history and timing of human impact on lake victoria pdf

7/27/2019 History and Timing of Human Impact on Lake Victoria PDF

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History and Timing of Human Impact on Lake Victoria, East AfricaAuthor(s): Dirk Verschuren, Thomas C. Johnson, Hedy J. Kling, David N. Edgington, Peter R.Leavitt, Erik T. Brown, Michael R. Talbot and Robert E. HeckyReviewed work(s):Source: Proceedings: Biological Sciences, Vol. 269, No. 1488 (Feb. 7, 2002), pp. 289-294Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/3067918 .

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290 D. Verschurennd othersHistoC fhumanmpactnLakeVictoria,astAfrica

Figure . LakeVictoria,tsdrainageasin nd coringtations. he 263000km2 asin omprisesortionsfUganda,Kenya,Tanzania,Rwanda ndBurundi, ith 1995populationf27.7millionUnitedNations 995).Highest opulationensitiesand agriculturalctivityccurnthedrainagesfKenyan, wandanndBurundiivershat ogetherontributea. 90%oftotal ivernput o LakeVictoriaBalirwa Bugenyi988).Core-siteymbolseflectherelativeualityf edimentecordsas determinedy ocalbottomynamics.losed ymbols,ndisturbededimentation;alf-openymbols,ioturbation;pensymbols,oth ioturbationnd frequentave urbulence.epth ontoursre nmetres.akeVictoriaathymetryreparedttheLarge akesObservatory,asedon DEAL projectchosoundingn1995and1996 mainlyffshorereas) ndBritishAdmiraltyaps nearshorereas);riverrainagesrom rul 1993); andpopulationatafrom eck 1998).value of 1.74pCig-' below a core depthof 37cm (n=3).Unsupported'0Pbwasfittedo a nonlinearegressionRobbinset al. 1978),takingnto ccount disconformityta coredepthof 25-26 cm (see 3). Solid-phasebiogenic ilica (Si) in thesedimentswas analysedby wetalkaline xtractionDeMaster1981), using a 0.5 m sodiumhydroxideolution t 80 C forsample digestion. issolvedSi in porewaterwas analysed yspectrophotometry,n aliquotsextracted rom uccessive oreincrementsmmediatelypon recovery. ossil diatomassem-blageswere analysed uantitativelyn aqueous suspensions fundigestededimentKling1998). Sampleprocessingor ossilmidge analysis followed standard techniques (Walker &Patterson985),using 100 um ieve o extract ossils rom hesedimentmatrix.

3. RESULTS(a) OffshoreedimentationynamicsDeep-wateredimentsn Lake Victoriafigurea)aresoft, ark-brownrganicmuds appedby a 1.5cmProc.R. Soc. Lod. B (2002)

2. MATERIAL NDMETHODSFieldworknLakeVictoriahatwas conducted ythe DEALprogrammeInternationalecade for he East African akes)in 1995and 1996recoveredurface-sedimentoresfromixoff-shore tations etween 8 and68 m water epth Verschurent

al. 1998). The 1996 cores, ncluding 96-5MC,were ollectedwith Hedrick-Marrsmulti-corersampling rea 4x71 cm2),that rovides ydraulicallyampened ediment enetrationftera support rame as come to rest n the akefloor.Of thefourcore replicates ecoveredt eachsite, wowere xtrudedn thefield n 0.5 cm increments ear thetop (0-10 cm), and 1 cmincrementsown-core. he third eplicatewas used forpore-water extractiont 2 cm (0-10 cm) and 4 cm intervals; hefourtheplicatewas cappedand stored ntact.Water ontentnd drymass dryweightml- wetmud)weredetellllinedydryingor 0 h at 105 C. Sedimentgeat depthand accumulationrates were determined y 2'0Pb dating(Robbins& Edgington 975). Unsupported '0Pb thefractionof 2'0Pb activity ot supported y ll SitU decayof 226Ra)wascalculated s thedifferenceetween otal '0Pbandits constant


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HistoCyfhumanmpactnLakeVictoria,astAfricaD. Verschurenndothers 291

