THE LIMNOLOGY OF

LAKE TITICACA (PERU-BOLIVIA),

A Large, H i g h A1 t itude Tropical Lake

Peter J. Richerson

Division of hv.1:ronmentaL Studies ard

Insti tute of EcoZogy Universi* of CaLi,com?ia, W i s

Carl Widmer

ELbert Cove12 College U n i u e r ~ i t y of the PackjYc Stock ton, CaZ-ifimCc

Timothy Ki ttel

Dfviston of ,%~iror twnta t Studies and

Ee"coZogy Graduate Group University of California, Davis

I n s t i t u t e of Ecology Pub? ication #14, June, 1979

- - Dedicamos esta monograffa a1 pueblo del A l t i p l ano , con una g r a t i tud profunda y sincera

- par su ayuda y p a c i m c i a para con nosotros, y a nuestros colegas del Instituto del M a r del

* Perb y de l a Unjversidad Nacional Tgcnica del A1 t i p l ano , s i n cuya colaborac i6n cont inua no

hubiera s ido pos i b l e es te t rabajo.

Estamos concientes de que estas personas viven, estud ian y enfrentan su realidad para

transformarla y enr-iquecerla acorde con e l proceso peruano nacional ac tua l . Querems ofre-

cer e s t a monograffa corn nuest ra p rop ia contribuci611, aun limitada, de p a r t i c i p a c i d n en

ese proceso.

The authors are g r a t e f u l f o r t he f i nanc i a l support prov ided by the National Geographic

Society, the Faresta I n s t i t u t e f o r Ocean and Mountain Studies, the Un i ve r s i t y o f Cal i fo rn ia

( through a Rockefel 1 er Foundation i n s t i t u t i o n a l development grant ) , the Jastro-Shiel ds

Scholarship Comi t t e e and t he U n i v e r s i t y sf the RacifSc. Widmer was supported by an Organ-

i z a t i o n o f American S ta tes fe l lowship and Richerson was p a r t l y supported by Nat ional Sci-

-\ ence Foundation grants GA-34099 and EMS 75-14273 during this study. We are a1 so indebted

- t o the I n s t i t u t o de l Mar del PerG, the Universidad Nacional Tecnica de l A1 t ip lano , the Ser- -

v f c i o Nacional de Meteorelogfa e Midro l ogfa, and t o i n d i v i d u a l s i n c l ud i ng V i c t o r i a Val cgr-

cel , Roger Srni t h , F l orenti no T i t o , Gerald F i sher , John Me1 ack , dswal do Zea , Linda Thorpe ,

T i m Long, Mayne Wurtsbaugh, Kenso Kawahira, Leigh Speichinger, Fathers Pa t r i ck , Eugene and

John, and S i s t e r Margaret. Antonio Landa C., i n charge o f Peruvian s tud ies on the lake,

was p a r t i c u l a r 1 y he1 pful i n many ways. H i s f i e l d personnel , Ale jandre A r d i l e s and Edgar

Farfan, prov ided impor tant l o g i s t i c a l servSces, and Sr. Farfan has done us several i noor -

t a n t favors s ince our leav ing the lake. Charles R. Goldman gave us va luable advice, c r i t i -

c ism, and the equipment t o count 14-C. Eleodoro Aquire was very helpful i n sending us

weather da ta . Vern Sco t t and the Independent Study Committee were inst rumenta l i n a l l ow ing

Ki t t e l t o spend an extended per iod i n Perd. Leonard Myrup and Tom Powel 1 gave us valuable

advfce regard ing t h e heat budget, and we thank them and Dan Godden f o r t he unptrblished Lake

Tahoe heat budget f igure. Matilde Lopez and A7 Mahood helped very s u b s t a n t i a l l y w i t h a lga l

species i d e n t i f i c a t i o n s . Ben Orlove furnished unpublished ca l cu l a t i ons o f f i s h harvest.

Uy-lhanh Ly d r a f t e d rn a n y of the f i g u r e s and Dolores Durnont, Wanda Greene, Lyn Schonewise,

and C l i f f Shockney typed t h e d r a f t s . The e d i t o r i a l e f f o r t s o f Geoffrey Handesforde-Smith

are g ra te fu l 1 y acknowledged.

i i

TARLE OF CONTENTS

Resumen - A b s t r a c t

I n t r o d u c t i o n

General Descr i p t i o n o f t a k e T i t i c a c a

Physical and Chemica l Limnology

P h y s i c a l Measurements

Cherni cal Measurements

R i o l o g i c a l Limnoloqy

Phytopl ank ton Popu la t i ons

loop1 ankton Ropul a t i ons

F i s h Fauna

Primary Product ion

Product ion o f Hiqher Trophic Levels

Concluding Discuss ion

Li tera ture C i t e d

RESUMEN

2 El lago T i t i caca es un gran lago de al tura(8.100 Km ; 281 m p ro f . m a x i 3.803 msnm)

en el A l t i p l a n o andino peruano-boliviano, ubicado a 16' 1at.S. Una se r i e de 21 mediciones

de va r iab les l i rnnol6gicas bdsicas fueron hechas en l a pa r t e honda cerca del pueblo de

Capachica, Perb, en 1973. Pas estaciones fueron muestreadas a i n t e r va l os de

aproximadamente dos semanas desde enero a diciembre. Las mediciones inc luyeron p e r f i l e s

de temperatura, profundidad-Secchi, penetracidn de l u r , pardmetros quTmicos especff icos

y producci6n p r imar ia . Mues tras de fi toplancton y zooplancton fueron co l ectadas y

enurneradas. Los da tos de 3973 junto a l a informaci6n recopi lada por o t r os investrigadores

fue usada para comparar l a 1 irnnologia del lago T i t i c a c a con lagos temperados y t rop ica les

de las t i e r r a s ba jas . E l lago T i t i c a c a es semejante a 10s lagos bajos t r op i ca l es en casi

todos 10s aspectos de su 1 imnologfa a pesar de s u temperatura f r i a . Sus perfiles termales

muestran un ep i 1 irnnion ampl i o , con re la t ivamente pequezas d i ferenc ias en t re temperaturas

ep i l imnPt icas (rnax. 16%) e hipo l imngt icas ( 1 1 . l 0 ~ j y un perTodo de remocibn mds o menos

cornpleto en l a estac ion seca, J u l i o a Septiembre. El balance c a l 6 r i c o es aproximadamente

equ i l i b rado durante todas l a s estaciones, pr inc ipa lmenre por l a evaporaciBn, l a que se

a proxima a Zm a%- ' . Consecuentemente, l a e s t r a t i f i c a c i d n es d 6 b i l pero pe r s i s t en te y l a rnagnitud de l a

rnezcla en profundidad v a r i a r i a substancialrnente de afio en azo. E l h ipol i rnnion no se

mezcl6 por debajo de l o s 150m en 1973. E l lago es bastante transparente parque l a

biomasa de f i top lancton c i r c u l a en un ep i l imn ion ampl i o y l a bjomasa de crus tzceos

predadores es pemanentemente grande; 1 as profundidades-Secchi fl uct 'uan entre 4.5 a 10.5rn.

E l lago es abundante en 561 idas d i sue1 tos (780 rng I-') debido a que su perdida de agua

es pr inc ipa lmente por evaporacihn. Bajo l a r condiciones c l i rn i t i cas ac tua les , l a s

concentraciones de s61 idos d i sue1 tos e s t h amen tando. Los cambios c l i m 5 t i c o s probable-

men t e a fec ta r ian t a l e s concentraciones substanci almen te . Las pobl a c i ones de f i toplancton

inc luyen pr ine ipa lmente algas uerdes y azules-verdes aunque l a diatamea Stephanodiscus

astraea es l a doininante durante l a estac i6n seca. La biomasa pramedio er 3.0 c j ~ m n * ~ . La

d ive rs idad ( H I ) es a1 rededor de 3 b i t s - i n d i v . -' , pero baja durante 10s pe rhdos de

"f lorecim-iento" (bloom). Porque e l 1 ago esta' regul armente mezclado b a j o de l a profundidad

de compensaci6n, hay poca d i f e r e n c i a c i h v e r t i c a l de l a s poblaciones. Algunas diferencias

son aparentes en t r e l a f lcra de Is73 y l a que ha sido desc r i t a p a r otros invest igadores.

Tasas de suces i6n es tac iona l son b a j a s . La fauna zooplanctUnica estd dominada po r el

cop6podo calanoide Boeckella t i t i c a c a e . La biomasa prornedio del sooplancton es alrededor

de 0 . 8 9 ~ ~ - r n - ~ . Se p r o d u c ~ n variaciones estac ionales en l a biornasa y en l a proporci6n de

10s d i s t i n t o s estados de 10s c i c l o s de vida ( l i f e h i s t o r y s t a q e s ) , pe r0 l a sucesibn

es tac iona l de las poblacinnes e s t i v i r t va lmen te ausente. La f auna de peces es pobre

en t a x a w p e r i o r e s y es tg dominada por un cardurnen end6mico del Cyprinodon genera

Orestias. Par l o menos T9 especies de e s t e g4nero es tan presentes. La riqueza aparente

de ~species es b a j a para un l ago t r o p i c a l de su t a m a i o .

E l ?ago T i t i c a c a cs roderadamente e u t r 6 f i c a son un proniedio d i a r i o de product iv idad

-2 -1 p r imar ia neta de 1 .45 gC-rn .dia . La va r fac i6n de l a produccidn primaria estacionaT

-2 . -1 f7uctGa en t r e aproximadamente 0.75-2.75 gCmm * d l a , y es d i f i c i l de exp l i ca r . La

produccihn n e t a ests corre lac ionada con l a produccibn p o r unidad de biomasa a1 nive7 de

saturac i6n de luz, t a l que e s t z implicado el estado de nu t r i en tes . La producci6n de 1 0 s

n i veTes tr6f icos superiores puede ser gruesarnente estirnada . E l rendimiento i c t i o l 6 g i c o r n i x i m o sos ten ib le puede ser del orden de l a $ 50.000

toneladas rn6tricas por ano.

Los contrastes es t r uc tu ra l es y funcionales ent re lagas temperados y trapicales

estan relacionados can 10s d i ferentes patranes de v a r i a c i e n ambiental. La es tac i ona l i -

dad e s t i suprimida en 1 0 s lagos t rop ica les , po r c o n s i g u i e n t e l a produccidn a n u a l y l a

biomasa no son detectab les. A una escala de tiernpo mayor, sin mbarqo, 10s lagos t r o p i -

ca les son prabablemente tanto o m5s var iab les que 10s ternperados. La e s t r a t f f i c a c i 6 n

termal d6bi 1 pero pe rs i s t en t e es l a que parece causar l a s diferencias de a50 en a60 en

las condiciones quirnicas de 10s nutrientes. Las a l t a s t a s a s de evaporaci6n hacen e l

agua y el balance de s6l idas disuel tos sensi bles a 1 as f l u c t u a c j o n e s de l cl ima. So-

larnente l a fauna i c t i o l 6 g i c a de los l agos tropicales responde a estos moldes con un

increment0 en l a diversidad y entre Tos peces como Pas de l lago T i t i c a c a l a s excepciones

ABSTRACT

Lake T i t i c a c a i s a l a r g e (8100 h2, 281 rn max. depth), h igh a l t i t u d e (3803 m a s l . )

l ake i n t he b l t i p l a n o o f the Peruvian and Bo l i v i an Andes a t 16's l a t i t u d e . A ser ies o f

21 measurements o f basic I imnologScal v a r i a b l e s were made i n deep water near the v i l l a g e

o f Capachica, PerG, i n 1973. The s t a t i ons were sampled a t approximately hi-weekly i n -

t e r v a l s from January t o December. Measurements inc luded temperature prof: 1 es, h c c h i

depth, l i g h t penetrat ion, se lected chemical parameters, and pr imary product ion. Phyto-

plankton and zaoplankton samples were co l l e c t ed and enumerated. The da ta from 1973,

together w i t h i nfarrrtati on co l Tected by o ther invest igators , is used t o compare the

Iirnnology of Lake T i t i c a c a t o temperate and lowland t r o p i c a l l akes .

