nutrients and plankton biomass in the rift lake sources of the weite
advertisement
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
Fisher ies Research I nstitute
P.O. Ror 343,Ji'tja, USanda
DONNA A, 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€sduring March 1995 with attenrionto water column
planktonbiomass,
chemistry,
andproduclionntes. Biomassof phytoplankron
exceeds
that of zooplanktonin borh rifr lakes,and rhe predictivereladonship
betwcenlighr
artenuationand particular€chlorophyllis consistentwirh thar of Late Victoria.
Cyclopoidcopepodsdominar€Albert and Edwardicalanoidsare rare (<l mr) and
Cladocem
represcnt
lessthantenpercentof zooplankron
biomass.Alrhoughpreviously
reporledto be pr€sentin olherfak€sin rheNile sysr.m,Themodiaptonus
galebi\as
not confirmedin l,ale Alben beforethisstudy. Adult malesandovigerousfemalesof
lhe spccieswere coll€ctcd in lnke Alb€n and immatuc femalcswer€ sampledin Lake
Edward. MesocyclopsoBlrrnur w.s coll€ctedftom kke Albert, which is a new rccord
for this speciesftom EastAfricanRift Vall€y lakes. Presentwarerlemperatures
of
Lake Alben sre elevated by about 0.5 .C compared vith historicsl records, a
circumstancewhich palallels changesrecordedfor kke Victoria. Bolh Alb€rr and
EdwardhavelowerlevelsofTotalP and nitrarethanhistoricalaccounts
havereported.
In shallowLake George,heterocysr-forming
cyanobacbria
ard c€n(ricdiaiomswer€
rareconparcd wilh historicalrecords,a recenrcondirionrhat may be the resuhof m€ral
contaminatiot,
especially
by copperfromminingacrivities
in theRuwenzoris.
l5l
LI Lzhna (.d,), Ewinw^tal
Chats. anaR6poN. i. E6t Ahica LnLt, l5t-172,
@ 1998Klwd Acod.hk Prblishe.s.P.int d in E N.h.tsM-
2. Inhodu€tion
The WestemRift of EastAfrica, nonh of the Virunga volcanoes,containstwo ancient
lakesthat attsactedscientiflc attentionsincetheh discoveryby Ewopeansmore than a
centuy ago. Underlain by sedimenisof gl€at antiquity, Lakes Albert and Edward
togetherwith Lake Victoria constitutethe major lake sourcesof the White Nile and
contribute to the dch limnology of centsalEast Afiica. These lakes have not been
subjectto comparativeexaminationof limnological propertiesfor over two decades'
despite changesin land use, fisheries exploitation, population gro\t'lh, and regional
climate. In order to identify modemconditions,we undertooklimnological sanpling
andproc€ssrateexpedmentsat theselakesin 1995
3roE
300E
Lake Alben is i
kn'? surfacearea,I
rnain inflow at tE
atrd \rhich river is
slopesof the Ru'*r
Dortiemmostend,I
Lake Edward i!
reachesits mariml
eradually to rhe U
Ruwenzoris,fiom t
Someinflow o€cu
deepandhypereur
studiedduringrhc
BiologicalProgra
of environmenhl
c(
fron mining opera
3. Methods
Iisf.
r, tilcs Alb.rt, Edwtrd, dd G.o!ge, wirh sanpling silesindicaM.
158
Lake Alb€rt was s:
Ugandain thenont
ro the mourhof rh.
rED2,I8 m andED
In situprofitcs€
Endeco,rySl
6000C
werecalibrared
ce
;alibraredagainsrs
ln siru temperaru
r.mp€ralure
coeffic
PAR)
was
ncas
,lI
gnples werecollc.
.t coll€ction. SR
'Ed'ods (Hach).Fi
rll and *€re shap
9O4t aceronci piS
tO.op'grn€nt. Bod
tt'or. Chlorid., r
\lt.linity was rrE:
:rljrum warenEaJu
rqE rrasurcd by I
trtrulatc P 'J.rc m
r ih. sumof rh. l*(
Zaoplankon *s,
iO sm apenurc. 0l
J.'.
.Lake Albert is a chuacFdstic half grabenhkc oi rnaximun deprh58 nr, dnd 530{)
k m ' s u r f a c c . v e a ,l y i n g a t s u r f a c ce l e v a t i o no l 6 1 5 I n ( F i S . l ) . T h e l a k e r e c e i l c s1 r \
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
s t o t e s o l r l r c R u $ c n z o r i s( V i n e r 1 9 7 5 ) . T h c v r c r o r i r N i l c e n t e r sL a k e A l b e r r , r s
n o r t h e r n n r oesnt d .! r h e r ei l J o i n sr h en o n h $ a r do u l t l o wo l l h e A l b e n N i t e .
L n k c L d \ r . d i s a s m a l l e r( 2 1 2 5k m r l a n d d c c p c r0 1 2 m ) r j h e d b a s i n . T h c l a k c
reachcsits nrrxinrunr deprh \rithin 5 krn ol rfic wcsrcrn (Congo) shore. and stoFs
g r a d u r l l yr o t h e U g a n d r s h o r e . T h e l a l i c r c c e r l c sd r r j n r 8 e f f o m r h c s o u r h w c s l c r r )
R u w c n z o r i sl ,l o n r t h e R w r n d ah i g h l a n d sr ,n d t i o l l r r h eV i r u n g av o t c a n o ctso l h c i ) u l t r .
