Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda - DR Congo) to climate warming. Presented by Hilde Eggermont by "Perth II: Global Change and the World's Mountains" conference in Perth, Scotland in September 2010.

1. Limnological and Ecological sensitivity of Rwenzori mountain lakes
(Uganda- DR Congo) to climate warming
Hilde Eggermont, James M. Russell, Leen Audenaert, Dirk Verschuren
Ghent University, Belgium
Royal Belgian Institute of Natural Sciences, Belgium
Brown University, Providence, US

3. DR Congo
UGANDA

5. Climate change and glacier retreat clearly constitute a major threat to the mountain
ecosystems/unique cold-water lakes located downstream from the glaciers
Increased glacier meltwater input may effect the thermal regime
Plant succession and soil development on previously glaciated terrain will influence
nutrient budget and productivity/biogeochemical cycles
 Warming may enhance the thermal stratification, resulting in deteriorating deep-water
oxygen supply
...

6. Observations of glacial termini confirm rapid glacial recession from 1906 to present
From R. Taylor et al. (2007; ECRC Research report N° 113).
Redrawn and adapted from Kaser and Osmaston (2002; ISBN 0 521 63333 8)
> At the current pace, all remaining glaciers are expected to disappear within the
next two decades

7. Controlling factors of deglaciation are still the subject of debate...
• Rising temperatures in recent decades
(e.g. Bradley et al. 2006; Thompson et al. 2006; Taylor et al. 2006a-b)
• Decrease in humidity at the end of the 19th century (ca. 1880)
(e.g. Kaser et al. 2004; Mölg and Hardy 2004; Mölg et al. 2006)
No information on when glacier recession actually started...
Unraveling the (recent) history of tropical African glaciers is vitally important for understanding long-
term tropical mountain ecosystem and glacier stability, the relative impacts of human-induced global
warming versus natural climate variability in tropical alpine environments (both terrestrial and
aquatic), and the climatic controls of glacial extent.
>> Lake sediment archives can provide the long-term historical perspective

8. Study sites
Fieldwork:
July 2005 (dry season)
July 2006 (dry season)
May 2007 (wet season)
Jan 2008 (dry season)
July 2009 (dry season)
Lakes
Pools
Eggermont et al. 2007. Hydrobiologia 592: 151-173

9. Lake Mahoma, 2990 m, 25.6 m depth “non-glacial” lakes East Bukurungu, 3801 m, 17.3 m depth
East Bukurungu, 3801 m, 17.3 m
Lake Bigata, 3998 m, 18.0 m depth depth Lake Batoda, 3890 m, 15.0 depth

10. Lake Bujuku, 3891 m, 13.5 m depth “glacial” lakes Lower Kitandara, 3989 m, 11 m depth
Upper Kitandara, 4009 m, 14.5 m depth Lac du Speke, 4235 m, 17 m depth

11. Recovery of short sediment cores of recent sediments (150-700 yrs old)
1. To assess the archival quality of the lake sediments
(>selection of good sites for long coring)
2. To assess the potential of Rwenzori mountain lakes to trace glacier recession
(i.e. (to assess their sensitivity to glacier retreat)
3. To assess the limnological and ecological sensitivity of Rwenzori mountain lakes
to climate warming
.

12. Paleolimnological records of recent glacier recession in the Rwenzori Mountains
Organic geochemical profiles – downcore trends
Atomic C/N ratios of organic matter do not show
clear differences between glacial and non-glacial
lakes, and imply that there have not been major
changes in the source of organic matter
δ15Norg profiles do not exhibit clear differences
between glacial and non-glacial lakes indicating that
recent glacial recession does not appear to have
strongly affected the nitrogen cycle in Rwenzori
lakes
3o/oo decline in δ13Corg in the glacial lakes suggesting
that glacier retreat is causing changes in the carbon
cycling in Rwenzori’s glacial lakes . Yet, trends in
aquatic ecosystem functioning are variable among
lakes and require more detailed analysis.
(Changes are probably driven by factors other than
primary productivity-presumably variations in
respiration and lake stratification)
Russell et al. 2009 Journal of Paleolimnology 41: 253-271

13. Paleolimnological records of recent glacier recession in the Rwenzori Mountains
Sedimentological profiles – dowcore trends in siliciclastic content
Siliciclastic content of the sediment in the
glacial lakes significantly decreases towards
the present, whereas non-glacial lakes
generally show weak trends over time
The magnitude of changes in siliciclastic
content can vary considerably between lake
basins despite similar magnitudes and rates of
glacier recession (i.e. glacial lakes can differ
dramatically in their sensitivity to glacier
fluctuations)
Changes in the siliciclastic content of glacial
lake sediment reflect fluctuations of glacial
extent
Signals of glacier dynamics can be isolated
through comparative studies
Russell et al. 2009 Journal of Paleolimnology 41: 253-271

14. TIMING AND CAUSES OF GLACIER RECESSION?
Stable, high siliciclastic concentrations for several
centuries prior to the late 19th century, under a
regionally dry climate
Reduction of siliciclastic content (documenting
glacial retreat) was underway by ~1870 during a
regionally wet episode
=> The influence of late 19th century reductions in
precipitation in triggering glacier recession in the
Rwenzori may be weaker than previously thought
Russell et al. 2009 Journal of Paleolimnology 41: 253-271

15. Recovery of short sediment cores of recent sediments (150-700 yrs old)
1. To assess the archival quality of the lake sediments
(>selection of good sites for long coring)
2. To assess the potential of Rwenzori mountain lakes to trace glacier recession
(to assess their sensitivity to glacier retreat)
3. To assess the limnological and ecological sensitivity of Rwenzori mountain lakes
to climate warming (= assess whether they are sensitive to climate-driven
environmental change of the same order of magnitude as that expected from
current and future anthropogenic global warming)
.

