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Response of Lake Kivu stratification to lava inflow and climate warming

  • University of Kaiserslautern-Landau (RPTU)


During the eruption of Nyiragongo Volcano in January 2002 about 10(6) m(3) of lava entered Lake Kivu. The high concentrations of CO2 and CH4 dissolved in the deep waters of Lake Kivu raised serious concerns about a potential gas outburst with catastrophic consequences for the population in the Kivu-Tanganyika region. Therefore, 3 weeks after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification. Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth; however, there was no substantial warming or destratification of the gas-containing deep layers below. The deep double-diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition, the recent measurements allowed for an update and gave new insight into the stratification and double-diffusive mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part of the lake of up to 0.5degreesC within the last 30 yr, which could be attributed to climate variability.
Limnol. Oceanogr., 49(3), 2004, 778–783
2004, by the American Society of Limnology and Oceanography, Inc.
Response of Lake Kivu stratification to lava inflow and climate warming
Andreas Lorke
Applied Aquatic Ecology, Limnological Research Center (EAWAG), CH-6047 Kastanienbaum, Switzerland
Klaus Tietze
PDT GmbH, Physik-Design-Technik, Sensorik und Consulting, D-29227 Celle, Germany
Michel Halbwachs
Universite´ de Savoie et Coordination de la Recherche Volcanologique, CNRS-INSU, F-73376
Le Bourget du Lac Cedex, France
Alfred Wu¨est
Applied Aquatic Ecology, Limnological Research Center (EAWAG), CH-6047 Kastanienbaum, Switzerland
During the eruption of Nyiragongo Volcano in January 2002 about 10
of lava entered Lake Kivu. The high
concentrations of CO
and CH
dissolved in the deep waters of Lake Kivu raised serious concerns about a potential
gas outburst with catastrophic consequences for the population in the Kivu-Tanganyika region. Therefore, 3 weeks
after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification.
Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth;
however, there was no substantial warming or destratification of the gas-containing deep layers below. The deep
double-diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally
driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition,
the recent measurements allowed for an update and gave new insight into the stratification and double-diffusive
mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part
of the lake of up to 0.5
C within the last 30 yr, which could be attributed to climate variability.
The Nyiragongo Volcano (Democratic Republic of Congo,
East-Central Africa) erupted on 17 January 2002. Volumi-
nous lava flows, originating from different eruptive vents,
destroyed the central part of the city of Goma and about one
million m
of lava entered Lake Kivu, which is located about
18 km south of the Nyiragongo Volcano in the western
branch of the East African Rift Zone (Fig. 1). The lake cov-
ers a surface area of 2,400 km
and has a maximum depth
of 485 m. There were serious concerns about a thermally
driven limnic gas outburst due to the high concentrations of
dissolved CO
and CH
at great depth in the permanently
stratified water column (Deuser et al. 1973; Tietze et al.
1980). It was feared that the hot lava could warm up and
locally destabilize the density stratification in those deep lay-
ers and, thus, trigger a massive gas outburst. Such a gas
outburst in Lake Nyos (Cameroon) in 1986 (Kling et al.
To whom correspondence should be addressed. Present address:
Limnological Institute, University of Constance, Mainaustrasse 252,
D-78464 Konstanz, Germany (
We thank Cle´ment Mudaheranwa (Kigali, Rwanda) for the logis-
tic support and for courtesy of the digital map (Fig. 1). We are
grateful to J. J. F. de Vos (Bralirwa Heinecken, Gisenyi) forhis kind
assistance in the field. We also acknowledge Martin Schmid’s com-
ments on an earlier draft of this manuscript. The Humanitarian Aid
Office of the European Community (ECHO) funded the study. A.L.
was supported by EAWAG.
1987; Tietze 1992), most probably triggered by rock fall,
released a dense cloud of carbon dioxide that flowed through
the surrounding valleys and asphyxiated approximately
1,800 people (Freeth et al. 1990). Taking into account that
the total volume of dissolved gas in Lake Kivu is about three
orders of magnitude higher than in Lake Nyos (3
compared to 3
, respectively) and given the con-
siderably larger population density at Lake Kivu, a cata-
strophic disaster could not be excluded a priori. Paleolim-
nological evidence for such catastrophic events in the past
5,000 years was found in sediment cores from Lake Kivu
by Haberyan and Hecky (1987).
Critical for a potential gas outburst is the relative satura-
tion of the dissolved gases, which reaches a maximum at
about 270 m depth with 8% carbon dioxide and 43% meth-
ane saturation relative to the hydrostatic pressure (Deuser et
al. 1973; Tietze et al. 1980). The gases remain in the deep
waters of the lake because the seasonal convective mixing
of the surface boundary layer extends only to about 50 m
depth (Tietze et al. 1980). Below, the water is anoxic and
concentrations of dissolved carbon dioxide and methane in-
crease continuously with depth. Although the carbon diox-
ide, which is thought to be mainly of mantle origin, enters
the lake (in dissolved form) by groundwater inflow, the
methane is thought to be produced within the lake by an-
aerobic bacteria, which can use both acetate from decom-
posing organic matter and magmatic CO
as a carbon source
(Deuser et al. 1973; Tietze et al. 1980; Schoell et al. 1988).