Lake T i t i c a c a resembles lowland t r o p i c a l l akes in almost a l l aspects o f i t s l imnology,

despi te cool temperatures. Its thermal p r o f i l e s show a t h i ck ep i l imnion, r e ' l a t i ve l y l i t t l e

d i f f e r e n c e between ep i l imne t i c (rnax. 1 6 * ~ ] and hypol imnet ic ( ~ 1 . 1 ~ ~ ) temperatures and a

per iod o f more o r less complete overturn i n the d ry season, Ju l y t o September. The heat

budget i s near1 y balanced dur ing a1 3 seasons, c h i e f l y by evaporat ion which approaches

2 rneyr-l. Consequently, s t r a t i f i c a t i o n i s weak but pe r s i s t en t and the ex ten t of deep

n i x i n g probably va r ies s u b s t a n t i a l l y from year t o year. The hypolimnion d i d no t m i x

be1 ow T 50 m i n 7 973. The l a k e i s qui te transparent because the phytoplankton biomass

c i r cu l a t es i n a t h i c k ep i l imn ion and crustacean grazer biomass i s pe renn ia l l y h igh ;

Secchi depths range from 4.5 t o 10.5 rn. The l ake i s h igh i n dissolved s ~ l i d s (780 mg - 1 - I )

because i t s l o ss o f water i s main ly by evaporation. Under present climatic condi t ions,

concentrat ions o f d isso lved so l i d s are increas ing. Changing c l imate probably a f fec ts

such concentrat ions subs tan t i a l l y .

Phytoplanktan populat ions inc lude mainly greens and b l ue-greens , a1 though the diatom

-2 Stephanodiscus astraea i s a dry-season dominant. Mean biomass i s 3.0 gC-rn . D i v e r s i t y

( H ' ) i s about 3 b i t sa ind i v . -1 but f a l l s du r ing bloom per iods. Because t he l ake is

usua l l y mixed t o below the compensation depth, t he re i s l i t t l e v e r t i c a l d i f f e r e n t i a t i o n

o f populat ions. Some d i f fe rences are apparent between the f l o r a o f 1973 and t h a t de-

scr ibed by other i nves t iga to rs . Rates o f seasonal succession a re low. The zooplankton

fauna i s dominated by the cal anoid cepepod Boeckel 1 a ti t i cacae. Zooplankton bjornass

averages about 0.89 qc*rnm2. Seasonal v a r i a t i o n i n biomass and i n the r a t i o of l i f e

h i s t o r y staues occurs, but seasonal successfon of popu la t i ons is v i r t u a l l y a b s ~ n t . The

f i s h fauna i s poor i n h i q h e r t a x a and 7 s dominated by an endemic flock of t h e Cyprinodont

senus Orestias. A t l e a s t 19 species of t h i s genus a r e present. The apparent species

richness i s low far a tropical l a k e o f i t s s ize .

l a k e T i t i c a c a i s moderately e u t r o p h i c w i t h a mean d a i l y net prirary p r o d u c t i o n o f

-2 1 .a5 g C rn day - I . The seasonal v a r i a t i o n o f p r imary p r o d u c t i o n ranges f rom about

0.75 to 2.75 g C . nm2 . day-', and the p a t t e r n i s difficult t o exp la in . Net production i s

correlated w i t h p roduc t i on p e r u n i t biomass a t l i g h t s a t u r a t i o n sa nu t r i en t s ta tus i s i m -

p l i c a t e d . Product ion o f h ighe r trephic l e v e l s can be roughly est imated. Maximum sur ta in-

abTe f i s h y i e l d s may be on the order o f 50 thousand met r i c tons per year .

The s t r u c t u r a 7 and funct ional cont ras t s between temperate and t rop ica l l a k e s are

r ~ l a t e d t o d i f f e ren t p a t t e r n s of env i ronmenta l v a r i a t i o n . Seasonality i s suppressed i n

t r o p i c a l lakes, so annual p roduc t i on and biomass v a r i a t i o n s a r e muted. On lanqer t ime

scales, however, t r o p i c a l lakes are probab ly as v a r i a b l e o r more v a r i a b l e than temperate

ones. Weak'but p e r s i s t e n t s t r a t i f i c a t i o n is l i k e l y t o cause substant ia l year t o year

d i f fe rences i n p h y s i c a l and n u t r i e n t chemical cond i t i ons . High evapora t i on r a t e s make

water and dissolved solids budgets s e n s i t i v e to f l u c t u a t i o n s i n c l i m a t e . Only t h e f i s h

fauna o f tropical 1 akes respond to these p a t t e r n s w i t h increased d i v e r s i t y , though examples

of low f i s h d i v e r s i t y , as i n T i t i c a c a , are c o m n .

INTRODUCTION

Unti l recen t l y , l imnology has been p r i n c i p a l l y a science of temperate lakes, and

work i n t r o p i c a l environments has lagged behind temperate ones i n both ex ten t and soph is t i -

cat ion. It i s impor tant t o r e c t i f y t h i s lack of informatlion because o f the i n t r i n s i c jn-

t e r e s t o f t r o p i c a l lakes and because the d i f f e r e n t condi t ions o f such hab i ta ts may prov ide

valuable cornparati ve data regard i ng broader eco log ica l questions. Th is general 1 i mnologi-

ca l desc r i p t i on o f Lake T i t i caca was undertaken t o charac te r i ze a system which i s unusual

not only because of i t s t r o p i c a l l a t i t u d e bu t a lso because o f i t s a l t i t u d e and s i z e . Lake

T i t i caca is subject t o a unique combination of s l i g h t seasonal j ty , low temperature, and

g r e a t depth and area. The paper i s presented in the s p i r i t of comparative natural h i s t o r y

Its a i m i s t o e l uc i da te the s i m i l a r i t i e s and d i f ferences behrreen Lake T i t i c a c a and o ther

temperate and t r o p i c a l lakes i n an e f f o r t t o shed l i g h t on the general problems of lacus-

t r i n e ecosystems.

Large freshwater t r o p i c a l 1 akes of lower elevat ions tend t o be character ized by several

p roper t ies which d i s t i n g u i s h them from temperate lakes of s i m j l a r area and depth. I n s t ra -

t i f i e d t r o p i c a l lakes, ep i l i rnn ia tend t o be t h i c k e r and transparency higher f o r a given

trophic s t a t e than i n temperate lakes (Ta l l i ng , 1969). Because seasonal i ty i s low, hypolim-

netic and e p i l i m n e t i c temperatures d i f f e r l i t t l e and arc usua l l y above 20°C. P r i m r y pro-

duct ion i s usua l l y high and r e l a t i v e l y aseasonal (B ry l insky & Mann, 1973). The phytoplank-

ton assemblages o f t r o p i c a l lakes are roughly comparable t o temperate ones i n instantaneous

d i v e r s i t y and few forms are r e s t r i c t e d t o t r o p i c a l environments. Seasonal succession o f

the p lank ton ic f l o r a i s more or less suppressed by the muted o r absent c l i m a t i c v a r i a b i l i t y .

The zooplankton and poss ib ly o ther inver tebrates a lso tend t o have only normal or lower

instantaneous diversi ty ((Green, 1972a; Hubendick, 1962) i n except ion t o the usual t a t f t u -

d i na l gradients of increased d i v e r s i t y i n the t rop ics . Seasonal changes are subs tan t i a l l y

reduced, g i v i n g p lank ton ic faunas a mnotonous character. F ish faunas are f requen t l y

character ized by impressive endemic species f l o cks and h igh t o t a l dli vers i ty (Lowe-McConnell ,

1975). Many components of the b i o t a are too poor l y known t o permi t genera l izat ion.

Research on t r o p i c a l lakes has accumulated i n we l l defined phases i n a l l par ts o f t h e

world. Scattered and fragmentary repor ts by t r ave l i ng n a t u r a l i s t s began t o appear by the

l a t e 1800%. Even t o t he present day, most in fo rmat ion from t r o p i c a l lakes i s o f t h i s

fom, a1 though t he v a r i e t y and soph i s t i ca t i on o f measurements has increased. For Lake

5

T i t i c a c a , t h i s phase i s represented by the work o f Agassis (1876) , Agass i t and Gaman (2876),

Neveu-Lemai r e (1 906), Schind l e r ( 1 955), and Qerkosch and LB f f l e r (1961 ) . The second phase

general I y c o n s j s t s of sho r t b u t i n t e n s f r e e x p e d i t i o n a r y i n v e s t i g a t i o n s which concent ra te on

surveys of phys i ca l cond i t i on , morphometry, and taxonomic coll ec t i ons . Juday's (191 5 ) and

Deevey'r (19571 work on Cent ra l American lakes, R u t t n e r ' s (1931, 1952) observat ions i n

fndonesi a d l r r i nq the Geman Sunda exped i t i on , Cunnington ' s (1920) e x p e d i t i o n t o the E a s t

A f r i c a n Lakes i n 1904-7905, and t h e Percy Sladen Trust Expedition t o take T i t i c a c a i n 1937

a r e examoles of this phase. Under the l e a d e r s h i p o f H. Cary G i l s o n (1939, 1960, 1 g 5 5 ) , t h e

Percy Sladen Exped i t i on spent s i x months, from A p r i l t o September, a t the l ake . Their

observat ions and c o l l e c t i o n s r e s u l t e d i n twenty reports o f which most are taxonomic s tud ies .

The t h i r d phase i s the c o l l e c t i o n of a complete s e r i e s of modern l i m n o l o g i c a l data f o r an

annual cyc le o r more. I n f o r m a t i o n o f t h i s type i s a v a i l a b l e f rom a few t r o p i c a l l akes

i n c l u d i n g bakes V i c t o r i a ( T a l l i n g , 1966) and George (Greenwood and Lund, 1973) i n East

A f r i c a and Lake Lanao (Frey, 1969; Lewis, 1973a, 1973) i n the Philippines. The E a s t A f r i c a n

work, e s p e c i a l l y a t Lake George, i s notable fo r the var je ty o f measurements obtained as p a r t

o f an I n t e r n a t i o n a l B i o l o g i c a l Programme p roduc t i on study. The work on Lake George perhaps

represents a fou r th phase, since i t inc luded exper imenta l i n v e s t i g a t i o n s as w e l l as desc r ip -

t i v e 1 imnalogy. Other reasonably ex tens ive s t u d i e s o f t r o p i c a l lakes i n c l u d e those on E a s t

A f r i c a n s a l i n e l akes (Melack and Kilham, 1974; La Barbera and Kilham, 1974; Hecky and Kilharn,

1!373), Lake Kariba (Balon and Coche, ?974), Lake bt i t l a n ( l l o r r i s and S u m r f e l d , 1969;

Dor r i s , 1972; Weiss, 1971), Lake I z a b a l ( b i n s o n and Nordl ie, 2 9 7 5 ) , and Lake Y o l t a (Biswas,

1969; Viner, 1970). A rather s u b s t a n t i a l l i t e r a t u r e of l e s s complete s tud ies and o f more

spec ja t i zed i nves t ioa t ions i s now a v a i l a b l e on a wide v a r i e t y of t r o p i c a l fresh w a t e r s , but

only f o r A f r i c a n l akes i s t h i s i n f o r m a t i o n surmnarized i n a conven ient form (Beadle, 1974).

T h i s paper r e p o r t s data on the physical and b i o l o g i c a l p r o p e r t i e s of Lake T i t i c a c a

c o l l e c t e d from January th rough December, 1973. Durina t h e year. a s e r i e s o f 21 excurs ions

i n t a the m a i n basin o f Lake Tit icaca were made a t ap~roximately two-week i n t e r v a 7 s t o

measure p r imary p roduc t i on and chemical condit ions and t o sample the p l a n k t o n . Most meas-

u r e ~ e n t s were made i n water of 100 meters depth a t a s t a t i o n ( I i n F igure 1 ) 250 meters t o

the eas t o f Isa:ata IsTand, near the v i l l a g e o f Capachica. Department o f Puno, Per6.

Because t h e b a s i n drops a u i c k l y t o cons ide rab le depths a t t h i s l o c a t i o n , e s s e n t i a l l y open

fake c o n d i t i o n s cou ld be sampled w i t h a small befit a s h o r t d i s tance from shore. This

l oca t i on was a l s o chosen becausp i t i s i n t h e same a r e a used by the Percy Sladen Trust

Exped i t i on a s a base f o r i t s i n v e s t i q a t i o n s (Gi lson, 1939). Samples f o r d i sso l ved oxygen

6

and measurements of water temperature and l i g h t absorpt ion were usually taken a t a deep-

- water s t a t i o n (O i n F igure 1 ) about 7 k i lometers east o f the Isa6ata s t a t i o n i n water of

175 t o 200 meters depth. The primary produc t i on measurements were made a t the I saza ta

s ta t i on because o f the danger of remaining t oo l o n g a t the deeper s t a t i o n i n a small boat.

A three-day c r u i s e o f the north end o f the lake was made t o es t ima te h o r i z o n t a l hetero-

geneity. No a t tempt was made t o sample the shallow water regions such as Puno Bay, which

contains e x t e n s i v e S c i rpus marshes, or Lago Pequeiio (Hui namarca) . These r a t h e r i s o l a t e d

basins are somewhat d i f f e r e n t ecosystems f rom the deep, steep-sided Lago Grande.

I.',

L - - P -.. -. - - "/.. PERU !; BOLIVIA $<- -

I

F igure 1. Map of Lake T i t icaca . Sampling s t a t i o n s used i n t h i s study are marked by c a p i t a l letters. Rout ine work was done a t s ta t ions I and 0.

