Content uploaded by George W. Kling
Author content
All content in this area was uploaded by George W. Kling on Dec 03, 2015
Content may be subject to copyright.
The Comparative Limnology of Lakes
Nyos and Monoun, Cameroon
George W. Kling, William C. Evans,
and Gregory Z. Tanyileke
Abstract
Lakes Nyos and Monoun are known for the dangerous accumulation of
CO
2
dissolved in stagnant bottom water, but the shallow waters that
conceal this hazard are dilute and undergo seasonal changes similar to
other deep crater lakes in the tropics. Here we discuss these changes with
reference to climatic and water-column data collected at both lakes during
the years following the gas release disasters in the mid-1980s. The small
annual range in mean daily air temperatures leads to an equally small
annual range of surface water temperatures (ΔT*6–7°C), reducing deep
convective mixing of the water column. Weak mixing aids the establish-
ment of meromixis, a requisite condition for the gradual buildup of CO
2
in
bottom waters and perhaps the unusual condition that most explains the
rarity of such lakes. Within the mixolimnion, a seasonal thermocline forms
each spring and shallow diel thermoclines may be sufficiently strong to
isolate surface water and allow primary production to reduce P
CO2
below
300 μatm, inducing a net influx of CO
2
from the atmosphere. Surface
water O
2
and pH typically reach maxima at this time, with occasional O
2
oversaturation. Mixing to the chemocline occurs in both lakes during the
winter dry season, primarily due to low humidity and cool night time air
temperature. An additional period of variable mixing, occasionally
reaching the chemocline in Lake Monoun, occurs during the summer
G.W. Kling (&)
Department of Ecology and Evolutionary Biology,
University of Michigan, Ann Arbor, MI 48109, USA
e-mail: gwk@umich.edu
W.C. Evans
U.S. Geological Survey, Menlo Park,
CA 94025, USA
e-mail: wcevans@usgs.gov
G.Z. Tanyileke
Institute for Geological and Mining Research,
Yaounde, Cameroon
e-mail: gtanyileke@yahoo.co.uk
D. Rouwet et al. (eds.), Volcanic Lakes, Advances in Volcanology,
DOI 10.1007/978-3-642-36833-2_18,
©Springer-Verlag Berlin Heidelberg 2015
401
monsoon season in response to increased frequency of major storms. The
mixolimnion encompassed the upper *40–50 m of Lake Nyos and
upper *15–20 m of Lake Monoun prior to the installation of degassing
pipes in 2001 and 2003, respectively. Degassing caused chemoclines to
deepen rapidly. Piping of anoxic, high-TDS bottom water to the lake
surface has had a complex effect on the mixolimnion. Algal growth
stimulated by increased nutrients (N and P) initially stimulated photo-
synthesis and raised surface water O
2
in Lake Nyos, but O
2
removal
through oxidation of iron was also enhanced and appeared to dominate at
Lake Monoun. Depth-integrated O
2
contents decreased in both lakes as
did water transparency. No dangerous instabilities in water-column
structure were detected over the course of degassing. While Nyos-type
lakes are extremely rare, other crater lakes can pose dangers from gas
releases and monitoring is warranted.
Keywords
Nyos Monoun Crater lakes Volcanic gas hazards Cameroon
Volcanic Line Tropical limnology
1 Introduction
Cameroon contains a large number of volcanic
lakes, but limnological studies prior to the 1980s
were fairly limited in scope (e.g., Hassert 1912;
Green 1972; Green et al. 1974). Kling (1987a,b,
1988) conducted a comprehensive study of 34
volcanic lakes and several other Cameroonian
lakes not known to occupy volcanic craters. This
work helped delineate the impacts of climate,
morphometry, sheltering, water transparency and
conductivity, and other factors on tropical lake
dynamics such as stratification and mixing,
thermocline depth, and oxygen profiles. The
research was timely because it was conducted
during the years when the disastrous gas releases
occurred at Lake Monoun (1984) and Lake Nyos
(1986), although pre-release data collection was
limited to a sample of surface water from Lake
Nyos in 1985 (Kling 1987a,1988).
The Lake Monoun gas release was described
by Sigurdsson et al. (1987), and the Lake Nyos
release was described by Tuttle et al. (1987),
Kling et al. (1987), Tietze (1987), Freeth (1992)
and several papers in Le Guern and Sigvaldason
(1989,1990). Numerous studies conducted at
lakes Nyos and Monoun in the years after the gas
releases focused on the evolution of the deep
water layers and on the natural recharge rate of
CO
2
near lake bottom (Nojiri et al. 1990,1993;
Evans et al. 1993). These studies were important
for understanding the cause of the gas releases
and the likelihood of recurrence. The demon-
stration that dangerous amounts of gas remained
in the lakes after the disasters, and that the
recharge rate of CO
2
was high enough to sub-
stantially increase the risk of recurrence on a
decadal timescale, provided stimulus for the
installation of pipes to safely remove the gas. The
rates of gas removal and changes in gas content
of the lakes have since been monitored (e.g.,
Kling et al. 2005; Kusakabe et al. 2008). How-
ever, even though degassing is well underway at
Lake Nyos and essentially complete at Lake
Monoun, there has been little published on how
the degassing has affected the surface water
limnology of these lakes.
This chapter discusses comparative limno-
logical aspects of Lakes Nyos and Monoun with
an emphasis on the characteristics and behavior
of the “mixolimnion”, the dilute water layer
overlying denser gas-rich water at depth (herein
called bottom water). The mixolimnion exhibits
diel and seasonal responses to climatic forcing at
402 G.W. Kling et al.
the lake surface, and at least episodically is
affected by some mixing with bottom water. The
mixolimnion is important because it keeps pres-
sure on deeper layers and allows the gas con-
centrations to build up. Conditions in the
mixolimnion that affect its chemical composition
and physical structure have a bearing on whole-
lake stability and thus on the hazard of uncon-
trolled gas release. We restrict our detailed dis-
cussion to Lakes Nyos and Monoun, but in a
final section we discuss other lakes that share
characteristics with Nyos and Monoun where the
comparison has relevance to gas release hazards.
2 Methods
Methods of water sampling, chemical analyses,
and many results are presented in Tuttle et al.
(1987), Kling et al. (1989,2005), and Evans et al.
(1993,1994). Dissolved organic carbon (DOC)
and total dissolved nitrogen (TDN) were filtered
through Whatman GFF filters (nominal pore size
0.6 µm), acidified to pH *2–3, and stored dark
at 4 °C until measurement on a Shimadzu TOC V
analyzer. Total dissolved phosphorus (TDP) was
measured on similarly filtered, treated, and stored
water using a persulfate digestion and subsequent
analysis of soluble reactive phosphate using a
molybdenum blue method on an Alpkem auto-
analyzer. Particulate C and N were filtered on
to GFF filters and analyzed in a Perkin-Elmer
2400 CHN elemental analyzer. Underwater light
extinction was measured using a Secchi disk and
a Licor LI-1000 meter with a LI-193 spherical
quantum sensor (PAR, 400–700 nm) following
methods and calculations in Kling (1988). Pro-
files of temperature were measured using a
recently calibrated Seabird SBE 19 CTD (0.01 °C
accuracy, 0.001 °C precision, conductivity
10 µS/cm accuracy) with a SBE 18 pH sensor.
Continuous measurements of lake temperature
were made from an anchored raft near the center
of the main lake basins using thermistor strings
containing YSI 46030 precision resistors (0.1 °C
interchangeability, thermometric drift < 0.01 °C
over 100 months, 2.5 s time constant) connected
to a Campbell Scientific CR23x datalogger
recording every 60 s and averaging into 30 min
intervals. At these same time intervals using the
CR23x datalogger, a meteorological station on
the raft measured air temperature and relative
humidity (Campbell HMP45C, accuracy 0.2 °C
and 2 % RH), wind speed and direction (MetOne
041A windset, accuracy 0.2 m/s, threshold
0.45 m/s), and downwelling solar radiation (Licor
LI-200 pyranometer, 400–1,100 nm, accuracy
3 %); sensor heights were *1.5–2 m above mean
water level.
3 Results and Discussion
3.1 Climate
The overall climate affecting Lakes Nyos and
Monoun is similar and is driven by seasonal
movements of the Inter-Tropical Convergence
Zone (ITCZ). In this part of Cameroon the ITCZ
movement creates a dry season with virtually no
rain in December–February and a monsoon sea-
son with heavy rains in June–September. Data
from the floating climate stations on both lakes
show similar patterns of the coolest air temper-
atures and lowest humidity during the dry sea-
son, the warmest air temperatures coming out of
the dry season at the end of February and into
March, and the highest humidity during the
summer monsoon (Figs. 1and 2). The lowest
maximum daily temperatures occur in June, off-
set several months from the period of lowest
minimum temperatures in late December through
early February. This time of lowest minimum
temperatures is also the calmest time of the year
with wind speeds rarely exceeding 5 m/s, and
incoming solar radiation at a minimum due to
atmospheric dust from the Harmattan winds
(Figs. 1and 2). Air temperature at both lakes
shows no trend since late 1989 when measure-
ments first began (data prior to 2000 not shown),
and none of the climate variables measured
shows a significant trend over time.
The Comparative Limnology …403
Fig. 1 Selected climatic variables recorded at 30 min
intervals on an anchored raft at Lake Nyos
Fig. 2 Selected climatic variables recorded at 30 min
intervals on an anchored raft at Lake Monoun
404 G.W. Kling et al.
3.2 Lake Morphology
and Hydrology
Almost all of Cameroon’s volcanic lakes,
including Nyos and Monoun, occupy maar cra-
ters (Kling 1988; Lockwood and Rubin 1989;
Aka et al. 2008). These craters are typically flat
bottomed rather than conical, and many are quite
deep in relation to their diameters (Kling 1988).
