NUTRIENTS AND PLANKTON BIOMASS IN THE RIFT LAKE SOURCESOF THE WEITE NILE: LAKFJ AI-BERT AND EDWARD
JOHN T. LEHMAN Deryrtnent oj Biology, Univercityof Michigan Ann Arbor, MI 4E109 USA
ARNI H, LITT Depa nent oltuoloqy, Univenity ofwashington Seattle,WA 98195 USA
ROSEMUCIDDE F ishe r ies Researc h I nstitute P.O. Ror 343, Ji'tja, USanda
DONNAA, LEHMAN Depannent o! Riolosy, Univerciryol Michisan AnnArbor,MI 48109 USA
l. Abstract
The rift lakcs Albcn and Edward, ss well as Lake Ccorge, USanda,were samplcdat nea$horeand offshoresir€s during March 1995 with attenrionto water column chemistry,plankton biomass, and produclion ntes. Biomassof phytoplankronexceeds that of zooplanktonin borh rifr lakes,and rhe predictivereladonship betwcen lighr artenuationand particular€chlorophyll is consistentwirh thar of Late Victoria. Cyclopoidcopepods dominar€ Albert and Edwardicalanoids are rare (
l5l LI Lzhna (.d,), Ewinw^tal Chats. ana R6poN. i. E6t Ahica LnLt, l5t-172, @ 1998Klwd Acod.hk Prblishe.s. P.int d in E N.h.tsM- 2. Inhodu€tion Lake Alben is i kn'? surfacearea, I The WestemRift of East Africa, nonh of the Virunga volcanoes,contains two ancient rnain inflow at tE lakes that attsactedscientiflc attentionsince theh discoveryby Ewopeansmore than a atrd \rhich river is centuy ago. Underlain by sedimenisof gl€at antiquity, Lakes Albert and Edward slopesof the Ru'*r togetherwith Lake Victoria constitutethe major lake sourcesof the White Nile and Dortiemmostend, I contribute to the dch limnology of centsalEast Afiica. These lakes have not been Lake Edward i! subject to comparativeexamination of limnological propertiesfor over two decades' reachesits mariml despite changesin land use, fisheries exploitation, population gro\t'lh, and regional eradually to rhe U climate. In order to identify modem conditions,we undertooklimnological sanpling Ruwenzoris,fiom t andproc€ss rate expedmentsat theselakes in 1995 Someinflow o€cur deepand hypereurl 300E 3roE studiedduring rhc BiologicalPrograrn of environmenhlc( fron mining operatr
3. Methods
Lake Alb€rt wass: Ugandain thenont ro the mourhof rh. rED2,I8 m andED In situprofitcs € Endeco,rySl6000 C werecalibrared ce ;alibraredagainsr s ln siru temperarure r.mp€ralurecoeffici ,lI PAR) wasncas gnples werecollc. .t coll€ction. SRt 'Ed'ods (Hach).Fil rll and *€re shapg 9O4t aceronci piSr tO.op'grn€nt. Bodl tt'or. Chlorid., r \lt.linity was rrE: :rljrum warenEaJu rqE rrasurcd by I trtrulatc P 'J.rc m r ih. sumof rh. l*( Iisf. r, tilcs Alb.rt, Edwtrd, dd G.o!ge, wirh sanpling silesindicaM. Zaoplankon *s, iO sm apenurc. 0l
158 .Lake Albert is a chuacFdstic half grabenhkc oi rnaximun deprh58 nr, dnd 530{) km'surfacc.vea,lying at surfaccelevation ol 615 In (FiS. l). The lake receilcs1r\ main inflow at rlre south fronl the Scmliki Riv$. Nhich desccndstrom Lake Ed$1rd. xnd $hich ri\er is borh augmenredand diluted bJ triburffy streamsftunr rhe no hc|1r stotesol rlrcRu$cnzoris (Viner 1975). Thc vrcrorir Nilc entersLake Alberr ,rs northernnrostend. !rhere il Joinsrhe nonh$ard oultlow ol lheAlben Nite. Lnkc Ld\r.d is a smaller(2125 kmrl and dccpcr012 m) rjhedbasin. Thc lakc J.'. reachcsits nrrxinrunr deprh \rithin 5 krn ol rfic wcsrcrn (Congo) shore. and stoFs gradurllyro the Ugandr shore. The lalic rccerlcsdrrjnr8e ffom rhc sourhwcslcrr) Ruwcnzoris,llonr the Rwrnda highlands, rnd tiollrrhe Virunga votcanocs to lhc i)ultr. Sonreinllos occursrhruugh the KazirSn Chrnnel tioln.r shallow(2 nt but (,flicx ! dcep rnd hypcreurrophicnorthcrstern basin. t-akc Ccorre Lake Ceorgehad bccn w. studicddLrrurS thc 1960sas fa of I RoyalSocict) romponenr of rhc Inlcrnrrionrl BroloricrlProgrrnnr)c (Burgis et al. l97l). Subscqucntlyrhc lrke hasbcconrc rn d$ oicnrironmcrlaleonccrn hecause ol hcaly m.(xl conr niinrtionby coptcr andcol\xlr konrnrininr ol)crlrlons in lheRu$cnzoris (t)cnn\' cr lt 1995).
