well as fluctuating microbial cell numbers. The links between hydrodynamic conditions, aggregation of planktonic detritus and its microbiome, as well as the accumulation of CH4 in the water column are further supported by high‐resolution echosounder revealing backscatter maxima at the top of the thermocline, where detritus is effectively trapped, and by oxic incubations showing that CH4 is produced in zooplankton detritus (0.046 nmol L−1 to 0.095 CH4 mg dry mass L−1 d−1). Our results also show that detritus‐hosted methanogenesis is stimulated through the addition of methylphosphonate, suggesting that zooplankton‐associated microbiomes exploit organic phosphorus compounds to release CH4. Understanding the variability of the methane paradox in relation to changing hydrodynamics and plankton communities will be crucial to predict the future role of lakes in the global methane budget.
Full-text available
Although lake sediments are globally important organic carbon sinks and therefore important habitats for deep microbial life, the deep lacustrine biosphere has thus far been little studied compared to its marine counterpart. To investigate the impact of the underexplored deep lacustrine biosphere on the sediment geochemical environment and vice versa, we performed a comprehensive microbiological and geochemical characterization of a sedimentary sequence from Lake Cadagno covering its entire environmental history since formation following glacial retreat. We found that both geochemical gradients and microbial community shifts across the ∼13.5 kyr subsurface sedimentary record reflect redox changes in the lake, going from oxic to anoxic and sulfidic. Most microbial activity occurs within the top 20 cm of sediment, where millimolar sulfate concentrations diffusing in from the bottom water are almost completely consumed. In deeper sediment layers, organic carbon remineralization is much slower but microorganisms nonetheless subsist on fermentation, sulfur cycling, metal reduction, and methanogenesis. The most surprising finding was the presence of a deep, oxidizing groundwater source. This water source generates an inverse redox gradient at the bottom of the sedimentary sequence and could contribute to the remineralization of organic matter sequestered in the energy-limited deep subsurface.
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Lake Cadagno, a permanently stratified high-alpine lake with a persistent microbial bloom in its anoxic chemocline, has long been considered a model for the low-oxygen, high-sulfide Proterozoic ocean where early microbial life gave rise to Earth’s oxygenated atmosphere. Although the lake has been studied for over 25 years, the absence of concerted study of the bacteria, phytoplankton, and viruses, together with primary and secondary production, has hindered a comprehensive understanding of its microbial food web. Here, the identities, abundances, and productivity of microbes were evaluated in the context of Lake Cadagno biogeochemistry. Photo-synthetic pigments and chloroplast 16S rRNA gene phylogenies suggested high abundances of eukaryotic phytoplankton, primarily Chlorophyta , through the water column. Of these, a close relative of Ankyra judayi , a high-alpine adapted chlorophyte, peaked with oxygen in the mixolimnion, while Closteriopsis -related chlorophytes peaked in the chemocline and monimolimnion. Anoxygenic phototrophic sulfur bacteria, Chromatium, dominated the chemocline along with Lentimicrobium , a newly observed genus of known fermenters. Secondary production peaked in the chemocline, suggesting anoxygenic primary producers depended on heterotrophic nutrient remineralization. Virus-to-microbe ratios spanned an order of magnitude, peaking with high phytoplankton abundances and at a minimum at the peak of Chromatium, dynamic trends that suggest viruses may play a role in the modulation of oxygenic and anoxygenic photo- and chemosynthesis in Lake Cadagno. Through the combined analysis of bacterial, eukaryotic, viral, and biogeochemical dynamics of Lake Cadagno, this study provides a new perspective on the biological and geochemical connections that comprised the food webs of the Proterozoic ocean. IMPORTANCE As a window to the past, the study offers insights into the role of microbial guilds of Proterozoic ocean chemoclines in the production and recycling of organic matter of sulfur- and ammonia-containing ancient oceans. The new observations described here suggest that eukaryotic algae were persistent in the low oxygen upper-chemocline in association with purple and green sulfur bacteria in the lower half of the chemocline. Further, this study provides the first insights into Lake Cadagno viral ecology. High viral abundances suggested viruses may be essential components of the chemocline where their activity may result in the release and recycling of organic matter. The framework developed in this study through the integration of diverse geochemical and biological data types lays the foundation for future studies to quantitatively resolve the processes performed by discrete populations comprising the microbial loop in this early anoxic ocean analogue.