S o n r ei n l l o s o c c u r sr h r u u g ht h e K a z i r S nC h r n n e lt i o l n . r s h a l l o w( 2 n t b u t ( , f l i c x !
dcep rnd hypcreurrophicnorthcrsternbasin.t-akc Ccorre Lake Ceorgehad bccn w.
s t u d i c dd L r r u r S
t h c 1 9 6 0 sa s f a o f I R o y a l S o c i c t ) r o m p o n e n ro f r h c I n l c r n r r i o n r l
B r o l o r i c r l P r o g r r n n r )(cB u r g i se t a l . l 9 7 l ) . S u b s c q u c n t rl yh c l r k e h a sb c c o n r cr n d $
o i c n r i r o n m c r l a le o n c c r nh e c a u s o
e l h c a l y m . ( x l c o n r n i i n r t i o nb y c o p t c r a n d c o l \ x l r
k o n r n r i n i n ro l ) c r l r l o n isn l h e R u $ c n z o r i s( t ) c n n \ c' r l t 1 9 9 5 ) .
3. Nlrlhods
I - r k c A l b c n w r s s n r t l c d n c a r s h o . c( 5 n r ) r n ( l 0 J t i l x n c( A I - l q 0 n , t i d n B L i r h i ,
l J 8 l r d r i n r h c n o r l h c l n c l r h a l l o l l h e l r k e . l . r l e l l d w l | r l tw l r ss a n r p l c ( l ' r e n r s t rcolrtc) s e
l o l l c n r o u l ho l l h o K / i D g aC h r n n . l ( [ D I , . 1 n r ) r s w c l l l s r r r w o o { t ! h o , ek ) ( r t i ( n s
{lil)2. l8 Ir rnd EDl, 25 nr).
I n s r l Ltrr ( n i [ s o l l c n r p c r a t u r o
c ,x ] g e n ,p H , n d e o n ( l u c n v i rw
y . r c o b r t u n e dw i t h r n
[ n d r c o / Y s I a n ) 0 0C I D c q u i p p e dw i r h i n l e r n r ld l h t o g g c r O x y g e nx n d p H c t c c l r ) d e s
N c r ec r h h r r t c dc r e h d a l o i u s c a n d i n s i r u t r o l i [ s o l d i \ v ) t l c d o x y S c n$ e r c t u r r h c l
. r l i b f u r c di g . r i n s ls u r t u c c$ r r c r n r c r s u r c n r c ' rht ys W r k t . r r i r r i . n . C o n d u e l i \ i r i f \] l
In srlu r'nrJicfutur.$
s c r c c r ) n \ c r l . d l r ) s f c c i l l ( . o n d u r l l l n c er r 2 5 . c b y u s r n gr
r . m t . r x r u r cc r t rl l l c i c n ro l : . : l ? ' C r ( T r l l i n r r n d ' t r l t i n s l , ) a r )5. t h . t $ $ r r e r i r r J i , u r . .
t U I j I r A R )w r s n r c r s u f c d\ ! i t h r I - i c o r e o s i r cc i , r r e ! l e d( t u r D t u Dsrc n \ o r . D i s c r c r c$ l | l c r
\ r r i \ r l e s$ c r c c o l l c c r c dh l V r D D o r n h o u l c . i j ) r t t c s
t l l l t r c d , ) n s h o r cw i l h i n . l h
"cre
i , 1 ! o l l c c r i o n . S I t l ' r n d S l t S ; l v c r c n r c x s u r c (i t r l e
d i . t e l ) b y s p c c t r o p h ( nroe l f i c
n r c l h o d(sH c h ) . I r i l r c l sl i n c h l o f o p h y lul r r (pl r u l i c u t r l c t r e n r i s l lw
y c r ed . i c d o ! e r r j t i . i l
rnd
wcrc
shittcLl
ro
A
n
Arbo.
thf
nrtlsis.
sel
Chlorr)drylt tilrers werc ground in
90'l acclurci tignrcr( \'rs mcasurcd hy cxlftrclcd ttL(tresccnce,corrcctcd tr)r
p h c o p i ! r r c n t . B o r h u n l i l t c r c dr , ' r r r . s a r D p l ersn d l i l r r [ . w . r e : r l s os h i p p e dl o A n f
, \ r b o r . C h l o r i d ! , o i l r i r c ,n i r r . l l c ,a n d s u l l r l c $ ! | c n r c r s u r . db ) i o n c h r o m l r o g r i t t h y .
. \ l k r l i n i t t w r s n r . r \ u r c d b y C r r n r i r r a r i o n . S o d i u r n .f o r r s s i u m .m a - ! n e s i u r rrrn, , l
r l l c r u n rs c r e n r c r N U r cbd) 1 l a m ea r o m i ca b s o r p r i osnp c c r r o s c o p yt r. o n a n d n r a n g r n c s c
trcreorcasurcd
b ! g n p h i ( e f u r n i c e a r o m i ci b s o r p r i o n I o r x l d i s s o t r c dp ( D p ) r n d
psniculalc P $crc nr.r\urcd afrer persulfateoxid ion rnd Torrl P (TP) was calcullrcd
rs lhe sum ol lhc two
Z o o f l a n k l o rw c r es x m p l c dw i l h l o u r . e p l i c a r cv c r r i c r lh r u l sb y n e t so f b o r h1 0 0 r d
6 0 [ m a p c n L l r l : , 0 . 5m m o u t h o p c n i n g ,a n d 5 : l a s J ' . c r l r i o ( R e s e a r c h
Ncls, tlrc.).
159
Zooplanhon dry massby taxon '*as estimat€dby drrwing individuals as edcountered
ftom platrktoncollections, rinsidg them in deiooizei water, atrd theo drying them to
const8ntmassar 60 "C. Massdeterminadonwasby elecEobalarce(Caho29).
Kinetics of phoslhate uplake were measu€d for nca$ho.e and offshore plahkton
assemblages.Inke waler aliquotsreceiv€dadditionsrangingfrom 0 to 2 UM NarHPOr
and conc€nhaiionchangesw€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).