(b)solid-phasec (%)0 5 10 15 20 25

(c)_ soft rganicmudunsupported10PbpCi g-'

0.1 1 10 100

'e'vax}-

O 1 2 3 4 5cumulativedrymassgcm- )

o.s l.o l.spore-water i (mM)

Figure . Palaeolimnological videncefor ake-wide cosystemhange n Lake Victoria. a) Sediment tratigraphy,umulativedrymass and 210Pb ctivityf core V96-5MC from 8 m waterdepth.Blackcircles, ata; solid ine,model;dashed ine,cumulative rymass (g cm-2). (b) Stratigraphyf biogenic i in thesolidphase (solid line) and dissolved n porewater blackcircles); hestippled one correspondso the nferredisconformity,epresentinghiatusofca.40 years ca. 1885 to 1925).(c) Down-coredistributionf thepelagic diatomsAulacoseirapp.,Cyclostephanosp., nd Nitzschiacicularis,eflectingchanges n their bsolute bundanceoverthepast 180 years;peak concentrationsn flocculenturfacemudsare diatomssubjectto furtherissolution efore ermanent urial. d)(i)Abundanceratioof themidge axaProcladiusnd ChironomusnLake Victoria urface ediments etween waterdepthof 48 and 68 m, reflectinghemodern-dayradientn the seasonalpersistencef bottom noxiafrom to approximately0 months r=0.98, p < 0.01); (ii) evolution f theProcladiuslChironomusbundanceratio t 68 m waterdepthoverthe past 180 years.thickurface orizon fflocculent udanda thin lanketof recently ettledalgae. Unsupported 1 Pb activitydecreases xponentially ith umulativerymassdown-core,whichndicates hat ediments aveaccumulatedta relativelyonstant ate (0.032+ 0.001 g cm-2yr-' or2.3 mmyr- ), and thatbothphysical ndbiologicalmix-ing of recentdepositsare insignificantRobbins t al.1978). A shift o lower2'0Pbactivityt a coredepthof25-26 cm reflects disconformityhatwe interpretohave resulted romedimentrosion uringn exception-allyviolent torm. he largewindfetchnd relativehal-lowness fLakeVictoriamake ts entire ffshoreottomsusceptible o thewave turbulence enerated y high-intensitytormsJohnson 980; Dearing1997). Only nthedeepest art fthe ake swave-inducedrosion rareevent, and has the sedimentrecord been preservedsufficientlyntact to reconstruct ecent lake history(Verschurent al. 1998). The effectsfwaveturbulenceand bioturbationn the rchivalualityf he edimentec-ord are increasinglyvident t depths f less than64m(figure ). In manyoffshorereasshallowerhan50 m,frequent aveturbulence reventshepermanent epo-sition f fine-grainedrganicmuds Scholz t al. 1991).(b) Historzcal iatomproductionThe record fbiogenic i in coreV96-5MC (figureb)andother eep-waterores Verschurent al. 1998)docu-ments hehistoryfpelagicdiatomproductionn LakeVictoria. eakSi concentrations21% ofdrymass) n theflocculentop 1.5 cmare recentlyettled iatoms ubjectto furtherissolution efore ermanenturial Conley&

