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

UGANDA

DR Congo

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

...

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

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

Study sites

Eggermont et al. 2007. Hydrobiologia 592: 151-173

Fieldwork:

July 2005 (dry season)

July 2006 (dry season)

May 2007 (wet season)

Jan 2008 (dry season)

July 2009 (dry season)

Lakes

Pools

East Bukurungu, 3801 m, 17.3 m

depth Lake Bigata, 3998 m, 18.0 m depth Lake Batoda, 3890 m, 15.0 depth

Lake Mahoma, 2990 m, 25.6 m depth East Bukurungu, 3801 m, 17.3 m depth “non-glacial” lakes

Lac du Speke, 4235 m, 17 m depth Upper Kitandara, 4009 m, 14.5 m depth

Lower Kitandara, 3989 m, 11 m depth Lake Bujuku, 3891 m, 13.5 m depth “glacial” lakes

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

.

Russell et al. 2009 Journal of Paleolimnology 41: 253-271

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)

Paleolimnological records of recent glacier recession in the Rwenzori Mountains

Sedimentological profiles – dowcore trends in siliciclastic content

Russell et al. 2009 Journal of Paleolimnology 41: 253-271

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

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

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)

.

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

Chironomids as paleothermometers

0

10

20

30

40

50

60

0 5 10 15 20 25 30

WA optimum (°C)

Taxo

n n

um

ber

Eggermont et al. 2010 J Paleolim 43: 413-435

MAT 5.5°C

3800 m

MAT 9.5°C

3000 m

Chironomids as paleothermometers

Eggermont et al. 2010 J Paleolim 43: 413-435

r² = 0.97

RMSEP = 1.62°C

Observed Mean Annual Air Temperature (°C)

Pre

dic

ted M

ean

An

nu

al A

ir T

em

pera

ture

(°C

)

Bato

da

Kopello

Big

ata

Afr

ica

Kanganyi

ka

Katu

nda

Low

er

Kachope

Mid

dle

Kachope

Upper

Kachope

Upper

Kitandara

Low

er

Kitandara

Buju

ku

Speke

Bukuru

ngu E

ast

Nsura

nja

Mahom

a

Infe

rred

cha

nge in M

AT

em

p (

°C)

-2.0

-1.0

0.0

1.0

2.0

3.0

Present-day warmer

Present-day colder

EL-EM

Ba

toda

Ko

pe

llo

Big

ata

Afr

ica

Ka

ng

an

yika

Ka

tund

a

Low

er

Ka

ch

ope

Mid

dle

Ka

ch

ope

Upp

er

Ka

ch

ope

Upp

er

Kita

nd

ara

Low

er

Kita

nd

ara

Bu

juku

Sp

eke

Bu

ku

rung

u E

ast

Nsu

ranja

Ma

ho

ma

Infe

rred

cha

nge in M

AT

em

p (

°C)

-2.0

-1.0

0.0

1.0

2.0

3.0

Present-day warmer

Present-day colder

Combined

Average chironomid-inferred historical MATemp change

for the 16 Rwenzori lakes between top and bottom

samples. Sites are arranged according to drainage

basin. Non-glacial lakes are shaded in black; glacial

lakes in grey. Dashed lines indicate the observed 20th

century regional MATemp change of 0.60 °C

Excluding the relatively unique mid-elevation lake

Mahoma (2990 m altitude), we find a three-to-one ratio

in cases of inferred warming versus inferred cooling,

A generalized linear mixed model analysis of the

combined result from all lakes except Mahoma indicates

significantly warmer MATemp (on average +0.38 ± 0.11

°C) at present compared to between ~85 and ~645

years ago.

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

Eggermont et al. 2010. Hydrobiologia 648: 123-142

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.

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.

Eggermont et al. 2010. Hydrobiologia 648: 123-142

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

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