01
abiotic turbidiry in I
Ceorge(2.54mr) fi,
backgound atenuar
water,independ€nlo
PAFiPerc€r )
110
Ftt
tl
MO
t99
Iirlr.
2. Photosyrthcticlly etivc iEldiu@
v.6u5 &pth $ FEa|Ig! of surfe B!din8r. for
ri*d Alb6l. Edsld, ud Gco'g.. 1995.
4. Resultj
4,I. PHYSICAL AND CHEMICAL PROPERTIES
4.1.1. UBht Att nuation
Vcrtical ar@nualionof light variesamonglakcs, sDdbctw€ennearshorc8nd offshore
Lakc Edward (Fig. 2). With thc crc€ption of Lalc Cco.gc, the raDgeof dtcnuation
coefhcicnts was similar lo observationsftom offshore llkc Victoria (Irhman cl al1998). The nearchorestationED I wasn€arthe mouth of th€ Krzinga Ch6hncl,Iinking
Edward with irs upstrearnhypcrcutophic extension,Lake Ceorge. Light attcnuation
was primadly a function of paniculatechlorophyll concc[t ation (Fig. 3) 6uch that a
conunonlinear model appli$ to the th!€€ large basinsAlbert, Edwrd, and Victoria.
Lighr attenuationcould lhus bc usedto Fedict algal biomassfor l.lEs€lakeswhcn direct
measurcments
are lacking. Light atl€nuationin kkc C.orge is considerably8.ca!er
thanpredictedby thc modcl lhat fits lhe derpcr la*es. this appers to rcsult ftom high
160
4.1.2.laLeAlbed
Measuremenbfor tll
'C warmerrhroughou
any other of his longil
di€l basisto a dcptho
spe€iic conduclanc
nrtate thanmeasurcd
Hislorical valucs
comparisor wirh o0t
a(enuationcocmcica
valuescite-dby Taliat
1961. Our estimarcf l
0.36II|'. As wc noG
chlorophyllftom tigh(
levelsin Marchl96t r
1995. If ir is uuc d
lariation in algat ahr
Alben havedcclind.
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
b a c k g r o u na
d t l c n u a r i o vna l u eo f 2 . 5 5 m ' t h a t G a n f ( 1 9 ? , 1r)e p o r r e df o r L a k e G e o f g e
Nrtcr. indepcndenrof irs chlorophyllcontenl.
E
o.035lchrFo28,2-oo+
5
0102030405060
m 0 C l r lr f - 3
/lgr.
J
Irk.\
Alhcn. Ed*r
V r n i ! r l r l r c n u r l i o n { , fI ' A R v { ^ c h L L r o t ' h y l ln t r
. (nlr|:(.. rrll ntr\lcrr Vi.({ir
MLnl.m v'.!r1r
drir
fror, l{
M0grLldc lL(hn r cr rl
t9981
i U c N u . c n r c n lfsu r L r k c A l b c f l a l l h c o i f s h o r es l N l i o rAr I - l r c y c a la h k . t h i ! a b o u l0 . 5
( $ n r c rl h l o u g h o u(l f - r g . 1 )t h a nm e a s u r c b
d ) l r l l i n g d u r i n gM a r c h l t l 6 l , o r d u r i n !
-lhclrkcrsNcll
x n \ o t h c r o lh i s l ( n g i t u d i n l a
l k es u n c l \ ( ' l r l l i n gl 9 6 l )
rixedonx
J r c l b r \ i s l o l d c p l ho i r b o u l l 5 n r . I n o t h c rr e s f ' c c r s . l hlcr k c i s n n t r ed i l u t ci n t c r m s , ) l
\ F c i l l c e o n ( j u c h n c cu n d c o n c e n r r a r i o n
o \l n l r j o r i o n s ,l n d l o s c r i r p h o s p h l e l n d
n n r d e r h t l l )n l c r s u r c dd . c r d c sa g ob ) T n l l i n gr n d o r h c r s( l r b l c l ) .
Hi\r(rical vrlucs li,r p.rrriculate
c h l { ) r o p h } l li n l - a t c A l h c , r c l a c k i n g l o r
. { r | f a r i s o n w i l h ( \ r r o ( ' d c r n p f o l i l c ( F i g 4 ) , b u r ' l r l l i r r ( 1 9 6 5 ) r e p o r ( e dl i g h r
nltenuatroncocllificnts litr g.ccD light, the n)ost pcnchlli g sl,cctrrl conponent. Thc
'
\ L r h r cesi r c db y T r l l i n g r . n g c l . o n r0 . 3 5l o 0 . 7 2m ' , i n c l u d i n S
0 . 5 0n r d u r i n gM n r c ho l
1 9 6 1 . O u r c s t i n . r l el b r l i g h r a t t e n u a t i o(nF i g . 2 ) d u r n r gt h e s a r n eo D n t h o f 1 9 9 5w a s
0.16 m r. As wc nolld above,rhe linear nodcl of Fig. I ean hc rcli)rmulrtcd to predict
.hlorophyll liom lrghr aucnurtion. Sucha fefonnulrted nrodclprediclsth.rtchlorophyll
l . ! e l s i n M a r e h l 9 6 l \ , , e r er b o u r6 . : 1m g m r . o r n e a r l yt h r e ct i r n e sh i g h c rt h a ni n M a r c h
1995. Ii it is truc rhat historical light rttcnL[tion in Lrke Alberi wrs controllcd b)
\ r r i . t r i o ni n a l r . l l l b u n d l n c e ,t h i sc o m p a r i s o inn d i c a r c tsh u rc h l o r o p h y llle \ e l s i n L * c
\lben halc dcclincd
t6l
TAALE I
iT
,-l)
:L ffi[
T[
;-11
:u_
3tl
rif!'.,/.