3. Nlrlhods
I-rkc Albcn wrs s nrtlcd ncarsho.c(5 nr) rn(l 0Jtilxnc(AI-l q0 n, tidn BL irhi, lJ8lrdr in rhcnorlhclnclr hall ol lhelrke. l.rle lldwl|rltwlrs sanrplc(l'renrstrorc clt)se lo llc nroulhol lho K /iDgaChrnn.l ([DI,.1 nr) rs wcll ls rr rwo o{t!ho,ek)(rti(ns {lil)2. l8 Ir rnd EDl, 25 nr). In srlLrtr(ni[s ol lcnrpcraturc,ox]gen, pH, nd eon(lucnviryw.rc obrtunedwith rn [ndrco/YsI an)00C ID cquippedwirh inlernrl dlh toggcr Oxygenxnd pH ctcclr)des Ncrecrhhrrtcd creh dal oi uscand in sirutroli[s ol di\v)tlcd oxyScn$erc turrhcl .rlibfurcdig.rinsl surtucc $rrcr nrcrsurcnrc'rtshy Wr kt.r rirr i.n. Condueli\irif\]l In srlu r'nrJicfutur.s$crc cr)n\crl.d lr) sfccill( .ondurlllncerr 25 .c by usrngr 'C r 't r.mt.rxrurccrtr lllcicnr ol :.:l? (Trllinr rnd rltins l,)ar5). th.t$$rrer irrJi,ur.. tUIj IrAR)wrs nrcrsufcd\!ith r I-icor eosircci,rre!led (turDtuDr scn\or. Discrcrc $l|lcr \rri\rles$crc collccrcdhl VrD Dorn houlc. ij )rttcs tllltrcd,)nshorc wilhin.l h "cre i,1 !ollccrion. SItl' rnd SltS; lvcrc nrcxsurc(ti rledi.tel) by spcctroph(norelfic nrclhods(H ch). Irilrclslin chlofophyllurr(l prulicutrlc ctrenrislly wcre d.icd o!er rjti.il sel rnd wcrc shittcLl ro A n Arbo. thf nrtlsis. Chlorr)drylt tilrers werc ground in 90'l acclurci tignrcr( \'rs mcasurcd hy cxlftrclcd ttL(tresccnce,corrcctcd tr)r phcopi!rrcnt. Borh unliltcrcdr,'rrr. sarDplesrnd lilrr[. w.re :rlsoshipped lo Anf ,\rbor. Chlorid!,oilrirc, nirr.llc, and sullrlc $!|c nrcrsur.db) ion chromlrogritthy. .\lkrlinitt wrs nr.r\urcd by Crrn rirrarion. Sodiurn.forrssium. ma-!nesiurrr, rn,l rllcrunrscre nrcrNUrcdb) 1lamearomic absorprion spccrroscopy. tron andnrangrncsc trcre orcasurcdb! gnphi(e furnicearomic ibsorprion Iorxl dissotrcdp (Dp) rnd psniculalc P $crc nr.r\urcd afrer persulfateoxid ion rnd Torrl P (TP) was calcullrcd rs lhe sum ol lhc two Zooflanklorwcre sxmplcd wilh lour.eplicarc vcrricrl hruls by netsof borh100 rd 60 [m apcnLlrl:,0.5m mouthopcning, and 5:l asJ'.crrlrio (ResearchNcls, tlrc.).
159 Zooplanhon dry massby taxon '*as estimat€dby drrwing individuals as edcountered abiotic turbidiry in I ftom platrkton collections, rinsidg them in deiooizei water, atrd theo drying them to Ceorge(2.54 mr) fi, const8ntmass ar 60 "C. Massdeterminadon was by elecEobalarce(Caho 29). backgound atenuari Kinetics of phoslhate uplake were measu€d for nca$ho.e and offshore plahkton water,independ€nl o assemblages.Inke waler aliquotsreceiv€d additions ranging from 0 to 2 UM NarHPOr and conc€nhaiionchanges w€re measuredover ca. 2 h al ambienttemp€ratur€ (c6. 25 oC) in subduedlight. Maximum rates of uptake and half-saturationconstant! werc determinedby nonlinear rcgrcssion(SYSTAT 5.03).
PAFiPerc€r ) 01 110
Ftt tl MO t99
Iirlr. 2. Photosyrthcticlly etivc iEldiu@ 4.1.2.laLe Albed v.6u5 &pth $ FEa|Ig! of surfe B!din8r. for Measuremenbfor tll ri*d Alb6l. Edsld, ud Gco'g.. 1995. 'C warmerrhroughoul any other of his longil di€l basisto a dcpthol 4. Resultj spe€iic conduclancc nrtate than measurcd 4,I. PHYSICAL AND CHEMICAL PROPERTIES Hislorical valucs comparisor wirh 4.1.1.UBht Att nuation o0t a(enuationcocmcical Vcrtical ar@nualionof light varies among lakcs, sDdbctw€en nearshorc 8nd offshore valuescite-d by Lakc Edward (Fig. 2). With thc crc€ption of Lalc Cco.gc, the raDgeof dtcnuation Taliat 1961. Our estimarc coefhcicnts was similar lo observationsftom offshore llkc Victoria (Irhman cl al- fl 0.36II|'. As wc noGC 1998). The nearchorestation ED I wasn€ar the mouth of th€ Krzinga Ch6hncl,Iinking chlorophyll ftom tigh( Edward with irs upstrearnhypcrcutophic extension,Lake Ceorge. Light attcnuation levelsin Marchl96t was primadly a function of paniculate chlorophyll concc[t ation (Fig. 3) 6uch that a r 1995. If ir is uuc conunon linear model appli$ to the th!€€ large basinsAlbert, Edwrd, and Victoria. d lariation in algat ahr Lighr attenuationcould lhus bc usedto Fedict algal biomassfor l.lEs€lakes whcn direct Albenhave dcclin d. measurcmentsare lacking. Light atl€nuationin kkc C.orge is considerably8.ca!er thanpredicted by thc modcl lhat fits lhe derpcr la*es. this appers to rcsult ftom high
160 rbiotic turbidiry in Lrke George. The fesidual dcli;ltion of lighr arenuarion in Lake ') G.orgc (l5.1 nr fron rhe linear model predicrionis sinrilar rn mrgnitude to the high backgroundatlcnuarion value of 2.55m' thatGanf (19?,1)reporred for Lake Geofge Nrtcr. indepcndenrof irs chlorophyllcontenl.