At 210 m, Lake Nyos is the deepest of the
Cameroonian lakes, but it is only 42 m deeper
than Lake Manengouba-Female. Several of the
lakes match or exceed the depth of Lake Monoun
at 100 m. Both Lakes Nyos and Monoun have a
surface area that is much larger than the diameter
of the main crater containing the gassy water.
Lake Nyos contains a small basin *48 m deep at
its northern end near the point of outflow, and a
shallow arm in the southern end near the inflow
stream. Lake Monoun contains a small cra-
ter *50 m deep just west of the main crater and a
relatively shallow arm further west near the
outlet (Sigurdsson et al. 1987).
Unlike many volcanic summit crater lakes, the
maar lakes of Cameroon are generally not fully
ringed by high rims. Ejecta ramparts extend only
part way around Lakes Nyos and Monoun (Sig-
urdsson et al. 1987; Lockwood and Rubin 1989).
Granitic bedrock forms part of the rim around
Lake Nyos, but both these lakes have low-
elevation rims on two sides that reduce sheltering
from the wind. Both maars erupted through
existing surface stream drainages, and the lakes
capture the inflow from drainage basins several
km
2
larger than the actual lake surface area.
Riverine inflow and outflow is perennial at Lake
Monoun. Only the inflow is perennial at Lake
Nyos, and surface outflow only occurs during the
rainy season. From 1987–2004 the outflow over
the spillway began on average in early July, and
ended by early December (Fig. 3). Annual dif-
ferences in climate can cause outflow to begin as
early as April or as late as August. Seepage from
Lake Nyos, which is perched on a mountain-side,
feeds large discharge springs just downslope. In
combination with lack of rain in the dry season
and evaporation from the lake surface, this
seepage causes lake level to drop below the
spillway each year (Tuttle et al. 1992; Evans
et al. 1994). The magnitude of lake-level drop
Fig. 3 Staff height (cm) at the spillway of Lake Nyos
representing lake level measured over the period 1987–
2004. Each point is a mean value for one month
calculated using all years of data. The average across all
years (circles) is shown with error bars representing
standard deviation of the total number (N) of measure-
ments across all years by month. The range in lake level is
taken as the minimum (triangles) and maximum (dia-
monds) value of the monthly mean lake level across all
years
The Comparative Limnology …405
below the spillway has reached 238 cm but varies
from year to year (Fig. 3). Interannual variation
in lake level is much less during the outflow
period from August into December.
Shallow inflows of dilute groundwater con-
stitute a significant part of the water budget at
Lake Nyos (Tuttle et al. 1992). Inflow and out-
flow of shallow groundwater have not been
investigated at Lake Monoun, but may not be
significant given the low gradient in topography
for many km upstream and downstream of the
lake. Inflow of groundwater that is rich in dis-
solved CO
2
and higher in Total Dissolved Solids
(TDS) than surface water or shallow groundwater
occurs at or near the bottom of both lakes, and is
the ultimate source of gas for the associated
hazard. Discrete inflow vents have never been
sampled or even precisely located, but the char-
acteristics of this inflowing water have become
better constrained over time through the
observed changes in bottom water temperature
and chemistry (Kling et al. 1989; Evans et al.
1993; Nojiri et al. 1993; Kusakabe et al. 2000;
Schmid et al. 2003; Kusakabe 2015).
3.3 Water-Column Structure
The overall density structure in lakes which are
permanently stratified (meromictic) includes an
upper zone called the mixolimnion which expe-
riences seasonal stratification and turnover, and a
lower zone called the monimolimnion which
stays permanently stratified. The separation
between these two layers is usually called the
main chemocline or pycnocline. The upper layer
is chemically dilute while lower layers can
accumulate salts or gases. In the upper layer there
is a thermocline, which is deeper in Lake Nyos
than in Lake Monoun due to the greater surface
area and water clarity at Nyos (Kling 1988).
Profiles of dissolved CO
2
,CH
4
, and dissolved
salts (usually plotted as conductivity) show
increasing concentrations with depth below the
chemocline. In addition, the shape of the depth
profiles evolved during the years following the
gas releases. The patterns of change have been
presented elsewhere (e.g., Kling et al. 2005;
Kusakabe et al. 2008 and updated in Kusakabe
2015). The most important and ominous change
was a clear increase in dissolved CO
2
over time
in both lakes.
Temperature also increases with depth below
the chemocline, and this in part offsets the
strength of stratification due to dissolved CO
2
and TDS because warmer temperature reduces
water density. The opposing effects of tempera-
ture and dissolved substances could allow dou-
ble-diffusive convection to govern transport
within the water column (e.g., Nojiri et al. 1993),
and this phenomenon was observed over a lim-
ited depth interval in 2002 in Lake Nyos (Schmid
et al. 2004). Water-column heat and mass fluxes
otherwise appear to be slow and diffusion con-
trolled (e.g., Kantha and Freeth 1996; McCord
and Schladow 1998; Kusakabe et al. 2000). The
inflow of high-TDS water near lake bottom may
actually be the main driver of net upward trans-
port in these lakes, consistent with modeling
results (Schmid et al. 2006) and observed profile
development (Kusakabe 2015).
This basic two-component model of the water
column accounts for the depth profiles of most
dissolved gases and ions, but complexities arise
regarding Fe and CH
4
. Transport of these species
involves solid and gaseous phases that can sink
or rise at much different rates than dissolved
substances. The lakes are essentially traps for Fe,
which is present as ferrous iron in anoxic bottom
water (Kling et al. 1989). Any mixing or
diffusion of dissolved salts up into overlying
oxygenated water converts the iron into ferric
oxy-hydroxide particles, which then sink instead
of being flushed from the lake. The precipitated
iron is then reduced and redissolved in anoxic
bottom water. The occurrence of this process
over many years of stratification explains why Fe
constitutes ≥40 % of the cations in bottom water
of Lake Nyos and ≥80 % in Lake Monoun. The
Fe cycling was discussed in detail by Teutsch
et al. (2009).
Along with iron precipitates, particulate
organic matter also settles down into bottom
water where it donates electrons to reduce ferric
406 G.W. Kling et al.
to ferrous iron and serves as a carbon or electron
source to promote production of CH
4
. Evans
et al. (1993) showed that the CH
4
in Lake Nyos
was produced at least in part from surficial
organic matter based on its high
14
C content
(41 % modern C) relative to that of the CO
2
(1–
2 % modern C). In the years after the gas relea-
ses, CH
4
concentrations in both lakes increased
much faster than did CO
2
concentrations, also
showing lacustrine production of CH
4
. The
greatest increase occurred just above lake bot-
tom, but CH
4
increases were substantial even at
depths where CO
2
and TDS concentrations
remained nearly constant (Kling et al. 2005).
Either some distributed CH
4
production occurs
within the water column, which contains <1 mg/L
SO
4
throughout (mean = 0.19 mg/L ±0.18 S.D.,
N = 34), or the CH
4
is transported in the lake
independently of dissolved species as a separate
gas phase. Probably the settling organic matter
supports methanogenesis even in the upper water
column on anoxic microsites (e.g., Karl and
Tilbrook 1994; Cabassi et al. 2013).
Evidence of a separate, gas-phase transport for
CH
4
came from trapping bubbles of CH
4
-rich gas
(47 %) in an inverted funnel suspended at the
chemocline in Lake Nyos (Kling et al. 2005). In
addition, depth sounders sometimes return ima-
ges that are interpreted as rising gas bubbles
(Sabroux 2001, pers. comm.). Gas saturation
apparently is reached at the sediment-water
interface or possibly on particles in the water
column, at least in local areas, allowing the
bubbles to form. Even though CH
4
constitutes
only *2 % (on a molar basis) of the total dis-
solved gas at the bottom of these lakes, it would
make up *50 % of the gas bubbles because of
its much lower solubility (Wilhelm et al. 1977).
3.4 Lake Stability
Dissolved CO
2
is a greater factor than TDS in the
increased density of bottom water that leads to
stratification in these lakes. However, as the
water-column stability imparted by dissolved
CO
2
increases, it becomes metastable when
saturation is reached because gas bubbles can
form. The presence of bubbles greatly reduces
bulk water density and the gas rises rapidly and
destroys the water column stability. This loss of
density structure and lake stability can lead to
spontaneous lake mixing and amplification of gas
exsolution to the point of creating massive,
dangerous gas releases.
Because of this non-linear effect of CO
2
on
water density, standard formulas for calculating
the energy required to overcome lake stratifica-
tion are inapplicable to gas-rich lakes. An alter-
nate parameter, E
*
, was developed to evaluate
lake stability and the potential danger of massive
gas bursts (Kling et al. 1994). E* decreases as the
pressure of dissolved gas (CO
2
+CH
4
+N
2
)
approaches hydrostatic (gas saturation), espe-
cially in situations with high (i.e., dangerous) gas
pressures. Profiles showing low E* indicate low
lake stability in gas-rich zones below the
chemoclines in Nyos and Monoun, even though
density gradients and conventional water-column
stability might be large. Increases in E* stability
due to controlled degassing were predicted and
later observed (Kling et al. 2005).