Schelske1989). Si concentrationselow that evelcorre-late tronglyith he ogarithmf otal ossil iatom bun-dance r= 0.89,p < 0.001, n = 20), implyinghat iatomsare theprincipalource f olid-phase iogenic i inthesesediments Conley 1988) Low, constantSi concen-trationsca. 5% ofdrymass) nthe ower alf f heprofileare similaro those n theupperportions fHolocenere-cords (Johnson t al. 1998), and reflecthemoderate,fairlytableoffshoreiatomproductionn Lake Victoriabefore human impactbecame noticeable. ncreasinglyhigher i concentrationseposited romhe1930s to themid- 980s reflect he greaterdiatomproduction hatresultedfroman increasedsupplyof growth-limitingnutrientsHecky1993). Cumulative oss of thisexcessdiatomSi to burial n deep-water ediments raduallydepleted akeVictoria's eservoirfdissolved i. Concen-trationsn theupperwater olumnduring tratificationhavedeclined rom 0-80 ,uM n 1960to lessthan8 ,uMin 1990 (Hecky 1993), and are now frequentlyelow1,uM a concentrationonsidered o severelyimitdia-tomgrowthSchelske t l. 1986). The slightlyower ia-tom-Si concentrationsepositedsince the late 1980s(figure b) reflecthe currentituationn whichdiatomproductions limited y thedissolved-Si oncentrationthat s realized hrough ecyclingfSi fromhallow-watersediments nd new inputsfrom he watershedhroughsoilrunoff.Changes n thepelagic diatomflora f Lake Victoriaover hepast 60years figurec) provide urthervidencefor ignificantistoricalhanges nnutrientoading.Thethinlyilicifiedpecies Nitzschiacicularisomprises 4%Proc.R. Soc. Lod. B (2002)

o/ -E 10- /= 15 _r 20:: 25_- 30-t 35_40


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292 D. Verschurennd others Historyfhumanmpactn LakeVictoria,astAfrica

- l | l l l l l l l l l l l l l

of totaldiatom bundance n recentlyettled lgae, con-sistentwith iveplankton ata (Kling et al. 2001) thatshow his pecies o be themost ommon elagicdiatomin Lake Victoria oday.A 30-folddecrease n its fossilabundancemmediatelyelow heflocculenturfacemuds(as comparedwith a. 1.5 times yclostephanosp. and ca.1.2 times orAulacoseirapp.) points o strong ifferentialdissolution fNitzschia rustulesefore ermanenturial.This is supported ythe depthprofile f dissolved i inpore water figure b),which ndicates ignificanti dis-solution nd diffusiono theoverlying ater olumn romtheuppermostew entimetresf ediment nly. ifferen-tialdiatom issolutionmplies hat he pecies bundancesin fossil ssemblages re not quantitativelyomparablewith ivefloralompositionFlower1993); however,pe-cies abundance rendsn permanentlyuried ssemblages(i.e. thosebelow heflocculenturfacemuds) can be con-sidered o reflect ealchanges n thepelagic-diatomom-munityhroughime Haworth 980; Leavitt t l. 1994).The fossil iatomdata (figure c) show that he pelagic-diatom ommunityfLake Victoriawas stablebetweenabout 1820 and 1940,withCyclostephanosnd Aulacoseiraco-dominantt ca. 80% and ca. 15%, and Nitzschiacicu-larzs bsent, r too rare o be preserved. he abundanceof all three iatom axastarted o increase etween bout1940 and theearly 960s,withNitzschia cicularzschiev-ing ca. 50% of fossil ssemblages eposited n the late1970s and early 980s;a level fdominance otobservedat any previous time during the past 12 400 years(Stager& Johnson000). Consistent ith hebiogenic-Sidata, the fossil-diatomecord hen ndicates reductionof diatomproductionn the ate 1980s, withdeclines nall threemajordiatom axa. The 2l0Pb-inferrediming fthis eductionmatches he ncreased ccurrence f mass-ivecyanobacterialooms fter 987 Ochumba& Kibaara1989). This indicateshat estructuringf the algal com-munityocyanobacteriaominance n themid-1980swasinfluenced y Si limitationf diatom rowth, ue to sea-sonaldepletion f diminishedissolved-Si eservoir. urfossildata also agreewith ong-term lankton urveys(Kling et al. 2001) that show the virtual limination fAulacoseirapp. overthe past 20 years. n Lake Victoriatodaythese diatoms re out-competed ecause of theirhigh-Si eeds nd high inking ate hroughhewater ol-umn Talling 1966;Kilham 1990).(c) Development f deep-water anoxiaFossil assemblages of midge larvae (Insecta:Chironomidae)n offshoreurface ediments re a spati-ally ntegratedeflectionf modern enthic ommunitiesacross a large area of the offshoreake bottom Frey1988). In Lake Victoria hey re rather oor in speciesdiversity, nd reflect oday's principal environmentalgradientn mud-bottomabitat rom dequateyear-roundoxygenation t shallow depths to persistent easonalanoxia up to 10months) n deep water.Abundance atiosof the anoxia-intolerantrocladius revipetiolatuso theanoxia-toleranthironomusmicola mong core stationscorrelate trongly ithwater epth figure d(i) r= 0.98,p < 0.01, n = 5), and with he ntensityf easonal noxia(r= 0.91, p < 0.01, n= 5), which s measured s theratiobetween heduration fbottom noxia < 1 mg 2 l-li inmonths) nd deep-water xygen ontent uring he mix-