E
o.035lchrFo28,2-oo+ 5
0102030405060 m0 Clrlrf-3
/lgr. J Vrni!rlrlrcnurlion{,fI'AR v {^chLLrot'hyllntr Irk.\ Alhcn. Ed*r . (nlr|:(.. rrll ntr\lcrr Vi.({ir MLnl.m v'.!r1r drir fror, l{ M0grLldc lL(hn r cr rl t9981
iUcNu.cnrcnlsfur Lrkc Albcflal lhc oifshoreslNliorr AI-l rcycala hk. th i! aboul0.5 ( $ nrcrlhloughoul (f-rg .1)than measurcd b) lrlling duringMarch ltl6l, or durin! -lhclrkcrsNcll xn\ othcrolhis l(ngitudinll lake suncl\('lrlling l96l) rixedonx Jrclbr\is lo l dcplhoirboul l5 nr. In othcrresf'ccrs.lhc lrkc is nntredilutc in tcrms,)l \Fcillc eon(juchnccund concenrrarion\ol nlrjor ions,lnd loscr ir phosphle lnd nnrde rhtll)nlcrsurcd d.crdcs ago b) Tnllingrnd orhcrs( lrblc l). Hi\r(rical vrlucs li,r p.rrriculatechl{)roph}ll in l-atc Alhc ,rc lacking lor .{r|farison wilh (\rr o('dcrn pfolilc (Fig 4), bur'lrllirr (1965) repor(edlighr nltenuatroncocllificnts litr g.ccD light, the n)ost pcnchlli g sl,cctrrl conponent. Thc ' \Lrhrcseircd by Trlling r.ngcl.onr 0.35 lo 0.72m', includinS0.50 nr duringMnrch ol 1961. Our cstin.rlelbr lighrattenuation (Fig.2) durnrgthe sarne oDnth of 1995was r. 0.16 m As wc nolld above,rhe linear nodcl of Fig. I ean hc rcli)rmulrtcd to predict .hlorophyll liom lrghr aucnurtion. Sucha fefonnulrted nrodclpredicls th.rt chlorophyll r. l.!els in Marehl96l \,,ererbour 6.:1 mg m or nearlythrec tirnes highcr than in March 1995. Ii it is truc rhat historical light rttcnL[tion in Lrke Alberi wrs controllcd b) \rri.trionin alr.ll lbundlnce,this comparison indicarcs thur chlorophyll le\els in L*c \lben halc dcclincd
t6l TAALE I (
Ks 0s) pH 3tl iT K ,-l) Mg CI sOa An(t&) sio, :L ffi[ SRP DP TP NO:
lvln ;-11T[ TAILE :u_ 25 lil{ |(il16l rif!'.,/. T.hpedrlE, oxysc. (ng Lr). chtoophyll(mgnr),6d ch.micdt PH pmFnis: b]. Albcrl, Mdch 1995. Ch.micdl unirsd !s itr Toble L K 4.1.3.Lake Etuatd Mg Ca Lake Edwardalso diffe$ from historicalrecord (Fig. 5, Table 2). Talling (1965) reponcdlight attcnuationof 1.3mr ftom a 5.5 m no(hcaststarion dear rhe Kazinga cl Channel. Our empirical relatioNhip betwccn auenuationand chlorophyll (Fig. 3) so implies that the valu€ coresponds wirh ca. 29 mg rrr Chl, similar to thc 22.7 rng r[] we foundat a 2-5 (at siq m stationirnmediar€ly offshorc of lhe Channelmouth l.ll ml SRP light attenuation).Offshore stations at 18 and25 m had Chl of 8.8 and 6.5 mg m'r, DP conespondingto measuredlight attenuationcoefficients of 0.63 and 0.58 m'r, T? respcctively. PresentP concentrationsin Lak€ Edwsrd are about one half of historical Nor reports and modemsulfate conccntratiorE arc lower, as well (Iablc 2). Lower nodern values for sulfate parallel obscrvationsmade by Hecky and Bugcnyi (1992) and by I-ehmanand Branstrator(1994) for Lake Victoria, wheremodem sulfate is lower lhan previoudy reported. Heclq and Bugenyisp€culate that someof the differencemay be amlytical,owing to modernmethods (ion cfuomatography).
162 TABLE L Cn.rical conpositionof tile Alben surfrcequter. Smples for 29 Msh 1995m rom offshoEslation ALI (40 m d.pth). Valu€s@ !M unlcs
l9:l' l95ll 1960.611 r9694 t995 Ks(!s) 825 801,8?0 110 660 7 8.9.2 9.t2 8.9.9.0 4224 1960 K 1690 1670 1333 1070 M8 l3t0 1300-t340 ll50 l0l0 230 225-210 260 2t6 Jt cl 850.1160 900 910.1080 110 SOi 260 280.4?0 290 195 Alk {FEq) 8070-8150 ?800 7250.?10 6t90 J500 sio: 57 0?.181 SRP 4.2 1.9.55 1.88 DP 2.11 TP 2.54 Nor 0.6 0.3.2.4 0.8.1.6 Mn
'FmmToltenhd.cn d bywo hrngron(1930) rFrcmElskens,.ncd byTdling (1961) 'FomTalling (1961). n= 5 smplingdaks. ed fonTrllins sd Trlling(1965)
TABLE2. ComrosnionoI Ljlc Edwo.dsdocc wolq. s{npks ror 25 Mdch 1995drc frcn m offshorestation 0l 0'12.596 S.29' 46.?99E(25 n dcprh)vulu.sandm.ftods N in Tublel.