T.hpedrlE, oxysc. (ng Lr). chtoophyll(mgnr),6d ch.micdt
pmFnis: b]. Albcrl, Mdch 1995. Ch.micdl unirsd !s itr Toble L
4.1.3. LakeEtuatd
Lake Edwardalso diffe$ from historicalrecord(Fig. 5, Table 2). Talling (1965)
reponcdlight attcnuation
of 1.3mr ftom a 5.5 m no(hcaststariondearrhe Kazinga
Channel. Our empirical relatioNhip betwccn auenuationand chlorophyll (Fig. 3)
implies that the valu€ corespondswirh ca. 29 mg rrr Chl, similar to thc 22.7 rng r[]
we foundat a 2-5 m stationirnmediar€ly
offshorcof lhe Channelmouth(at l.ll ml
light attenuation).Offshorestationsat 18 and25 m had Chl of 8.8 and 6.5 mg m'r,
conespondingto measuredlight attenuationcoefficientsof 0.63 and 0.58 m'r,
respcctively. PresentP concentrationsin Lak€ Edwsrdare about one half of historical
reportsand modemsulfateconccntratiorEarc 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, wheremodemsulfate is lower lhan
previoudy reported. Heclq and Bugenyisp€culatethat someof the differencemay be
amlytical,owingto modernmethods
(ioncfuomatography).
162
(
Ks0s)
pH
K
Mg
CI
sOa
An(t&)
sio,
SRP
DP
TP
NO:
lvln
TAILE
25 lil{
|(il16l
PH
K
Mg
Ca
cl
so
siq
SRP
DP
T?
Nor
TABLE L Cn.rical conpositionof tile Alben surfrcequter. Smples for 29
Msh 1995m rom offshoEslationALI (40 m d.pth). Valu€s@ !M unlcs
l9:l'
Ks(!s)
78.9.2
K
M8
cl
850.1160
SOi
Alk {FEq)
8070-8150
sio:
SRP
DP
TP
Nor
l95ll
1960.611
825
9.t2
4224
1690
l3t0
230
801,8?0
8.9.9.0
1960
1670
1300-t340
225-210
900
260
?800
57
4.2
910.1080
280.4?0
7250.?10
0?.181
1.9.55
0.6
0.3.2.4
0.8.1.6
t995
660
r9694
110
1333
ll50
260
110
290
6t90
1070
l0l0
2t6
Jt
195
J500
1.88
2.11
2.54
Mn
'FmmToltenhd.
(1930)
cn d bywo hrngron
rFrcmElskens,.ncd
byTdling(1961)
'Fom Talling(1961).
n=5 smplingdaks.ed fonTrllins sd Trlling(1965)
TABLE2. Comrosnion
oI Ljlc Edwo.d
sdoccwolq. s{npks ror
25 Mdch 1995drcfrcn m offshore
station0l 0'12.596 S.29'
46.?99E(25n dcprh)vulu.sandm.ftods
N in Tublel.
1969'
pH
K
Mg
cl
sor
r0J1
339
48?0
2010
t850
242
l0l I
9l
4?80
2lt0
1990
I0
760
l0t0
165
120
10100 9850
lt
108
l0
06
t026
508,580
91
798.2
24SO 2115.1262
2t20
818-94r
811-962
280
130-:140
12-35
410,4?4
150..100
9900 4070.47'10
110.1?0
DP
Nor
|6
4.1
1.1
4,2
<02
'!om Elstcns,
ci|.d by TallinS( I963)
'Fmm Talling
ed Tallir8 ( I965)
163
1995
880
8.9
l?50
1620
1610
3r0
44
580
250
1410
Il0
0.6
0.3
0.06
Lake Albert, despirepri,
Dumont1984).
'"[i""[f
'i
|ll
,il1
il
t|t\ t/ il.
I
ll
l1
!tl I
_[ L]*L-I tl tt
tI
t7
ti tl I
ll ll t\
V
LL-
L_t
tl_
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.
by Cre€n(1967,
monachdrtc Fese in Lakc Alb€n, consistentwith observalions
l97l), Thc sp€cieswasrarc in Lake Edward,andsmallerDaphnialon|ispina occnrTed
there instead. ThenDdiaplomus galebi is very rarc in both lakes; only immature
specimenswcrc found in Lak. Edwrd despi@cxhaustivescarching. Wc assumethat
the immatureindividualsare L galrbi b€.ausepreviousauthoritiesidcnlified the laxon
fron the lakc (Dussanl9E9). We found ovigerousI. farebt f€mal€sand adult malcsin
Lake Albert
r,lesrsts€(5 trt 1710r{
Ofidto.e (aO m) 6860 d!
\
Fl8r.6. (obov.) R.Lrn
w.td colum bionB b
tua in L.Ic Alb.n Ss
in Fig. L
fisur.7. (tisht) ^'Fts 6
kke Albe( despiteprior lit€mtue ass€donsthat caluoids w€re absent(Verhcyeand
Dumotrt1984).
Lake Edward
Leke Albe(
r.le{shoE (6 m) 1710 m! Dw m-2
NeardBo EDl (4 trD 1220 trE Cr,Vn-2
olEs€ (,|o m)58€OmcDd 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) Rclotivccontriboon 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
llaoc!.lopt
Th.tu
'6-.
tlBr
!.lo\
zoool{tton 13& in t lq Albcn sd Edw.d, Mach 1995
a.quabnalk a.quahnois
n.ELctu
M*octclop! a.quatoridlh tinilk
M. soct clopt d. quak nait hybtid
Th.mqclopt obldSotB
Th. N.t cIopt. ont ini li t
Th.nnoniaptffi\t gdhbi
Th.tudiottow
Doplsio lMho|zj Mcha
Dd?tuialu,rtola) wha
DiaphuosoM noryoliaaun
Diopht olotu ^o^BoII6u6
Cciodaph a comutl ige.li
165
(sat bi)
4,3, PHOSPHATE
I
Size distsibutionsof the dominantcyclopoid copepodassemblages
in the two lakes
are similar, but amlysesot ChaoborusaadDaphniapopulationsrcvealthat animalsare
larser in Lake Alben thanin l,ake Edward(Fig. 8). Moreover,Candina waspresenritr
offshoreplankon colections ftom l,ake Albert, but the skimp was not encounteredin
Lake Edwardcolections. Rotifersmadea tdvial contributionto zooplankon biomass.