agriculturalroduction% of1960value)100 150 200 2501996

1 80 -1960 .1940 -1 201 900 -18801 60 -1 40 -1 20

/ / pelagic yanobacteriabloom/ < Nile perch opulationurges,/ , indigenousish tocks ollapse/ r lake-level ise loods iparian etlands

/ Nile perch ntroducedrailroad rrivesn Kampala,Uganda

| Ugandan otton xportsg railroad rrivesnKisumu, enya| foundingfKampala, uganda

Arab radewith uganda| discovered y European xplorerslI Buganda ingdoml* W w * 1 1 F I | I * * *, I0 10 20 30catchmentopulationmillions)

Figure . Principal vents n the recent nvironmentalhistory f Lake Victoria, n relation o human-populationgrowth nd agriculturalroduction n itsdrainage asin.ing eason data from ecky t al. 1994;Ochumba1996).Changes n theProcladiuslChironomusatio ffossilmidgefaunas eposited hroughime tV96-5MC (figure d(ii))indicate hat, fter t least 140years of adequate year-round ottom xygenation,hedeep-waterxygen egimeofLake Victoria tarted o deterioratenthe early 960s.Seasonally ersistent eep-water noxia appears o havereached tscurrentpatial xtent y the ate 1970s. Thesefossildata agree with fragmentaryistorical issolved-oxygen ata from he 1920s, 1960s and 1990s (Hecky tal. 1994), and imply hat the intermittenteep-wateranoxia first bserved n 1960-1961 (Talling 1966) rep-resented he arliest tage f utrophication-inducedeep-water xygen oss n Lake Victoria.4. DISCUSSION

The combined ossil vidence ndicates hathistoricalchanges n phytoplanktonroductivityndcompositionfLake Victoriahave been caused mainly y bottom-upeffectsfexcessnutrientoading, nd essso by food-webalterationsfter he 1980s upsurge fNile perch.Timingand progress f the nferredroductivityncreasematchhuman-populationrowth nd agriculturalctivityn theLake Victoria asin (figure ). Quantitativestimates ftotalhistoricalnd current utrientxports o the ake donot exist Scheren t al. 2000), but strong orrelationfpopulation ize with griculturalroduction ver hepast40 years 1965-1991: r= 0.97, p < 0.001, n = 27) justifiesthe use of historical opulation ize as a proxy ndicatorfor nthropogenicoil disturbancend itseffect n nutri-ent fluxes n catchment unoffMeybeck1982; Caraco1995). The population f the Lake Victoriabasin hasgrown rom .6 million n 1932 to 27.7million n 1995(UnitedNations1995). Between 900 and a. 1930 thadnot increased appreciablybeyond pre-colonial evels

Proc. R. Soc. Lond. B (2002)