1969' 1995 r0J1 l0l I t026 508,580 880 pH 339 9l 91 798.2 8.9 48?0 4?80 24SO 2115.1262 l?50 K 2010 2l t0 2t20 818-94r 1620 Mg t850 1990 811-962 1610 242 I0 280 130-:140 3r0 12-35 44 cl 760 l0t0 410,4?4 580 sor 165 120 150..100 250 10100 9850 9900 4070.47'10 1410 lt 108 110.1?0 Il0 l0 06 0.6 DP 0.3 4.1 Nor | 6 1.1 4,2 0.06 <02
'!om Elstcns,ci|.d by TallinS( I963) 'Fmm Tallinged Tallir8 ( I965)
163 Lake Albert, despirepri, Dumont1984). '"[i""[f Lake Albert 'i r,lesrsts€(5 trt 1710r{ |ll t|-il ,il1 t\ t/ il. !tl l1I ll I Ofidto.e (aO m) 6860 d! _[ L]*L-I tltt
\ t7ti tItl I Fl8r.6. (obov.) R.Lrn w.td colum bionB b) ll ll t\ tua in L.Ic Alb.n Ss in Fig. L
V fisur.7. (tisht) ^'Fts 6 LL- L_ttl_ Fig@ J, Dktribltion of t mp.6tuG, orygcn,chloophyll, ed ch.micd pop.ni.sin btcE!*&d, Moch1995. Unit!s1.Pi8.4.
4.2, PLANKTON COMMIJNTI'IES
Cyclopoid cop€podsdominrtc kkes Alben (Fig.6) and Edward (Fig.7). The lakes erhibit similar sp€cies .ichocss of €rustaceanzooplankton, but diffe. in spccies alsemblAges(Table 4). In panicular, lalge unh€lmcttcd Daphnia lunholtzi \ar. monachdrtc Fese in Lakc Alb€n, consistentwith observalionsby Cre€n(1967, l97l), Thc sp€cieswas rarc in Lake Edward,and smallerDaphnia lon|ispina occnrTed there instead. ThenDdiaplomus galebi is very rarc in both lakes; only immature specimenswcrc found in Lak. Edwrd despi@cxhaustive scarching. Wc assumethat the immatureindividuals are L galrbi b€.auseprevious authorities idcnlified the laxon fron the lakc (Dussan l9E9). We found ovigerousI. farebt f€mal€sand adult malcsin kke Albe( despiteprior lit€mtue ass€donsthat caluoids w€re absent(Verhcye and Dumotrt1984). Leke Albe( Lake Edward r.le{shoE (6 m) 1710 m! Dw m-2 NeardBo EDl (4 trD 1220 trE Cr,Vn-2
olEs€ (,|o m)58€O mc Dd m-2 Ofiltlo.! ED€ (25 nt 2450 rE D,V m-2
Ottshdo ED2 (18 Fr) 41gO rl9 D/v E-2
a,t!r.6, (.bovc) Rclotivccontribo on to rotdl watd collmn biohas by mojo. eoplantton taxa in t*c Albcn, SdmplinSsiGs indicaicd inFig,l.
Fi8!.. Z (ddt A5Fis.6,rorrlrc Edw{d.
TABLE 4. Cdslled zoool{tton 13& in t lq Albcn sd Edw.d, Mach 1995
llaoc!.lopt a.quabnalk a.quahnois M*octclop! a.quatoridlh tinilk M. so ct clop t d. quak nait hybtid Th.tu !.lo\ n.ELctu Th.mqclopt obldSotB Th. N.t cI opt. on t i ni li t Th.nnoniaptffi\t gdhbi Th.tudiottow (sat bi) '6-. Doplsio lMho|zj Mcha Dd?tuialu,rtola) wha
DiaphuosoM noryoliaaun Diopht olotu ^o^BoII6u6 tlBr Cciodaph a comutl ige.li
165 4,3, PHOSPHATEI Size distsibutionsof the dominantcyclopoid copepodassemblages in the two lakes are similar, but amlysesot Chaoborusaad Daphnia populationsrcveal that animalsare Uptake tinetics for ! larser in Lake Alben than in l,ake Edward(Fig. 8). Moreover,Candina waspresenr itr for Lake Vicloria. M offshoreplankon colections ftom l,ake Albert, but the skimp was not encounteredin ambientphosphate ar Lake Edward colections. Rotifers madea tdvial contributionto zooplankon biomass. uptakefunctions (VJ They we.e virtually absentfrom the plankton of both rift lakes,except for occasional specimensof Ke.atella tropica in Lake Edward. TABLE 5. PhosplBb q Water column inventoriesof phytoplanktonand zooplanktonbiomass arc plott€d in EDI dd NPG (Napokq Figure 8 for both shallownearshore and deeperoffshorc stations of both lales. Algal C e fron t*hlfu ard B. wasestinated ftom chlorophyll by calculatingC:Chl = 100:l by mass. ZooplanktonC I*e si€ O was calculaled as 50% of dry mass. Phytoplanktonbiomass exceedszooplankon ng greater Edqdd EDI biomassin both basins,by a ratio in Lake Edwardthan in Lake Alben. Ed*ard ED3 Vicio.i! NIC I victori BC
5. Discussion
f01 Zooplanlron in t akc I aft dominatedover*t Mesocrclops. BotIt l2 two Diaphanosoma. Korovchinsla,persoo Fi8u. 8. Si& ii.lu.ncy distdbutiotu ol cyclopoid copcpodids. chdoboB twn , 6d Victoria,where only c Darho trcm otrshoc sr.tioN of L!k. Alb.rl and ktc Pdward. Iak€sa.re otherwise sir diaptomidsare pres.nr typicalsituation in lar! Despite superficial Lake Edward comp& intensein Edward.Th to phytoplankronbior perch, in Lake Albcn occurin thatlake. fu int€nsityof pr€darionh Zooplankronbiorru exceed$e seasonal! (1996),bur rhe vatt'.! Victoria. ID atl rhrE approxtmately2 ro 6 4m 1Bm 25m 5m 40rn amongthe lakes,ho\&, Edward Albert in Victoria. As a cont FiSuz 9. Totd bions ofphytopletto! sd eopl&tto! ranges, comPubd voft at n6hoe aod ollshoe stltions in Lka Albcrr md Rlward than in rhe orb
166 13. PHOSPHATEUPTAKE
Uptakckinelics fttr phosphdtc.rereponed in Table5, logetherwjth conrparisondata lor LakeVicLoria. Maximum uptake rates scaled to chlorophyllare inversely rclaled to ambienrphosphate .nd appearro be a usclLrlcompararive index. lnirial slop|sof rhc uptlkefunctrcns (vn,/kj) vary in\erseiy wrlh lake phosphate as wcll.