They we.e virtually absentfrom the planktonof both rift lakes,except for occasional
specimensof Ke.atella tropica in Lake Edward.
Water column inventoriesof phytoplanktonand zooplanktonbiomassarc plott€d in
Figure 8 for both shallownearshoreanddeeperoffshorcstationsof both lales. Algal C
wasestinated ftom chlorophyll by calculatingC:Chl = 100:l by mass. ZooplanktonC
was calculaled as 50% of dry mass. Phytoplanktonbiomassexceedszooplankon
biomassin both basins,by a greaterratio in Lake Edwardthanin Lake Alben.
Uptake tinetics for !
for Lake Vicloria. M
ambientphosphatear
uptakefunctions(VJ
TABLE 5. PhosplBb q
EDI dd NPG (Napokq
e fron t*hlfu ard B.
I*e
si€
O
ng
Edqdd
Ed*ard
Vicio.i!
victori
EDI
ED3
NIC
BC
I
5. Discussion
f01
Fi8u.
Darho
8. Si& ii.lu.ncy distdbutiotu ol cyclopoid copcpodids. chdoboB
trcm otrshoc sr.tioN of L!k. Alb.rl andktc Pdward.
4m 1Bm 25m 5m 40rn
Albert
Edward
FiSuz 9. Totd bions ofphytopletto! sd eopl&tto!
at n6hoe
aod ollshoe stltions in Lka Albcrr md
166
twn , 6d
Zooplanlron in t akc I
aft dominatedover*t
Mesocrclops. BotIt l2
two Diaphanosoma.
Korovchinsla,perso
Victoria,whereonly c
Iak€sa.reotherwisesir
diaptomidsarepres.nr
typicalsituationin lar
Despite superficia
Lake Edward comp&
intensein Edward.Th
to phytoplankron
bior
perch, in Lake Albcn
occurin thatlake. fu
int€nsityof pr€darion
h
Zooplankron
biorr
exceed$e seasona!
(1996),bur rhe vatt'.
Victoria. ID atl rhrE
approxtmately2 ro 6
amongthe lakes,ho\&,
in Victoria. As a cont
ranges,comPubd voft
Rlward than in rhe orb
13. PHOSPHATE
UPTAKE
Uptakckinelicsfttr phosphdtc.rereponedin Table5, logetherwjth conrparison
data
lor LakeVicLoria.Maximumuptakeratesscaledto chlorophyllareinverselyrclaledto
ambienrphosphate
.nd appearro be a usclLrlcompararive
index. lnirial slop|sof rhc
(vn,/kj)varyin\erseiywrlh lakephosphate
uptlkefunctrcns
aswcll.
TAILE 5 Phosphar.
upl*e pannrrcb rar Llkc Eiqdd rn(l ncanhoreb\c vicrunr. Mrnh 1995
E D I a n l N P C( N r ! . l c . n C u l i a k n c r E h o r\ui r e \ E O l e d B C ( t l u g r i r ) r E o 8 h o r c \ i k s l ) d i f t t r B C
E f r o n L h m f f a t r d l k r s t r a r o r ( l 9 9 l r D r h n n N P Gr e l r o n I r h n r n e rr l ( 1 9 9 6 )
Lllle
Sile
Edsrnl
EdSJTI
EDI
ED]
Chld
r!n'
7:
69
l8I
V,
v,,,
sRl'
uru nnDlPl.hL nmo[,{u!chl)rhi
OIO
11117)
l9 (:.1)
05',7
13 17)
16 (l o)
0o1
t 3 0( : l )
96 (l:)
2 0 (0.J)
ll.1tdlssr
661i85S)
J . 1 7( 1 6 l l )
n00 (600)
00ll
o 017
0.101
0 019
Z o o p h o k t o ni n I - i k c A l b c ( a n dl - a k cl d r v r i ( l a r e s u r c r l i c i a l l yv c r ! s i m i l a . B o t h l a k c s
r r e d o n r n a r c do \ c f \ \ h c l n r i n ! l yb ) a s u r t c( n c y c l o t o n l\ I ) c c i c s i.r c l u d i n gl h c p r e d a r ( t r
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 . ! c h r n s t ) . t c r n n n l c o n n n u n i c l r i o n )r .n d i s d i l l e r ! n t l r o n t h c s l r u r t i { ) ni n L a r c
Y r c t ( ) r r as.h e r c o n l y o n c s p c c i c \i s p r c s . r l ( l - c h t n . t r1 9 9 6 ) I h c c o n n n u n i r i e. si l h c s r
l x k c sr r c o r h c r w i s cs i n r p l c r h r n L . r k cV i c r l ) r i Lw
r ,h c r cf i \ c c y c l o t o i ds p c c i c sr n d t h r c c
{ l i r p r ( ! r r i ! lrsr c f r c \ e n t o i l l s l x r r(el l r a n s t r r ( , rc l r l . 1 9 9 6 ) .I t o l i l e r s . r r cf r r c , w h i c hi s r h c
t y p i c r l s i l u r t i o ni r l i r i : c A f r i c r n l x l c \ d o l l r i n a r . db r c v c l o p o i ds f c c i c s( L c h n i r n 1 9 9 6 )
D c s p i r cs u p e r l r c i r sl n r i h r i r i c s ,r b e s n H l l c r s i l c o l n o n c t c l o p ( r d z o o p h n k r o rr n
L r k e U ( l w r r d c o n r p n r e dw i t h L a k c A l b c r l s u g g c ( s l h r t v i s t x l f l a n k t i v o r y i s [ r o f c
i n t e n s ci r [ d w r f t l . T h r r . o l i o I i s r c i n l i ) r c e d
h y l h c l o $ c r r r t r c o l l o o p h n k l r ) nh x r n r r \
r r i p h \ k , t l l n k t o n h r o r r r u si sn L r k c E d \ v x i ( l P r e s e n c(cn r n c f l c c r i \ c p i s r i ! o r c . \ i l ! fishtl l
t . r c h , i n L r k c A l b c n m d y c r L r s fr c d u c l i o l si n t h c n u l r b c r so l f h D l l i l o u s
, ) c c u ri n r h r l h k c . A s c r r l y . N l h c 1 9 6 0 sG
. r c c r )( 1 9 6 7 )i r g u e d r h r l l h c r cw N f c d u c c d
i n l e n s i l y , )fl r e d a t i o nh y l i s h ( r / o o p h n k t r Di n r n i d l . k c r e g i o n so l L r k c A l h c r t .