779Lake Kivu stratification
Fig. 1. Map of Lake Kivu with sampling stations and the lo-
cation of the Nyiragongo volcano (18 km north of the city of
Goma). The two bold and numbered CTD stations in the center of
the northern basin are referenced within the text.
Fig. 2. Profiles of potential temperature, electrical conductivity
corrected to 20
C, and light transmissivity, LT, from a CTD cast in
the center of Lake Kivu northern basin (CTD Sta. 1 in Fig. 1).
A short-term investigation was carried out on 7–10 Feb-
ruary 2002 to assess the potential of a gas outburst and to
investigate the impact of the lava inflow on the density strat-
ification of Lake Kivu. A SeaBird-19 conductivity–temper-
ature–depth (CTD) probe was used to measure vertical pro-
files of electrical conductivity, temperature, and light
transmissivity (at a wavelength of 660 nm along a light path
of 0.1 m). Profiles were measured off the city of Goma in
the vicinity of the lava inflow, as well as along a 26-km
transect from Goma to the island of Idjwi in the southern
part of the lake (Fig. 1).
Water samples were collected for calibration of the con-
ductivity measurements at 10, 25, 150, and 450 m depth,
using a 5-liter Niskin bottle; a remotely operated underwater
camera (ROV) was used to visually inspect the lava intru-
sions at different locations and depths.
The general features of the CTD profiles are illustrated in
Fig. 2, showing a cast from the center of the northern basin
(Fig. 1). The stratification is characterized by a seasonal ther-
mocline within the uppermost 50 m depth and a permanently
stratified hypolimnion with increasing temperature stabilized
by increasing salinity with depth. Below 200 m, the density
stratification is further stabilized by increasing concentra-
tions of dissolved CO
and weakly destabilized by increasing
concentrations of dissolved CH
(Degens et al. 1973). The
salinity was estimated to vary between 1.15 g kg
at the
surface and 4.47 g kg
at 450 m depth, based on the ionic
composition of the water samples and following the proce-
dure described by Wu¨est et al. (1996). An interesting feature
of the overall stratification is the step-like structure of the
profiles within the permanently stratified part of the water
column, showing three distinctive mixed layers (200–250 m,
275–300 m, and 320–355 m) separated by strong gradients.
At least the lowest two mixed layers must have been actively
mixed very recently and do not exhibit any diffusion-in-
duced gradients of the measured parameters.
The light transmission has a constant bulk value of about
80% transmissivity (at 660 nm over 10 cm path length) and
strong peaks of higher turbidity (i.e., lower light transmis-
sivity) at each density gradient. However, based on our data
it is not clear whether these turbidity peaks are related to the
volcano eruption and subsequent lava inflow or whether they
are a rather typical feature of Lake Kivu, since no detailed
turbidity profiles were measured in the lake before. Lake
Kivu has no significant river inflows (only one outflow, the
Ruzizi River, which discharges into Lake Tanganyika); riv-
erine intrusions can be excluded as alternative sources of the
observed turbidity layers.
A signature of the lava inflow, however, is the thin layer
of slightly increased temperature at about 80 m depth (Fig.
2). The layer thickness is approximately 5 m, and the tem-
perature increase is less than 0.05
C. Temperature and light
transmissivity profiles along the transect are shown in Fig.
3a and 3b, respectively. The temperature profiles close to the
lava inflow show large fluctuations between 50 and about
120 m depth that resulted from the local heating by the lava
and subsequent advective isopycnal transport. The observed
780 Lorke et al.
Fig. 3. Successive profiles of (a) temperature and (b) light transmissivity following a north-to-south transect from the lava inflow in
Goma to the island of Idjwi (Fig. 1). Successive profiles are offset by 0.21
C and 2.2%, respectively. The numbers above the profiles
indicate the distance from the lava inflow in kilometers. The inset plot in Fig. 3b shows the changes of the bulk values of the light
transmissivity as a function of distance from the lava inflow. The bulk values were estimated as the mean background light transmissivity
below 50 m depth (disregarding the fluctuations). All profiles were measured within 4 h, and temporal changes of the water column
characteristics can hence be neglected.
temperature inversions do not cause local convection, since
the strong salinity gradient within that depth range stabilizes
the density stratification of the water column. Farther away
from the lava inflow, the temperature fluctuations are in-
creasingly smoothed out, and only one distinct peak of high-
er temperature remains. This peak is most pronounced in the
profile taken 14.1 km away from the lava inflow with a tem-
perature anomaly of
C and disappears completely in
the profiles measured farther away. This strongest peak can
be related to the earliest and major inflow events. Taking
into account the time between the Nyiragongo eruption and
the profile measurements (23 d) and setting 14 km and 20
km as the lower and upper limits for the horizontal disper-
sion results in a horizontal advection velocity between 0.7
, respectively. This is in good agreement with
tracer measurements in several lakes of comparable and
smaller size (Peeters et al. 1996). Neither the main structure
of the vertical stratification nor the heat content changed
significantly as a result of the lava inflow.