7

GENERAL DESCRIPTION OF LAKE T I T I C A C A

Lake Titicaca i s located near t h e narthern end o f a l a r g e , h i s h a l t i t u d e i n t e r n a l

dra inage basin, the Altiplano, which i nc ludes p a r t s of PerG, B o l i v i a , Chile, and Argentina.

The m idpo in t o f the l a k e i s about 1 5 ~ 4 5 ' S l a t i t u d e and 69" 25'W l o n g i t u d e , The lake

i t s e l f i s shared between PerG and B o l i v i a . The l a k e sur face i s about 3803 m as1 and has an

2 area o f 8100 km . Gf? son (1939, 7964) prepared a morphometrie map o f the l a k e based on

soundings from t h e Percy Sladen T r u s t E x p e d i t i o n and those of Agassiz (1876) and Neveu-

2 Lemaire (1906). The l a k e i s composed of a l a rge , deep main basin, Lago Grande (6315 km ),

2 2 and two s m a l l e r , sha l lower basins, Lago Pequeiio (1260 kin ) and Puno Bay (525 km ) (F igu re

1). The mean depth and maximum depth o f the 1 ake a r e 107 rn and 281 rn, r e s p e c t i v e l y . The

t o t a l volume o f t h e l a k e is thus 866 krn" The ehhorel i n e l e n g t h o f the l a k e i s 1141 km,

i n c l u d i n g i s l a n d s . Shore l i ne development i s 4.2. The hypsographic curve o f the l a k e ' s

bas in i s g i ven i n F igu re 2. A11 l eng th , area and volume measurements were made w i t h a

d i g i t i z i n g t a b l e from N e w e l l ' s (1949) map whose soundings a r e based on Gi l son (1939).

G i l s o n (1939, 1964) reported somewhat d i f f e r e n t areas and mean depths. Our area c a l c u l a -

t i o n compares very closely t o Monheim (1956). The area o f t h e catchment b a s i n i s about 8

t imes t h e t o t a l su r face area of the l a k e .

The c l i m a t e o f t h e Lake T i t i c a c a r e g i o n i s cool and semi-arid, The weather p a t t e r n

near t h e lake a t Puno, PerG, f o r 1973 i s presented i n Tab le 1 . Mean month ly temperatures

v a r i e d f rom 5.7' C i n July t o 10 .7 '~ i n January and November; month ly ra in fa17 va r ied f rom

none i n June t o 238 m i n January. To ta l p r e c i p i t a t i o n fo r the year was 797 m. The rainy

season (months w i t h mre than 100 m o f r a i n f a l l ) u s u a l l y begins in December and lasts

u n t i l March bu t i s q u i t e v a r i a b l e . The C a k e f i t i c a c a r e g i o n has a d i s t i n c t seasonality of

temperature. However, t h e t y p i c a l d a i l y ranqe i n temperature greatly exceeds the v a r i a t i o n

i n ~ n t h l y means. Mean d a i l y ranges of temperature v a r i e d f r o m 8 . 8 ' ~ i n January t o 1 4 . 9 ' ~

-1 - i n June i n 1973. Average wind speeds ranged from 0.E9 rnesec-I i n March t o 1.3 mosec ~n

August, 1973. Monheim (1 956) repo r ted t h a t winds a t Lake T i ticaca seldom surpass Beaufort

3-4. However, the authors have f r e q u e n t l y observed s t r o n g a f te rnoon and evening winds up

t o 10-14 mmsec-' (Reaufort 6-7). Figure 3 summarizes, in the form o f a cl imograph, mean

monthly weather data f r o m Puno, f o r the years 1964-1 973. A d d i t i o n a l weather data a r e

sumarired by Kessler and Monheim (196R).

The dominant n a t i v e t ~ r r e s t r i a l vege ta t i on i n the v i c i n i t y o f t h e l a k e i s locally known

A R E A ~ r n ~

1000 3000 5000 7000 9000 I I I I

20 - 4 0 - Lago Grande 60 - 8 0 - I00 -

Figure 2. Hypsographic curve f o r Lake T i t i caca .

as puna. Th is vegetat ion Cs dominated by 1 arge bunch-grasses , espec ia l l y S t i p a , Ee_s_txa,

and Catamagrostis species. tower elevat ions around t he l ake are ex tens ive ly cu l t i va ted .

Potatoes, oca, quiiiua, bar ley , and broad beans a r e the p r i n c i p a l crops. Sheep, c a t t l e ,

llamas and alpacas a r e grazed on fa1 f ow f i e l d s and a t higher elevat ions. The rural pepu-

l a t i o n i s dense i n the regions ad jacen t t o the lake. Extensive use i s made of l a ke re-

sources i n c l ud i ng n a t i v e and in t roduced f i s h and t o t o r a (Scirpus t o to ra ) , which i s used

f o r cons t ruc t ion and fodder. See h izarraga -- e t a?. (1965) and Bertholet (1969 ) fo r descr ip-

tions of the l o c a l environment, economy and soc ia l system and Laba (1977) f o r a h i s t o r y o f

t he devel opmen t o f 1 a ke resources.

The geolog ica l formation of Fakes r e l a t e d t o the modern system cou ld have begun a s

e a r l y as t he Miocene (Moon, 1939). P a r t o f an o l d peneplain was ra ised and de l im i ted by

overthrusts to form an intermontane basin, the A1 t i p lano , i s o l a t e d from ocean drainage. In

the l a t e s t Pl iocene-early Pleistocene, Lake B a l l i v i s n formed as a massive l ake t h a t included

both the present day Lake T i t i caca and Lake Poop6 basins (Newell, 1949). Based on remnant

wave cut ter races, Newel l concludes t ha t , du r ing the Pleistocene, Lake Ball i vidn reached a

level of more than 100 rn above the present l e v e l o f Lake Titicaca and remained for a time

a t the 85 rn level. The drop o f the l ake t o form Lake T i t i c a c a near i t s present l e ve l

occurred r a p i d l y , a s no in termediate terraces are d i scernabl e. P l e is tocene o r Recent r e -

- newed block f a u l t i n g deepened the 1 ake f l o o r t o i t s present depth. See James (1971) f o r a

modern i n t e r p r e t a t i o n o f Central Andean geology. The i s o l a t i o n o f the a1 t i p l a n o lakes from

o ther drainage systems since the PI iocene expla ins t he subs tan t ia l endemism o f the b i o t a

(Brooks, 1950).

The l e v e l of Lake T i t i c a c a f luc tua tes substantially and i t s water budget has been the

subject o f several s tud ies (Monheirn, 1956 ; Kessler and Monheim, t 968; Kessler, 3970, 1974 ;

Cehak and Kessler, 1976). A r e l i a b l e d a i l y record o f the fluctuations a t Puno since 1912

i s ava i l ab l e (Monheim, 1956; Del C a s t i l l o , 1977). Annual f l u c t u a t i o n s o f 0.5 - 1.0 meter

are super impred on longer term f l uc tua t i ons which inc lude a record maximum i n 1933 o f

about 1.2 m above the a r b i t r a r y ze ro mark an the Puno gauge and a minimum i n 1943 o f -3.7 rn.

In recen t decades the record inclvdes l e v e l s near the h i s t o r i c maximum i n 1963 and a -1 -7 m

minimum i n 1970. The average l e v e l i n 1973 stood a t about -0.7 rn; approx imate ly the

h i s t o r i c average. The variance spectrum of the record shows no well-defined cycles other

than the annual one (Cehak and Kessler, 1976). Kessler (1974) a t t r i b u t e s changes i n JeveT

t o v a r i a t i o n i n r a i n f a l l r a t he r than evapora t ion , s ince the annual average evaporation,

ca lcu la ted f rom water budgets (1 957-1 961 ) i s r a t h e r constant, compared t o r a i n f a l l

v a r i a t i o n and changes i n l a k e level. During t h i s same f ive-year perfod evaporat ion account-

ed f o r 985: o f water losses and r i v e r ou t f low v i a the R i o Desaguadero to Lake Poop6 on l y 2Y7.

Incoming water was est imated t o be 58% from d i r e c t p r e c i p i t a t i o n and 425 from stream i n f l o w

( K e s s l er and Monheim, 1968).

Given t h i s regime of high evaporation, it i s not s u r p r i s i n g t h a t the l a k e i s high in

disso lved sol i d s (780 mg/l , G i l son, 1964). G i 1 son ( 1 964) gave a water residence t i m e for

Lake T i t i c a c a o f approximately 280 yr . , but a ca l cu l a t i on from the wa te r budget da ta of

Kessler and Moqheim (1968) and our es t ima te of volume g ives a water residence t i m e of 70 yr.

and a conservat ive n o n - v o l a t i l e cons t i tuen t residence t ime a f 3,440 yr. Newel 7 (1949) gives

evidence t h a t the l a k e has been s t a b l e near t he present l e v e l f o r some t ime. The separa-

t i o n of Lago Umayo and Laguna Arapa from Lake T i t i caca probably occurred w i th an & rn drop

i n the l ake l e v e l i n r e l a t i v e l y recen t times. Newel1 s t a t e s t h a t t h i s sh r ink ing o f t he

l ake was probably caused by t he cutting o f the basin r i m a t Desaguadero and observes t h a t

the o u t l e t i s not c u r r e n t l y being degraded. Monheim (1956) a lso concluded tha t the l eve l

of the l a k e i s presently s t a b i l i z e d by the f l o w regirne o f Rio Desaguadem.

PHYSICAL AND CHEMICAL LIMNOLOGY

Physical Measurements

Methods

Temperature measurements were made w i t h a thermistor (February-May , September-December)

and a water bottle thermometer (January, June-August ) both cal i b r a t e d aga ins t a laboratory

standard thermometer. Temperature p r o f i l e s were adjusted t o a 1 1 . 1 'C hypo1 imnet ic m i n i m u m

temperature t o remove s m a l l inconsSstencies in the measurements. Heat content w a s calcula-

ted from these temperature d a t a and from the hypsographic curve a s t h e heat required t o be

last t o cool t h e l a k e to O'C.

A secchi disk and a homemade photometer, w i t h a s e n s i t i v i t y peak i n the green, were

used t o make rough measurements of light penetrat ion. L i g h t e x t i n c t i o n coef f ic ients were

then determined f r o m the slope of l i g h t penetrat jon vs. depth p l o t t e d on semi-log paper

(Hutchinson, 1 9 5 7 ) .

12. IY, 16, 12. IY. 16. 12. I'I. 16. 32. 14. 16. 12. 19. 16. 12. 19. 16.

F igu re 4 . Temperature profiles from Lake T i t i caca , 1473.

Thermal Regime

Lake T i t i c a c a is a w a r m monomictic lake according t o Hutch inson 's (1057) system of

thermal types. During the summer and f a l l , the lake was stratified w i t h a 40 m t h i c k

epilimnion in March deepening to 70 m in June (Fiqure 4). Durinq mid-winter (July 301,

the l ake became nearly isothermal at 11.1 to 1 1 . 2 ~ ~ . Weak sha l l ow ep i l i rnn ia were o f t e n

present i n the least stratified per iod and on no measurement d a t e w a s the l a k e completely

i so thermal . These data and t h e chemistry proffles discussed below ind ica te t b a t Lake

Titicaca mixed to at leas t 100 rn by July 30 and that thorough deep rnixtnw did not occur in

7973. Stronger stratification was re-established in September and continued into t h e

f o l l o w i n g summer.

During s t r a t i f i c a t i o n , there was only a small difference between epilimnetic

(12.0 - 15.7'~) and hypal i rnnet ic (11.1 '~) temperatures. A t t imes, multiple-stepped

thermoclines occurred in the epilimnion (February 26, March 11, and November 30). Th is was

soon followed by atelomixis (March 27 and December 14), the mixinq of thermally-divergent

layers i n t h ~ epilimnion wi thou t erosion o f the main therrnecl ine ( L ~ w i s l 9 7 3 a ) .

Heat Budget

The heat budqet of l akes i s im~ortant for t w o reasons: one, the p a t t e r n o f heat

q a i n and loss controls stratificatfon which in turn sets a p h y s i c a l framework for chemical

and biological processes in the lake, and two, t h e heat budget f s intimately related to

evaporation rates which a f f e c t the geochemistry of the lake and the climate of the lake

shore area.