Zool)lonkron
h i o n r ! \ \ a t o l l s h o i c\ t a r i o n \ i n b o r h L r k c A l b c r r r n d L a l c l r l w l r d
c x c c c d l h c s c a s o n r ln r e a n\ r l u c s r e p o r r c ( 1
c lr u l
l ( , rL r k c V i c l o r i . rh y B r a D s t r L u o
(1996), bur the v.rlucs are wirlrin tlre rlnge of rnnurl varialion observcd lir Lake
V i c t o r i a . I n x l l r h r e c o l r h c h k e s . r o o t h n k t o n h i o n r i l s sl ; i l l s r n r r l n s e I i o n r
, r t p r o x i n n r c l )2 r o 6 r D W n r . W a r c r c o l u m nd c p r h st h a t \ ! c r c \ a n r p l e dr i i l l t r c d
rrnong thc hkcs, horwlcr, and nngcd lionr 25 m in Edwrrd o 40 In rn Albcrr, ro 50 rr
r n V i c t ( n i r . A s . r c o n s c q u c n coel r v e r r ! i n g t l ) er r c . r la h u n d a n c cosl c r d i f l t r c | r d c p t h
r r n g e s .c o m p u r e d! o l u m e r i c . r b u n d a n c cosl z o o p l a n k r o na r e s o D r c $ h a h
t i i r h e ri n
E d $ a r dt l n n i n d r e o r h c .l l v o l a f t c s .W e d i . l n o t i n v c s l i g r t e$ c \ c r l i c a l d i s r r b L r l j o on l
167
are only approximated
the planktonin the lakes,and thus actualin situ concentsations
by watercotunn averages.
Previous literahrre rcpotu t\at Diaptonus is not present in Lake Albert are
enoneous. It is unlikely that our discoveryreFesentsa new colonizationevent. The
animals are extsemelyrare, on the order of one individual per cubic meter. We
confimed their presenceonly by diligent andsustainedsearchof large samplevolurnes.
An immature diaptomid copepod had already been discovered in a Lake Albert
coll€dion during 1993 with an identical net (V. Kiggundu and J. l,ehman, personal
obseNation),but its speciesidentity could not be esrablishedat that time. The fact lhat
calanoidsarc so scarcein theselakes, comparedwith t ake Victoria, points again to
higher l€velsof visual pla*tivory in the rift lakes.
Lake Albert seemsto be morc Eansparcntnow than30 yearsago,evidenlly because
of rcduced algal chlorophyll. Phosphatecorcentrationsare also lower, but that is
probably not the cause of increas€dEansparency. Talling had repo(ed extreme
phosphateconcentrations(Table 1), and stoichiometricconsiderationsdictate that P
could not be a limiting factor in biomassproduction, then or now The elevaled
t€mperatur€of the lake wateris intriguing, becauseit is reminiscentof similar warming
of Lake Victoria (Hecky 1993;Lehman1996). Whetheror how alteredmixing in
responseto the thermalregimemay haveaffect€dlake production,nutdent suppliesto
theepilimnion,andalgalbiomass
cannotbeanswered
by our limitedtemporaldata.
We found lower values of Total P in Lake Edward than previous reports had
indicated. We cannotyet determineif the reductiofiis paralleledby chlorophyll
becausehistorical dataa.e not readily availablefor comparison.All tkee lakesexhibit
mod€rnvaluesof sulfatelower thanhistoricalreports. This apparcntreductionis a
regional featurein colunon wilh Lake Victoria. Il is nol yet clear how much of this
differenceis rcal, a consequence
of lessened
inputsor possiblyincreasedchemical
reductionin the sediments,
or how muchis the resultof improvemenls
in analytical
methodology.