TAILE 5 Phosphar.upl*e pannrrcb rar Llkc Eiqdd rn(l ncanhoreb\c vicrunr. Mrnh 1995 EDI anl NPC(Nr!.lc.nCuliak ncrEhoru\ire\ EOledBC(tlugrir)rEo8horc\iks l)difttrBC EfronLhmffatrdlkrstraror(l99lr Drh nn NPGre lron Irhnrn errl (1996) Lllle Sile Chld sRl' V, v,,, r!n' uru nnDlPl. hL nmo[,{u!chl)rhi Edsrnl EDI 7: OIO 11 117) l9 (:.1) ll.1tdlssr 00ll EdSJTI ED] 69 05',7 13 17) 16 (l o) 661 i85S) o 017 l8I 0o1 t30(:l) 96 (l:) J.17(16ll) 0.101 2 0 (0.J) n00 (600) 0 019
Zoophoktonin I-ikc Albc( andl-akc ldrvri(lare surcrlicially vcr! simila. Bothlakcs rre donrnarcdo\cf\\hclnrin!ly b) a surtc(n cyclotonl\I)ccics. ircluding lhc predar(tr J/.r,.r./r?r. Borh llkcs hivc x spccics(l l)uph|ut, twti C.rruliplr,ti, and rhc sa rc t\rr' I)it)lkutoyrna. Cocxistcrcc of Dtr1r,.rf,ro,zr snccicsis nol unconrrr)n (r.v M K.i.!chrnst). tcrnnnl connnuniclrion).rnd is diller!nt lron thc slrurti{)nin Larc Yrct()rra.sherc onlyonc spccic\is prcs.rl (l-chtn.tr1996) Ihc connnuniries.i lhcsr lxkcsrrc orhcrwiscsinrplcr rhrn L.rkcVicrl)riLr, whcrc fi\c cyclotoidspccics rnd thrcc {lirpr(!rri!lsrrc frc\entoillslxrre (llranstrr(,r cl rl. 1996).Itolilers.rrc frrc, whichis rhc typicrlsilurtionir liri:c Africrnlxlc\ dollrinar.dbr cvclopoidsfccics (Lchnirn 1996) Dcspircsuperlrcirl snrihrirics, rbe snHllcrsilc ol nonctclop(rd zoophnkrorrn Lrke U(lwrrdconrpnred with Lakc Albcrl suggc(s lhrt vistxl flanktivoryis [rofc intenscir [dwrftl. Thrr .olioI is rcinli)rcedhy lhc lo$cr rrtrcol loophnklr)nhxrnrr\ rri ph\k,tllnkton hrorrrussin Lrkc Ed\vxi(l Presencc(n rn cflccri\c pisri!orc. \il!- t.rch, in Lrkc Albcn mdy crLrsfrcducliols in thc nulrbcrsol fhDllilous fishtl l ,)ccurin rhrl hkc. As crrly.N lhc 1960s.Grccr) (1967) irgued rhrl lhcrcwN fcduccd inlensily,)lfredation hy lish(r /oophnktrDin rnidl.kc regionsol Lrkc Alhcrt. Zool)lonkronhionr!\\ at ollshoic\tarion\ in borh Lrkc Albcrr rnd Lalc lrlwlrd cxcccdlhc scasonrlnrean \rlucs reporrc(l1(,r Lrkc Viclori.rhy BraDstrLuorcl ul (1996), bur the v.rlucs are wirlrin tlre rlnge of rnnurl varialion observcd lir Lake Victoria. In xll rhrec ol rhc hkes. roothnkton hionrilssl;ills rn r rlnse Iionr ,rtproxinnrcl)2 ro 6 r DW nr. Warcr columndcprhs that \!crc \anrpledriilltrcd rrnong thc hkcs, horwlcr, and nngcd lionr 25 m in Edwrrd o 40 In rn Albcrr, ro 50 rr rn Vict(nir. As.r conscqucnceol rverr!ing tl)errc.rl ahundanccs olcr difltrc|r dcpth rrnges.compured !olumeric .rbundanccsol zooplankronare soDrc$hathiirher in Ed$ardtlnn in dreorhc. llvo laftcs.We di.l not invcsligrte$c \crlicaldisrrbLrljon ol
167 the plankton in the lakes,and thus actualin situ concentsationsare only approximated which link ceorge to E by water cotunn averages. the Semliki River (Buga Previous literahrre rcpotu t\at Diaptonus is not present in Lake Albert are The IBP investigario enoneous. It is unlikely that our discoveryreFesents a new colonization event. The establishedthat nitrogen animals are extsemelyrare, on the order of one individual per cubic meter. We community respiration. confimed their presenceonly by diligent and sustainedsearch of large samplevolurnes. and Viner 1973). Nitr An immature diaptomid copepod had already been discovered in a Lake Albert fixation by diazorrophic coll€dion during 1993 with an identical net (V. Kiggundu and J. l,ehman, personal the Ruwenzoris (Viner obseNation),but its speciesidentity could not be esrablishedat that time. The fact lhat represenlativeof headwa in lakes, comparedwith t ake Victoria, points again to calanoidsarc so scarce these The hish pH of I_ pla*tivory higher l€velsof visual in the rift lakes. ph,.toplanlrcn (Canf aj Lake Albert seemsto be morc Eansparcntnow than 30 yearsago, evidenlly because pllorosynthesiselevales of rcduced algal chlorophyll. Phosphatecorcentrations are also lower, but that is variation ft om approxin! probably not the cause of increas€dEansparency. Talling had repo(ed extreme I L2 has b€enreponed 6 phosphateconcentrations (Table 1), and stoichiometricconsiderations dictate that P pH l0 (Dennyet at. lq could