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Historyfhumanmpactn Lake Victoria,astAfrica D. Verschurennd others 293becauseofhighnaturalmortalitynder arly olonial ule,exacerbatedby the toll of exotic diseases (Kuczynski1949) Completion of the Uganda railroad by 1930opened heLake Victoria egion o settlementy timula-tingplantation gricultureor he export f commercialcrops Maxon 1990). Rapid population rowth hroughimmigrationnd improved ealth onditions hen tartedthe pattern f large-scale eforestationnd agriculturalconversion hat ontinues o the present ay.The occurrenceof seasonallypersistent eep-wateranoxia incethe ate 1970s that s inferredrom ur datasupports he hypothesisKaufman & Ochumba 1993;Hecky r al. 1994) thatdeep-water xygen oss in LakeVictoriamayhave facilitatedhe decimation f demersalhaplochromineish tocksby Nile perch,by eliminatingthe deep-water efugiumhathad protected hese fishesfrom uch excessivepredation ntil then. This idea isattractiveecause tmight otentiallyxplainwhy heNileperchpopulation xploded uddenly, ome 25 years fterits introductionrom akes Albert nd Turkana Lowe-McConnell 1987). However, he evolution f fish peciescompositionn experimentalrawls incethe early 970s(Witte r al. 1995) does not how vidence hat xpansionof seasonaldeep-waternoxiacaused characteristiceep-water haplochromines o migrate o shallowerdepthranges nd augment he food base of Nile perch. Alsobefore heNile perch psurge, aplochromineensities tdepthsof more than 30 m had been significantlyowerthan n shallower reas Kudhongania Cordone 1974).Hence, although he development f deep-water noxiamusthavecontributedo thedemise fcertain aplochro-mine fish axa that were dependent n the deep-watermud habitat,t probably id not trigger'he 1980s popu-lation xplosion f Nile perch.Our palaeolimnologicalata establish strong hrono-logical ink betweenhistorical and use and algal pro-duction n Lake Victoria,which ndicates hat andscapedisturbance ather han food-web lterations r climatechange s the dominant ause of the ongoing utrophi-cation.With current stimates rojecting doubling fthe regional umanpopulation o 53 million y the year2020 (UnitedNations 1995), furtheregradation f theLake Victoria cosystem an be countered nly f and-managementtrategieshat everelyestrictutrientnputto the ake and ts ributariesre mplementedn a multi-national, asin-wide cale.The authors hank . W. B. Bugenyi, . Chan,T. I=rdal,G. Ngobi,B. Odhiambo nd M. Rosenmeieror ssistance,theNational griculturalesearch rganisationfUganda orresearch ermissionnd access to the R. V. Ibis, andD. Conley, . Cumming,. Schelske,. Schindler,. . TallingandF. Witte or iscussionnd omments.hiswork as up-ported ytheUS National cience oundationT.C.J.),USNational ceanic nd Atmosphericdministrationnd Fundfor cientificesearch-FlandersD.V.), Norwegian esearchCouncil M.R.T.) and NaturalSciences nd EngineeringResearch ouncil fCanada P.R.L.). This s IDEAL contri-bution o. 127.REFERENCESBalirwa,J. S. & Bugenyi, . W. B. 1988 An attempt o relateenvironmentalactors o fish cology n the otichabitats flake Victoria.Verh.nt. Ver. imnol.3, 1756-1761.