The heat content of Lake T i t i c a c a declined as the lake became isothermal and later

increased dur ing restratification (Table 2 ) . Approximately 19,300 cal .crnm2 of heat was

Tost from the February maximum to the July minimum. Because of decreased seasonality from

temoerature to equatorial l a t i t u d e s , Lake Titicaca's total annual heat budget i s lower

than that o f larse temperate lakes such as B a i k a l , 65,500, Mich isan, 52,40D, and Tahee,

34,800 cal.crnw2 (Hutchinson, 1957 ) bu t i s h igh compared to that o f equatorial Lake Victoria

(0' 05' S ) , 9.000 - 11,000 cal-cm- ' (Tall ing. 1966). The T i t i c a c a budqet resembl er more

t h a t of tropical Lake AtitIan (Guatemala, 14' 40' N ) , 22.1 10 cal .ern-? (Hutchinson, 1957). Using d a t a from the Capachica weather station, 30 km northeast of Puno. an ~ s t i r n a t ~

Date Heat content Secchi depth L i gh t ex t inc t ion - l m coefficient, rn

8 Feb. 168,700 4 . 5 - 24 Feb. 169,100 4.75 0.17

11 Mar. 1 65,400 4.5 0.13

27 Mar. 164,800 5.8 0.13

11 Apr. 165,300 6.5 0.10

2 May 165,300 7.4 0.07 18 May - 7.0 0.07

2 June - 8.25 0.07 22 June 7 55,300 8.5 0.07

15 Ju ly 151,900 8.75 0.06

30 July 149,800 10.0 0.07 15 bug. 150,300 10.5 0.08

4 Sept . 150,200 10.5 0.05

28 Sept. 1 56,500 8.75 0.05

14 Oct. 153,600 9.25 0.05

30 Oct. 157,100 9.25 0.04

16 Nov. 156,500 7.5 0.04

30 Nev. 1 58,600 6.2 0.06 14 Dec. 162,900 6.0 0.06

28 Dec. 1 56,900 6.0 0.07

T a b l e 2. H e a t content and l i g h t penetration parameters i n Lake T i t l i caca , 1973.

was made sf t h e var ious components o f the heat budget . The p r i n c i p a l methods used are

summarized i n Neumann and Pierson (1966). The b a s i c equat ion f o r t he heat budget o f a

water body i s

S = R ( I - a ) - 1 - H - L E + h t

where 5 i s storage rate, R t i s t o t a l i n c i den t s o l a r r ad i a t i on , a i s surface albedo, I i s

net long-wave r ad i a t i on , H i s sens ib le heat t rans fe r , SE is e v a p o r a t i v e hea t Toss, and

h i s a res idua l term. This equat ion neglects advect ive e f f e c t s due t o in f low, outflow,

and r a i n f a l l . Monthly averages o f 5. were calculated f rom the h e a t content data by assumina

t h a t storage changed a t a constant d a i l y rate between measurements o f heat content. The

average monthly r a t es were taken t o be the number of days a t each r a t e i n t h a t month times

that r a t e , summed, and then d i v i d e d by the number o f days i n the month. Na correction w a s

made f o r t he f i r s t week i n February ngr t he l a s t th ree days o f December. R t was ca lcu la ted

by rnu l t i p l y i nq the t heo re t i ca l value of so l a r radjation from L i s t (1951) t imes the Y of

t heo re t i ca l i n s o l a t i o n r ~ p o r t e d from t h e Puno weather s t a t i o n . The surface albedo was a s -

- S R+(3-a) I & - I + - LE -H A B E E pan Wa

Feb -14 442 651 -788 -267 -58 +6 -22 4 .6 5.3 3.0 Mar -103 440 616 -795 -271 -67 -26 - 2 5 4.5 4.2 2.8 Apr +I2 506 617 -783 -269 -63 4-4 .23 4.6 4.4 2.9 May -187 531 548 -376 -300 -69 -1 15 - 2 3 5.1 4.2 3.0 3un -186 478 519 -759 -258 -66 -100 - 2 5 4.4 4.5 2.6 Jul -138 501 513 -747 -372 -79 -14 .25 5.3 4.1 3 . 4

+12 530 549 -754 -333 -69 +89 -21 5.7 5.4 3.7 Sep +24 511 589 -757 -308 -67 +56 .21 5.2 4.9 3.7 Oct +I97 621 615 -774 -335 -52 +I22 .16 5.7 6.2 3.5 NOV +51 559 620 -777 -369 -55 +73 .15 6.3 6.5 3.8 Dec -96 486 617 -776 -372 -65 +I4 -17 6.3 5.7 4.0

T a b l e 3a. Heat Budget f o r Lake T i t icaca, 1973. Negative values represent loss o f

hea t from the l ake . Energy f l u x e s are given in calmcrn-Z . day-'. B i s

t h e Bowen's r a t i o , and E i s the calculated estimate o f d a i l y evaporat ion .

Monthly mean w ind speed, %. (rn sec-' ), and pan evaporat ion , E pan,

(rrm day*' ) , d a t a f rom the Capachica station are a l so given.

1. F' +Ri n -E -Rout - A V = Residual

+I016 +724 -1900 -31 -375 y -566

2. P +in - A V = E

+I016 +724 -31 -375 = 1334

Table 3b. Water Rudge t for Lake T i t i c a c a , 1973. Values are i n m-year-'. 1: Evaporation term from heat budget. 2 : Evaporat ion es t imated as water budget residual ,

Sensible heat transfer is small and a loss throughout the year, w i t h little or no

fluctuation. Th is results from t he l a k e be ing consistently warmer than the mean a i r temp-

erature for all months. Evaporat ive lass i s high and relatively constant during the year.

The pattern of LE only loosely follows Rn (r = .60, p - .05, n = 2 1 ) , and i s not s i g n i f i -

cantly related to t he s t o r a g e term Cr = .36, p > .05, n = 11). Because of t h i s p a t t e r n and

t h e high magnitude o f LE, the storage f l u x results mainly from the small imbalances of LE

and Rn.

The storage term is c o r r e l a t e d t o the sum of source and sink terms, Rn - LE - H (r =

-84, p c .01, n = 11). During some months the res idual i s high r e l a t i v e t o o ther budget

terns. Th is may be i n p a r t t h e r e s u l t o f some a f t h e s imp l i f y ing assumptions. A d v e c t i v e

heat flux f rom r i v e r s , advect ion from precipitation, and the change i n heat content per

un i t surface area caused by the seasonal f l u c t u a t i o n of lake volume are probably each re-

sponsi b i e for no more than i 5 cal day-' . The res idua l t e r m i s co r r e l a t ed w i t h

the storage term ( r = .91, p < .01, n = l l ) , which ind ica tes t ha t , w h i l e the pat tern o f the

f l u c t u a t i o n o f S i s explained by the source and sink terms, t he magnitude o f the f l u c t ua -

t i o n i s not accurate ly est imated. The res idua l i s probably largely the r e s u l t o f having

t o use western shore-s ta t ion weather data r a t h e r than accurate average overwater data t o

est imate t he terms. Temperature, hurnidity,annd windspeed are in f luenced by topography and

t he land sur face 's heat budget. E r ro rs or b iases may a l so a r i s e fram apply ing Jacob's

(1951) evaporat ion equation t o a h igh e leva t ion l ake and from the f a c t t h a t r e l a t i v e l y few

temperature p r o f i l e s were used t o es t imate S . Residuals i n heat budgets can usua l l y be

made small o n l y by using longterm averages o f parameter estimates (T. Powell , personal com-

municat ion).

Water Budget

A water budget f o r Lake T i t i c a c a was estimated f o r the year 1973. The budget i s given

as a balance of inputs, ouptuts , and change i n l a k e l e v e l :

P + R. - E - Rout - bV = 0 i n

where P i s p r e c i p i t a t i o n on t he l a k e surface, Rin i s r i v e r i n f l ow , E i s evaporation, Rout

i s r i v e r outflan, and A Y i s the net change i n l a k e l e v e l . Ground water inpu t and output

were assumed t o be neg l i g i b l e .

P r e c i p i t a t i o n was est imated from the average of annual r a i n f a l l a t th ree near-shore

s ta t ions , Capachica, Puno, and JuF i . Kessler and Monheim's (1 968) mean est imate o f river

input and output f o r t he years 1957-1961 was used. Since Kessler and Monheim's extens ive

summary of r a i n f a l l data c l e a r l y ind ica ted t h a t the west shore of t he l ake i s d r i e r than

the east, t h e i r cha r t was used t o ad j us t the data f rom our three s t a t i o n s upward t o r e f l e c t

p r e c i p i t a t i o n onto the l a k e surface as a whole. Likewise, s ince 1973 had s l i g h t l y higher

r a i n f a l l than Kessler and Monheim's average of 5 years, r i v e r in f low was ad justed propor-

t i o n a t e l y from t h e i r mean. The lake level a t Puno was measured t o have increased by 14 3/4

inches (375 mn) dur ing 1973 (Oswal do Zea R. , personal comuni ca t i o n ) . Annual evaporation

was ca lcu la ted from the heat budget values. The monthly evaporat ion f o r January was e s t i -

mated assuming t he same d a i l y rate i n February. I n add i t i on , evaporat ion was estimated

fram the res idua l o f the other terms. The water budget i s sumnarized i n Table 3b.

Evaporation estimated as the budget res idua l , 1334 m, i s much lower than t h a t

ca lcu la ted from the heat budget but only a 1 i t t l e lower than Monheim and Kess le r ' s water

budget est imates. The water budget r e l i e s heav i l y on an adequate es t imat ion of r i v e r i n -

18

f l o w and ou t f l ow data which are n o t ava i l ab l e t o us f o r 1973, Kessler and Monheim's anal -

ys i s o f the water budget f o r the per iod 1957-1961 ind ica tes t h a t v a r i a t i o n i n annual out-

f l ow v i a Rio Desaguadero, Rout' was small : ? 11 m. Kessler (1970) a lso showed a cons i -

derable discrepancy between 5 year average est imates of evaporat ion based on a heat budget

method (1 714 m) and the water budget method (1480 m), a1 though the d i f fe rence i s less

than ours.

L igh t Penetrat ion

Secchi d isk depths and l i g h t e x t i n c t i o n c o e f f i c i e n t s a r e shown i n Table 2. Secchi

depth was 4 . 5 - 4.75 m i n February. As the r a i n y season ended, c l a r i t y increared and the

Secchi depth gradually went t o a maximum o f 10.5 rn in August. With the onset o f l i g h t

sp r ing r a i n s i n September, fo l lowed by an increase of phytoplankton biomass, Secchj

depth decreased t o 6 m by December. Because of the s e n s i t i v i t y of the photometer i n the

green, e x t i n c t i o n measurements have o n l y r e l a t i v e value. E x t i n c t i o n ctpeff i c i e n t s followed

a pa t t e rn in f luenced by r a i n f a l l du r ing the f i r s t h a l f of the year and by phytoplankton

biomass du r i ng the l a t t e r ha l f =

Regression Analysis

Pai rwise simple co r re l a t i ons were ca lcu la ted between most physical measurements ob-

ta ined i n 1973. Two phys ica l parameters, the e x t i n c t i o n c o e f f i c i e n t and the depth o f the

epi l imnion, were i nves t iga ted us ing m u l t i p l e regression t o determine t h e i r s t a t i s t i c a l re-

l a t i onsh ips t o o ther physical and b i o l o g i c a l cond i t i ons . De ta i l s an the method o f comput-

i n g phytoplankton biomass i s g iven on page 30. The results o f the simple co r re l a t i ons

among phys ica l f a c t o r s are shown i n Table 4 , (see a l so Table 17).

The equations for the l i g h t e x t i n c t i o n c o e f f i c i e n t and the depth o f m ix ing are h i g h l y

s i gn i f i can t . Ra in fa l l and phytoplankton biomass account f o r 75.5% o f the var iance o f l i g h t

ex t i nc t i on . The t w o independent va r iab les are n o t s i g n i f i c a n t l y co r re l a t ed ( r = 0.31,

p < -05, n = 21). The equat ion i s i n t e rp re tab l e : l i g h t e x t f n c t i o n increases w i t h t u r b i d i t y

caused by sediment load (brought i n by r u n o f f ) and by algal biomass. The correlat ion i s

somewhat s t ronger w i t h r a i n f a l l than w i t h biomass, which i s rather surp r i s ing . Apparently,

the small f l u c t ua t i ons i n biomass combined w i t h h igh t u r b i d i t y o f i n f lows dur ing t he r a i n y

season cause the higher dependence on r a i n f a l l compared t o biomass. The authors on one

occasion i n May observed a sediment plume from t he Rio Aamis reaching several km i n t o the

l ake .

The depth o f m ix ing can be pred ic ted e m p i r i c a l l y by a i r temperature and wind speed

w i t h 7 6 . 1 % o f the var iance explained. A i r temperature and wind speed a re e s s e n t i a l l y inde-

pendent ( r = 0.06, p x- .05, n = 21). Pow a i r temperatures may cool the l a k e surface,

causing convect ive mixing, and strong winds across the l a k e crea te water currents t ha t may

cause v e r t i c a l turbulence. The regress ion equat ion g ives a i r temperature as being the p r i -

mary factor although t u rbu l en t mixing i s generally considered t o be mare impor tant than

t hemal convection.