Kineticcharacteristics
is not
of the algaein Lak€ Edwad suggestthat phosphat€
gro'r,th liniting, Inverse vadation of both Vn and Vm& with SRP in thesetropical
lakesis consistcntwith cxpectationsftom tempera(elakcs. valucs for Lake Bdwardale
intennediatebetweeninshoreand offshoreLake Victoria, wherebioassayexperimenh
(L€hman and Branstrator 1993, 1994) demonstated that P is not the proximate
production.
limitationfor biomass
5.I, CON'DITIONOF LAKE CEORCE
Lake George,as well as much of easleroLakc Edward, lies within Qu€enElizabeth
(Ruwenzori) National Park. Restorationof polilical stability to Uganda in the late
1980safter a deaadeof strife hasstimulatedconservationefforts for wildlife within the
NationalPa*. In addition,coppermining hasrecentlyresumedalong lhe Nyanwamba
River, which flows into Lake George. The Kilembe mines first began operation in
were
1956,and sho(ly beforcthey closedin 1979 elevatedcopperconcentmtions
measuredalong a gradient extendingfron Lake George,along lhe Kazinga Channel
168
which link ceorge to E
the Semliki River (Buga
The IBP investigari
establishedthat nitrogen
communityrespiration.
and Viner 1973). Nitr
fixation by diazorrophic
the Ruwenzoris (Viner
represenlativeof headw
The hish pH of I_
ph,.toplanlrcn (Canf aj
pllorosynthesiselevales
variation ft om approxin!
I L2 has b€enreponed6
pH l0 (Dennyet at. lq
chemicalweaftering of !
high€r silica concenr-a
amorphoussilica by diar.
b€comepanly inhibilcd
Copperis knownto inhit
TABLE ]
30! ll t7 |
K! (!s)
PH
Na
K
M8
cl
sor
siol
SRP
DP
TP
Nor
Mr
$hlch links G€orgero Edlvard,rnd then acrossnorthernLakc Edwnr{tlo (s outflow as
r h eS e n i i k i R i l e r ( B u g e n ! i1 9 7 9 )
T h e I B P i n v e s r i g a r i o nosf r h c 1 9 6 0 sf ' r e c c d e de v i d c n c eo i m e r n lp o l l u r i o n . t h e y
. s t a b l i s h e rdh r t n i l r o g e nw , r sa l i i r i n g r c s o u r c .i o r h e a l g a e o f t h el . r k e .a n d r h a tr o t i l
connnunity .espirarionafproximrlely bxlancedphorosynlhesison I 21_h b,rsis(Crnf
a n d V i n e r 1 9 7 3 ) . N i r r o g c ni n . o m e w r s d o n i n r t e d h y i t v j r l i n f u t s a n d n i t n ) g e n
tr\u|on b) diazorrophicclanobr.rerir. Hydrologic inpul i5 domiD ed b] flow from
r h e R u w e n z o r i s( V i s e r n d S n r i r h 1 9 7 3 1 ,l n d t h c c h e n r i s t r yo t t h e s r r e a n l si s
r e p r e \ e n t a i i vocf h c . r d w a l e rdsr a i n i n gt r . d o m i n r n rs i t i c . r er o c k s( K i t h r m 1 g i l { ) .
The hiSh pH of L.rkc ce('ge is lhe .csutr ot photosynttrcsisby suspcndcd
f h r t o p l a n l t o n ( c ! f l i a n d H o n r c l 9 ? 5 ) w i r h d r a $ t o f ( . O r | r ( ) m r h c $ , . u c rh \
\ ^ r ' a I n h u : r . e l . \ r t e , r l , c p H D r r i , t l h e l , , o U \ .r t . c t d \ r ( r h r l - r l . d J r L I n r t
t.H
"
y i r i r t i o n f r o n a p p r o x i n l c l y 8 . 7 5t o 9 . 9 8( c a o t 1 9 7 2 ) .
M o r c r c c e n r t yp. H . r \ h i l h a s
L l . l h r s b c c nr e p o r t e df r o m n e a r s h o rsea i e r s ,$ i r h o t c n l a k c v a t u c sc o n s i s r c n r n
t )e r r
t l i l 0 ( D c n n y e r r l . 1 9 9 5 ) B y c t c v . r i S r h c f H . t h o l o s l , n t h e s ci si r n a c c e l c r ! ( en c r
r h c n r i c a$
l e l l h e r i n go f s i l i c ak o m s e d i n r e n r sn. d l h c r c a p p c n r sr ( j b c a l r e n dr o s i l l d
h r l : h c rs i L c r c o n c 0 n r r a r i ( nirns r c c e n ty c a r s ( T a b t c 3 ) B i ( , g e n i . p r c c i t i r r l i d r o f
. r D o r p h o Lsr isl i c . b v d i a r o n s .l p r ) c e s st h r t t v t i c u r r c d u c e s t i c a t c \ c t si n t r k c s . r r \
F . . ( l D c t r n l ) i n h r h i r e db y r o x i c c i f c c t so f r h c p o L I i n g n r c r r l s .c l n c c i r l i y c o t f c r
( o t ) t ) cirs k n o w nk ) n r h i b i ls i l i c . u p t l k cI r d s i t r e i t i c a r i o n
h y d i r b m \ ( R c u r c rt ( ] 8 t )
TAllLli :l
( lrcnnfrl .otrUr^(ur
S r , i t L c ! l d : . 1 N i r ( [ l { ) 9 5{ ! (
:l{) ll 17 E(l!rdcUh)
.J ttrtc (;.d!c
rlrh.L
l n ] c f t d n n i L l t l i cr r 0
$, cl
0tl)N.
Vtu!\ rtu }rlv u!tc$ iDdrcrrr(l
l l.1t62
tlJ
!ll
cl
Allr|[q)
1781)t290
t7:11)
ll
'TrlLrnB
& d ' l i l l i n 8i l e 6 5 )
r V r n c (r 1 9 7 5 ) r e p o n s
N o r a sn 0 rd . r . d l l h t e
169
Copp€r also inhibits the nitsogen-fixationFocesses of diazotrophic, heterocystformins cyanobacteria(Elder and Horne 1975), includins 6e Iara Anabaena and
Aphanizonenon,.,r,:.ichforme.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
cyanobacteriacan fix niEogenin the dark whenthereis no Foduction of nitrogenasedisablingoxygenby photosynthesis,
or in the light either by spatial separationbetween
aerobicand anerobicportions of coloniesor by temporalseparationof photosynthesis
and nitrogen fixation in different phasesof the cell cycle (Carpenterand Price 1976;
StalandKrumbein1985;Mitsuiet al. 1986). SmaI pennatediatomsarepresent,but
not asbiomassdominants.Centricdiatoms,esp€.iallyAulacosein (formerly Melosna)
species,were rare. A similar situation was reported for Lake Edward during l9?2
(Hecky and Kling 1987); Ctuoococcalesdominatedthe phytoplanktonand diatoms
were significandy abundant oDly at a southem site, remote ftom the metal
contaminationlater identifiedin the north.