3.5 The Mixolimnion
3.5.1 Seasonal Patterns at Nyos
Temperature variations in the mixolimnion and
their relation to climate have been discussed by
numerous authors (e.g., Kling et al. 2005; Sch-
mid et al. 2006) and are shown in Fig. 4for
several annual cycles. Surface water shows a
seasonal variation of *6°C with a period of
pronounced warming in the spring months
March–May following increases in air tempera-
tures (Fig. 1). The onset of the summer rainy
season brings some cooling of the surface layer,
but subsurface layers continue to warm. During
this period there is heat loss from surface water to
the atmosphere, and downward heat transfer
from the surface to lower layers by mixing. As
the strength of the thermocline decreases, there is
more heat transfer to greater depths; for example,
The Comparative Limnology …407
water at 10 m depth warms most rapidly in June
as surface water cools, and water at 20 m depth
warms later, in August through October, as
temperatures at 10 m depth decline (Fig. 4). The
near-constant temperature at 60 m depth, imme-
diately below the chemocline, during the pre-
degassing period indicates no significant upward
heat transfer across this boundary.
Early fall (September–November) brings a
slight increase in the shallow water temperature
gradient, perhaps due to the declining frequency
of major storms, but the low humidity increases
evaporation and the coldest night-time tempera-
tures of the year cause a rapid cooling of surface
water in December–February (Fig. 4). The
cooling over this period gradually deepens the
upper mixing layer until it eventually reaches
the chemocline before springtime warming
begins again. Thus, the annual pattern of mixo-
limnion dynamics can be divided into three main
parts: a period of strong stratification in spring, a
period of weak stratification in summer, and a
period of deep mixing in winter.
Temperature and conductivity profiles with
depth (CTD) typical of each of the three periods
are shown in Fig. 5, and they can be compared to
the annual cycle shown in Fig. 4. The April
profile (Fig. 5a) shows the strong spring-time
stratification, with the sharp seasonal thermocline
beginning at about 8 m depth. The temperature
gradient is fairly small between 20 and 40 m
depth, where the chemocline begins as seen in
the specific conductance profile. Temperatures
begin to increase at the chemocline and continue
to warm all the way to lake bottom. The early
November profile (Fig. 5b) was actually col-
lected at the end of the rainy season, but it shows
the cumulative effect of the preceding months of
wet and windy weather. Thermal stratification
is much weaker than in April but persists as is
shown by a relatively uniform temperature gra-
dient from the surface down to about 40 m depth.
The late January 2001 profile (Fig. 5c), the last to
be collected prior to the start of degassing,
reflects the deep mixing of wintertime. Water
temperature is nearly uniform from 5 to 42 m
depth. The weak thermocline in the upper few
meters reflects daytime solar heat inputs and
disappears most nights, as seen in Fig. 4.
Specific conductance profiles help to under-
stand mixing dynamics in the mixolimnion. The
January profile (Fig. 5c) shows the uniformity of
specific conductance from 0 to 40 m during deep
wintertime mixing. This uniformity mostly per-
sists during the height of springtime stratifica-
tion, even when the thermocline prevents mixing
between the over- and under-lying water masses
(Fig. 5a). A gradient in specific conductance
(above 30 m depth) is only seen in the November
profile (Fig. 5b).
Superimposing the specific conductance pro-
files from all three dates highlights the subtle
differences in structure and shows that water in
the uppermost *25 m was significantly diluted
in November relative to the earlier or later dates
(Fig. 6). The November chemocline was also
slightly less sharp than on the other dates. The
surface dilution likely reflects the effects of
the preceding months of the rainy season and
some wind-driven eddy mixing down to *25 m,
as discussed above. The slight weakening of the
chemocline is more interesting. Based on the
annual patterns shown in Fig. 4, a profile col-
lected in November would follow an 8-month
period of stratification in the mixolimnion since
the preceding winter mixing. The weakening of
the chemocline seen in Fig. 6documents some
limited mixing across this boundary that must
have occurred during the preceding 8 months.
Downward mixing of surface water appears to
stop at 25 m depth, but the violent storms of
Fig. 4 Lake Nyos water temperatures at fixed depths
(relative to mean lake level) recorded at 30 min intervals
408 G.W. Kling et al.
Fig. 5 Profiles of the upper 60 m of Lake Nyos obtained
by CTD equipped with pH and O
2
probes. “Temp”
(diamonds) is temperature in °C, “SC”(squares) is specific
conductance in μS/cm, pH (triangles) in log units, O
2
(circles) in mg/L. Dotted line shows the log of Henry’s law
CO
2
pressure (in atm) estimated from pH and our
observation that for SC <200 μS/cm, HCO
3
(mg/L) =
6.68 + 0.530 ×SC (µS/cm) (N = 44; R
2
= 0.94). Solid “O
2
sat”line is the calculated concentration of oxygen in water
(mg/L) at the measured temperature in equilibrium with the
atmosphere at lake surface elevation. aProfiles in April
1998. bProfiles in November 1999. cProfiles in January
2001 (O
2
determined on water samples retrieved to lake
surface)
The Comparative Limnology …409
summer could still potentially transfer energy to
the chemocline depth and promote some mixing
through shear or wave breaking on internal
seiches.
The concentrations of O
2
and CO
2
also reflect
mixing processes, but unlike the major ionic
species that behave nearly conservatively, these
gases have independent sources and sinks such
as photosynthesis and respiration plus exchange
with the atmosphere. Thus their profiles differ
from those of specific conductance. Our CTD
casts did not directly measure CO
2
, but it can be
estimated from pH and specific conductance as
discussed by Kusakabe et al. (2008).
As expected for a lake with anoxic bottom
water, O
2
profiles show decreasing concentra-
tions with depth and always drop below detection
at the top of the chemocline. Dissolved CO
2
profiles show increasing concentrations with
depth; pH profiles show the mirror image with
decreasing values from the surface down to the
chemocline as seen clearly in Figs. 5a and b.
In April, both O
2
and pH profiles show a
conspicuous jump to higher values in surface
waters above the thermocline (Fig. 5a). In fact,
O
2
concentrations in this layer reach 120 % of
saturation with respect to the overlying atmo-
sphere. Similar O
2
oversaturation has been
observed during spring months in other years.
These high O
2
concentrations, and the corre-
sponding decrease in CO
2
(increased pH) are
clearly caused by photosynthetic activity in the
surface waters (as discussed below). The strong
springtime thermocline allows these conditions
to develop by effectively isolating the surface
water from the underlying CO
2
source and O
2
sink.
By November, thermal stratification has
greatly weakened and O
2
concentrations in the
surface water have dropped to just below satu-
ration (Fig. 5b). Both the O
2
and pH profiles
have become fairly smooth curves exhibiting
little correlation to the temperature profile. Deep
mixing in winter reduces surface O
2
concentra-
tions below saturation but does not completely
eliminate the gradient in either O
2
or pH
throughout the mixolimnion, at least during the
CTD cast in late January (Fig. 5c). This reduction
in O
2
is likely due to upward mixing of anoxic
water from beneath the chemocline as the ther-
mocline deepens in winter. Therefore, while
some deep mixing to the chemocline might occur
nightly (as indicated in Fig. 4), the complete
homogenization of the mixolimnion implied by
the temperature and specific conductance profiles
(Fig. 5c) is a less frequent event.
The January O
2
profile (Fig. 5c) was derived
from measurements in water samples retrieved to
the surface and protected from atmospheric
contact. The pH was also determined on these
samples, and alkalinity was determined within
several hours after collection, allowing the CO
2
Fig. 6 Lake Nyos specific
conductance profiles in
April 1998 (crosses),
November 1999 (solid
line), and January 2001
(triangles)
410 G.W. Kling et al.
partial pressure to be accurately calculated. The
CO
2
pressures for these and similarly processed
samples from other dates are shown in Fig. 7.
The P
CO2
values show a large seasonal range
at the lake surface, from about 10,000 µatm
to <100 µatm. The highest surface P
CO2
was
found in January 2001, near the time of the
profile in Fig. 5c when surface pH was low and
deep mixing had brought some gas-rich bottom
water into the surface water. The lowest P
CO2
was found in early May 1998, just days after the
profile in Fig. 5a.
The April and May surface P
CO2
values are
well below saturation with respect to the overlying
atmosphere (atmospheric saturation *330 µatm
at lake elevation using 390 ppmv in the
atmosphere). The strong springtime temperature
stratification that traps photosynthetically-
produced O
2
above the thermocline also blocks
upward transport of CO
2
into surface waters so
effectively that invasion of CO
2
from the atmo-
sphere must occur to support photosynthesis. That
this lake, made famous by the enormous amount of
CO
2
stored in bottom waters, should at times be a
net sink for atmospheric CO
2
is remarkable and
highlights the role of limnological processes in
concealing the hazard at depth.
3.5.2 Seasonal Patterns at Monoun
Annual cycles of temperature variations in the
mixolimnion of Lake Monoun (Fig. 8) are similar
to those of Lake Nyos, although the upper mixing
layer in Monoun is shallower. Both lakes show a
period of springtime warming, and the onset of
the summer rainy season in late June brings
cooling of the surface layer. However, in contrast
to Lake Nyos, summertime mixing can involve
the entire mixolimnion as seen for example in the
small temperature gradients near the surface and
the abrupt temperature drop at 15 m depth during
August 2000. This deeper mixing in late summer
to early fall has also been observed in crater lake
Barombi Mbo in southwestern Cameroon, and
may play a role in the timing of the Nyos and
Monoun gas releases which both occurred in
August (Kling 1987). Weak temperature stratifi-
cation continues through the summer and fall
until the lowest temperatures of the upper water
Fig. 7 Calculated P
CO2
values for different depths and
dates. The “atm”line shows the expected P
CO2
of water in
equilibrium with the atmosphere at lake surface condi-
tions. The “hyd”curve shows hydrostatic pressure, the
maximum P
CO2
that could exist in a stable solution.
aLake Nyos. bLake Monoun
Fig. 8 Lake Monoun water temperatures at fixed depths
(relative to mean lake level) recorded at 30 min intervals
The Comparative Limnology …411
column are reached typically in late December
and January. These lowest water temperatures are
due to low humidity and cold nighttime air tem-
peratures, the same factors affecting Lake Nyos
surface water temperature at this time of year.