Discussion

Most t r o p i c a l lakes of in termediate depth appear t o have a thermal p a t t e r n s i m i l a y t o

Lake T i t i caca , more o r l e ss subs tan t ia l t y m ix ing once annual ly ( T a l l i n g , 1969). Lake T i t i -

caca i s perhaps a marginal member of t he o l i g o m i c t i c c lass o f l akes defined by Hutchinson

and t o f f l e r (19561, but i t i s probably best categorized as p r i m a r i l y rnonomictjc w i t h incorn-

p l e t e c i r c u l a t i o n i n some years. Apparent ly no really good example has y e t been recorded

o f o l i gom ix i s uncomplicated by density s t r a t i f i c a t i o n of the deeper waters by e l e c t r o l i t e s

as i n t he cases of Tanganyi ka and M a l a w i or by smaf 1 s i z e and g rea t p ro tec t i on as i n Bun-

yon$ (Ta l l i ng , 1969; Baxter e t a l . , 1965; Eccles, 1974).

The heat budget o f Lake T i t i c a c a i s i n s t r i k i n g con t r as t t o temperate lakes. Godden

(1976) has sunmarired the information f o r several such lakes, a l l o f which show very l a r g e

storage terms on the same order a s a,., and smaller and seasonal ly f l u c t u a t i n g LE. Godden's

budget for Lake Tahoe, which i s more or less t y p i c a l of temperate monomici t ic lakes, i s

shown i n Figure 6 f o r comparison w i t h Lake T i t i c a c a . H and LE are both much less var iab le

i n Lake T i t i c a c a than i n temperate lakes. In T i t i caca , sens ib le heat always represents a

loss term, wh i l e i n temperate lakes i t i s a source o f heat in the summer. Rn and LE are

both l a r g e and nearly balance each other i n every month, hence the storage r a t e term and

t o t a l storage remain small. Other t r o p i c a l lakes w i 7 l probably be found t o e x h i b i t a s i m i -

l a r pat te rn , a l though i t w i l l be su rp r i s i ng i f lowland lakes do not usua l l y gain sensible

heat. High Rn and LE throughout the year, perhaps combined w i t h genera l l y low mean wind

speeds, a r e probably responsib le fo r the stepped thermocl ines dur ing the sumner months and

fo r the pe rs i s tence of weak s t r a t i f i c a t i o n during the w in te r .

Because o f the large seasonal changes i n R and a i r temperature and the h igh s p e c i f i c n

heat of water , temperate 1 akes 1 ag considerably behind the seasonal weather. Trop ica l

l a k e s , w i t h much less seasonal environments, are always much closer t o thermal equil i br ium

w i t h e x i s t i n g condi t ions. I n add i t i on , the r e l a t i v e l y small t o t a l heat storage i n tropical

20

I AUG- MOV- FEE- MAY- AUG- UCT JAN APR JULY QCT

Figure 6 . Summary of t h e analytical heat budget for Lake Tahoe. Seasonal averages based on 38 months of data . Courtesy o f D.R. Godden (1976), L.O. Myrup, and T.M. Powell.

lakes r e s u l t s i n weak s t r a t i f i c a t i o n . Consequently, r e l a t i v e l y small changes i n the large

and nearly balanced values of LE and Rn may e a s i l y d is rupt or n o t i c e a b l y strengthen s t r a t i -

f i c a t i o n i n a short per iod o f time. The depth of mixing during the winter approach t o i s o -

themy may then be very dependent on the particular weather conditions i n a given year.

A1 though t r o p i c a l l akes are much l e s s seasonal than temperate ones, the pat tern o f strati-

f i c a t i o n may be h i g h l y var iab le from y e a r to year i f weather patterns vary much between

years. Hence, i n some respects, t r o p i c a l 1 akes may be physically less s t a b l e systems than

temperate ones.

The general c h a r a c t e r i s t i c s of Lake T i t i c a c a ' s water budget are c l e a r from Kessler and

Monheim's (1968) data. The g r e a t bulk o f water (approximately 98Z) i s lost by evaporation

rather than by outf low v i a the R i o Desaguadero. Nevertheless, the substant ia l disparity i n

21

both our and Kessl el-' s (1 970) ca l cu l a t i ons between evaporat ion est imated by water budget

and heat budget methods begs f u r t he r a t t en t i on .

Chemical Measurements

Methods

Routine chemical measurements were performed on samples taken a t S t a t i o n I inc l ud i ng

oxygen, a 1 ka1 i n i t y , pH, s i l i c a t e , phosphate, n i t r a t e , calcium, magnesium, ch lo r ide , and

s u l f a t e . These measurements were made w i t h a Hach DR-EL, whose methods a r e adopted from

those recornended in Standard Methods of Water and Waste Water Analysis ( A . P . H . A . , 1971)

(Tab le 5). The approximate accuracy o f t h e methods a r e indicated i n t h e t ab l e . Accuracy

was general l y acceptable f o r a1 1 measurements except n i tra te and phosphate whose concentra -

t i o n s were normal ly too l o w f o r r e l i a b l e results w i t h the Hach k i t . One analys is o f c a l -

cium, magnesium, copper, and z i n c was performed using a tom ic absorpt ion techniques by G .

Smith a t t he Un i ve r s i t y aF Ca l i fo rn ia , Davis, on a sample o f water concentrated by evapo-

r a t i o n .

Resu l ts

The ep i l imn ion pH was around 8.6 fo r most of t he year and decreased t o 8.5 a t the

t ime of isothermy. Likewise, the pH of the deeper water was s l i g h t l y under 8.4 except

dur ing the isothermal pe r iod when i t rose t o 8.5. Most o f the ino rgan ic carbon ava i l ab l e -

t o photosynthesis i s present i n t he form of bicarbonate, MC03 . To ta l a l k a T i n i t y remained

very constant a t 120 m g * l m l CaC03 throughout the year and d i d not va ry w i t h depth.

The s i l i c a data a re shown i n Figure 7. From January t o the end o f May, s i l i c a concen-

t r a t i o n s i n the ep i l imn ion were observed t o ranqe from 0 .49 to1 .18 rng-l-'. Shor t l y there-

a f t e r , concentrations f e l l t o low l e v e l s (0.06-0.18 rnq.1-'1. Thrmghout t h i s t ime (January-

Ju l y 15), hypolimnetic s i l i c a a t 100 meters depth ranged from 1.82- 2 .60mg* l - l . As t he l ake

became isothermal a t t he end of July , t he epil irnnetic s i l i c a concentrat ion rose t o between

0.25 and 0.46 rng.lm', and the concentrat ion i n the hypolimnion f e l l t o 0.34 m g - l - ' a t 100

meters. A f t e r this period, ep i l imne t i c s i l i c a cancentrat ions again became lower before

s t a r t i n g a slow bui ld-up. S i l i c a concentrat ions a t 100 m and deeper f o l l owed a s i m i l a r

p a t t e r n bu t increased t o considerably higher values dur ing t h i s per iod. A t t he end o f the

year . d i s s o l v e d s i l i c a had risen t o 3.7 mg-lwl, i tr h ighest value, a t 150 meters depth.

Other reported values i n Lake T i t i c a c a are i n the 0.5-1.0 rng- l - ' range (LGf f le r , 1960;

Rohrh i r sch - e t -- 3 7 . - 1969). The concentrat ions o f s i l i c a i n t h e e p i l imnion repor ted he re

could be J i m i ting t o diatom g row th dur ing c e r t a i n seasons of t h e year (Hughes and Lund,

22

E x t ~ n c t i o n Secchl Daily Hours A i r Relat ive Wind Mixed Coef f ic ient Depth I nso la t i on o f Sun Tmp. Humidity Speed Ra in fa l l S i 0 2 Depth

Ext inc t ion Caef.

Secchi Depth

D a i l y Inso la t ion

A i r Temp.

Rel. Humidity

U i n d Speed

Ra in fa l l

Si02

Mixed Depth

Tabl e 4. Pai rwi se pmduct moment c o r r e l a t i o n coef f ic ients between se lected physical and chemical measurements. One aster isk ind icates a c o e f f i c i e n t s i g n i f i - cantly di f ferent from 0 a t the .05 level , and two as te r i s ks i nd i ca te the -01 1 eve1 .

Total A l k a l i n i t y was masured t i t r i m t r l c a l l y d t h standard s u l f u r i c ac id uslng Bromcresol Gwen-Methyl Red i nd i ca to r . 1 Phenolphthalein a l k a l l n l t y was absent. The r e s u l t s were expressed i n mg CaCO per l i t p r . t2 rng-l- . 3

Chlor jde was determ$ned by t i t r a t t n g wjth standard mercuric n i t r a t e t o the dl phenylcarbatone endpolnt . Results were ex-

pressed as mg chlarlde ton per l i t e r . $5 rng.1-'.

Calcium expressed as mg CaC03 per I 1 ter was detemined by t i t r a t l o n a t pH 12-13 w l t h a standard soTutjon o f the sodlum

s a l t o f ethylenediamine te t ra-acet ic acid (EOTA) t o the Cal Ver I1 Calcium Ind i ca to r endpoJnt (red t o blue).

~5 mg-I-'.

AagnesIum expressed as mg CaC03 per l i t e r was determined by the difference between calcium hardness and t o t a l hardness.

The l a t t e r was m a ~ u r e d by t i t r a t i o n a t pH 10.1 w i t h standard EOTA (Sbdtum s a l t ) t o the Calmaglte endpoint ( r e d t o 1 blue) . Calmagite and C a l Ver I 1 Calcium Indicator are s t r u c t u r a l l y r e l a ted azo dyes. !I0 mg-1- .

N i t r a t e was determined by a modificatton of the cadmium reductton mth& I n whEch the n l t r a t e f o m d s t o i c h i m t n c a l l y

i s measured by d iazo t i za t i on of s u l f a n i l i c ac id and coupl ing w i t h gen t i s i c acid t o g ive a dye uh ich i s w a s u w d

c o l o r i m t r i c a l l y . Results Here expressed ar mg n l t rogen-n i t ra te i on per l i t er . 7.03 mg-1".

Otssol ved O x y ~ was determined by a modi f ica t ion o f the Winkler nethod i n wh ich MTI'~ , converted to manganese hydroxide,

reacts w ~ t h O 2 t o fonn manganese dioxide. This oxIdIzes iodlde t o elemental iod ine in acid so lu t ion . The iodine

i s t f t r a t e d w i t h a standard reducing so lu t i on o f phenylarslne o x i d e (P4O). The resu l t s are glven I n terns o f nq O2

per l i t e r . t.2mq-I-'.

@ was determined calorimetrically using a narrow range i nd i ca to r . Khynml Blue. :.05 pH u n i t .

Ortha-Phosphate was masured c o l o r i ~ t r i c a l l y by the amunt o f heteropoly b lue f o m d a f t e r treatment w i t h mlybdate and

ascorbic acid. Results were given I n rng phosphate per 11 t e r . * .03 m . 1 - l ,

S i l l c a was determined c ~ l o r i m e t r i c a l l y by the helert lpoly blue method whlch e n t a i l s e l im ina t i ng mlybdophosphoric ac id

w i t h oxa l i c ac id pr Io r to reductton with 1-amino-2-naphthol-a-sulfonic acfd solution. Stllca was reported as

mg 510 per l i t e r . 1.05 rng.l-ll. 2 Sulfate was measured turbidlmetrlcally accordfnq t o the amount of barium su l fa te p rec jp i t a ted From the water under spec{-

1 f ied condit ions and reported as rg s u l f a t e per l i t e r . !10 mq.1- .

Tabl e 5. Methods used fo r chemical measurements and t h e i r approximate accuracy.

1962). Diatoms are not a very important component o f t he plankton except dur ing the autumn

and w i n t e r months when t he dominant phytopl ankter i s Stephanodi scus astraea. Th is species

has a d e f i n i t e seasonal succession pa t t e rn (Figure 9 ) which probably in f luences the season-

a1 changes i n s i l i c a t e concentrat ion. Talling (1966) observed t h a t dlatom p o p u l a t i o n s i n -

creased markedly in Lake V i c t o r i a during t imes of isothermal m ix ing t h r o u g h o u t t he water

column, and that the increase i n diatoms depleted t h e surface concentrat ions o f s i l i c a . A

reg ress ion a n a l y s i s o f v a r i a t i o n s i n silicate concentration showed a hjghly s i g n i f i c a n t re-

lationshjp t o r a i n f a l l and a s i g n i f i c a n t relationship ta m i x i n g depth ( f a b l e 4 ) . The most

impor tant source o f s i lSca te during 1973 appears t o have been runo f f du r i ng the rainy sea-

son, w h i l e the recycling dur ing mixing was o f less s ign i f icance. D i a t o m uptake of the si-

1 i c a t e renewed by w i n t e r overturn along w i t h the near absence of diatoms from the plankton

during the summer r a i n y season may be p a r t l y responsible f o r t h i s pa t te rn .