The heat input of the 10
of hot lava flowing into the
lake can be estimated to about 2.4
J, assuming typical
values (Dragoni et al. 2002) for temperature (1,000
C), heat
capacity (850 J kg
), and density (2,800 kg m
). The
assumption that this heat was solely distributed within this
thin, 5-m thick layer of water over an area spanned by a
segment of a circle centered in Goma with an opening angle
of 130
(following the shorelines) and a radius of 17 km
results in a temperature increase of 0.3
C. This estimate is
in reasonable agreement with the observed maximum tem-
perature increase of 0.14
C, if we take into account that a
significant amount of heat was probably lost to the atmo-
sphere due to evaporation or was mixed into the rest of the
water column. Since a local destabilization of a 5-m thick
layer within the respective depth range would require a tem-
perature increase of 0.5
C, even the total amount of heat
introduced by the lava was not sufficient to force convective
mixing of this layer over the observed spreading area.
The transmissivity profiles along the CTD transect are
shown in Fig. 3b. Again, the profiles close to the lava inflow
show strong fluctuations, which become smoother with in-
creasing distance. A direct correlation between the strength
of the well-defined temperature peaks and the peaks in the
transmissivity could not be observed. However, in contrast
to the temperature, the bulk or background transmissivity, as
estimated from the profile sections, which show constant
transmissivity values, increases with distance from the city
of Goma (inset plot in Fig. 3b). Since this bulk transmissiv-
ity shows no vertical structure, contrary to what would be
expected for settling particles, the changes can be attributed
to the dilution of the particles only, with settling not being
a dominant factor.
Several peculiarities have made Lake Kivu the subject of
comprehensive investigations in the past. These include the
high concentrations of dissolved gases (Tietze et al. 1980)
and their origin (Deuser et al. 1973; Tietze et al. 1980;
Schoell et al. 1988), the resulting density stratification (New-
man 1976; Tietze 1981), the special underground geological
situation (Degens et al. 1973), and the potential exploitation
of the dissolved methane (Tietze 2000). In addition, double-
diffusive mixing, caused by the geothermal heating at the
bottom of the lake, was observed and analyzed (Newman
1976; Tietze 1981). All of those investigations were made
more than 25 yr ago and provide the opportunity for a long-
term comparison with the recent profiles.
A comparison of the recent temperature profile with the
one measured by Tietze in 1975 (Tietze 1981) in Fig. 4 re-
veals a distinct warming of up to 0.5
C within the perma-
781Lake Kivu stratification
Fig. 4. Comparison of one recent (February 2002) temperature
profile and one measured by Tietze in 1975 (Tietze 1981). Both
profiles were measured with a vertical resolution of a few decime-
ters and a proven temperature calibration. Fig. 5. Detailed structure of the temperature stratification
around the mixed layers. The two profiles were measured 11 (CTD
1) and 17 km (CTD 5) off Goma (see Fig. 1) with 3 days time
difference in between. The inset plot shows the double-diffusive
staircase in CTD 1.
nently stratified part of the water column between 50 and
250 m depth. Both profiles were measured with a proven
accuracy better than 0.01
C and a high vertical resolution of
10 cm. A similar increase in temperature was already ob-
served by Deuser et al. (1973) by comparing their profiles
to earlier profiles of Damas (1937); however, their accuracy,
which was stated to be ‘‘within a few tenths
C’’ in the
original publication (Degens et al. 1973), is not sufficient to
allow for a direct comparison. In contrast to Fig. 4, the for-
merly observed warming affected the entire water column,
and Deuser et al. (1973) proposed the energy released by
the formation of methane to be responsible. However, since
the methane concentration increases significantly only below
a depth of 250 m, where no recent temperature increase
could be observed from Fig. 4, and the amount of warming
increases toward the surface, this recent increase in temper-
ature below the depth of seasonal mixing can most probably
be attributed to climate variability or change. Comparable
climatic warming was observed in other African lakes (Half-
man 1993; Hecky 1993; Plisnier 2000; O’Reilly et al. 2003;
Verburg et al. 2003), as well as in other lakes around the
world (e.g., Scheffer et al. 2001).
Interestingly, the near-bottom temperature of the lake has
not changed at all in the past 25 yr, indicating a stable equi-
librium between the geothermal heat flux into the lake and
the vertical transport within the water column. However, the
stratification did change slightly. The most obvious change
occurred between 250 and 300 m depth, where a well-mixed
layer has been established in a former gradient region. The
two other well-mixed layers, at 200–250 m depth and 320–
350 m depth, became thicker compared to 25 yr ago. At
least these two layers do not exhibit any gradients of the
measured physicochemical parameters, as one would expect
due to the smoothing effect of diffusion. Hence they can
only be sustained by active mixing. The only possible mech-
anism for the local production of the necessary mechanical
energy is convection, which may be driven by double dif-
fusion or by layer merging of thin, 1- to 2-m thick double-
diffusively mixed layers as shown in the inset plot in Fig.
5. The spikes in the recent profiles at the edges of those
mixed layers (at 170 and 250 m depth in Fig. 5) also indicate
convective activity, although they are stabilized by salinity.
An unusual peculiarity is that the double-diffusive mixing in
Lake Kivu is not solely characterized by the classical coun-
tergradients of temperature- and salinity-related density pro-
files, but also the increasing concentrations of dissolved gas-
es must also be taken into account: whereas the carbon
dioxide increases the density, the dissolved methane decreas-
es the density with increasing gas concentration.