Thus, the annual cycle of the mixolimnion at
Lake Monoun can be divided into two main
periods: a period of strong stratification in late-
winter and spring, and a period of deep mixing or
very weak stratification in summer to early
winter.
The period of weak thermal stratification is
typified by the November profile (Fig. 9a)
showing a thermocline at about 2 m depth. The
water column is nearly isothermal below the
thermocline down to the top of the chemocline
at *17 m depth, although there is still some
chemical structure to the water column as seen by
the conductivity profile. The May profile (Fig. 9b)
shows a stronger and deeper thermocline, and the
temperature gradient continues down to the
chemocline where it reverses and temperatures
get warmer all the way to lake bottom (Kling et al.
2005).
Annual changes in chemistry at Lake Monoun
are in general similar to those at Lake Nyos. For
example, the strong springtime thermocline
allows the pH in overlying water to shift to
higher values (Fig. 9b) because the effect of
photosynthetic production of O
2
is restricted to
the surface water layer where there is sufficient
light and nutrients. Based on limited available
data, it appears that the volumetric photosyn-
thetic production of O
2
in surface water from
0–3 m depth is similar at Lake Monoun and Lake
Nyos. The mean O
2
production (light-dark bottle
method) in Monoun was 0.14 mg O
2
/L/h (±0.04
S.D., N = 1 date and 3 depths) and at Nyos it was
0.11 mg O
2
/L/h (±0.08 S.D., N = 3 dates and
10 depths). Oxygen oversaturation in Monoun
Fig. 9 Profiles of the
upper 25 m of Lake
Monoun (see Fig. 5caption
for more details).
aObtained in November
1999 by CTD equipped
with pH and O
2
probes.
bDetermined on samples
retrieved to lake surface in
May 1998
412 G.W. Kling et al.
surface water was observed in April 1992, when
the O
2
concentration at the lake surface reached
115 % of saturation. Surface water temperatures
and thermocline strength at that time were com-
parable to those shown for May 1998 (Fig. 9b).
Profiles of CO
2
show the drawdown that occurs
in surface waters during the spring thermal
stratification due to photosynthetic uptake
(Fig. 9b). Undersaturation in CO
2
observed in
March 1992 (Fig. 7b) coincided with O
2
over-
saturation and a rise in surface water pH to 8.67.
Unlike Lake Nyos, where some dissolved O
2
can be detected below the thermocline and
occasionally as deep as the top of the chemocline
(e.g., Fig. 5c), water below the thermocline in
Lake Monoun is normally anoxic and we have
never detected O
2
at depths >10 m (e.g., Fig. 9a).
This likely reflects a greater oxygen demand in
Lake Monoun relative to Lake Nyos, for example
by bacteria using the high DOC concentrations as
a substrate for respiration or by greater demand
for oxidation of Fe(II). Mixing across the
chemocline may be more important at Lake
Monoun because of its much shallower chemo-
cline depth. Water below the chemocline is
extremely rich in ferrous iron, and thus any input
of this water would create a huge oxygen demand
in the overlying water.
3.5.3 Short-Term Variability at Both
Lakes
The most obvious short-term change is the reg-
ular diel variation in surface water temperature,
which produces the high-frequency signal at 1 m
depth in Figs. 4and 8. The diurnal range in
temperature is commonly about 1 °C in both
lakes, but occasionally reaches 2 °C in Lake
Nyos and 3 °C in Lake Monoun with its shallow
thermocline and lower light transparency.
Diurnal variations in specific conductance are
near the detection limit, but changes in dissolved
O
2
and CO
2
can be large during daylight hours as
photosynthesis proceeds. The three CO
2
values for
surface water at Lake Monoun in May 1998 (from
right to left in Fig. 7b) were measured at one-hour
intervals beginning at 09:45 on a morning with
full sun and no wind. The progressive decline in
CO
2
over two hours is significant in comparison to
the seasonal variability.
3.6 Chemocline Evolution
The chemocline represents the plane of greatest
resistance to vertical mixing in the lakes, and its
depth constrains the total amount of gas that may
accumulate in bottom waters. Thus an under-
standing of the controls on chemocline depth and
its evolution are important for the process of
controlled degassing and the cycle of gas buildup
and release in the lakes. The mixing of bottom
water up to the surface during the gas releases
disrupted pre-existing chemoclines in both Lake
Nyos and Monoun. After the events, the chemo-
cline redeveloped, deepening and strengthening
in both lakes during the years after the gas
releases (e.g., Kusakabe et al. 2008).
In Nyos, this redevelopment began soon after
the 21 August 1986 gas release. Water samples
collected just two weeks later in early September
show the largest gradient in TDS between 2.6 and
5.4 m depth (Tuttle et al. 1987; Kling et al. 1987),
and by October a sharp chemocline had devel-
oped at 7 m depth (Kanari 1989). By December of
1988 the chemocline had deepened to 30 m
(Kusakabe et al. 2008), and it reached 47 m depth
by November 1993 and remained near this depth
for many years (Kling et al. 2005). In Lake
Monoun a similar deepening occurred from a
chemocline depth of 8 m in October 1986 to 17 m
in 1993 and to 23 m in 1999 (Kusakabe et al.
2008). Profiles of conductivity between 1999 and
2003 suggest only a slight deepening (*1m)
over that time period (Kling et al. 2005).
The specific conductance of surface water in
both lakes as a function of time is shown in
Fig. 10, as is the range in specific conductance of
the main inflows based on rainy season and dry
season samples at each lake. One pre-release
surface sample at Lake Nyos was collected in
1985 (Kling et al. 1987a), and it shows that
specific conductance increased during the gas
release by more than a factor of five and
remained at this high level for several months.
The overlying water became more dilute in
The Comparative Limnology …413
subsequent years as the chemocline deepened, and
prior to the start of degassing the surface waters
were similar to the pre-release value. The specific
conductance of surface water has always been
slightly higher than in the inflow stream, and the
difference is more significant considering that the
inflow stream is more dilute at high flow during
the rainy season. This effect probably reflects
some bottom water mixing across the chemocline
both before and after the gas release. High-
conductivity water input to the deepest levels of
the lake (e.g., Kusakabe 2015) would require
some throughput to the surface unless a matching
level of seepage occurred below the chemocline.
At Lake Monoun, the specific conductance of
surface water prior to the start of degassing was
always near to that of the inflow. This likely
reflects the greater impact of the Panke River on
the relatively small lake. Specific conductance
was probably elevated for a brief period follow-
ing the gas release, but that period had passed
before the first measurement of specific conduc-
tance ten months later (Kling 1987a). In sum-
mary, the CTD profiles, the time course of
conductivity (Kling et al. 2005), and the time
series of specific conductance in surface water all
indicate that chemocline depth and the chemistry
of the mixolimnion had stabilized in both lakes
prior to the onset of degassing. One indication of
this stabilization is the fact that the profiles of
TDS and gas measured at Lake Nyos in January
2001 show that the temperature of surface water
would have to drop to <20 °C to achieve a
density great enough to sink and penetrate the
chemocline. Such temperatures do not occur
anywhere in our multi-year record. A similar
situation applies to Lake Monoun.
A numerical model presented by Schmid et al.
(2006) reproduced fairly well the chemocline
deepening that was observed prior to the onset of
degassing at Lake Nyos, but the predicted final
chemocline depth (without degassing) of
60–70 m is deeper than the stable depth
of *50 m observed prior to degassing (Fig. 6).
Either the “equilibrium”mixing depth was not
reached by 2001, or one-dimensional models
cannot completely account for some factors that
might influence chemocline depth, such as
bathymetry effects and the depth distribution of
groundwater inflows and outflows.
Groundwater inflow and outflow are known to
be important to the water budget at Lake Nyos
(Tuttle et al. 1992; Evans et al. 1994). Such flows
may affect chemocline depth by altering the
Fig. 10 Specific conductance of surface water through
time at Lake Nyos (blue diamonds) and Lake Monoun
(red squares). Dashed horizontal lines show the specific
conductance range in the main inflows, based on rainy
and dry season samples (highest in dry season). Dashed
vertical lines show timing of gas releases and of initial
pipe installation
414 G.W. Kling et al.
density gradient in the lake. For example, the
density gradient is strengthened by addition of
CO
2
and high-TDS water beneath the chemo-
cline or by addition of dilute water above the
chemocline, and surface stirring processes may
not be able to mix water as deeply against the
stronger gradient, and the chemocline would rise.
Similarly, outflow of gas-rich, dense water from
60 m depth could deepen the chemocline, but
outflow of dilute water from 40 m depth would
not. However, groundwater inflow and seepage
are unlikely to influence chemocline depth at
Lake Monoun where the surrounding terrain is
relatively flat and there is only a small gradient in
groundwater head. In terms of bathymetry, both
lakes have a surface area that is much larger than
the diameter of the main crater containing the
gassy water. The lip of the main crater is
at *40 m depth in Lake Nyos and *20 m depth
in Lake Monoun. The submerged lip effectively
reduces the fetch and thus the transfer of mixing
energy from the wind to the crater area compared
to the entire surface area of the lake.
The chemocline depth before the 1984
(Monoun) and 1986 (Nyos) gas releases is diffi-
cult to predict. In this monograph, Kusakabe
(2015) proposes that the pre-release chemoclines
in both lakes were very deep, near 55 m in Lake
Monoun and 110 m in Lake Nyos. On the basis
of depths of mixing recorded in other tropical
lakes (Kling 1988) it is unlikely that surface
mixing would help to establish a chemocline at
such great depths. However, it is possible that
chemoclines could form at any depth depending
on the vertical distribution of inputs of dense,
gas-charged groundwater, and that a gradient
zone would exist above a deep chemocline up to
the depth of typical seasonal mixing such as
observed at 50–60 m depth in Lake Nyos.