not be a limiting factor in biomassproduction, then or now The elevaled chemicalweaftering of ! t€mperatur€of the lake water is intriguing, becauseit is reminiscentof similar warming high€r silica concenr-ari of Lake Victoria (Hecky 1993;Lehman 1996). Whetheror how alteredmixing in amorphoussilica by diar. responseto the thermalregime may haveaffect€d lake production,nutdent suppliesto b€comepanly inhibilcd theepilimnion, and algal biomass cannot be answered by ourlimited temporal data. Copperis knownto inhit We found lower values of Total P in Lake Edward than previous reports had indicated. We cannotyet determineif the reductiofiis paralleledby chlorophyll TABLE ] becausehistorical dataa.e not readily availablefor comparison. All tkee lakes exhibit mod€rnvalues of sulfatelower thanhistorical reports. This apparcntreduction is a 30!ll t7 | regional feature in colunon wilh Lake Victoria. Il is nol yet clear how much of this differenceis rcal, a consequenceof lessenedinputs or possiblyincreased chemical K! (!s) reductionin the sediments,or how muchis the resultof improvemenlsin analytical PH methodology. Na phosphat€ K Kineticcharacteristics of the algaein Lak€ Edwad suggestthat is not M8 gro'r,th liniting, Inverse vadation of both Vn and Vm& with SRP in these tropical lakesis consistcntwith cxpectationsftom tempera(elakcs. valucs for Lake Bdwardale intennediatebetween inshore and offshoreLake Victoria, wherebioassay experimenh cl sor (L€hman and Branstrator 1993, 1994) demonstated that P is not the proximate limitationfor biomassproduction. siol SRP 5.I, CON'DITIONOF LAKE CEORCE DP TP Nor Lake George,as well as much of easleroLakc Edward, lies within Qu€enElizabeth (Ruwenzori) National Park. Restorationof polilical stability to Uganda in the late Mr 1980safter a deaadeof strife hasstimulated conservation efforts for wildlife within the National Pa*. In addition, coppermining hasrecently resumed along lhe Nyanwamba River, which flows into Lake George. The Kilembe mines first began operation in 1956,and sho(ly beforcthey closedin 1979elevated copper concentmtions were measuredalong a gradient extendingfron Lake George,along lhe Kazinga Channel
168 $hlch links G€orgero Edlvard,rnd then acrossnorthern Lakc Edwnr{tlo (s outflow as rheSeniiki Riler (Bugen!i1979) The IBP invesrigarionsof rhc 1960sf'reccded evidcnce oi mernlpollurion. they .stablishedrhrt nilrogenw,rs a li iringrcsourc. io rhealgaeofthel.rke. and rhat rotil connnunity .espirarionafproximrlely bxlancedphorosynlhesis on I 21_h b,rsis(Crnf and Viner 1973). Nirrogcnin.ome wrs doninrted hy it vjrl infuts and nitn)gen tr\u|on b) diazorrophicclanobr.rerir. Hydrologic inpul i5 domiD ed b] flow from rhe Ruwenzoris(Viser nd Snrirh 19731,lnd thc chenristryot the srreanlsis repre\entaiivcofhc.rdwalers draining tr.dominrnr sitic.re rocks (Kithrm 1gil{). The hiSh pH of L.rkc ce('ge is lhe .csutr ot photosynttrcsisby suspcndcd fhrtoplanlton (c!fli and Honrc l9?5) wirhdra$ t of (.Or |r()m rhc $,.ucrh\ \^r'aIn hu:r. el.\rte, rl,c pH Drri ,t lhe l,,oU\.rt.c td\r (rhrl-rl.d JrLInrt yirirtion " t.H fron approxin lcly 8.75to 9.98(caot 1972).Morc rccenrty.pH.r\ hilh as Ll.l hrs bccnreported from nearshoresaiers, $irh otcn lakc vatucsconsisrcnrt) nerr (Dcnny tli l0 er rl. 1995) By ctcv.ri S rhc fH. tholosl,nthesiscirn accelcr!(e ncr rhcnrical$ellhering of silicakom sedinrenrs.nd lhcrcappcnrs r(j bc a lrendrosilld hrl:hcrsiLcr conc0nrrari(nrsin rccentycars (Tabtc 3) Bi(,geni.prccitirrlidr of .rDorphoLrssilic. bv diarons.l pr)cessthrt tvticu r rcduces ticatc\cts in trkcs. rr\ F..(lDc trnl) inhrhiredby roxic cifcctsof rhc po LIing nrcrrls.clnccirliy cotfcr ( ot)t)cris knownk) nrhibilsilic. uptlkcIrd sitreiticarionhy dirbm\ (Rcurcrt(]8t)
TAllLli :l ( lrcnnfrl .otrUr^(ur .J ttrtc (;.d!c rlrh.L $, cl Sr,itLc! ld :.1Nir([ l{)95{!( ln]cftd nni Lltlicrr0 0tl)N. :l{) ll 17 E(l!rdcUh) Vtu!\ rtu }rlv u!tc$ iDdrcrrr(l
tlJ l l.1t62 !ll
cl
Allr|[q) 1781)t290 t7:11)
ll
'TrlLrnB &d'l illin8i le65)
rVrncr (1975)reponsNor asn0r d.r.dllhte