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This paper reports a study of oxygen and redox conditions, trophic status, and phytoplankton community in the meromictic Lake Idro (Italy) from 2010 to 2014. The sequence of causes and effects of meromixis are also evaluated by comparing recent research with studies conducted from the late 1960s to the mid-1990s. In the last half century, Lake Idro was steadily meromictic due to solutes which accumulated in its deep waters, along with both dissolved nutrients and chemically reduced substances produced by the anaerobic microbial metabolism. These substances were retained in bottom waters and made unavailable to upper layers until stratification broke. Mixing episodes occurred in 2005–2006 altering stratification, and oxygen and nutrient distribution within the lake. The potential full overturn effects were also evaluated as potential oxygen consumption due to the oxidation of reduced substances to forecast possible oxygen exhaustion and collapse of biological communities. Finally, meromixis is discussed as a potential threat for deep perialpine lakes using Lake Idro as a reference to comparatively evaluate the present status and possible future trends.
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Transport phenomena are among the most important processes in natural systems. Chemical compounds, the constituents of biogeochemical systems, are in continual motion in all parts of the earth. The thermal motion of atoms and molecules is perceived on the macroscopic level as molecular diffusion i.e., as the slow but persistent movement “down along the concentration gradient.” Although the average speed of the atoms is on the order of tens to hundreds of meters per second, the net transport is small, because the molecules do not maintain the same direction long enough. Thus, typical molecular diffusion coefficients of solutes in water are approximately 10-9 m2s - 1 corresponding to characteristic annual transport distances of approximately 20 cm. In solids the diffusion coefficients even drop to values as low as 10-14m2s-1 or less.
The organic carbon dynamics of Lake Cadagno, a meromictic alpine lake, was studied considering the POC, DOC and chlorophyll spatial and temporal distribution in the whole water column during the ice-free period in 1994, 1995 and 1996. A well defined layer of purple sulphur bacteria was the source of 63-83% of total organic carbon which was therefore segregated in the chemocline. The maximum concentration of POC was found in October 1994, and was 6 mg C l-1; in the same year the chlorophyll reached the peak of 37 μg l-1. In the mixolimnion the phytoplankton and the autotrophic (APP) and heterotrophic (HPP) picoplankton were enumerated and their activity measured. The number of taxa identified was rather high but the biomass was low, as typically happens in alpine lakes. The carbon fixation was low near the surface because of PAR and UV photoinhibition; and reached values characteristic of a mesotrophic lake (7.7 mg C m-3 h-1) at 2 m. APP production was 13% of total phytoplanktonic production with picocyanobacteria reaching a maximum density of 32000 cell ml-1. The HPP biovolume, although mirroring closely the POC vertical distribution, provided a little contribution to the organic carbon pool.
With a newly developed fluorescence probe, fluorescence induction kinetics of phototrophic bacteria have been measured in situ in a meromictic alpine lake. The results indicate that the physiological adaptation of the cells towards varying environmental conditions is faster than these changes actually occur in the environment of the cells, as a consequence of either their tactic behavior or of their displacement by biofluctuation within the whole phototrophic bacterial population.
Lake Cadagno is a meromictic lake of 21-m depth in the central Swiss Alps at 1,923 m asl. Its mixolimnion reaches from the surface down to 10-m depth. A dense population of phototrophic bacteria dominated by Chromatium okenii is located in the subsequent 2-m layer containing up to 200 mg bacteriochlorophyll (Bchl) m-3. Maximum cell concentration was found at 10.8-m depth during the summer season. The light intensity at the upper edge of the bacterial layer averages 4.7% of the subsurface radiation (at 0.05 m depth) and at the density peak of the layer it averages 0.4%. Variations over the summer season in the photosynthetic properties of the bacterial population at the depth of highest cell density have been observed in in situ 14C-CO2 incorporation experiments. These changes are random, and no photoadaptation effect was found. Thus, the population of phototrophic bacteria was physiologically uniform during the investigation period and it had the following photosynthetic properties (medians for the 25th and 75th percentiles are in parentheses): P(max), 0.154 mg C (mg Bchl)-1 h-1 (0.048, 0.174); I(k), 0.016 mol quanta m-2 h-1 (0.012, 0.019); α, 7.7 mg C (mg Bchl)-1 h-1 (mol quanta m-2 h-1)-1 (4.8, 11.6). The specific dark production rate was 0.016 mg C (mg Bchl)-1 h-1 (0.014, 0.023). The average quantum yield for CO2 assimilation at the depth of maximum cell density, φ(z), was low at 0.012 (0.007, 0.020). The cells were sensitive to light intensities higher than the optimum found at ~0.036 mol quanta m-2 h-1. Maximum inhibition by excessive light was determined to be 80% at light intensities >0.108 mol quanta m-2 h-1 (=30 μmol quanta m-2 s-1) when cells were exposed for periods of 2.5 h or longer. The bacterial biomass increment assuming no losses, μ*, was calculated to be 0.05 d-1 (0.03, 0.08) and the loss rate, λ, to be 0.03 d-1 (-0.10, 0.06).