3.7 Effects of Degassing
The intent of installing pipes is to remove the gas
from these lakes, but the pipes actually function
as circulators that bring bottom water to the lake
surface where the gas can escape to the atmo-
sphere. Complete degassing of a lake requires
that all of the gas-rich water below the chemo-
cline be brought up and discharged onto the lake
surface. The stability of stratification must be
maintained throughout the degassing process in
order to keep the remaining gas trapped in
solution and prevent a spontaneous release. Ini-
tial concerns that the density stratification in the
lake would be destroyed by (1) the sinking of
degassed but still salt-laden and dense water
released at the surface from the pipes, (2) the
lateral flow of water to the pipe intake creating
sheer and turbulence, or (3) density currents
formed by cooling of the pipe during exsolution
of gas, all proved groundless. Several modeling
studies concluded that disruption of lake stability
from sinking of dense water released at the sur-
face was unlikely (McCord and Schladow 1998;
Kusakabe et al. 2000), and temperature monitor-
ing near the pipe intake during the initial tests of
degassing at Lake Monoun in 1992 (Halbwachs
et al. 2004) showed no evidence that the second
and third processes occurred. In fact, during
degassing there was remarkable preservation of
water column structure (Halbwachs et al. 2004;
Kling et al. 2005; Kusakabe et al. 2008). Entire
water layers have been removed sequentially as
overlying layers have lowered, analogous to the
removal of cards from the bottom of a deck.
The main effects of degassing on the upper
water column are (1) changes in thermal struc-
ture, (2) a redistribution of salts from bottom to
surface waters and a deepening of the chemo-
cline in each lake, (3) increases in dissolved
nutrients (especially N and P) from bottom
waters, which stimulated algal growth and oxy-
gen generation, (4) consumption of oxygen in
surface waters due to the oxidation of reduced
iron brought from depth to the surface, and (5) a
reduction in light penetration. As expected, the
specific conductance of surface water in both
lakes increased with the onset of degassing
(Fig. 10). The effect is less pronounced at Lake
Monoun and quickly diminished with time
because of greater flushing and dilution from the
Panke River compared to the inlet stream of Lake
Nyos. In contrast, the specific conductance of
surface water at Lake Nyos has remained ele-
vated over time, in part due to the installation of
The Comparative Limnology …415
two more degassing pipes in 2010. The surface
water conductivity reached a peak in June 2012
of *280 µS/cm, higher than the values just after
the gas release in 1986 (Fig. 10), and then
decreased to *200 µS/cm in January 2013.
3.7.1 Changes in Thermal Structure
The main changes in thermal structure of both
lakes during the first 1–3 years of degassing were
noticed near the depths of the upper chemoclines.
The upper water column temperatures are domi-
nated by seasonal and annual changes in climate
(Figs. 4and 8). Independent of these natural
variations, at Nyos the water temperatures at the
60 and 80 m thermistor depths, around the 23 °C
isotherm, started to decrease *1 year after
degassing started in early 2001 (Fig. 4; Kling
et al. 2005). Cooling at 60 m began in April 2002
and at 80 m in September 2002, and temperatures
decreased by *0.2 °C at both depths by early
2004 (Fig. 4). Because TDS at these depths did
not change initially, and only gradually decreased
over time (Kling et al. 2005; see discussion
below), Schmid et al. (2004) proposed that dou-
ble-diffusive convection observed at these depths
in 2002 was responsible for the cooling.
At Lake Monoun similar decreases in water
temperatures at the 45 and 55 m thermistor depths
occurred within *1 month of the start of degas-
sing in early 2003 and continued through 2004
(Fig. 8; Kling et al. 2005). Surface water tem-
peratures down to 25 m were slightly cooler in
2003 and 2004 compared to prior years, and
deeper mixing of these cooler waters may have
contributed to the declining temperatures at 45
and 55 m (Fig. 8). However, in the case of Mo-
noun and as discussed in detail in the next section,
deep-water removal from the degassing pipe was
likely the main cause of chemocline lowering
resulting in cooling temperatures at mid-depth.
3.7.2 TDS Redistribution
and Chemocline Deepening
During the degassing operation deepening of the
chemocline occurred in both lakes by a combi-
nation of processes. First, the chemocline is
lowered as water beneath it is piped to the sur-
face. Second, increases in surface water con-
ductivity reduce the density gradient across the
chemocline and allow for deeper mixing. For
example, in Lake Nyos average surface water
conductivity increased from 61 µS/cm in January
2001 to 87 µS/cm in January 2004, during which
time the chemocline was lowered by *13 m
(Kling et al. 2005). Because the deep chemocline
around 180 m depth in Lake Nyos was only
lowered by 3–4 m during this same time period
(Halbwachs et al. 2004; Kling et al. 2005), most
of the drop in the upper chemocline must have
been due to deeper mixing. Despite the combined
effects of double diffusive convection and
increased TDS in surface water to weaken the
chemocline, it remained above 60 m depth until
after January 2004 (Kling et al. 2005) indicating
that the lower water column was still strongly
stratified and stable during the degassing.
In Lake Monoun, measurements in 2004
showed that the inflowing Panke River (106.5 μS/
cm) and the pipe discharge (*2,300 μS/cm)
mixed to form surface water with a specific con-
ductance of 140.4 μS/cm in the main crater near
the pipe platform. Surface-water values in the
neighboring middle basin and in the shallow
western arm of the lake were similar, 139.0 and
138.8 μS/cm, respectively, showing uniform
horizontal mixing across the lake surface. From
the start of degassing in January 2003 until
January 2004 the main chemoclines at 25 and 55
m were each lowered by *7 m (Kling et al.
2005). The fact that both chemoclines were
lowered the same amount indicates that chemo-
cline subsidence was caused more by deep-water
removal than by enhanced mixing and erosion at
the surface. In early 2003 the depth of surface
mixing clearly reached the 25 m thermistor
(Fig. 8). As 2003 progressed and especially into
early 2004, the temperatures at 45 and 55 m depth
cooled in response to deep water removal.
3.7.3 Nutrients and Oxygen
Total dissolved phosphorus (TDP) concentra-
tions in January of 2005 were less than 0.1 µMin
the surface waters of Nyos and Monoun, but
416 G.W. Kling et al.
were at least 10-fold greater near lake bottom
(Fig. 11). Differences in surface and bottom
water concentrations of total dissolved nitrogen
(TDN) were also striking, and pumping of
nutrient-rich bottom water to the surface during
degassing increased TDN in surface waters over
time (Fig. 12). For example, in Lake Monoun in
January 2003 before degassing, the average TDN
concentration in surface waters (0–7 m) was
21 µM, and one year after degassing started the
average concentration increased to 85 µM. In
Lake Nyos prior to degassing in January 2001
the average TDN concentration in surface waters
was 4 µM(0–40 m) and in 2004 it had increased
to 208 µM (Fig. 12).
These increases in surface-water dissolved N
and P concentrations would be expected to
increase phytoplankton growth rates and increase
the production of dissolved oxygen. The dis-
solved O
2
concentrations are a net of production
and consumption, and the degassing pipes also
brought water with high dissolved ferrous iron to
the surface. The Fe(II) rapidly oxidizes at surface
conditions to form reddish hydroxy-oxide pre-
cipitates, and the oxidation process consumes
oxygen according to:
Fe2þþ1=4O2þHþ¼Fe3þþ1=2H2O
Fe3þþ3H2O¼Fe(OHÞ3þ3Hþ
where each mole of O
2
consumed can produce 8
H
+
, converting 8 HCO
3
−
to 8 CO
2 (aq)
+8H
2
O.
At Lake Nyos in October 2001, after degas-
sing had started, dissolved O
2
in surface water
was the highest ever measured at the lake, at
Fig. 11 TDP concentrations in Lakes Nyos and Monoun
in January 2005
Fig. 12 TDN concentrations in Lake Nyos in January
2001 and January 2004 and Lake Monoun in January
2003 and January 2005
The Comparative Limnology …417
times exceeding 150 % of saturation (Fig. 13).
We interpret this high O
2
concentration to result
from photosynthetic production that was stimu-
lated by increased dissolved nutrients brought to
the surface from the degassing pipe. At this time
the surface water had turned a greenish color
from an algal bloom. Dissolved oxygen con-
centrations dropped quickly with depth however,
reaching a low value at just 10 m. Such a sharp
drop was abnormal compared to pre-degassing
profiles (e.g., Fig. 5). In addition, although we
have no post-degassing data on phytoplankton
biomass, profiles in both Nyos and Monoun
indicate that chlorophyll aand total particulate
carbon concentrations were highest in the near-
surface waters (Fig. 14). Particulate C produced
at the surface settled through the water column
and concentrations often increased again at depth
approaching the pycnocline, but there was little
chlorophyll content at depth (Fig. 14). In addi-
tion, particulate C:N molar ratios in both Nyos
and Monoun in surface waters were similar to
ratios expected in phytoplankton (mean C:
N = 8.6 ±3.5 in Nyos and 11.7 ±3.5 S.D. in
Monoun; expected value of 8.3, Sterner et al.
2008). Thus we assume that the phytoplankton
bloom responsible for the high surface O
2
buildup was limited to the very upper water
column, and the rapid decline in oxygen with
depth was likely due to respiration of the organic
matter produced by the algal bloom. Overall, the
lake contained less total O
2
on this date than ever
before.