The density ratio R
, an indicator for the susceptibility of
the stratified water column to double diffusion, is defined as
the ratio of the stabilizing forces and the destabilizing forces
and can be expressed in this special case as
[H CO ]
[CH ]
1G 2
where Sand Tdenote salinity and temperature, zis the depth,
are the coefficients of thermal and haline expan-
sion, [H
] and [CH
] are the dissolved gas concentra-
tions, and
the respective expansion coefficients
(note that
is negative), whereas
denotes the adiabatic
lapse rate. The ionic carbon species (HCO and CO ) are
included in S, and [H
] was estimated following Schmid
et al. (2003), whereas [CH
] was taken from Tietze et al.
(1980). Double-diffusive staircases have been observed in
systems with 1
10, but the susceptibility to double
diffusion increases with decreasing R
until the density ratio
approaches 1, where the stratification becomes unstable
(Turner 1973).
Similar to that found by Schmid et al. (in press) in Lake
Nyos, the density ratio is below 10 throughout the entire
water column, but it falls below 3 within two depth ranges,
at 130–170 m and 380–400 m depth. Double-diffusive stair-
782 Lorke et al.
cases could only be observed within the upper range (see
Fig. 5) and were best resolved in the profile shown in Fig.
2 (CTD Sta. 1 on the map in Fig. 1). This profile shows a
typical double-diffusive staircase with layer heights between
2.4 and 4.3 m, increasing with depth. Applying the semi-
empirical flux law from Kelley (1990) results in an average
double-diffusive heat flux of 0.03 W m
. Although using
the original flux law from Turner (1973) results in a higher
heat flux of 0.05 W m
, these values are more than one
order of magnitude smaller than those found by Newman
(1976). Within the same depth range he estimated an average
heat flux of 1.6 W m
, which could be attributed to the
much smaller layer thicknesses of 0.85 m. Although in dis-
crepancy with former measurements, the observed upward
heat flux within the double-diffusive staircase is within the
wide range of bottom heat flux estimates of 0.017–0.17 W
determined by Degens et al. (1973).
However, relating the bottom heat flux to the double-dif-
fusive heat flux between 130 and 160 m depth raises ques-
tions about the heat transport within the intermediate depth
range, where no small-scale staircases could be observed.
The apparent diffusivity of heat K
within the double-dif-
fusive regime can be estimated by applying Fick’s law to
the observed heat flux and the local temperature gradient and
results in K
, about one order of mag-
nitude greater than the molecular diffusivity. Since it can be
assumed that turbulent mixing in the interior of a strongly
stratified and deep lake is comparably weak (Schmid et al.
in press; Wu¨est and Lorke 2003), with vertical diffusivities
close to the molecular ones, the enhanced vertical transport
within the double-diffusive depth ranges could become
transport-limited from below.
The continuing double-diffusive transport with limited
heat flux at the lowest interface could lead to progressive
layer merging until the entire depth range becomes mixed.
This process could be responsible for the formation and
maintenance of the special pattern in the Lake Kivu strati-
fication, showing these three mixed layers separated by
strong gradients (Fig. 2). In fact, the three mixed layers ob-
served in the recent measurements correspond to the three
distinct depth ranges with strong double-diffusive staircase
formation in the measurements of Newman (1976). Further
indication of active convective mixing across these layers is
given by the observation of spikes (which are stabilized by
salinity) in the temperature profiles adjacent to the layers
(Fig. 5). The oppositional (bimodal) occurrence of these
spikes in the two profiles shown in Fig. 5 reveals the either
temporal or spatial dynamics of the fine structure of the strat-
ification in Lake Kivu.
Beyond all the unique findings, it is important to point
out that the overall stability of Lake Kivu is relatively high
and that a much stronger impact than that of the January
2002 lava inflow would be necessary to trigger a gas out-
burst from the deep waters. Clear signatures of the lava in-
flow were found down to 100 m depth and as far as 14 km
away from the location of the lava inflow, but the stability
and the heat content of the gas-rich layers below were not
affected. The observed general increase in temperature with-
in the upper part of the permanently stratified part of the
water column is related to long-term changes, which are
most probably caused by climate variability. Even the dou-
ble-diffusive structures, which are based on a small-scale
equilibrium of molecular processes, were well preserved.
Therefore, besides the acquisition of new concentration pro-
files of dissolved gases, future research should focus on the
potential impact of lava outflows from submersed vents as
well as on the potential impact of baroclinic dislocation in
the water column triggered by the enhanced seismic activity
at the lake bottom.
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Received: 5 March 2003
Amended: 10 December 2003
Accepted: 11 December 2003
... Fluxes, turbulent and double diffusion Schmid et al. 2004Schmid et al. , 2005Schmid et al. & 2010Pasche et al. 2009Pasche et al. & 2010Wüest et al. 2012;Sommer et al. 2013a & b;Hirslund 2012;Schmid and Wüest, 2012;Schmid et al. 2010;Carpenter et al. 2012a & b;Katsev et al. 2014;Ross et al. 2015a;Toffolon et al. 2015;Sommer et al. 2014Sommer et al. & 2019. Warming and climate Lorke et al. 2004;Borges et al. 2011;MacIntyre, 2013;Katsev et al. 2014;Thiery et al. 2014aThiery et al. , 2014bKranenburg et al. 2020. Hydrology Bergonzini 1998Bergonzini et al. 2002;Munyaneza et al. 2009;Habiyaremye et al. 2011;Muvundja et al. 2009. ...