Barel,C. D. N., Dorit,R., Greenwood, . H., Fryer, .,Hughes,N., Jackson, . B. N., Kanawabe,H., Lowe-McConnell, . H., Witte, . & Yamaoka, . 1985Destruc-tion ffisheriesn Africa'sakes.Nature 15, 19-20.Beck, .1998 n NewMacmillanocial tudies tlas orUgandaKampala, ganda:Macmillan.Caraco,N. F. 1995 nfluencefhuman opulationsn phos-phorus ransferso aquatic ystems: regionalcale studyusingarge ivers.n Phosphorusn he lobal nvironmented.H. Tiessen), p. 235-244.New York:Wiley.Conley, . J. 1988 Biogenic ilica s an estimatef iliceousmicrofossil bundance in Great Lakes sediments.BiogeochemistC, 161-179.Conley, . J. & Schelske, . L. 1989 Processes ontrollingthebenthic egenerationnd sedimentaryccumulationfbiogenicilica n Lake Michigan. rch. ydrobiol.16,23-43.Crul,R. C. M. 1993 Limnologynd hydrologyfLake Vic-toria. n UNESCOlInternationalydrologicalrogramme-IVProject -5.1 'Comprehensivend comparativetudy f theGreatLakes. Nijmegen, he Netherlands: rul Con-sultancy.Dearing,J. A. 1997 Sedimentaryndicators f lake-levelchanges n the humid emperateone: a critical eview.. Paleolimnol.8, 1-14.DeMaster, . J. 1981 The supply nd accumulationf ilicain themarine nvironment.eochim.osmochim.cta45,1715-1732.Flower,R. J. 1993 Diatom preservation:xperimentsndobservationsn dissolutionndbreakagenmodernndfos-sil material. ydrobiologia69/270,73484.Frey,D. G. 1988 Littoral nd offshoreommunitiesfdiatoms, ladocerans nd dipterous arvae, and theirinterpretationn paleolimnology.Paleolimnol., 179-191.Goldschmidt,., Witte, . & Wanink, . 1993 Cascadingeffects f the introduced ile perch on detritivorous/phytoplanktivorouspecies n the ublittoralreasof LakeVictoria. onserv.iol.7, 686.Haworth,. Y. 1980 Comparisonfcontinuoushytoplank-ton ecords ith hediatom tratigraphyn the ecentedi-ments fBlelham arn.Limnol. ceanogr.5, 1093-1103.Hecky, . E. 1993The eutrophicationfLakeVictoria. erh.Int. Ver. imnol. 5, 39-48.Hecky, . E. & Bugenyi,. W. B. 1992Hydrologyndchem-istry ftheAfrican reat akes nd water ssues: roblemsand solutions. itt. nt. Ver. imnol. 3, 45-54.Hecky, . E., Bugenyi,. W. B., Ochumba, ., Talling, .F.,Mugidde, ., Gophen,M. & Kaufman,. 1994Deoxygen-ation f he eepwater fLakeVictoria,astAfrica.imnol.Oceanogr.9, 1476-1481.Johnson,. C.1980 Sedimentedistributionywavesn akes,reservoirsnd mbayments.roc. SCE Symp. uCface aterImpoundments-9, 1307- 317.Johnson,. C., Chan, ., Beuning, ., Kelts, ., Ngobi,G. &Verschuren,. 1998 Biogenic ilicaprofilesn Holocenecores romakeVictoria:mplicationsor ake-levelistoryand nitiationftheVictoria ile. n Environmentalhangeand responsen EastAfricanakes ed. J.T. Lehman), p.75-88. Dordrecht,he Netherlands:luwer.Kaufman,. & Ochumba, . 1993Evolutionarynd conser-vation iology f cichlid ishes s revealed y faunal em-nants n northernake Victoria. onserv.iol.7, 719-730.Kilham, .1990 Ecology fMelosirapeciesn heGreat akesofAfrica.n Large akes: cologicaltructurend unctioned.M. M. Tilzer C. Serruya),p.414-427. Berlin: pringer.Kling,H. J.1998 A summaryfpast nd recent lanktonfLake Winnipeg,Canada using fossil algal remains.. Paleolimnol. 9, 297-307.Kling, . J.,Mugidde, . & Hecky, .E.2001 Recent hanges

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294 D. Verschurennd others HistoryfhumanmpactnLake Victoria,astAfricain the phytoplankton ommunity f Lake Victoria inresponseto eutrophication.n The great akes of the zvorld(GLOW): food-zveb,ealth nd integrityed. M. Munawar&R. E. Hecky), pp. 47-65. Leiden, The Netherlands:Backhuys.Kuczynski, . R. 1949 Demographicurvey f he ritisholonialempire. ew Jersey: xfordUniversity ress.