Oxygen profiles for January sampling from
successive years can be compared without con-
sidering seasonal effects. In general the profiles
are similar to the January 2001 profile, obtained
just prior to degassing, and indicate that the
overall effect of degassing on lake productivity
(as estimated by the oxygen-change method) was
fairly small. The situation at Lake Monoun was
much different. All profiles measured after the
start of degassing show greatly reduced O
2
rel-
ative to the January 2003 profile, collected just
prior to degassing (Fig. 13b). The 2005 profile
shows O
2
dropping to low levels within the
upper 0.5 m of the water column. Bottom water
in Lake Monoun is richer in ferrous iron, relative
to Lake Nyos, and oxidation of this iron was
apparently a major factor in the large reduction in
dissolved O
2
.
3.7.4 Light Penetration
Pumping of bottom water to the surface
decreased water clarity and light penetration in
both lakes. This decrease was due to a combi-
nation of increased iron hydroxide floc, increased
dissolved organic carbon concentrations, and
likely increases in algal biomass due to nutrient
additions to surface waters. The average Secchi
depth in Lake Nyos prior to degassing from
1989–2001 was 4.09 m and has been reduced to
an average of 1.73 m since degassing began
(Table 1). Within the period of degassing the
light penetration has tended to decrease, in part
due to the addition of two more degassing pipes
in 2010. The shallowest Secchi depths recorded
in Nyos are 0.21 m in June 2012 and 0.34 m in
January 2013. The highest light extinction coef-
ficient (K) measured in Nyos prior to degassing
Fig. 13 Dissolved oxygen for different depths and dates
after the start of degassing. aIn January 2001 at Lake
Nyos. bIn January 2003 at Lake Monoun
418 G.W. Kling et al.
was 0.33 m
−1
in December 1989, and the lowest
is 0.73 m
−1
in January 2005 during the degassing
(no measurements available after that time). In
Lake Monoun the water transparency has also
decreased from a mean Secchi depth of 1.49 m
before degassing to 0.41 m during the piping
operations. However, after the degassing opera-
tion ceased in late 2008, the Secchi depth
increased to 1.57 m in June 2012 and 1.16 in
January 2013. The light extinction coefficient
was 1.23 m
−1
prior to and 5.23 m
−1
during the
degassing.
Dissolved organic carbon (DOC) is often the
most important light-absorbing component in
lake water. Bottom water concentrations of DOC
are high in Lakes Monoun (5,200 µM at 95.4 m
Fig. 14 Chlorophyll
a(top) and particulate
carbon concentrations
(bottom) in Lakes Nyos and
Monoun in May 1998,
November 1999, and
January 2001
Table 1 Changes in Secchi depths (m) in Lakes Nyos and Monoun prior to, during, and after degassing
Lake Monoun Secchi depth (m)
Pre-degassing During degassing Post degassing
Dec 1989–Jan 2003 Jan 2005-Jan 2006 Jun 2012–Jan 2013
Mean 1.49 0.41 1.37
SD 0.26 0.17 0.29
Range 0.90–1.85 0.27–0.70 1.16–1.57
N10 5 2
Lake Nyos Secchi depth (m)
Pre-degassing During degassing
Dec 1989-Jan 2001 Jan 2001–Jan 2013
Mean 4.09 1.73
SD 1.27 1.16
Range 2.90–6.20 0.21–3.64
N8 9
SD is the standard deviation of the mean, range gives the minimum and maximum values, and Nis the number of
unique sampling dates or locations
The Comparative Limnology …419
depth in January 2003) and Nyos (3,400 µMat
205.5 m depth in January 2001). In Monoun the
mean DOC value in 0–15 m water was 240 µM
and 320 µM in January 2001 and January 2003,
respectively. Surface water concentrations
increased to 460 µM in the upper 7 m of Monoun
in January 2004 one year after degassing started.
In Lake Nyos, the DOC concentration was 60 µM
in surface waters (mean of 0–40 m), and after
degassing started the concentrations in the upper
mixing layer (mean of 0–50 m) increased to 80 µM
in January 2003 and to 210 µM in January 2004.
Thus at least part of the decrease in water trans-
parency in these lakes results from colored DOC
being brought to the surface during degassing.
3.8 After Degassing Completion
At the end of the degassing process, gas-rich
high-TDS water will occupy only the bottom few
meters of the lake. The entire overlying water
column will likely consist of a nearly homoge-
neous mixture of degassed water from depth and
the original surface water, modified by inflows
and outflows. Modeling indicates that this thick
layer could mix every year or every few years
down to within a few meters of the lake bottom
(Schmid et al. 2006), and this would serve to
flush some gas from the lake and prevent a gas
buildup. Without any pipes in place however, the
inflowing gassy water would likely begin to refill
the main crater and raise the chemocline depth
over time, but how rapidly this occurs and how
much gas is ultimately stored in the lake is linked
in part to the equilibrium position and strength of
the chemocline. The design and urgency of future
mitigation efforts would in part depend on this
information.
It is unclear whether either lake would return
to the same water column structure and gas
content that existed prior to the gas releases.
There is some evidence that the rate of gas input
to bottom waters of Lake Nyos changes over
time (Evans et al. 1993; Kusakabe et al. 2000)
and it is conceivable that pre-release lake struc-
ture and gas content reflected inflow rates much
different from those observed during the rela-
tively short period of observation.
3.9 Global Implications
Although the gas releases from Lakes Nyos
and Monoun helped fuel a global study of
volcanic crater lakes, the two Cameroonian
lakes *100 km apart appear to be unique. They
constitute the “bursting bicarbonate volcanic
lake”category in the classification scheme of
Pasternack and Varekamp (1997).
Several attributes of Lakes Nyos and Monoun
likely contribute to the dangerous gas buildup,
and may explain why more lakes are not gas-
charged (Kling 1987b; Kling et al. 2005). The
lakes must be deep enough to contain gas at high
pressures, and large enough to hold substantial
amounts of CO
2
in solution. The lakes must be
strongly stratified and maintain that stratification
for multiple years to allow the buildup of gases.
Such strong stratification is aided in Nyos and
Monoun by submerged steep-walled main craters
that inhibit penetration of wind-driven mixing
and by the small seasonal cooling of the tropical
climate that may limit deep convective mixing.
Finally, there must be a strong local source of
CO
2
, which occurs in Cameroon in the form of
low-temperature, CO
2
-charged soda springs that
are widespread along the Cameroon Volcanic
Line (Tanyileke et al. 1996).
Still, the global absence of known analogues
among crater lakes remains a bit curious because
CO
2
sources are common in many volcanic
regions, CO
2
of presumed magmatic origin is
now known to vent into many crater lakes (e.g.,
Mazot and Taran 2009), and a recent estimate of
the global emission of CO
2
through volcanic
lakes is surprisingly large (Pérez et al. 2011). In
addition, many crater lakes are deep and steep-
walled. However, meromictic lakes are relatively
rare, even in the tropics, and the unlikely com-
bination of a deep meromictic lake and a gas
source feeding directly into the bottom waters, is
probably the main factor limiting the occurrence
of lethal gas-charged lakes worldwide.
420 G.W. Kling et al.
The characteristics of the CO
2
source are also
important for the scarcity of Nyos and Monoun
analogues. For example, if the inflowing CO
2
is
in gas phase, then bubbles can create plumes that
convey the gas to the lake surface where it is lost
to the atmosphere, especially if the gas inflow is
located in a relatively shallow part of the lake.
Examples include Lake Quilotoa (Aguilera et al.
2000), the Laacher See (Aeschbach-Hertig et al.
1996), and Ruapehu (Christenson 1994). At low
inflow rates, gas bubbles could dissolve com-
pletely and raise the CO
2
concentration of bot-
tom water, but the correspondingly small density
increase might then be insufficient to prevent
mixing and flushing of the CO
2
to the atmo-
sphere during seasonal cooling and lake turn-
over. A gas supply rate sufficient to enhance
perennial density stratification, but not large
enough to produce a buoyant plume, may be an
unlikely occurrence.
The bubble plume is avoided if the CO
2
gas
first dissolves in groundwater, which then flows
into and gradually fills up the deeper part of the
lake over time, the process thought to occur at
Lakes Nyos and Monoun (Kling et al. 1987;
Nojiri et al. 1993). Buoyancy remains an issue
because CO
2
-rich groundwaters that are sub-
stantially warmer and thus less dense than ambi-
ent lake water should rise in the lake to their point
of neutral buoyancy and would not become
trapped near lake bottom. Nyos-type lakes require
the coincidence of a strong magmatic gas source
that is also depleted in heat. However, the density
increase in cold groundwater that absorbs CO
2
will also reduce its tendency to move upward into
a lake basin. External hydraulic head can drive
the upflow of CO
2
-rich water into Lakes Nyos
and Monoun, but not at summit crater lakes or
lakes that are perched above the local water table.
The source strength of CO
2
, heat from
magma, and other factors like water table depth
or climate can change over time, and conditions
may be right for gas buildup in a crater lake for
only a small window of its existence. Several
Italian crater lakes are located in areas of strong
CO
2
emission (Chiodini and Frondini 2001), and
magmatic gas components have been found in
lake waters (Carapezza et al. 2008; Caracausi
et al. 2009; Cabassi et al. 2013). The lakes cur-
rently pose no hazard of a catastrophic release,
but unusual events that might have involved gas
releases are recorded in ancient Roman writings
(Cioni et al. 2003). Results of a multi-year study
of Lake Albano indicate that an anomalous spike
in CO
2
input accompanied a seismic swarm in
1989 (Chiodini et al. 2012). Carapezza et al.
(2008) suggest that a prolonged warm spell
without winter-time mixing might allow initia-
tion of stable stratification and gas buildup in this
lake, the reverse of a proposed weakening of
stratification in Cameroonian lakes due to an
unusually cool period in the mid-1980s (Kling
1987b).