Most of the phosphate determinations were near the 1 i m i t of s e n s i t i v i t y o f the method.

However, re1 a t i v e l y high val ues were obse~ved i n deeper water. averaging 35 pg-l- ' PO4-'?.

The da ta i n Table 6 suggest a p a t t e r n s i m i l a r t o t h a t o f s i l i c a , w i t h a sho r t - l i ved bu i ld -

- -

p g ~ 0 3 - ~ 1 -' mg S ~ O ~ - I - ' Date Sha l low Deep Shallow Deep Shal l ow Deep

26 Jan

8 Feb

24 Feb

11 Mar

27 M a r

11 Apr

3 May

18 May

2 Jun

22 Jun

1 5 Jul

30 Jul

15 Aug

4 Sep 28 Sep

1.4 O c t

30 Oct

15 Nov

30 Nov

14 Dec

28 Dec

Tab le 6 . Average shallow (0-30m) and deep water (>30m) concentrat ions o f phosphate, n i t r a t e , and s i l i c a i n Lake T i t i caca , 1973.

24

up i n concentrat ion at t he tine o f mixing. The values obtained a r e cons is ten t w i t h those

of Rohrhirsch -- e t a ? . (1969) i n Lake T i t i c a c a who recorded values from 7.8 t o 62 p9=1-' i n

surface water.

Concentrations of n i t r a t e measured were a lso too Pow t o be very reliable given the

l i m i t e d method. Rohrhirsch -- e t a l . were not ab le t o de tec t n i t r a t e i n T i t i c a c a . If only

the averages of ve ry deep measurements (100 m o r deeper) are used for the est imate, the

U:P r a t i o i n Lake T i t i c a c a Ss low, approximately 4:1 by weight or a 10:1 atomic r a t i o .

NO3-N averages 170 ug-1-' a t these depths and POq-P averages 38 ~ ~ - 1 - l .

Average values, based on a t l e a s t one e p i l i rnnet ic and one hypol imnet ic sample on 21

measurement dates, f o r the concentrat ions o f chloride, sul f a t e , calcium, and magnesi urn

ions, were 260, 282, 66, and 34 rng.1-' , respective1 y. G i l son (1964) repor ted 250 eg. l - l

o f ch l o r i de , 246 rng-l- ' o f su l f a t e , 65 mg-l-' o f calcium, and 35 m p l - ' o f magnesium.

Analysis f o r calcium and magnesium by atomic absorption on the sample o f wa te r concentrated

by evaporat ion contained 64.0 m g - l w ' o f ca lc ium and 36 rng* l - ' o f magnesium ion. The zinc

concentrat ion was 28 vg- l - ' , and the copper concentrat ion was a t leas t 2.5 ug.l"' -

Table 7a shows an apparent increase i n the concentrat ions o f some major ions between

1937 and 1973. The v a r i o u s pas t measurements are d i f f i c u l t t o compare c r i t i c a l l y because

of di f ferences in methodoTogy, l o c a t i o n s from which samples were taken, and number o f sam-

p les obta ined. The values repor ted by G i l son (1964) a r e the mast representat ive of the

main basin, but are based on a s i ng le mixed sample o f water from various depths except fo r

C1- for which he reports 20 determinat ions. Our values represent averages o f approximately

70 determinations f o r each ion but are 1 i m i ted by the accuracy and p rec i s i on of the Hach

k i t . Since the l a k e ' s l o s s of water i s p resen t l y l a r g e l y by evaporation, i t s chemical com-

pos i t i on may be inc reas ing under present c l i m a t i c cond i t i ons and the apparent increase,

p a r t i c u l a r l y i n the conservat ive anions 1 so4=) could be real . Using the water budget given by Kessler and Monheim (1968) and the chemistry data f o r

Lago Pequefio and the Rio Rarnis and Rio Huancan4, g iven i n very sketchy terms by G i l son

(1964), an es t ima te o f t he s a l t balance i s poss ib le able 7b). The years 1957-1961 used

by Kessler and Monheim i n t he es t imat ion o f the hydrological budget were during a per iod

when the l ake stood near i ts h i s t o r i c mean level and over which the ne t change i n l a ke l eve l

was a modest 0.42 rn. The agreement rri t h the observed increase i s s u r p r i s i n g l y c lose f o r C1-,

1903 %lfr08 1 fr37 1954 9 973 Reveu-hemaire Posnansky G i 1 son h l f f f l e r Authors

(1906) (1911) 11964) ( 1 960)

Table 7a. Ma jo r i o n concentrat ions recorded since 1906 i n Lake T i t i c a c a . Neveu-

ternaire and Posnansky mixed samples, i n c l u d i n g w a t e r from Laqo Pequeh.

L o f f l e r ' s sample was from Puno Bay. G i l son ' s and the authors ' samples were

f r o m Lago Grande. A l l u n i t s rng.l-'. Gilsan's data here are f a r one sample

date except f o r C1- which i s a mean of 4 d a t e s ( 5 depths each date) ,

Standa rd errors are ind ica ted f o r anions where m u l t i p l e determinat ions a r e

a v a i l a b l e .

I. tlm concentrat ion i n 1937 was 247 rng.l-' i n the main b a s i n and 274 r n g * l - l i n

Lago Pequefio ( G i 1 son, 1964) . 2. Rio Desaguadero f low (Kess ler and Monheim, 1968) = 2 .5 x l o 8 m3 - y-'.

~ 1 - output v i a R i o Desaguadera = (274 g*m-3)(2.5 x lo8 rn3 y-') = 6.9 x 10 10

-1 9 ' Y V -

3 . CI- input v i a Cncoming r i v e r s : 3 Rio Ramis f l o w = 82 rn3*sec-I, Rio Huancane f low = 16 rn -set-' (Kess ler & Monheirn,

1968) -1 Rio Ramis [C?-] = 45 mg-I-', Rio Huancane [ C l - ] = 161 nq.1 (Gilron, 1964)

Estimated m e a n i n ~ u t concentrat ion:

8 3 -1 Tota l r i v e r i n f l ow (Kessler 8 Monheim, 1968) = 53.9 x 10 m *yr

C t - r i v e r i n e i npu t : 8 3 (64 9.m3)(53.9 x 10 m Vyr-') = 34.5 x 10" g - y i '

- 1 4 . N e t annual increase i n ~ 1 - = Input - Output = 27.6 x 1 0 ~ ' gmyr .

Lake volume = 866 krn 3

Annual increase i n the concentrat ion o f C1-:

- I -1 5. Expected increase i n [ ~ l - ' 1 1937 - 1973 = (0.32 rng-1 -yr ) ( 3 6 yrs) = 11.5 rng.l-'

Apparent observed increase i n [ ~ l - I ] 1937 - 1973 = 260 m s * l =' - 2 67 mg.l-' =

13 mq-I-'.

Tab le 7b. S a l t balance ca l cu l a t i ons fo r ch l o r i de ion .

26

given the c r u d i t y of the var ious data and the s h o r t span of t ime s ince 1937 f o r observing -

changes. No i n f l o w SO4- data a re ava i lab le , so no check i s poss i b l e us ing t he second ion.

The general conclus ion that the d isso lved s o l i d s concentrat ion of Lake T i t i c a c a i s f a r out

of equ i l i b r i um wi th present c l i m a t i c cond i t i ons i r f a i r l y s t rong however, since the water

budget and i on concentrat ions are u n l i k e l y t o be i n error by so large an amount as com-

p l e t e l y negate t he f i v e f o l d d i f ference between i n f l o w and outflow mass o f cI-. The l a k e ' s

~ 1 - budget w i l l apparent ly balance under the present r a i n f a l l and geochemical regime when

the Lago PequeRo concentration r i s e s t o about 1400 mg-1-' . The approach t o equil ibriun i s

f a i r l y r a p i d on a geelogical t i m e scale. After 500 years the concentrat ion would r i s e t o

422 mg.1-I.

DissaIved oxygen curves (F igure 83 present a rather complex p i c t u r e . A t 140 meters

and belaw, oxygen concentrat ions (2.4-4.8 mg*l- ' ) are about h a l f t h a t on the surface. Mix-

ing of oxygen down t o 100 meters had taken p lace by the midd le of August. A t t he usual

mid-day temperatures of t he surface waters , around 13. ~ O C , s a t u r a t i o n w i t h oxygen occurs

a t about 6.6 rngml-', so t h a t the somewhat higher valuer often observed represent supersat-

u ra t i on . Since sampling was done a t about mid-day, i t j s reasonable t o suppose t h a t t h i s

supersaturat ion i s a result o f photosynthesis. A t t he end o f November, a no tab le decrease

i n oxygen concen t ra t ion had occurred a t a l l depths, The lowes t concentrat ion was 2.4

rng.l-' a t 150 meters. T h i s event coinc ides with a t ime o f i nc reas ing phytoplankton biomass

and product ion f o l l o w i n g a Spring minimum {F igure 16) . Much d e t r i t u s was observed on m i -

croscope s l i d e s prepared from ma te r i a l co l l e c t ed dur ing th i s season, and it i s probable

t ha t the low d isso lved oxygen r e f l e c t s decomposition of organic matter.

The oxygen d e f i c i t i n the hypol imnion can be roughly r e l a t e d t o pr imary product ion.

In June, j u s t before over turn, the hypo1 imnetic O2 concentrat ion was about 2.5 mg-1-I

-3 (2.5 g-m ) less than the @pi l i m n e t i c concentration. I f t h e resp i ra t i on c o e f f i c i e n t de-

r i v e d from metabol iz ing ep i l imne t i c organic mat ter i s 1.2, about 3.0 grams o f oxygen will

be required to minera l i ze a gram of carbon. Since the hypol imnion w i l l average about 100

-2 -2 rn th i ck , the t o t a l oxygen d e f i c i t i s about 250 gO -m or 83 gC*m ox id ized. This amount

2 o f carbon represents about 57 days of average I4t pr imary production (see below). A1 - though t h i s simple c a l c u l a t i o n neglects a number o f cons iderat ions inc luding t h e ac tua l

oxygen concentrat ion a t great depth, sedimentary storage of carbongand l i t t o r a l and alloch-

thonous inputs, i t s t rong ly suggests t h a t on l y a b u t 205 of net annual product ion is min-

eral ized below the t h e m c l ine .

n I . ? . ~ . : I : . 7 , 3 o ! , r . ? . 3 . o . 1 . ? , I r % r l . r r ~ z ~ . ; r+-+ :T..-. I \

R :. :I I

:'! Z O . l % , ? ? : ' 1 ? . ~ , 7 3 "

1 \ r Ti! , , 1

:st \

m i 8 > t I \\

Figure 7 . Silica p r o f i l e s from Lake T i t i c a c a , 1973.

r. I ';s;~-:!EI: nvV::frl R'. 0: ' I

Figure 8. Dissolved oxygen profiles from Lake Titicaca, 1973.

28

Qi scussi on

The chemistry d a t a r a i s e a number o f i n t e r e s t i n g p o i n t s . One i s t h a t Lake t i t i c a c a ,

i n comon w i t h many other t r o p i c a l lakes, has a low N:P r a t i o . Tall i ng (1 966) remarks on

the same cond i t i on i n Lake Vic to r ia , and Lewis (1974) r epo r t s an extremely low r a t i o o f 0.2

by weight i n the ep i l i rnn ion o f Lake Llano. Of the well stud ied t r o p i c a l lakes, only Lake

George, i n whish N f i x a t i o n i s impor tan t (Horne and Viner, 19711, has a near l y normal r a t i o

of N: P (8:1, Dunn e t a 1 , , 1969). Low N:P r a t i o s may prove t o be general fea tures of t r o p i -

ca l l akes .

A second i n t e r e s t i n g fea tu re o f the chemical regime i s t h e f a i l u r e o f complete mix ing

o f t h e lake in 1973, conf i rming t h e i n d i c a t i o n s based on temperature p ro f i l e s . Low concen-

t r a t i o n s of oxygen pers is ted below 100 rn i n the p r o f i l e s from Ju l y 15 and September 28

(F igu re 8). On August 15 and September 28, t h e 100 rn measurements approach s a t u r a t i o n , bu t

the d e f i c i t a t 170 rn was s t i l l about 2 rng.l-' on September 28 and increased thereafter. I n

1974, by c o n t r a s t , Wayne Wurtsbaugh (personal comun ica t ion) observed saturated values a t

150 m dur ing isothermy.