16. Recovery of short sediment cores of recent sediments (150-700 yrs old)
Top-Bottom design:
By comparing in 16 lakes the species assemblages of larval chironomid
remains (non-biting midges) deposited recently in lake sediments with those
deposited at the base of short cores, dated to within or briefly after the Little
Ice Age.
By comparing temperature reconstructions (estimates) for top and bottom
sediments using fossil chironomids
No info on the timing or rate of observed ecosystem change, nor on the
causes (natural vr anthropogenic); but this apparent weakness is
compensated by the ability of the approach to simultaneously assess a large
number of sites

17. Chironomids as paleothermometers
0
10
20
Taxon number
30
40
50
60 MAT 5.5°C MAT 9.5°C
3800 m 3000 m
0 5 10 15 20 25 30
WA optimum (°C)
Eggermont et al. 2010 J Paleolim 43: 413-435

18. Predicted Mean Annual Air Temperature (°C)
Chironomids as paleothermometers
r² = 0.97
RMSEP = 1.62°C
Observed Mean Annual Air Temperature (°C)
Eggermont et al. 2010 J Paleolim 43: 413-435

19. Average chironomid-inferred historical MATemp change
for the 16 Rwenzori lakes between top and bottom
samples. Sites are arranged according to drainage 3.0
3.0
Present-day warmer
Present-day warmer EL-EM
basin. Non-glacial lakes are shaded in black; glacial Combined
lakes in grey. Dashed lines indicate the observed 20th
Inferred change in MATemp (°C)
Inferred change in MATemp (°C)
century regional MATemp change of 0.60 °C 2.0
2.0
Excluding the relatively unique mid-elevation lake 1.0
1.0
Mahoma (2990 m altitude), we find a three-to-one ratio
in cases of inferred warming versus inferred cooling,
0.0
0.0
A generalized linear mixed model analysis of the
combined result from all lakes except Mahoma indicates
-1.0
significantly warmer MATemp (on average +0.38 ± 0.11 -1.0
°C) at present compared to between ~85 and ~645
years ago. Present-day colder
Present-day colder
-2.0
-2.0
Kopello
Batoda
Bigata
Africa
Katunda
Kanganyika
Middle Kachope
Speke
Bujuku
Mahoma
Bukurungu East
Nsuranja
Lower Kachope
Upper Kachope
Upper Kitandara
Lower Kitandara
Kopello
Batoda
Bigata
Africa
Katunda
Speke
Kanganyika
Bujuku
Middle Kachope
Bukurungu East
Nsuranja
Mahoma
Lower Kachope
Upper Kachope
Upper Kitandara
Lower Kitandara
Inferred temperature changes are independent of
whether lakes are located in glaciated or non-glaciated
catchments, and of basal core age, suggesting that at
least part of the signal is due to relatively recent,
anthropogenic warming.
Eggermont et al. 2010. Hydrobiologia 648: 123-142

20. Is the shift in species composition similar in
each lake (i.e. always the same species that
increase or decrease in abundance towards the
present?)
Historical (core top/bottom) change in the
percent abundance of common chironomid taxa
in 16 Rwenzori study lakes
Some taxa show a clear decrease (e.g.
Diamesa type East Africa) or increase (e.g.
Polypedilum type Bandasa) towards the
present. However, trends are more variable for
other taxa, without any obvious relationship with
lake type, catchment vegetation or elevation.
Eggermont et al. 2010. Hydrobiologia 648: 123-142

21. Main pattern of faunal change in each lake in the context of the chironomid taxa-environment relations in
the Rwenzori inferred on the basis of the calibration data set
Eggermont et al. 2010. Hydrobiologia 648: 123-142
The direction of faunal change at the lakes in relation to established species-environment relationships
suggests that part of the observed shifts in species composition reflect lake-specific evolution in habitat
features other than temperature, such as nutrients, pH or oxygen regime, which co-vary with temperature to
greater or lesser extent.
Yet, the fairly uniform and marked historical warming trend in Rwenzori lakes documented by this study
highlights their ecological vulnerability, and their value as early-warning systems for detecting the limnological
and ecological effects of global warming.

22. Conclusions
...Rwenzori’s high-elevation lakes are highly sensitive to alpine glaciation and constitute a unique laboratory to
assess relationships between glacier extent, Afroalpine ecosystem processes, and (long-term) changes in
central African climate
Avenues for future research
... Long-term climatic, limnological and ecological monitoring
... Optimalisation of existing climate proxies, and development of new ones
... Multi-proxy study of long sediment cores (~mid- to late Holocene) from selected lake sites
... Paleogenetic work (i.e. genotyping DNA of dormant eggs to reconstruct long-term changes in population
dynamics and genetics of zooplankton in isolated mountain regions)
... Similar work on Mt Kenya and Bale Mts