Lake Kivu in eastern Africa is often consid-
ered the closest analogue to Nyos and Monoun
even though it does not occupy a volcanic crater.
This huge rift lake certainly contains a vast
quantity of dissolved CO
2
in stratified water
layers at depth, and the associated hazard was
recognized soon after the disasters in Cameroon
(Kling et al. 1987; Tuttle et al. 1990; Tietze
1992). The lake consists of several basins, of
which Kabuno Bay appears to present the greatest
hazard of an uncontrolled gas release (Tassi et al.
2009). Methane constitutes a larger fraction of the
dissolved gas, relative to the Cameroonian lakes,
and will become a greater factor in the hazard if it
continues to increase at present rates (Pasche et al.
2011). Experience at Lakes Nyos and Monoun
continues to inform plans to remove the gas from
Lake Kivu, especially the procedure for safe
discharge of the degassed water (Kling et al.
2006; Hirslund 2012).
Gas releases have been invoked at other lakes,
such as in Lake Voui based on an observed color
change (Bani et al. 2009), in Lake Quilotoa based
on historical records (Aguilera et al. 2000), and
in Lake Kivu based on unusual sediment layers
(Haberyan and Hecky 1987). A confirmed gas
release from a crater lake atop Mount Chiginagak
volcano in 2005 differed in many respects from
the Cameroonian disasters but showed some
The Comparative Limnology …421
similarities. As elaborated by Schaefer et al.
(2008), this 105 m deep meltwater lake formed
quickly in response to renewed upflow of heat
and gas into a snow- and ice-filled crater. Water
eventually burst through a cavity in the glacial
ice impounding the upper *45 m of the lake and
cascaded down the steep slopes into the valley
below. The rapid depressurization and agitation
released sulfur-rich gas, likely as an aerosol,
which flowed as a cloud down the valley.
The Chiginagak region is remote and unin-
habited, but vegetation bleached and killed by
the acidic cloud preserved a record of its 29 km
2
area and height (Schaefer et al. 2008), which
indicate a volume comparable to the Lake Nyos
gas cloud (*0.2–1km
3
). The huge size of the
cloud suggests that the drop in lake level may
have induced additional gas to exsolve from the
lake water remaining in the crater or even vent
rapidly from the underlying volcanic conduit.
This latter process is a concern at other crater
lakes where rim failure could cause a large drop
in lake level and thus pressure on underlying gas
stores, even if the lake itself does not contain
large amounts of gas. At Lake Nyos where a
fragile dam impounds the upper 40 m of water
(Lockwood et al. 1988), a significant amount of
gas may be contained in the sediments and would
remain a concern after the lake itself is degassed
(Freeth 1994). Thus the current effort to
strengthen the natural dam to prevent flooding
(Aka and Yokoyama 2013) has a double benefit.
4 Conclusions
The water columns of Lakes Nyos and Monoun
evolved over time into structures consisting of
dilute surface waters and high-TDS bottom
waters separated by a sharp chemocline. The
strength of this boundary allowed climate,
inflows and outflows, and biological processes to
completely control short-term and seasonal pat-
terns in surface waters; impacts from the under-
lying bottom waters were almost imperceptible.
Piping of bottom water onto the lake surface
during degassing has caused complex but
predictable changes to surface waters and has led
to expected deepening and in Monoun weaken-
ing of the chemocline. The likely evolution of
lake structure after degassing is somewhat
uncertain but could trend back toward dangerous
gas accumulation.
Nyos-type lakes can form only when several
conditions are simultaneously met: great depth,
large volume, CO
2
input, and persistent stratifi-
cation to allow CO
2
buildup beneath one or more
chemoclines. Nyos-type lakes are rare because
most lakes apparently fail to meet one of these
necessary conditions. Over geologic time how-
ever, it seems unlikely that Lakes Nyos and
Monoun would be the only crater lakes to cycle
through gas buildup and massive release. As
conditions change, perhaps even subtly, deep
crater lakes could turn into Nyos-type lakes
and become dangerous (see Pasternack and
Varekamp 1997; Aguilera et al. 2000). Thus some
program of occasional monitoring is worthwhile,
especially for those lakes in regions of volcanic
activity or known to have magmatic CO
2
inputs.
Gas release hazards can exist at lakes much
different from Lakes Nyos, Monoun, or even
Kivu. For example, crater lakes at active volca-
noes might be warm and acidic but have such a
strong input of gas that buildup can outpace
convective flushing to the atmosphere, especially
if salts or suspended solids contribute to any
density gradient in the water column. Although
gas pressure might never become sufficient to
drive a Nyos-type overturn and violent gas burst,
depressurization of lake water through crater
breaching could allow dangerous amounts of gas,
including sulfur gases, to be rapidly released.
Acknowledgments Funding and support came from
U.S.AID-OFDA grant AOTA-00-99-00223-00, the
Cameroonian Government (MINRESI-IRGM), the U.S.
and Japanese Embassies in Cameroon, the U.S. Geolog-
ical Survey, the Japanese SATREPS project of JST-JICA,
and the Japan Society for the Promotion of Science. We
thank Karen Riseng, Ibrahim Issa, Aka Festus, Nia Paul,
Chris Wallace, Mark Brahce, Amanda Field, Sara Fortin,
Susanna Michael, Mark Huebner, and Keisuke Nagao for
field or laboratory help. Aka Festus, Minoru Kusakabe,
and an anonymous reviewer provided helpful comments
on the draft.
422 G.W. Kling et al.
References
Aeschbach-Hertig W, Kipfer R, Hofer M, Imboden DM,
Wieler R, Signer P (1996) Quantification of gas fluxes
from the subcontinental mantle: the example of
Laacher See, a maar lake in Germany. Geochim
Cosmochim Acta 60:31–41
Aguilera E, Chiodini G, Cioni R, Guidi M, Marini L,
Raco B (2000) Water chemistry of Lake Quilotoa
(Ecuador) and assessment of natural hazards. J Volc-
anol Geotherm Res 97:271–285
Aka FT, Yokoyama T (2013) Current status of the debate
about the age of Lake Nyos dam (Cameroon) and its
bearing on potential flood hazards. Nat Hazards
65:875–885
Aka FT, Yokoyama T, Kusakabe M, Nakamura E,
Tanyileke G, Ateba B, Ngako V, Nnange J, Hell J
(2008) U-series dating of Lake Nyos maar basalts,
Cameroon (West Africa): implications for potential
hazards on the Lake Nyos dam. J Volcanol Geotherm
Res 176:212–224
Bani P, Oppenheimer C, Varekamp JC, Quinou T, Lardy
M, Carn S (2009) Remarkable geochemical changes
and degassing at Voui crater lake, Ambae volcano,
Vanuatu. J Volcano Geotherm Res 188:347–357
Cabassi J, Tassi F, Vaselli O, Fiebig J, Nocentini M,
Capecchiacci F, Rouwet D, Bicocchi G (2013)
Biogeochemical processes involving dissolved CO
2
and CH
4
at Albano, Averno, and Monticchio mero-
mictic volcanic lakes (Central–Southern Italy). Bull
Volcanol 75:683. doi:10.1007/s00445-012-0683-0
Caracausi A, Mario Nuccio P, Favara R, Nicolosi M,
Paternoster M (2009) Gas hazard assessment at the
Monticchio crater lakes of Mt. Vulture, a volcano in
Southern Italy. Terra Nova 21:83–87
Carapezza ML, Lelli M, Tarchini L (2008) Geochemistry
of the Albano and Nemi crater lakes in the volcanic
district of Alban Hills (Rome, Italy). J Volcanol
Geotherm Res 178:297–304
Christenson BW (1994) Convection and stratification in
Ruapehu Crater Lake, New Zealand: implications for
Lake Nyos-type gas release eruptions. Geochem J
28:185–197
Chiodini G, Frondini F (2001) Carbon dioxide degassing
from the Albani Hills volcanic region, Central Italy.
Chem Geol 177:67–83
Chiodini G, Tassi F, Caliro S, Chiarabba C, Vaselli O,
Rouwet D (2012) Time-dependent CO
2
variations in
Lake Albano associated with seismic activity. Bull
Volcanol 74:861–871
Cioni R, Guidi M, Raco B, Marini L, Gambardella B
(2003) Water chemistry of Lake Albano (Italy).
J Volcanol Geotherm Res 120:179–195
Evans WC, Kling GW, Tuttle ML, Tanyileke G, White
LD (1993) Gas buildup in Lake Nyos Cameroon: the
recharge process and its consequences. Appl Geochem
8:207–221
Evans WC, White LD, Tuttle ML, Kling GW, Tanyileke
G, Michel RL (1994) Six years of change at Lake
Nyos, Cameroon, yield clues to the past and cautions
for the future. Geochem J 28:139–162
Freeth SJ (1992) The Lake Nyos gas disaster. In: Freeth SJ,
Ofoegbu CO, Onuoha KM (eds) Natural hazardsin West
and Central Africa. Vieweg, Braunschweig, pp 63–82
Freeth SJ (1994) Lake Nyos: can another disaster be
avoided. Geochem J 28:163–172
Green J (1972) Ecological studies on crater lakes in West
Cameroon. the zooplankton of Barombi Mbo, Mboan-
dong, Lake Kotto and Lake Soden. J Zool Lond
166:283–301
Green J, Corbet SA, Betney E (1974) Ecological studies
on crater lakes in West Cameroon. Debundsha Lake.