F i n a l l y , t he h j g h evaporat ion and small ou t f l ow make the lake" dissolved s o l i d con-

cen t r a t i on sensi t ive t o cl ima t i c f l uc tua t ions . The climatic f l uc tua t ions o f the P l e i r t o -

cene p a r t i c u l a r l y may have caused severa l - fo ld changes i n s a l t concen t ra t i on , even w i t h o u t

m a j o r l a ke - l eve l changes. Such long-term i n s t a b i l i t i e s i n chemical segjme are well known

for Af r i can 1 akes (Livingstone, 1975).

BIOLOGICAL LIMNOLOGY

Phytoplan ktan Populat ions

Methods

Phytoplankton were co l l ec ted from the same Van Dorn b o t t l e samples as t he product ion

and chemistry samples. Usual 1y n i ne samples were obtained from the euphot ic zone and

samples were co l l e c t ed from depths up t o 100 m on some occasions. 125 m l samples were pre-

served w i t h Lugo l ' s f i x a t i v e and f i f t y ml were filtered onta 0 .45 p m i l l i p o r e f i l t e r s which

were mounted on s l i d e s us ing the method of Dozier and Richerson (1975). I d e n t i f i c a t i o n s

were made us ing s e t t l e d ma te r i a l and a Wild i n ve r t ed microscope t o supplement t he s l i d e

mounts. The s l i d e s were o p t i c a l l y good, bu t t he re was some c e l l d i s t o r t i o n and the pigments

were severely bleached. Unfor tunate ly , no I i v i n g mate r ia l cou ld be exarni ned.

Organisms on the s l i d e s were enumerated by count ing from 50 t o 100 randomized f i e l d s

a t 1200 x us ing phase cont rast , represent ing a volume of about 0.05 - 0.7 m l . Except i n

very sparse samples f r o m below the euphot ic zone, a minimum o f 250-300 c e l l s were counted.

P a r t i c u l a r l y when the l a rge co lon ies o f Anabaena sphaerica were abundant, we l l ove r 1000

c e l l s were f requent ly enumerated. Biovolume and bioarea o f the phytoplankton populat ions

were determined by approximating each species by one o r a few simple geametr ical forms. For

each species, a sample o f c e l l s izes was assembled by measuring a number o f cel ls (up t o

about 100 for t he dominant organisms) from d i f f e r e n t experiments, and computing an average

volume. Care was taken t o ensure that any s i ze d i f fe rences between dates d i d no t unduly

a f f e c t the average, bu t no major changes i n t he c e l l s i ze o f species from t ime t o t i m e was

observed.

The carbon content o f the biomass was est imated by th ree methods. M u l t i p l i c a t i o n of

biovolume by 0.1 g i ves t he convent ional l i n e a r approximation o f carbon content. I n addi-

t i o n , the regress ion equat ion of Mul l i n , Sloan and Eppley (1966) was used t o est imate car-

bon content f o r i n d i v i d u a l spec ies . the equat ion used i s a f rac t iona l power func t ion ,

LoglO C = 0.76 LoglO V - 0.29

where C i s biomass per c e l l i n picograms carbon and V i s c e l l vatume i n cubic micrometers.

The t h i r d es t ima te o f biomass was made us ing t he r e l a t i o n s h i p repor ted by Mu l l i n , Sloan and

Eppley (1966) between c e l l surface area and carbon content: 0.18 t imes c e l l surface area i n

2 - v gives c e l l carbon i n picograms.

A1 though Mu1 1 i n , Sloan and Eppl ey's regression equations are based on marine diatoms,

+ the power f unc t i on equat ion i s most I i kely a b e t t e r est imate of carbon biomass than the

l i n e a r b i o v o l u m based equat ion f o r the T i t i c a c a species as well. The power funct ion

equat ion takes account o f the f a c t t h a t smaller cells have smal ler o r absent vacuoles and

hence a g rea te r p ropor t ion o f carbon biomass per u n i t volume. The averall average biomass

est imates compare f a i r l y well, but there are systematic d i f fe rences . The area based e s t i -

mate i s about 10% higher than t h a t based on the volume power funct ion equation. The l i n e a r

volume est imate i s much lower than t he o ther two i n the s u m r when c e l l s are small, but

equals or exceeds them i n the w in te r when large c e l l s predominate. The est imate used f o r

f u r t h e r ca l cu l a t i ons and i n graphs is t h a t der ived from the M u l l i n , Sloan and Eppley volume

equat ion unless otherwise noted.

Community pa t te rns i n the phytoplankton were examined by several methods. The d i ve r -

s i t y o f the e p i l imnet ic assemblage (down t o 30 m) was ca lcu la ted f o r each measurement date.

The index employed i s the in format ion t h e o r e t i c s t a t i s t i c , ca lcu la ted us ing t he unbiased

es t imato r (Pie1 ou , 1969), as we1 1 as the convent ional Shannon-Weaver f o m u l a. The d i f f e r -

ences between these est imators i s q u i t e small f o r these data. D i v e r s i t y spectra, an index

o f t he s p a t i a l heterogenei ty of the assernbl age (Margalef , 1969), were computed over depth

by unaveraged and averaged methods. The unaveraged method begins w i t h a given sample ( the

sur face) , f o r which t he o rd inary Shannon-Weaver index i s ca lcu la ted. Then the data of

succeeding depths are added t o the sample and t he index reca lcu la ted . The average p l o t s

f i r s t t he average index of a l l s i n g l e samples, then of a l l adjacent pa i r s , then t r i p l e t s ,

u n t i l the whole se t i s combined t o ca l cu l a t e the same f i n a l va lue a s the unaveragd

method. I f much between-habitat d i f f e rences i n assemblages ex i s t s , i n i t i a l d i v e r s i t y w i l l

be low, but will increase r a p i d l y as samples a r e added. Conversely, r e l a t i v e l y f l a t

divers i ty spectra i nd i ca te less between-habi t a t di f ferences.

Two d i f f e ren t succession indfces were computed t o quan t i f y seasonal changes i n com-

munity composit ion. One index, f i r s t used by Armstrong (1969), i s based on t he changing

con t r i bu t i on o f species t o t he Shannon-Weaver index divers i ty , and can be i n t e rp re ted as a

f r a c t i o n a l r a t e o f change o f i n fo rmat ion :

where Sab i s the d a i l y rate of movement o f the cornun i t y through d i v e r s i t y space, (b-a) i s

the t ime i n t e r v a l i n days,and fia and fIb are the f r a c t i o n a l con t r ibu t ions o f species i t o ,-

diversity on the days o f succeeding measurements such t h a t

- - log f. = X- x-

H -

where xi is the numbers o r biomass o f species i a t date a o r b, X - i s the t o t a l numbers or -

biomass on the corresponding date, and H - Ss the in fo rmat ion t heo re t i c measure o f d i ve rs i t y

en t h a t date.

The second index was developed by Jassby and Goldman (1974a) and i s the r a t e o f change

of biomass compos i t i on :

where

and bi i s the biomass o f t he i t h species a t a par t icular time, and a-b i s the t ime i n t e r - - val between two separate measurements. S . i s analogous t o the A m t r o n g index, b u t does

J n o t w e i g h t a species con t r i bu t i on t o the change jn community compos i t ion by a d ivers i ty

function.

Resul ts

The c o m n e r and more e a s i l y i d e n t i f i e d phytoplankton species occur r ing i n Lake T i t i-

caca dur ing 1973 are l i s t e d i n Table 8. A l s o l i s t e d jn t he table are some comparissns w i t h

ear l i e r r e p o r t s of a7gal occurrences i n Lake T i ticaca based on the diatom determinat ions o f

Frenguell i (1939), the net plankton co l 1 ect ions of the Percy S l aden Trust Expedit ion

(Tu t in , 1940), and Thornasson's (1956) l i s t f rom Puno Bay. The patterns o f biomass change

of dominant and comnon organisms are also shown in Figure 9.

The species composit ion i n 1973 was subs tan t i a l l y d i f f e r e n t from 1937 as judged from

the l argely m in te r - t ime col 1 ect ions of the Percy 51 aden T r u s t Expedi t ion. For example, the

overwhelrni ng dominant o f 1937, Botryococcus Brauni i , was probably absent i n 1973, and

Stephanodiscus astraea repor ted as rare f r o m the 1937 ma te r i a l , was the dominant dur ing the

w i n t e r o f 1973. U6no (1967) a l s o repor ted t h a t . R . - B raun i i dominated the phytoplankton

Other Occurrences - .

Species rCI

Cyanophyta

1. Lyngbya vacuol i f e r a Skuja

Abundance 1973

Dominant September t o November.

None repor ted f r o m T i t i caca . (Tutin, 1940: L n b a aes tuar i i (Mert*i~mann i n Lake Poop6.)

Co-dominant January - February. Dominant November - December.

None reported. 2. Anabaena sphaer ica var. a t tenua ta Bharadwaja

3. Anabaena sp. Uncommon. Thomasson (1 956) : Ana baena sp. i n Puno Bay.

4. Nodul a r i a Harveyana (Thw. ) Thuret

Tu t i n (7940): f a i r l y frequent.

Comon. January - March, November - December.

5. Gloeothece i n c e r t a Skuja Perennial. Common, but n o t abundant.

None reported.

6. G1 oeocapsa puncta t a Naegel i Comon, June - August None repor ted.

1. E l aka to th r i x c f . v i r i d i s (Snow) Printz

Corrrmon, never abundant. None reported.

Tu t i n (1940): comnn. 2. Ulothrix s u b t i l issima Rabenhorst

Perennial , dominant or co-domi nan t .

3. Schroederia s e t i g e r a (Schroed.) Lemmermann

Common, never abundant, None reported.

4. Pediastrum du l e x var . cl athraturn &am)

Rare. Thomasson (1956): i n Puno Bay.

5. Pedi as t r u m c f . Kawra i s kyi Schmi d l e

Rare. Tutin (1940): present.

6. Pediastrum Boryanum (Turp in) Meneghini

Rare. T u t i n (1 940) : uncorrrmon . Thomasson (1 956) : in Puno Bay.

T u t i n (1 940) : uncommon. Thomassan ( 1 956) : i n Puno Bay.

7. Coet astrum c f . mi croporurn Naegel i

Rare .

Tu t i n (1940) : Oocystis gigas Archer var. Bor e i Lemmermann , (synonym -f . Thomasson (1 956) : i n Puno Bay.

Tu t i n (1940) and Thornasson (1956): i n Puno Bay.

8. Oocyst is Borgei Snow Perenni 91 , common t o co-dominant.

9. Ankis trodesmus f a 1 catus var. acicul a r i s ( A . n ) G . S . West

Perennial , common, never abundant .

Tu t i n (1940): Anki s trodesmus Ion i ssima (Lemm.1 WiI le, &

10. C los te r i ops i s longissima v a r . t r o p l c a W. and G.S. West

Perennial , uncommon.

None reported. 11. Selenastrum minutum (Naeg. ) Col 1 i n s

Comon, but not abundant, June - Sept

J u t i n (1 940 ) : Mougeotia sp. 12. Mou e o t i a cf. v i r i d i s h i t t r o c k ( S t e r i l e material only)

Cornon t o subdominant except January; October - December.

Chloraphyta (cont inued)

Tu t i n (1940): Closterium acerosum (Shrank) Ehrenb, f rom wetted mud. Thomasson (1956): 3 species i n Puno Bay.

1 3 , Closterium sp. Perenni a1 , moderately abundant.

14. Staurastrum g r a c i l e Ral f s Perenn ia l , cormnon. Tu t in (1940) : Staurastrum -. .

paradoxurn Meyer, synonm QY

similar tax. r a ~ i l e sccordi t o S k u i ? o - rnasson 18956) : taurartrum sp. i n Puno Bay.

1 5 . Eotr ococcus Brauni i Kits ing Very rare or absent d d o u b t f u l col any observed)

Tut in (1940): heavi ly dominant, June and July. Thomasson (1956): lin Puno Bay.

Chrysophy t a

1. tri bonema ambi guum Skuja Uncommon, August t o December.

T u t i n (1 9401 : T r i bonema sp. from wet ted mud, Capachica stream.

Bac i 1 lariophyta

1 . Cyclotella W h i n i a n a Kijtzing

Frenguel l i (1939). Tho- masson (1956) : i n Puno Bay, a l s o Cyclotella sp.

Frenguelli (1939).

Common, May - June

2. Cyclotel la s t e l l i g e r a C 1 . & Grunow

Corrmon, May - June

3. Stephanodiscus a s t r a e a var. minutula (m Grunow

Cumon t o dominant, March - September.

Frenguel l i (1939). Tu t i n (7940): rare p lank te r .

4. F r a g i l a r i a sp. Rare. Frenguef 1 i (1939) : several species from Puno Bay.

5. Achnanthes l a n c e o l a t a v a r , dubia Grunow

Rare. None repor ted.

6. Cacconei s 1 acentul a var . m x & J m F - -. -

Rare. Frenguel 1 i (1939).