... Many authors have reported that part of the interannual variability in precipitation over East Africa is related to sea surface temperature perturbations over the Equatorial Pacific (El Niño Southern Oscillation) and Indian Ocean (Indian Ocean Dipole) (Khaki and Awange, 2021;Delvaux et al., 2015;Anyah and Semazzi, 2006;Bergonzini et al., 2004;Nicholson, 1996). Lorke et al. (2004) reported an increase of 0.5°C in the lake surface waters due to global warming. Some contradicting studies exist on the prediction of precipitation trends around Lake Kivu. ...
Lake Kivu is one of the African Great Lakes lying in the Albertine Rift. It provides livelihoods to 5.7 million people living in the two riparian countries of the Democratic Republic of the Congo and the Republic of Rwanda. Lake Kivu is currently experiencing numerous stressors, including fish habitat destruction, pollution, invasive species, weak governance and law enforcement as well as conflict between riparian countries. One of the biggest challenges on Lake Kivu is the limitation of coordinated and consistent research on the lake. Scientific attention to large lakes is often not seen as a high enough priority by the riparian countries, despite the lake sustaining millions of people’s livelihoods, and contributes to the GDP of both countries. Although we have a fair understanding of the basic geology, physics, chemistry, and biology of the lake, there is a need for stronger long-term monitoring and research frameworks to gain more comprehensive understanding of the changes resulting from human uses and global warming. These would be needed to develop good policies and management decisions for sustainable and long-term health and use of the lake’s resources. This manuscript presents an opinion of experts on what is known about the current lake’s current status and its resources as well as about what should be done. It highlights key threats, issues and gaps that needs to be urgently addressed, and provides specific and strategic ways forward for long-term monitoring and management, essential to achieving a healthy Lake Kivu, able to sustain its dependents.
... There is evidence that global warming is already affecting African lakes and reservoirs [129]. Lakes Malawi [130], Kivu, [131] and Tanganyika [124] appear to have strengthened stratifications. Lake Kariba still becomes isothermal in winter, and mixing occurs despite its warming [129]. ...
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This chapter reviewed phytoplankton communities in African freshwater lakes and reservoirs and further assessed the latitudinal diversity gradient (LDG) which has been used to explain species variations in other taxonomic groups. The chapter also identifies freshwater lakes and reservoirs on the continent that have been heavily impacted by anthropogenic impacts and assesses how these have led to variations and/or changes in phytoplankton communities. From the systematic review, phytoplankton information was available for fifty-one reservoirs in Africa. A total of 1633 freshwater phytoplankton species belonging to nine taxonomic groups were recorded from the fifty-one reservoirs. Bacillariophyta were the most abundant taxonomic group whilst Synurophyta were the least abundant. There was strong evidence that supports LDG with respect to phytoplankton species richness whereby the number of species increased from the poles towards the equator. Species that highly occurred in reservoirs from the three regions include Microcystis aeruginosa, Cyclotella meneghiniana, Merismopedia tranquilla and Aulacoseira granulata. Despite the basal trophic importance of phytoplankton, undesirable phytoplankton blooms have been reported from several reservoirs on the continent. The increase in human activity is causing an increased industrial, agricultural and wastewater deposition into African reservoirs thereby enriching them with nutrients resulting in the proliferation of harmful algal blooms (HAB), particularly Cyanophyta which are a global problem. The cyanotoxins produced by HAB have had lethal effects on various animals including humans in Africa. Besides nutrients, increasing water temperatures are driving HABs development in African reservoirs. Increased temperatures as a result of climate change could therefore favour the growth of HAB, thus augmenting the risks associated with the blooms. Measures that reduce nutrient loading in freshwater systems should be put in place by responsible authorities to prevent biodiversity loss as well as serious human health issues. The issue of climate change which affects phytoplankton as discussed in this review, can be addressed collectively worldwide to reduce global warming mainly by reducing the emission of greenhouse gases to prevent the predicted catastrophic impacts. Phytoplankton monitoring and assessments should be periodically conducted in African aquatic systems to provide insights into the changes over a period of time, while assessment indicates the status of these ecosystems.
... per decade (data provided by Woolway and Maberly). The modeled expectations align well with in situ data on water temperature in several lakes within the LVB ecoregion, including Lake Edward (WWF, 2006), Lake Kivu (Katsev et al., 2014;Larke et al., 2004), and Lake Victoria Sitoki et al., 2010). Ogutu-Ohwayo et al. (2020) presented mean annual air temperature and summer lake water temperature for several lakes of Africa between 1985 and 2010, based on a global database of summer surface water temperatures (Sharma et al., 2015). ...