J Zool Lond 173:199–223
Haberyan KA, Hecky RE (1987) The late Pleistocene and
Holocene stratigraphy and paleolimnology of lakes
Kivu and Tanganyika. Palaeogeogr Palaeoclimatol
Palaeoecol 61:169–197
Halbwachs M, Sabroux J-C, Grangeon J, Kayser G,
Tochon-Danguy J-C, Felix A, Béard J-C, Villevieille
A, Vitter G, Richon P, Wüest A, Hell J (2004)
Degassing the “Killer Lakes”Nyos and Monoun,
Cameroon. EOS 85(281):285
Hassert K (1912) Seenstudien in Nord-Kamerun. Z Ges
Erdk Berl 1912:7–41,135-144,203-216
Hirslund F (2012) An additional challenge of Lake Kivu
in Central Africa—upward movement of the chemo-
clines. J Limnol 71:45–60
Kanari S (1989) An inference on the process of gas
outburst from Lake Nyos, Cameroon. J Volcanol
Geotherm Res 39:135–149
Kantha LH, Freeth SJ (1996) A numerical simulation of
the evolution of temperature and stratification in Lake
Nyos since the 1986 disaster. J Geophys Res 101
(B4):8187–8203
Karl DM, Tilbrook D (1994) Production and transport of
methane in oceanic particulate organic matter. Nature
368:732–734
Kling GW (1987a) The comparative limnology of lakes in
Cameroon, West Africa. PhD thesis, Duke University
Kling GW (1987b) Seasonal mixing and catastrophic
degassing in tropical lakes, Cameroon, West Africa.
Science 237:1022–1024
Kling GW (1988) Comparative transparency, depth of
mixing, and stability of stratification in lakes of
Cameroon, West Africa. Limnol Oceanogr 33:27–40
Kling GW, Clark MA, Compton HR, Devine JD, Evans
WC, Humphrey AM, Koenigsberg EJ, Lockwood JP,
Tuttle ML, Wagner GN (1987) The 1986 Lake Nyos
gas disaster in Cameroon, West Africa. Science
236:169–175
Kling GW, Tuttle ML, Evans WC (1989) The evolution
of thermal structure and water chemistry in Lake
Nyos. J Volcanol Geotherm Res 39:151–165
Kling GW, Evans WC, Tuttle ML, Tanyileke G (1994)
Degassing of Lake Nyos. Nature 368:405–406
Kling GW, Evans WC, Tanyileke G, Kusakabe M, Ohba
T, Yoshida Y, Hell JV (2005) Degassing Lakes Nyos
and Monoun—defusing certain disaster. Proc National
Acad Sci 102:14185–14190
The Comparative Limnology …423
Kling, GW, MacIntyre S, Steenfelt J, Hirslund F (2006)
Lake Kivu gas extraction: report on lake stability.
Report produced for the World Bank and the
Government of Rwanda, p 104
Kusakabe M (2015) Evolution of CO
2
content in Lakes
Nyos and Monoun, and sub-lacustrine CO
2
-recharge
system at Lake Nyos as envisaged from C/
3
He ratios and
noble gas signatures. doi:10.1007/978-3-642-36833-2
Kusakabe M, Tanyileke GZ, McCord SA, Schladow SG
(2000) Recent pH and CO
2
profiles at Lakes Nyos and
Monoun, Cameroon: implications for the degassing
strategy and its numerical simulation. J Volcanol
Geotherm Res 97:241–260
Kusakabe M, Ohba T, Issa Yoshida Y, Satake H, Ohizumi
T, Evans WC, Tanyileke G, Kling GW (2008)
Evolution of CO
2
in Lakes Monoun and Nyos,
Cameroon, before and during controlled degassing.
Geochem J 42:93–118
Le Guern F, Sigvaldason GE (1989) The Lake Nyos event
and natural CO
2
degassing I. J Volcanol Geotherm
Res 39:95–275
Le Guern F, Sigvaldason GE (1990) The Lake Nyos event
and natural CO
2
degassing II. J Volcanol Geotherm
Res 42:307–401
Lockwood JP, Rubin M (1989) Origin and age of the
Lake Nyos maar, Cameroon. J Volcanol Geotherm
Res 39:117–124
Lockwood JP, Costa JE, Tuttle ML, Nni J, Tebor SG (1988)
The potential for catastrophic dam failure at Lake Nyos
maar, Cameroon. Bull Volcanol 50:340–349
Mazot A, Taran Y (2009) CO
2
flux from the volcanic lake
of El Chichón (Mexico). Geofis Int 48:73–83
McCord SA, Schladow SG (1998) Numerical simulations
of degassing scenarios for CO
2
-rich Lake Nyos,
Cameroon. J Geophys Res 103(B6), 12355−12364
Nojiri Y, Kusakabe M, Hirabayashi J, Sato H, Sano Y,
Shinohara H, Njine T, Tanyileke G (1990) Gas
discharge at Lake Nyos. Nature 346:322–323
Nojiri Y, Kusakabe M, Tietze K, Hirabayashi J, Sato H,
Sano Y, Shinohara H, Njine T, Tanyileke G (1993) An
estimate of CO2 flux in Lake Nyos, Cameroon.
Limnol Oceanogr 38:739–752
Pasche N, Schmid M, Vazquez F, Schubert CJ, Wüest A,
Kessler JD, Pack MA, Reeburgh WS, Bürgmann H
(2011) Methane sources and sinks in Lake Kivu.
J Geophys Res 116:G03006. doi:10.1029/
2011JG001690
Pasternack GB, Varekamp JC (1997) Volcanic lake
systematics I; physical constraints. Bull Volcanol
58:528–538
Pérez NM, Hernández PA, Padilla G, Nolasco D, Barrancos
J, Melían G, Padrón E, Dionis S, Calvo D, Rodríguez F,
Notsu K, Mori T, Kusakabe M, Carmencita Arpa M,
Reniva P, Ibarra M (2011) Global CO
2
emission from
volcanic lakes. Geology 39:235–238
Schmid M, Lorke A, Wüest A, Halbwachs M, Tanyileke G
(2003) Development and sensitivity analysis of a model
for assessing stratification and safety of Lake Nyos
during artificial degassing. Ocean Dyn 53:288–301
Schmid M, Lorke A, Dinkel C, Tanyileke G, Wüest A
(2004) Double-diffusive convection in Lake Nyos,
Cameroon. Deep-Sea Res I 51:1097–1111
Schmid M, Halbwachs M, Wüest A (2006) Simulation
of CO
2
concentrations, temperature, and stratification
in Lake Nyos for different degassing scenarios.
Geochem Geophys Geosyst 7:Q06019. doi:10.1029/
2005GC001164
Schaefer JR, Scott WE, Evans WC, Jorgenson J,
McGimsey RG, Wang B (2008) The 2005 catastrophic
acid crater lake drainage, lahar, and acidic aerosol
formation at Mount Chiginagak volcano, Alaska,
USA: field observations and preliminary water and
vegetation chemistry results. Geochem Geophys Geo-
syst 9:Q07018. doi:10.1029/2007GC001900
Sigurdsson H, Devine JD, Tchoua FM, Presser TS,
Pringle MKW, Evans WC (1987) Origin of the lethal
gas burst from Lake Monoun, Cameroon. J Volcanol
Geotherm Res 31:1–16
Sterner RW, Andersen T, Elser JJ, Hessen DO, Hood JM,
McCauley E, Urabe J (2008) Scale-dependent carbon:
nitrogen:phosphorus seston stoichiometry in marine
and freshwaters. Limnol Oceanogr 53:1169–1180
Tanyileke GZ, Kusakabe M, Evans WC (1996) Chemical
and isotopic characteristics of fluids along the Cam-
eroon volcanic line, Cameroon. J Afr Earth Sci
22:433–441
Tassi F, Vaselli O, Tedesco D, Montegrossi G, Darrah T,
Cuoco E, Mapendano MY, Poreda R, Delgado
Huertas A (2009) Water and gas chemistry at Lake
Kivu (DRC): geochemical evidence of vertical and
horizontal heterogeneities in a multibasin structure.
Geochem Geophys Geosyst 10:Q02005. doi:10.1029/
2008GC002191
Teutsch N, Schmid M, Müller B, Halliday AN, Bürgmann
H, Wehrli B (2009) Large iron isotope fractionation at
the oxic-anoxic boundary in Lake Nyos. Earth Planet
Sci Lett 285:52–60
Tietze K (1987) Results of the German-Cameroon
research expedition to Lake Nyos (Cameroon) Octo-
ber/November 1986. Interim-Report, Bundesanstalt
für Geowissenschaften und Rohstoffe Archive no.
100470, 84p
Tietze K (1992) Cyclic gas bursts: are they a ‘usual’
feature of Lake Nyos and other gas-bearing lakes? In:
Freeth SJ, Ofoegbu CO, Onuoha KM (eds) Natural
hazards in West and Central Africa. Vieweg, Braun-
schweig, pp 97–107
Tuttle ML, Clark MA, Compton HR, Devine JD, Evans
WC, Humphrey AM, Kling GW, Koenigsberg EJ,
Lockwood JP, Wagner GN (1987) The 21 August
424 G.W. Kling et al.
1986 Lake Nyos gas disaster, Cameroon. US Geol
Surv Open-File Rep 87–97:58
Tuttle ML, Briggs PH, Evans WC, Kling GW, Lockwood
JP (1992) Influence of mafic minerals on water
chemistry and water column stability of Lake Nyos,
Cameroon. In: Kharaka Y, Maest A (eds.) Water-rock
interaction, Vol. 1, A.A. Balkema, Rotterdam,
pp 449–452
Tuttle ML, Lockwood JP, Evans WC (1990) Natural
hazards associated with Lake Kivu and adjoining areas
of the Birunga Volcanic Field, Rwanda and Zaire,
Central Africa. US Geol Surv Open-File Rep 90–
691:47
Wilhelm E, Battino R, Wilcock RJ (1977) Low-pressure
solubility of gases in liquid water. Chem Rev 77:219–
262
The Comparative Limnology …425