7 . Navicula radiosa Kiltzing Rare. Frenguel 1 i (1939) : i n Puno Bay and Lago PequeEo.

8. Amphiprora -- a l a t a Kijtzing

9. Epithemia argus Kutzing

Raw.

Rare.

None repor ted.

Frenguel l i (1939): i n Puno Bay.

Ryrrophyta

1 . W n o d i n i u m sp. Perennial, common.

Raw -e.

None repor ted.

Tutin (1940): f a i r l y common deep p lank te r . Thomasson (1 956) : Peri -

2. Peridiniurn spp. ( a t least one larse and

dinium ~ i l i e i and m- -- dinium sp. i n Puno Bay. --

Tab le 8. Planktonic algal species i d e n t i f i e d from 1973 Lake T i t i c a c a samples and t h e i r re1 a t i ve numerical abundance. Approximately 200 ramp1 es were examined. Previous reports o f occurrence i n t h e T i t i c a c a p lank ton and nearby hab i t a t s are a l so given.

Anoboena sphoerica

120 Ulolhrix subtilissima

EO O

80

60 - g? 40-

2" 20- +-. Y m 7 40-

Lyngbya vocuolifera 20 L

60b Maugeatio cf. viridis 1 d

Stephanodiscus as traea - - -

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

$ 2E 5 - 20 15-

Figure 9. Patterns o f mean epilimnetic ( top 30 m) biomass fluctuation of the more important species o f planktonic a lgae i n Lake Ti t icaca, 1973. Vertical bars are +I S. E . based on enumeration error and vertical heterogeneity. Note scale differences between the two sets of species.

- Closterium sp - - - - - - A - - * - - Nodularra Morveyono -

5

15 10 5

- Staurestrum grocl le -

7

- - - A = 1 - Floeocapso punctato - - - - -

1 1 1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Mev Dec

during A p r i l , 1961. Some important species are cornon t o the two years, however, i n c l u d i n q

l l lo th r ix subtil i ssima, Oocystis Borgei , Closteriopsis l o n ~ i s s i m a , Mougeotia s p . , and

Stauras trum graci 1 e.

The pa t te rn of d i v e r s i t y o f the e p i l imnetic cornuni ty dur ing 1973 i s shown i n Figure

10. Divers; t y is low during the per iods o f dominance o f Anabaena sphaerica and Ulothr ix

subt i l iss i rna i n the l a t e sp r ing and early sumer period. Values a r e rather cons tan t dur ing

the fa1 1 and winter, though there i s a no t i ceab le d e c l i n e dur ing the period o f deep mixing.

The annual pa t te rn i s most pronounced fo r c e l l numbers because o f the s t rong e f fec t of the

numerous small A. sphaerica c e l l s during the summer.

oL 1 m a I r m r r r r 1 1

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure TO. Variat ion o f epf 1 imnetic phytoplankton d i v e r s i t y . 1973.

Few s ign j f i cant c o r r e l a t i o n s were found between d ivers i ty stat isti cs and o the r var i - ables. Some s i g n i f i c a n t r e l a t i o n s h i p s were found w i t h weather v a r i a b l e s , r e f l e c t i n g the

surrmer low and f a l l -w in te r maximum o f d i v e r z i t y . For example, d i v e r s i t y based on numbers

has a c o r r e l a t i o n w i t h a i r temperature o f r = -.66 ( p < -01, n = 21). Corre la t ions with

f a c t o r s of presurnabl y more d i r e c t causal s i g n i f i c a n c e 1 i ke nutrient concentra t ion. mixing

depth and primary production s t a t i s t i c s are genera l ly weak. Table 9 shows some 05 these

c o r r e l a t i o n s . The expected negat ive r e l a t i o n s h i p t o production s t a t i s t i c s and biomass i s

36

present but i s s i g n i f i c a n t only f o r t h ree of the four comparisons wi th Mullin, Sloan and

Eppley biomass est imates. The major v a r i a t i o n i n d i v e r s i t y was caused by the midsumer

dominance o f the f l o r a by Anabaena sphaerica and U l e t h r i x sub t i l issirna-, bu t the causes o f

t h i s dominance p a t t e r n appear t o be obscure. The most t h a t can be sa id i s t h a t there i s a

weak but gene ra l l y de tec tab le tendency f o r h i gh d i v e r s i t y t o be assoc ia ted w i t h more 07 iga-

t m p h i c per iods. Th is is the expected re1 a t i onsh i p between d i v e r s i t y and a1 igotrophy

according t o Margalef ( 1 968).

ZP ZP n n Hours

o f Dajly Secchi H l xed (day - ' I (hr. ' I ) En B 1 0 v o l m Eloarea B t m a s s PI5 P/B/R, C Sun I n s n l a t ~ o n Depth Ralnfall SfO, Depth

Table 9 . Product-mment correlations beweween the Shannon-Weaver divers1 t y Indlces based on eel 1 rider and b i m s s and selected bloloqical and phys~cal v a r i a b l e s . One asterlsk ~ n d i c a t e r a sign1 f i c a n t d i f ferencr fmm 0 d t the .05 l e v e l , and wo asterirks I n d i c a t ~ the - 0 1 level. Symbols are d~srrlbed i n th* t e x t .

The diversity spectra are f a i r l y f l a t on almost a l l dates (Figure 11). This confirms

t h e idea gained from inspec t ion of t he ran biomass data t h a t r e l a t i v e l y l i t t l e vertical

structure i s present i n the phytoplankton assemblage (Table 17). Individual species never

show marked differences with depth in the euphotic zone.

The succession rate patterns i n Lake Ti t i caca are shown i n F igure 12. The substitu-

t i o n o f numbers f o r biomass causes some a1 t e r a t i o n o f patterns and the d i f f e rence between

the two indices is subs tan t i a l , although all curves have some features i n comon. Table 10

shows the co r re l a t i on between indices. I n the early part o f the year, p a t t e r n s d i f f e r sub-

s t a n t i a l l y between the Amstrong and Jassby-Goldman indices, wh i l e la ter i n the year both

indices show a peak i n August, a minimum i n September, and a second peak i n October or

November, The August peak i s associated with the drop i n production a t t he beginning o f

August and the change i n dominant populations from Stephanodt scus ast raea and Mougeoti a- c f .

v i r i d i s t o Lyngbya - vacuoll i f e ra . The second peak i s associated w i t h t h e decl fne o f L. -

v a c u o l i f e r a a n d the bloom o f Anabaena sphaerica. The generally h igh succession rates early

i n the year, ev iden t i n a17 analyses except the Jassby-Goldman biomass index, are associated

w i t h blooms and declines o f Ulothr ix swbtilissima and Anabaena sphaerica. The low or de-

cl i n j n q rate t h r o u g h the f a l l t o m i d - w i n t e r period i s associated w i t h s m o t h e r and more

modest changes in the popu la t ion o f most species. On the whole, the Jassby-Gwldman index

i s more var iab le, w h i l e the log - func t iona l nature o f Amstrong's index produces a lower-

variance pattern .

- - I I-, 4----- i 't. -1 .: . .

. - 7 - .1- 'I

- - *-*.-- -+.. a-,

_. ..-- -

F i qure 11 . Divers i ty spectra , showing how divers i ty changes as sample s i z e i s increased by adding successive depths beginning a t the surface (unaveraged method).

Jassby-Go1 dman Index

Biomass Number

Jassby-Go1 dman Index

Biomass 1 .OO

Number .41 1.00

Arms t rong Endex

Biomass .35 -38

Number .22 .34

Armstrong Index

'Biomass Number

T a b l e 10. Product moment correlation c o e f f i c i e n t s o f the d i f f e r e n t succession ind ices Double a s t e r i s k indicates s i g n i f i c a n t di f ference from zero a t the .O1 level o f s ign i f i cance . The c r i t i c a l value for the -05 l e v e l i s . 4 4 .

=' JAN F E D MPR &PA MAY JUN JUL RUG SEP 3 C T NW DEC JAN FER MbR APR MAY JUN Jut AUG SEP OCT W3.I Mt

Figure 12. Succession rate per day o f the t a k e T i t i caca phytoplankton assemblage, 1973. Upper graphs i n each case are Jassby-Goldman index and the lower a r e t h e Armstrong index.

Discussion

The phytoplankton assoc ia t ion o f Lake T i t i caca i s not s t r i k i n g l y d i f f e r e n t f r o m other

t r o p i c a l lakes, o r even from temperate lakes f a r t h a t m a t t e r . The species which make up

the assemblage are widespread outs ide Lake T i t i c a c a and n e i t h e r t h i s lake nor o t he r t r o p i c a l

lakes, except perhaps Lake Tanganyika (Cunnington, '1920), appear t o be character ized by a

highly spec ia l i zed t r o p i c a l f l o ra . Nor i s the instantaneous d i v e r s i t y o f the phytoplankton

assemblage outs ide t he range c o m n l y encountered i n temperate f l o r a s .

Some features of the T i t i caca f l o r a do r e f l e c t i t s t r o p i c a l environment. The rela-

t i v e l y deep ep i l imn ion o f the l a k e (almost always deeper than the compensation depth) pre-

vents sys temat ic vertical d i f f e r e n t i a t i o n o f t he p lankton hab i t a t . I n m 5 t temperate lakes

as t ransparent a s Lake T i t i caca , some production would occur below the top o f the thermo-

c l i n e , pe rm i t t i ng a r e l a t i v e l y s tab le v e r t i c a l mu1 t i p 1 i c a t i o n of hab i ta ts . The complex,

sometimes a t e l om ic t i c , s t r uc tu re o f the T i t i c a c a ep i l imn ion i s n o t s u f f i c i e n t l y s t ab l e t o

produce marked populat ion d i f f e r e n t i a t i o n w i t h i n the euphot ic zone, a1 though i t might he lp

diversi fy the assemblage by the non-equi 1 i br ium rnechanl'sms proposed by Hutchinson ( I 961 )

and Richersan , Amtstrong and Go1 dman (1 970).

The sharp di f ferences between the f l o r a observed by t h e Percy S l aden Trus t Expedi t ion

(Tu t in , 1940) and our study i n 1973 could have resu l t ed from e i t h e r a long-term secular

t r end i n l a ke cond i t i ons or f rom the year-to-year v a r i a t i o n o f cond i t i ons r e s u l t i n g from

weak s t r a t i f i c a t i o n . As discussed earl ier , the l a t t e r hypothesis i s q u i t e p l a u s i b l e f o r

t r o p i c a l lakes i n general, a1 though more extensive da ta from t r o p i c a l lakes than i s

presen t l y a v a i l a b l e w i l l be required t o prov ide a d e f i n i t i v e answer. Lake l eve ls were very

s i m i l a r i n 1937 and 1973, although 1937 was i n the middle of a 10 year f a l l i n g and 1973

a t the beginning o f a 6 year r i s i n g trend.

The seasonal changes i n t he phytoplankton assemblage are marked and, by some indices,

the rate o f change i s somewhat var iab le seasonally. Nevertheless, i n so fa r as q u a n t i t a t i v e

comparisons with other 1 akes are poss ib le using previous ca l cu l a t i ons of W i 11 i a m s and

Goldrnan(1975), T i t i c a c a ' s succession rate i s genera l l y low and w i thou t seasonal d i f fe rences

compared t o typical temperate and a r c t i c lakes. However, i t appears t o have a higher and

mere seasonal l y va r i ab l e rate o f succession than Lake V i c t o r i a , and more closely resembles

Lake Tahoe, another deep m n o m i c t i c lake. Wil l iams and Goldman's (797'5) est imate o f

Average Re1 . % Hum. Average Theo- 07. 13. Wind retical Precipi- - 7

Air Temp .(*C) 19 hrs . Speed Insola- t a t i o n Evaporation Month Max. Lfl. 9; - - (m/sec) t i o n (mm) Piche (m) Tank (mn)

Jan. Feb. Mar. dpr. May June July bug - Sept. Oct. Nov . Dec. MEAN:

4 0 4 9 5 2 6 8 8 7 79 8 1 7 7 6 5 7 1 6 1 5 2 - 65 TOTAL:

Tab1 e 7 . Weather summary for Puno, per;, 1973.

Lake Titicaca: Puno, Per6 Altitude 3825 m Mean a i r temperalure 8.5-C Mean onnuol precipita!ian 630 mm Meon annual evoporot ion 1655 mm (PI, 1995 rnm (TI

i?

Evaporation (PI P".\ ,.f=h..n,." x. \

Figure 3 . Climograph f o r Puno. The d a t a p l o t t e d represents the monthly averages fo r the ten-year period 1964-1 973 (1 965-1 973 for evaporation d a t a ) , Evapora t ion g iven for Tank method ( T ) and Piche evaporometer ( P ) . All weather d a t a courtesy o f Servi c i o Naci ona l de Meteorologfa e Hidralogfa , Puno.