Nilrogen fixalion formeJly provided nearly half of the niEogen income for Lake
George(Home and Vin€r l97l). Unlike regenerated
nudents,such"new" nutrient
permib net biological production and expon of biogenic rhaterial from th€ water
column1othe sediments.The prospecr
ihatmetalpollutionmay increase
with mining
developmentsuggeststhe needfor additionalanalysesof the productiveecosystem.
6, Conclusiotrs
Themosrconspicuous
changes
10lakesAlber(andEdwsd du ng theprevious30 years
appearto have been (l) reductionsin algal biomass,measuredas pafliculate
chlorophyll,and (2) reducedwatercolumninventories
of P and inorganicN. These
changes
areconsiltentwith increased
temperature
of LakeAlbert,6ndsugg€sr
reduced
mixing of the surfacelayer,possiblyunderinfluenceof r€duc€dwind sress, Oiven
sufJicient
silicato supportfast-sinking
physicalstabilitycanleadto
diatoms,increased
accelerated
s€dimentation
of particulate
matter,includingChl, P, andN, from sudace
layerc. This tendencyis opposedoDly whenthereis a floristic changeto more buoyant
taxa, as in modemLake Victoria. We regardclimafic variationto be a master
controlling variable for lakes of East Allica, and its signal may be registeredthrough
changes
in prirharyproduction
andalgalbiomass
aswell asnutrientconditions.
7. Acknowledgmentj
We thanktbegovernment
of Ugandafor its counesyandp€rmission
to investigate
these
lakes. We are gratetul for assistanceof M. Mugumba,V. Kiggundu, and staff of the
Ecology Instiote at Mwey4 Uganda and lhe FisheriesResearchInstitute (FIRI) at
Jinja, Uganda. Our Feliminary id€ntificatiols of Diaphan^onu species were
conirmed by Prol N. M. Korovchinsky,and our idenrifrc nons ot Ce odaphnia were
contuned by Dr. D- Bemer. A. J. Home and C. D. Sandgrenprcvided comnents
170
aDoutnutrientdynami
by the U. S. NarionajC
8. R€fercncer
Badle, L C. 1966. p,otd
Ugdd4 conp@d virh L
BmI6tor, D. l<., NdaE\
33?-155./,1 C. ,obM
th. E€$AfriolaLs
CBugrni, F. W, B, 1979. C
Hy{'rohiologia64:9t5
Burgis,M. ,,, L C. Dln C
Sodid on ! ftpi.d fB!
Podoctivity pr.blcc d F
Clrpcnie!, E. J, &d C. C p
nnrog.n fiiarjon widbu h
D.my, P,, R, Bajtcy,E. Tut
(Uedda) !d ib q.dan&
Du$an, B, t989, CNrr
Eldcr,,. F,, A, r. sd Hom
Enviromnhl Mtu8ctE
O&f, O. G, l9?2. Th. ESd
708. /, K. K!j* ed ^
sci.niilic Prbticdrions.
Cei O. C, l9?4. tnci<tctr
chlorophylt
d coDrc of. r
609.
C4nr, c. O. l9?4b. phrogt
Ugd.bl Oeotog,a !6: e,:
Oanl G, C., ud A. J. Hd
shallow,.qurorisl bI. (Ll
Canl O. O,, and A. A v,G
Ugod!). Pbeedings of ttE
c@n. J 196?, Tnc dntriblo
fish pEdaLjonin blc Atbd!
Gtan, L l97l. Ass@iaritud
ofZoology165:3?1,4r!
Heky, R, E. t991. n.tuh
nir Linuologie 25: j9-38
H€ty, R. E., dd H. J. Kliq
I
Atuca AJchiv fii. Hydrcbiol
l|. ly, R. E. ud F, W A. suF
qoaliy issu.s: p,obtctu .,n i
trboutnuiricnt dyoamicsth improved rhe paper. This rcsearchwas supfo.ted in part
l c i c n c eF . u n d a t i o n .
b \ I h e U S . N a l i o n a l G e o s r l p h iSc o c i e t )i n d r h cU . S . N a r i o n aS
8. References
3.3dlc.L c 1966. ProlongcLj
rrurililarionind ,i.,)tJ_genr(ion
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L l' n n o l o g r) n { lo . c s o - q n p h \1 l l 5 l . l 6 l
Lgoda.compred\ridLnic\tlun)oniandEd\rn
'l'
B.trnntu$. D K, Nd3*ulr. L lil andkhmrn. J
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of
:17155 ,r T. c Johnn rDd E o odadalcds.l,The limnoloSyclinrlolosy Md prl.ocl'huloloEjrheEi( Arnranhlies Cod)n id Belch Pull'drr\
ii!:.nyl. I: w B. 1919. copprr ron disribu(i,rnIn rhe surfrcesir.a rf Lr\e5 Ceorfc xnd Id1Ah'n
H \ d r r h o l o 8 r631 9 l 5
Btrrlr. irl i.L C Dunn.G C CaDl.L M Ilc(nitrM, andr\ I Vrr.r 1971 tnlc Ccor!! Urandr
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