Kudhongania& Cordone 1974 Past trends, resent tocks ndpossiblefuture tateof the fisheries f the Tanzania partofLake Victoria.Afr. t. Trop.Hydrobiol.ish 3, 167-181.Leavitt, . R., Hann, B. J.,Smol, J. P., Zeeb, B. A., Christie,C. E., Wolfe, B. & Hecky, R. E. 1994 Paleolimnologicalanalysis f whole-lake xperiments:n overview f resultsfrom xperimental ake Area ake 227. Can. 3t. ish.Aquat.Sci. 51, 2322-2332.Lowe-McConnell,R. H. 1987 Ecologzcaltudiesn tropicalfishcommunities.ambridgeUniversity ress.Maxon, R. M. 1990 Themes n Kenyan historyed. W. R.Ochieng),pp. 29-35. Nairobi: Currey.Meybeck,M. 1982 Carbon, nitrogen nd phosphorus rans-portby worldrivers. m. 7. Sci. 282, 401-445.Mugidde,R. 1993 The increase n phytoplanktonrimary ro-ductivitynd biomass n Lake Victoria Uganda). Verh. nt.Ver.Limnol. 5, 846-849.Ochumba,P. B. O. 1996 Measurement fwater urrents,em-perature, issolved xygen nd winds on the KenyanLakeVictoria. n The imnology,limatologynd paleoclimatologyftheEast Af7icanakes ed. T. C. Johnson E. Odada), pp.155-167. Newark:Gordon & Breach.Ochumba, P. B. O. & Kibaara, D. I. 1989 Observations nblue-green lgal blooms n the open waters fLake Victoria,Kenya. Afr. Ecol. 27, 23-24.Ogutu-Ohwayo, . 1990 The decline of the nativefishes fLake Victoria nd Kyoga (East Africa) nd the impact ofintroduced pecies,especially he Nile perch,Lates niloticusand theNile tilapia,Oreochromisiloticus.nviron. iol. Fish.27, 81-86.Robbins,J. A. & Edgington, . N. 1975 Determination frecent edimentation ates n Lake Michigan using Pb-2l0and Cs-137. Geochim. osmochim. cta 39, 285-304.

Robbins,J. A., Edgington, . N. & Kemp, A. L. W. 1978Comparative l0Pb, l37Cs, and pollen geochronologies fsedimentsfrom akes Ontario and Erie. Quat. Res. 10,256-278.Schelske, C. L., Stoermer,E. F., Fahnenstiel,G. L. &Haibach, M. 1986 Phosphorus nrichment,ilicautilization,and biogeochemical ilicadepletion n theGreatLakes. Can.. Fish. Aquat. Sci. 43, 407-415.Scheren, . A. G. M., Zanting,H. A. & Lemmens,A. M. C.2000 Estimation fwater ollution ources n Lake Victoria,East Africa: pplication nd elaboration f the rapid assess-mentmethodology. Environ.Mngmt 8, 235-248.Scholz, C. A., Rosendahl,B. R., Versfelt, . W. & Rach, A.1991 Results f high-resolutionchosounding f Lake Vic-toria. 7. Afr. arth ci. 11, 25-32.Stager,J. C. & Johnson, . C. 2000 A 12,400 l4C yroffshorediatomrecordfrom ast-central ake Victoria, ast Africa.T Paleolimnol.3, 373-383.Talling,J.F.1966 The annual ycle f tratificationnd phyto-planktongrowth n Lake Victoria East Africa). nt. Rev.Ges. Hydrobiol.1, 545-621.

United Nations 1995 Worldpopulation rospects: he 1994revision. ew York: United Nations.Verschuren, ., Edgington, . N., Kling, H. J. & Johnson,T. C. 1998 Silica depletion n Lake Victoria: edimentarysignals t offshoretations. . Great akes Res. 24, 118-130.Walker, . R. & Patterson, . G. 1985 Efficienteparation fsubfossil hironomidae rom ake sediments.Hydrobiologia122, 189-192.Witte, F., Goldschmidt,T., Wanink, J., Van Oijen, M.,Goudswaard,K., Witte-Maas, . & Bouton, N. 1992 Thedestructionf an endemic peciesflock: uantitative ata onthe decline of the haplochromine ichlids f Lake Victoria.Environ. iol. Fish. 29, 1-28.Witte, ., Goldschmidt, . & Wanink, . 1995 Dynamics f hehaplochromineichlid auna nd other cological hanges nthe Mwanza Gulf of Lake Victoria. n The mpact f pecieschangesn Africanakes ed. T. J. Pitcher& P. J. B. Hart),pp. 83-110. London: Chapman & Hall.

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