Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda - DR Congo) to climate warming [Hilde Eggermont]
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Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda - DR Congo) to climate warming [Hilde Eggermont]

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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" ...

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.

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Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda - DR Congo) to climate warming [Hilde Eggermont] Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda - DR Congo) to climate warming [Hilde Eggermont] Presentation Transcript

  • 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
  • DR Congo UGANDA
  • Climate change and glacier retreat clearly constitute a major threat to the mountainecosystems/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 influencenutrient budget and productivity/biogeochemical cycles Warming may enhance the thermal stratification, resulting in deteriorating deep-wateroxygen supply...
  • Observations of glacial termini confirm rapid glacial recession from 1906 to presentFrom 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 thenext 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 globalwarming versus natural climate variability in tropical alpine environments (both terrestrial andaquatic), and the climatic controls of glacial extent.>> Lake sediment archives can provide the long-term historical perspective
  • 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
  • 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 mLake Bigata, 3998 m, 18.0 m depth depth Lake Batoda, 3890 m, 15.0 depth
  • Lake Bujuku, 3891 m, 13.5 m depth “glacial” lakes Lower Kitandara, 3989 m, 11 m depthUpper Kitandara, 4009 m, 14.5 m depth Lac du Speke, 4235 m, 17 m depth
  • 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.
  • 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
  • Paleolimnological records of recent glacier recession in the Rwenzori MountainsSedimentological 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
  • 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 thoughtRussell et al. 2009 Journal of Paleolimnology 41: 253-271
  • 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 20Taxon 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
  • 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
  • Average chironomid-inferred historical MATemp changefor the 16 Rwenzori lakes between top and bottomsamples. Sites are arranged according to drainage 3.0 3.0 Present-day warmer Present-day warmer EL-EMbasin. Non-glacial lakes are shaded in black; glacial Combinedlakes 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 2.0Excluding the relatively unique mid-elevation lake 1.0 1.0Mahoma (2990 m altitude), we find a three-to-one ratioin cases of inferred warming versus inferred cooling, 0.0 0.0A generalized linear mixed model analysis of thecombined result from all lakes except Mahoma indicates -1.0significantly warmer MATemp (on average +0.38 ± 0.11 -1.0°C) at present compared to between ~85 and ~645years 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 KitandaraInferred temperature changes are independent ofwhether lakes are located in glaciated or non-glaciatedcatchments, and of basal core age, suggesting that atleast part of the signal is due to relatively recent,anthropogenic warming.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.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 inthe 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.
  • Conclusions...Rwenzori’s high-elevation lakes are highly sensitive to alpine glaciation and constitute a unique laboratory toassess relationships between glacier extent, Afroalpine ecosystem processes, and (long-term) changes incentral African climateAvenues 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 populationdynamics and genetics of zooplankton in isolated mountain regions)... Similar work on Mt Kenya and Bale Mts