Freshwater organisms face multiple threats associated with habitat degradation, pollution, and eutrophication, in addition to overharvesting and species invasions. Furthermore, there is mounting evidence that freshwaters are highly sensitive to climate change. This chapter provides an overview of contemporary environmental changes in inland waters of the Lake Victoria Basin (LVB) ecoregion of East Africa with a focus on climate change, eutrophication, and land use. Case studies of fishes in the Lake Victoria basin and swamp-river systems of Western Uganda are used to explore potential effects of these stressors on morpho-physiological, performance, and fitness-related traits. Overall, fishes in the LVB ecoregion show acclimation capacity in upper thermal tolerance and aerobic performance, and adaptive plasticity in traits related to hypoxia tolerance (e.g., gill size). However, a trait-based climate change vulnerability assessment revealed that over 70% of LVB ecoregion fishes are vulnerable to climate change; and fish kills associated with turnover events or anoxic upwellings highlight the danger of rapid change in dissolved oxygen for some species. Plasticity may allow some fishes to persist in the face of multiple stressors in the LVB ecoregion. However, there may be consequences for fitness-related traits such as body size that could affect demographic stability and contributions of fish to food security.
... The methane deposit is of great economic value and is being exploited for the production of energy Schmid et al., 2019;Bärenbold et al., 2020a). The resulting immense dissolved gas pressures have been feared to have the potential to trigger a limnic eruption (Boehrer et al., 2021a), as it happened at Lake Nyos and Lake Monoun in Cameroon in 1986 and 1984, respectively, and therefore pose a threat to over 2 million local inhabitants of the region (Kusakabe, 2017;Sigurdsson et al., 1987;Kling et al., 1994;Lorke et al., 2004;Boehrer et al., 2021b). The recent Nyiragongo volcano eruptions of January 2002 and May 2021 reminded the world of the precarious situation: magma coming into contact with the deep waters of Lake Kivu could result in local heating and the formation of bubbles which -in the worst case -could trigger a large scale ebullition. ...
Due to their biological and chemical inertness, noble gases in natural waters are widely used to trace natural waters and to determine ambient temperature conditions during the last intensive contact with the atmosphere (equilibration). Noble gas solubilities are strong functions of temperature, with higher temperatures resulting in lower concentrations. Thus far, only common environmental conditions have been considered, and hence investigated temperatures have almost never exceeded 35 °C, but environmental scenarios that generate higher surface-water temperatures (such as volcanism) exist nonetheless. Recently published measurements of noble gas concentrations in Lake Kivu, which sits at the base of the Nyiragongo volcano in East Africa, unexpectedly show that the deep waters are strongly depleted in noble gases with respect to in-situ conditions, and so far no quantitative explanation for this observation has been provided. We make use of recently published noble gas solubility data at higher temperatures to investigate our hypothesis that unusually high equilibration temperatures could have caused the low measured noble gas concentrations by applying various approaches of noble gas thermometry. Noble gas concentration ratios and least squares fitting of individual concentrations indicate that the data agrees best with the assumption that deep water originates from groundwater formed at temperatures of about 65 °C. Thus, no form of degassing is required to explain the observed noble gas depletion: the deep water currently contained in Lake Kivu has most probably never experienced a large scale degassing event. This conclusion is important as limnic eruptions were feared to threaten the lives of the local population.
... The methane deposit is of great economic value and is being exploited for the production of energy Schmid et al., 2019;Bärenbold et al., 2020a). The resulting immense dissolved gas pressures have been feared to have the potential to trigger a limnic eruption (Boehrer et al., 2021a), as it happened at Lake Nyos and Lake Monoun in Cameroon in 1986 and 1984, respectively, and therefore pose a threat to over 2 million local inhabitants of the region (Kusakabe, 2017;Sigurdsson et al., 1987;Kling et al., 1994;Lorke et al., 2004;Boehrer et al., 2021b). The recent Nyiragongo volcano eruptions of January 2002 and May 2021 reminded the world of the precarious situation: magma coming into contact with the deep waters of Lake Kivu could result in local heating and the formation of bubbles which -in the worst case -could trigger a large scale ebullition. ...
... It was feared that the hot lava could sink to the deeper strata of Lake Kivu, and heat up water to an extent that it could rise to a level where the hydrostatic pressure is no longer sufficient to keep the gases trapped. However, Lorke et al. (2004) showed that the influence of the lava on the lake was minimal, and they concluded that subaerial volcano eruptions are not likely to trigger a limnic eruption of Lake Kivu. In May 2021, Nyiragongo erupted again but without apparently disturbing the lake's stratification, although the observations indicate the formation of a new dyke beneath the lake (Jones, 2021). ...
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Lake Kivu is a 485 m deep, Central-East African rift lake with huge amounts of carbon dioxide and methane dissolved in its stably stratified deep waters. In view of future large-scale methane extraction, one-dimensional numerical modelling is an important and computationally inexpensive tool to analyze the evolution of stratification and the content of gases in Lake Kivu. For this purpose, we coupled the physical lake model Simstrat to the biogeochemical library AED2. Compared to an earlier modelling approach, this coupled approach offers several key improvements, most importantly the dynamic evaluation of mixing processes over the whole water column, including a parameterization for double-diffusive transport, and the density-dependent stratification of groundwater inflows. The coupled model successfully reproduces today's near steady-state of Lake Kivu, and we demonstrate that a complete mixing event ∼2000 years ago is compatible with today's physical and biogeochemical state.