169 Copp€r also inhibits the nitsogen-fixationFocesses of diazotrophic, heterocyst- aDoutnutrient dynami< formins cyanobacteria(Elder and Horne 1975), includins 6e Iara Anabaena and by the U. S. NarionajC Aphanizonenon,.,r,:.ich forme.ly were important elemenlsof the flora (Canf 1974b). Our recent sanples were domhated by colonial coccoid cyanobacteria (Chroococcales), which do not Foduce heterocysts, but nonheterocystous 8. R€fercncer cyanobacteriacan fix niEogenin the dark when there is no Foduction of nitrogenase- disablingoxygen by photosynthesis,or in the light either by spatial separationbetween Badle, L C. 1966. p,otd aerobicand anerobicportions of coloniesor by temporalseparation of photosynthesis Ugdd4 conp@d virh L and nitrogen fixation in different phasesof the cell cycle (Carpenterand Price 1976; BmI6tor, D. l<., NdaE\ Staland Krumbein 1985; Mitsui et al. 1986). SmaI pennatediatoms are present, but 33?-155./,1 C. ,obM not as biomassdominants. Centric diatoms,esp€.ially Aulacosein (formerly Melosna) th. E€$AfriolaLs C- species,were rare. A similar situation was reported for Lake Edward during l9?2 Bugrni, F. W, B, 1979. C (Hecky and Kling 1987); Ctuoococcalesdominated the phytoplankton and diatoms Hy{'rohiologia64:9t5 were significandy abundant oDly at a southem site, remote ftom the metal Burgis,M. ,,, L C. Dln C contaminationlater identified in the north. Sodid on ! ftpi.d fB! pr.blcc Nilrogen fixalion formeJly provided nearly half of the niEogen income for Lake Podoctivity d F Clrpcnie!, George(Home and Vin€r l97l). Unlike regeneratednudents, such "new" nutrient E. J, &d C. C p permib net biological production and expon of biogenic rhaterial from th€ water nnrog.n fiiarjon widbu h column1o the sediments.The prospecrihat metal pollution may increase with mining D.my, P,, R, Bajtcy,E. Tut (Uedda) developmentsuggests the needfor additionalanalyses of the productiveecosystem. !d ib q.dan& Du$an, B, t989, CNrr
6, Conclusiotrs Eldcr,,. F,, A, r. sd Hom Enviromnhl Mtu8ctE O&f, Themosr conspicuous changes 10 lakes Alber( and Edwsd du ngthe previous 30 years O. G, l9?2. Th. ESd appearto have been (l) reductionsin algal biomass,measured as pafliculate 708. /, K. K!j* ed ^ chlorophyll,and (2) reducedwater column inventories of P and inorganicN. These sci.niilic Prbticdrions. changesare consiltent with increasedtemperature of LakeAlbert, 6nd sugg€sr reduced Cei O. C, l9?4. tnci
8. References
3.3dlc.L c 1966. ProlongcLjrrurililarion ind ,i.,)tJ_genr(ionin to|1t31 ltlics I c rrr litli. Ntugulc Lgoda.compred\ridLnic\tlun)oniandEd\rnlL'nnolog) rn{l o.cso-qnph\ 1l l5l.l6l 'l' B.trnntu$. D K, Nd3*ulr. L lil andkhmrn. J 1996 ZooplMllondynimics rn l:kr v,norir r :17155 ,r T. c Johnn rDd E o odadalcds.l, The limnoloSyclinrlolosy Md prl.ocl'huloloEj-of rheEi( Arnranhlies Cod)n id Belch Pull'drr\ ii!:.nyl. I: w B. 1919. copprr ron disribu(i,rnIn rhesurfrce sir.a rf Lr\e5 Ceorfc xnd Id1Ah'n H\drrholo8r361 9l5 Btrrlr. irl i.L C Dunn.G C CaDl.L M Ilc(nitrM, andr\ I Vrr.r 1971 tnlc Ccor!! Urandr Sxrdicson trtorical fEshwnr$ croslnem. p 1l)1.r09hK KdirkrndA Hillbnfhlllro\*\l ledsI. PrcdrdiviryPn)blemsinFrcshwrc^P!lishScicnrihr Puhl1.tr!onr cirpcnrcr li J md C C rt'cc 1976 N\ntin. (^1ittdrtu (Ttith.\l't !r,, c\thDxri,r nn ierohk' trrki!cDlirfion*rrhour h!r!o(J_!s Screnc.l9l ll73 1180 l)rnn). I'> li Br[y E Tulrxhxai. Md P i!tu,rl]i l9'rl Hca\\ nnrl ronrimin3honof LL. Croryc LLlgrn{13)rDd 1l\ sctlnnd\ lltdn)holo!D 29? .:lL)l}) lrNsin. ll lelie. Cn^hels codpodcs..rhnrr[.s rlcs 3itrx , frictrrc\ Ar.lritrcs cnrlrcrrr iiJ!r.J lj..\ J indHom. l97ri Cofi^-r.\rlc\Md CtrSOJrlg!'dxl!{ror) m trtuCJlLrt)hir lrl,c\ ljo\ mnnrtrhl l\lan3!rtrRr 2 l7-11) i.!nl C (i le?l ncrLAuhriu ofnct piNrytrolucr(rn 'n ljkc Ocorls,llSrndr.Hr! Alir.r p 691 rl)8 ,i K. (rrI'k and A llill[(.nLilk0qs]1, lcrlsl, ttodtr.rirlly ltohlcns 'n 'ic\1,*nlc's I'olirh S.,ch!lrcI'1rhlr.r!.n: ,r. C (; lr71 hcnl.nr $Lr rtrJJ,rnr. rn'l trnifusJkr lilht f4nrt ofl !s f:'rort.onkDllrn-grhr rL,nph\ll d ..nr.nI of ! \h!lhi{ .qu3roadlrlu rl:rl. Cc.rlc. Lrt[n(lr) J.umrl ol LrolLJ!)6] 591
