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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" conference in Perth, Scotland in September 2010.

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  • 1.Limnological and Ecological sensitivity of Rwenzori mountain lakes (Uganda- DR Congo) to climate warmingHilde Eggermont, James M. Russell, Leen Audenaert, Dirk VerschurenGhent University, BelgiumRoyal Belgian Institute of Natural Sciences, Belgium Brown University, Providence, US

2. DR Congo UGANDA 3. Climate change and glacier retreat clearly constitute a major threat to the mountainecosystems/unique cold-water lakes located downstream from the glaciersIncreased glacier meltwater input may effect the thermal regimePlant 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... 4. 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 5. 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; Mlg and Hardy 2004; Mlg 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 6. Study sites Fieldwork: July 2005 (dry season) July 2006 (dry season) May 2007 (wet season) Jan 2008 (dry season) July 2009 (dry season)LakesPoolsEggermont et al. 2007. Hydrobiologia 592: 151-173 7. 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 depthdepth Lake Batoda, 3890 m, 15.0 depth 8. Lake Bujuku, 3891 m, 13.5 m depth glacial lakes Lower Kitandara, 3989 m, 11 m depthUpper Kitandara, 4009 m, 14.5 m depthLac du Speke, 4235 m, 17 m depth 9. 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. 10. Paleolimnological records of recent glacier recession in the Rwenzori Mountains Organic geochemical profiles downcore trendsAtomic C/N ratios of organic matter do not showclear differences between glacial and non-glaciallakes, and imply that there have not been majorchanges in the source of organic matter15Norg profiles do not exhibit clear differencesbetween glacial and non-glacial lakes indicating thatrecent glacial recession does not appear to havestrongly affected the nitrogen cycle in Rwenzorilakes3o/oo decline in 13Corg in the glacial lakes suggestingthat glacier retreat is causing changes in the carboncycling in Rwenzoris glacial lakes . Yet, trends inaquatic ecosystem functioning are variable amonglakes and require more detailed analysis.(Changes are probably driven by factors other thanprimary productivity-presumably variations inrespiration and lake stratification)Russell et al. 2009 Journal of Paleolimnology 41: 253-271 11. Paleolimnological records of recent glacier recession in the Rwenzori MountainsSedimentological profiles dowcore trends in siliciclastic contentSiliciclastic content of the sediment in theglacial lakes significantly decreases towardsthe present, whereas non-glacial lakesgenerally show weak trends over time The magnitude of changes in siliciclasticcontent can vary considerably between lakebasins despite similar magnitudes and rates ofglacier recession (i.e. glacial lakes can differdramatically in their sensitivity to glacierfluctuations)Changes in the siliciclastic content of glaciallake sediment reflect fluctuations of glacialextentSignals of glacier dynamics can be isolatedthrough comparative studiesRussell et al. 2009 Journal of Paleolimnology 41: 253-271 12. TIMING AND CAUSES OF GLACIER RECESSION?Stable, high siliciclastic concentrations for severalcenturies prior to the late 19th century, under aregionally dry climateReduction of siliciclastic content (documentingglacial retreat) was underway by ~1870 during aregionally wet episode=> The influence of late 19th century reductions inprecipitation in triggering glacier recession in theRwenzori may be weaker than previously thoughtRussell et al. 2009 Journal of Paleolimnology 41: 253-271 13. 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). 14. 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 chironomidremains (non-biting midges) deposited recently in lake sediments with thosedeposited at the base of short cores, dated to within or briefly after the LittleIce Age.By comparing temperature reconstructions (estimates) for top and bottomsediments using fossil chironomidsNo info on the timing or rate of observed ecosystem change, nor on thecauses (natural vr anthropogenic); but this apparent weakness iscompensated by the ability of the approach to simultaneously assess a largenumber of sites 15. Chironomids as paleothermometers 0 10 20Taxon number 30 40 50 60MAT 5.5CMAT 9.5C 3800 m 3000 m0 5 10 1520 25 30 WA optimum (C)Eggermont et al. 2010 J Paleolim 43: 413-435 16. Predicted Mean Annual Air Temperature (C) Chironomids as paleothermometers r = 0.97 RMSEP = 1.62C Observed Mean Annual Air Temperature (C) Eggermont et al. 2010 J Paleolim 43: 413-435 17. Average chironomid-inferred historical MATemp changefor the 16 Rwenzori lakes between top and bottomsamples. Sites are arranged according to drainage 3.03.0 Present-day warmer Present-day warmerEL-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 2.02.0Excluding the relatively unique mid-elevation lake1.01.0Mahoma (2990 m altitude), we find a three-to-one ratioin cases of inferred warming versus inferred cooling,0.00.0A generalized linear mixed model analysis of thecombined result from all lakes except Mahoma indicates-1.0significantly warmer MATemp (on average +0.38 0.11-1.0C) at present compared to between ~85 and ~645years ago. Present-day colder Present-day colder-2.0-2.0 KopelloBatoda BigataAfricaKatunda Kanganyika Middle KachopeSpeke BujukuMahomaBukurungu East Nsuranja Lower KachopeUpper KachopeUpper Kitandara Lower Kitandara KopelloBatoda BigataAfricaKatundaSpeke Kanganyika Bujuku Middle KachopeBukurungu East NsuranjaMahomaLower KachopeUpper KachopeUpper KitandaraLower 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 18. Is the shift in species composition similar ineach lake (i.e. always the same species thatincrease or decrease in abundance towards thepresent?)Historical (core top/bottom) change in thepercent abundance of common chironomid taxain 16 Rwenzori study lakesSome taxa show a clear decrease (e.g.Diamesa type East Africa) or increase (e.g.Polypedilum type Bandasa) towards thepresent. However, trends are more variable forother taxa, without any obvious relationship withlake type, catchment vegetation or elevation.Eggermont et al. 2010. Hydrobiologia 648: 123-142 19. 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 setEggermont 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. 20. Conclusions...Rwenzoris 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 (~m