... More than 1700 humans perished in these natural disasters. Also at other places, extreme gas pressures have been detected and assessed for their danger to the local population (Lake Kivu: Lorke et al., 2004;Schmid et al., 2005;Guadiana Pit Lake: Sánchez España et al., 2014;Boehrer et al., 2016;Lake Vollert-Sued: Horn et al., 2017;Lago Monticchio Piccolo: Caracausi et al., 2009;Cabassi et al., 2013). All these lakes are meromictic, and accumulation of solutes in the deep water can occur over long periods (e.g., Boehrer et al., 2013;Boehrer et al., 2021). ...
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Gases dissolved in the deep water of lakes can pose a hazard when extreme concentrations are reached. A sudden release of large amounts of gas can cost the lives of humans living in the neighbourhood, as happened at Lake Nyos in 1986. Since 2001, the gas risk at Lake Nyos has been mitigated by induced degassing, but the lake continues to be supplied by CO 2 , and a regular survey needs to be implemented to guarantee safe conditions. Frequent sampling of this remote lake requires an enormous effort, and many analytical techniques are very difficult to run at the lake site. In this contribution, we combined a commercially available sound speed sensor with a CTD (electrical conductivity, temperature, depth) probe to obtain an indirect but quantitative estimate of carbon dioxide concentrations with fine depth resolution (decimetre scale). Dissolved carbon dioxide increases sound speed but does not contribute to electrical conductivity. Hence the difference between measured and calculated (on the base of electrical conductivity, temperature and pressure) sound speed gives a quantitative indication of dissolved carbon dioxide. We infer the vertical distribution of dissolved CO 2 and hence continue the survey of the progress of the intended degassing. In conclusion, we present an easy to implement method for very high CO 2 concentrations in deep lakes, and we highly recommend the implementation of the sound speed-CTD probe combination at Lake Nyos and at other gas-laden volcanic lakes, as such an approach could safeguard the people living in the area with acceptable cost and effort for the operators. In this manner, alarming CO 2 concentrations in deep parts of lakes can be detected in a timely fashion.
... Catastrophic events of spontaneous gas ebullition from deep waters (Lake Nyos and Lake Monoun-both in Cameroon, Africa) have cost the lives of many humans in single events [12][13][14][15]. Since then, a number of other lakes with gas pressures of concern have been reported in the literature (e.g., Lake Kivu: [16]) and assessed for the danger of limnic eruptions (Lake Kivu: [17], Guadiana pit Lake: [18]). ...
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Dissolved gases produce a gas pressure. This gas pressure is the appropriate physical quantity for judging the possibility of bubble formation and hence it is central for understanding exchange of climate-relevant gases between (limnic) water and the atmosphere. The contribution of ebullition has widely been neglected in numerical simulations. We present measurements from six lacustrine waterbodies in Central Germany: including a natural lake, a drinking water reservoir, a mine pit lake, a sand excavation lake, a flooded quarry, and a small flooded lignite opencast, which has been heavily polluted. Seasonal changes of oxygen and temperature are complemented by numerical simulations of nitrogen and calculations of vapor pressure to quantify the contributions and their dynamics in lacustrine waters. In addition, accumulation of gases in monimolimnetic waters is demonstrated. We sum the partial pressures of the gases to yield a quantitative value for total gas pressure to reason which processes can force ebullition at which locations. In conclusion, only a small number of gases contribute decisively to gas pressure and hence can be crucial for bubble formation.
When a sediment laden river flows into a stratified water body, the water mass can either intrude as an overflow, interflow or underflow, depending upon the density contrast. Different modes of sediment driven convection occur in each case. For the case of overflows, convective sedimentation occurs beneath the plume, whereby sediment rich plumes rapidly transport fine materials to depth. If underflow of dense sediment laden waters initially occurs, then after sediment has been deposited, the light interstitial material can subsequently loft and potentially mixes the entire water column. For an interflow, both lofting and sediment driven convection can occur above and below the pycnocline. All of these different regimes can be described in terms of two dimensionless parameters: namely RS = DrS / DrC and RA = DrA /DrC, where DrA is the density contrast between the upper layer and the river inflow (due to just salinity or temperature differences), DrC is the density contrast due to sediment between river and upper‐layer, and DrS is the density contrast between upper and lower layers (due to just salinity or temperature differences). Laboratory experiments were used to describe the vigour of convection in terms of these dimensionless parameters, which then allows behaviour of various inflows to be predicted. In most cases the convective velocities observed were an order of magnitude faster than Stokes settling velocities. These observations are also applied to predict how a turbidity current could lead to lofting and possible overturn of the stratification of Lake Kivu, a large meromictic lake between Rwanda and the Democratic Republic of the Congo.