,xl C C ll)?th Phl'rrtlnkrr! lrnnxrs dd dNtrrl,ulonn x s[rlltN .uto!l!c LI'|c(l-1lie Ccorgd, l grndr) Occol0g,r16:9:t i,i (i (i. Md A I tl,,nic 1tr75 l)iumrl lrrrii.xn.n phon\en'h.\n Ml nnrcs(r ll(ton 'n x .r.Jl,\\.cqurnlnrl ljl. lLx[. (;cD.!c l]smdr) F$hsJr.r lriol0g)5 r r re I C a;. rDi A ts Vnkr lt)71 Ecollginrl nrb,hrJ n r s[rlltitr .qud!n]l [[c ]l]*,: O.or!c. !. dr) l)r0..cdin8solrhc li.tdl Sociery.ll-o or Ill31:l2l.r.16 :.,. i l')(,7 lhc dktihLrr[ ti vridion ol /rtll,Jir lrrrlltrr lcrtrlr..r ClMoccrr)rn r.hrbn 10 ' .)r 0rgirlronm L*c Alb.11.llitrtAfric! Joumnlor Toologtl5l 1811(rr :r::l LrTl ,\*ocitrtions.i( h,io.ertrinlltr hrtl$ltlon olrhehkc \otrr..r ofrhe \\'hircNLld iidmrl lffl(i!\ 165 :l7l..ll1 .r\ R D 1!9:l The eurrothrcrrn)nofL[c vicritr,3 vc'hrndlurs.i Jer Inredr3riotrr[v.reinigung r.LrDr)l08'u25:l9 '18 .rr R E.M!ll J Klirg 1tr87Ph)ropl3nkrdn (f.lL)!y oarhe Ccrr Lll'.r in rh.nli v. l.!sotcentnl :a..r 1r.h'f lir Htdnrhrn.!1. Erg.bnisscLmnoLo!rc:j l9?.llli ' R E rnll F \\' B Btrg.nl, l99l H)d!,1o3\rnJ.lr.nnnn rflhc AlncanCrcrr lrlr\ bd {xrc. . .Jn\ nrr.r: plrblemsud nn,u,nr Nliueilunr.nV.r.inigungInrltNronnleLinnolog!:ll 15 51
l7l Hom., A. J,. ed A. B. vinc., 1971, Nfoc.n fixarionsd ils sirnincrlc. in tlpic.l tjtc Cslgc Ug$dr Nare 232:417-418, Kitllb. P. [email protected] y of Afrid lat6 ed nvcB. P't.D.rt sir, Du& Uriw6ity. KII![ !. 1984.Surat in Aliicd inlud vaEn: Sutftrcbchloridcntior. VdltDdllntd btdatiodc vcrdlisus UtMlosi. 22:29G102. COMPARISON rrhelil, J.t 1996,Pcllgic food v.b! oftn. Africs GMt t t6,p.2El-301./rT.C.Joh6@,!dE,O CAICITE FRO Od!d! l.dcl, Ttu liNolosy, cliDtolosy sd prl4liEtolos' or tlE E$t Atics lat6. Godon sd
[email protected]. t, dd D, K D6sir{or, 1993,E Lct5 ofNld.nlr ud gruing on ih. phtlodmkon ofhr. P.I\ Vicio.i& V6hrndlug6 llEdanoMlc V.GinigungLimlogi. 25:E50-855, Dcg' Lhru. J- 1, ed D. K, Bmsi6to.. 1994,Nltdcnt dynmi.r ed tunovd nt6 of phdphit sd sulh& Dul ir l*! victoria"Eat Atric!, Limlogy 6d O@eo86phy39: 227.233, Mibui, A,, S. Xul|@qa" A. Tstlhahi. H. Ik mto, S. C!o, sd T, Ani, 1986, SE cgt by which M,\ nift8cn-firing unic.lluhr cy!$bacEriagFw phobNrohphicdly. NltuE 323r?2G722. Drp R.utq, r. C. h 1983.Effet of@pp.! on sDwth,3ilicicrcid uprrL Md 3olublcp@k olsilicic acidin thc NiF diltoh.I/ulanloJ'e r.itbSil (Brcillaiophy@). toumd of Phycolo&'19: l0l-1o4. St l, L, Dd W. E, Knn$cln. 19E5, Ni&og.na&!.tiliry in dtc noducrocystour.ydnob&t riium THO or.i[dtofid rp. gown un&r dt.rmthg lidt-d$t cycl.s. A!.hiv flt Mi@biolosi. r{3r 67-71. Ia.g Trlling,r. F. 1963.Th.origh otitnttfic{ion h & Aftiu dthkc UMdoE/ sd OcdosnpnyEr68- 74. Trllin8. ,. E 1965.Thc pholo6ynthctic elivity ol phtloplstlo. in th. Ed Afdce l.ls. hGmanonrL KEIT R4!c t..mi.n Hld'obioloslc50: l-32. Stabt. T.[n& ,. F., lnd T.Iin& I A. 1965. TtE chmicd comgoliliotrot africM llk pltcB. IrtcoationaL R.vo. dq s.s$En HydDbiolod. tot 4zt-463. v4t r, H. M., .d H. ,. Dsnont. 1984.c.linoid @p.podiof $. Nil. ry8t!m. Hy.lrobiolodaUor l9l - 2t2. l. vi't.,r" B. 1969.TIF clFhistry of in *|crofblc cior8., Uglrda. vcrh&dhn8.n d.rht mltio l. Abstrrca [email protected] fflr Utmlod. U: 289-295. Thc viM, A. B. 1975. Rchtimhip! of nitDg.tr |rd pho5ph@rto r tDpicrl phytoplskionpopul ioa. oryScn isolop ,os6acods) gydlobioloda!2: 1E5.195. in moda ;arLonates vik, A.4., rnd t. R. Snith. l9?!. G@F9hicd, hilidicd sd phFicd uFlls of lrlc G.o!t. ap[rcai lo rt precipilating Pro...{titr$ ofilE loyd S@ictyoft do! E lta:235.270, a! O, lorming wod[itr$oq E. B. 1930.Obs 'lid 6 th. tdnpcnr@, hydbgd'io c.l16oition,6d otlr.i physicd rhcirshclls , e. cxleor c@dfioB of dE Vicbrit od Albcrr Ny|nar. ht.llutiomlc l.vu. dtr Scsmt n Hydrobiologi.,4: of inr€radi ur 32E-357. mcritc and i'l on dE lalc suggcststhrl Pdcocnvironrnenhli, Covariaice of 5n r.mo ybp wficn bL rt!.r 4,000 ybp, a da r $?t.r coluhn thl iE cad)onaEsarc dq I e n. p*t"'n ( Ll invaslonduc ro I L.f Turkau ha, I I .rI bc a.hicvcd ody t12 :t Lt- t.d ). F&'r . rqtL* A.&