Heavy metals are among the pollutants that threaten living organisms including human beings. Heavy metals in water are of great concern due to their toxicity and ability to bio-accumulate in aquatic organisms. There is a need to regularly monitor their concentration in an aquatic medium. The present study was conducted to evaluate the level of heavy metals in lake Kivu. Water samples from lake Kivu were taken from three sites, namely: Rusizi, Karongi, and Rubavu. Heavy metals were analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Copper, lead, cadmium, chromium, manganese, mercury, and arsenic were analyzed in the water samples. The concentration of copper ranged from 3.240 to 10.011 µg/L, the concentration of lead varied from 8.81 to 37.44 µg/L, cadmium ranged from 5.014 to 14.012 µg/L. Chromium concentration was between 139.5 and 226.6 µg/L, and that of manganese was between 598.3 and 795.7 µg/L, mercury concentration ranged from 0 to 0.047 µg/L while Arsenic was not detected. Thus, except for arsenic, the concentrations of heavy metals in Kivu lake waters were above the Environmental Protection Agency (EPA) maximum permissible limit for class III surface water intended for fish consumption, recreation, propagation, and maintenance of a healthy population of fish and wildlife. There is a need to further establish the sources of lake water pollution and limit the amounts of heavy metals entering lake Kivu to avoid the excess heavy metals beyond the maximum tolerable limit.
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The possible effect of recent climate change on Lake Tanganyika limnology had not previously been investigated, although the abundance of pelagic fishes is changing that is not necessary linked to fisheries changes. Data collected in the framework of the Lake Tanganyika Research project (LTR) of FAO/FINNIDA allow some preliminary analyses on recent climate–limnology changes. Plisnier, P.-D., 1997, Climate, limnology, and fisheries changes of Lake Tanganyika. FAO/FINNIDA Research for the Management of Fisheries on Lake Tanganyika.GCP/RAF/271/FIN-TD/72 (En) 38p.
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Geophysical, geochemical and biological data are integrated to unravel the origin and evolution of an unusual rift lake. The northern basin of Lake Kivu contains about 0.5 km of sediments which overlie a basement believed to be crystalline rocks of Precambrian age. Volcanic rocks at the northern end of the lake have created large magnetic anomalies of up to 300γ. Heat flow varies from 0.4 to 4 hfu. The extreme variability may be due in part to sedimentation or recent changes in the temperature of the bottom water. Sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers. Such convecting cells are the result of the increase in both temperature and salinity with depth.
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Large amounts of methane and carbon dioxide, among other gases, are dissolved in the deep water of Lake Kivu. There is no dispute about the primarily magmatic origin of the carbon dioxide, but models of the genesis of the methane have been contradictory up to now. They have been based on too few and partly too inaccurate data. On the basis of new measurements obtained from gas and sediment samples, some of the old concepts have been further developed to a new model. According to this model, the methane is generated mainly by bacteria from the organic carbon of the sediment. It probably also contains minor amounts of thermocatalytic methane. About 70% of the organic carbon of the upper sediment is derived from mainly magmatic carbon dioxide (“old” carbon), which enters the biozone of the lake from the deep water by eddy diffusion and is assimilated there. The remaining 30% comes from atmospheric carbon dioxide (“young” carbon) assimilated in the biozone. But because methane also migrates into the lake from deeper sediment, the14C-content in the methane dissolved in the lake water is not 30% modern but only ca. 10% modern. More isotopic measurements on plankton, methane, carbon dioxide and sediment samples are necessary to support this model.
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Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.
The phenomena treated in this book all depend on the action of gravity on small density differences in a non-rotating fluid. The author gives a connected account of the various motions which can be driven or influenced by buoyancy forces in a stratified fluid, including internal waves, turbulent shear flows and buoyant convection. This excellent introduction to a rapidly developing field, first published in 1973, can be used as the basis of graduate courses in university departments of meteorology, oceanography and various branches of engineering. This edition is reprinted with corrections, and extra references have been added to allow readers to bring themselves up to date on specific topics. Professor Turner is a physicist with a special interest in laboratory modelling of small-scale geophysical processes. An important feature is the superb illustration of the text with many fine photographs of laboratory experiments and natural phenomena.
The isotopic composition of Lake Kivu methane (, ) and its 14C activity can be explained by the simultaneous formation of methane through fermentation and CO2 reduction. Culture experiments with Lake Kivu sediments were only successful for acetic acid fermenting Sarcina-type bacteria. The methane produced in deuterated water equivalent to Lake Kivu water had a δ13C- and δD-value of −45‰ and −316‰, respectively. Using the culture data as being representative for the fermentation-derived methane, it can be calculated that approximately one-third of the methane in Lake Kivu is derived from an acetate fermentation process and two-thirds from a CO2-reducing bacterial process which uses the dissolved CO2 in the lake water as a source. This dual bacterial origin satisfactorily explains the lower 14C activity in the methane compared to the sediment. He enriched in 3He and CO2 which are also found in the dissolved gases are most likely of magmatic origin. Lake Kivu is, therefore an excellent example how gases of very different origin (i.e. bacterial and magmatic) can become mixed in the same system.
Vertical profiles of temperature microstructure in Lake Kivu were obtained with ''mini-microstructure recorders''. The profiles revealed three depth intervals containing many isothermal layers typically 0.25 to 2 m thick and of increasing temperature increments 0.01 to 0.03 C from layer to layer. Approximately 150 such layers appeared in a single profile. Double-diffusive convection was assumed, and previously reported results were applied to calculate an upward heat flux of 0.71 to 1.6 w/sq m and a corresponding upward salt flux equal to one-fifth of the average salt output of the lake's only outflow. The chief source of heat and salt is probably geothermal springs in the lake bottom.