limnological studies on a shallow reservoir in western venezuela (tule reservoir).pdf

8/9/2019 Limnological Studies on a Shallow Reservoir in Western Venezuela (Tule Reservoir).pdf

1/7

L i m n o lo g ic a 3 1 ( 20 01 ) 1 39 -1 45 L I M N O L O G I C A

http://www.urbanfischer.de/j ournals/limno

by Urban & Fischer Verlag

D e p a r t a m e n t o d e B i o l o g f a , F a c u l t a d E x p e r i m e n t a l d e C i e n c i a s , U n i v e r s i d a d d e l Z u l i a , M a r a c a i b o , V e n e z u e l a

L i m n o l o g i c a l S t u d i e s o n a S h a l l o w R e s e r v o i r in W e s t e r n V e n e z u e l a

T u l 6 R e s e r v o i r )

RITA PAEZ, GILIA RUIZ, ROMULOMARQUEZ, L u z MARINA SOTO, MARYNES MONTIEL CARLOSLOPEZ

With 6 Fig ures and 2 Tables

Ke y words: Lim nology, neotropics , reservoir , nutr ients , Venezuela , zooplankton, ph ytoplankton prim ary productivity

bstract

Lim nologic al features o f Tul6 Reservoir , a large and shallow reser-

voir in western Venezuela were s tudied from May 1992 to Decem ber

1993. This reservoir is polym ictic , with a low water transparency.

Leve ls o f n i t rogen were h igh and phy top lank ton p roduc t iv i ty was

low. The phy top lank ton p roduc t iv i ty va lues m ay be exp la ined by the

low concentrations of orthophosphate during the larger part of the

samp ling period and the frequent m ixing and resuspension of sedi-

ments . Hetero trophic bacteria were between 1651.67 and 4365.00

Colo ny-form ing Units /ml. Species compo sit ion of the crustacean

zooplankton community was s imilar to other neotropical eutrophic

reservoirs .

In troduct ion

L i m n o l o g i c a l c h a r a c t e r i z a t i o n i s a f u n d a m e n t a l t o o l f o r d e -

s i g n i n g o f t h e c o n s e r v a t i o n a n d m a n a g e m e n t o f la k e s a n d

r e s e r v o i r s , H o w e v e r , e x c e p t f o r B r a z i l i a n r e s e r v o i r s ( T U ND IS I

e t a l. 1 9 9 3 ) , t h e k n o w l e d g e a b o u t n e o t r o p i c a l r e s e r v o i r s i s

g e n e r a l l y v e r y l i m i t e d .

I n V e n e z u e l a , t h e b u i l d i n g o f a r t i f i c i a l r e s e r v o i r s h a s

r a p i d l y i n c r e a s e d i n r e c e n t d e c a d e s ( C R E S SA e t a l . 1 9 9 3 ) , b u t

o u r k n o w l e d g e o n t h e s e w a t e r b o d i e s h a s p r o g r e s s e d v e r y l i t-

t l e s i n c e t h e p r e l i m i n a r y d a t a o f L E W IS & W E I B E ZA H N 1976).

T h e n u m b e r o f V e n e z u e l a n r e s er v o i rs f r o m w h i c h l i m n o l o g i -

c a l i n f o r m a t i o n s a r e a v a i l a b l e i s v e r y l o w ( SO R IA N O & C R E S -

SA 1989 ; CRESSA & SENIOR 1990 ; GONZALEz e t a l . 1991 ;

SOTO et a l . 1 994; INFANTE82; 1NFANTE 19 94; VEG AS-VILA RRU -

B~A 1995 ; SOTO & LOPEZ 1996 ; GONZALEZ & ORTAZ 1998 ;

M O a A L E S & L O P E Z 1 9 9 8 ; M A R I N e t a l . 1 9 9 9 ) , r e p r e s e n t i n g

l e s s t h a n 5 % o f t h e t o t a l n u m b e r o f a r t i f i c ia l r e s e r v o i r s i n t h e

c o u n t r y ( C m c s s A e t a l . 1 9 9 3 ) . O n t h e o t h e r h a n d , m o s t a v a i l -

a b l e d a t a a r e f o r l a r g e a n d d e e p r e s e r v o i r s .

I n t h i s p a p e r , w e d e s c r i b e t h e l i m n o l o g i c a l fe a t u r e s o f a

l a r g e b u t s h a l l o w r e s e r v o i r ( T u l 6 R e s e r v o i r ) a n d t h e i r s p a t i a l

a n d s e a s o n a l v a r i a t i o n s .

Study Si te

T u l 6 R e s e r v o i r i s a l a r g e ( 5 6 .6 k m 2) a n d s h a l l o w w a t e r b o d y

( m e a n d e p t h 7 . 2 m a n d m a x i m u n d e p t h 9 .7 m ) , l o c a t e d a t

3 4 m a b o v e s e a l e v e l i n w e s t er n V e n e z u e la ( 1 0 5 3 ' N -

7 2 0 8 ' W ) ( F i g . 1) . C o n s t r u c t i o n w a s c o m p l e t e d i n 1 97 1 a n d

t h e i m p o u n d m e n t i s u s e d t o s u p p l y w a t e r t o t h e m a i n c i t i e s o f

t h e r e g i o n f o r i r r i g a t i o n a n d f o r f l o o d c o n t r o l .

T h e m a i n t r i b u t a r i e s a r e t h e C a c h i r f R i v e r , C o l o r a d o

S t r e a m a n d t h e d i s c h a r g e c h a n n e l f r o m S o c u y R e s e r v o i r . T h e

b e d r o c k o f th e d r a i n a g e a r e a i s h e t e r o g e n e o u s . T h e s u r r o u n d -

i n g v e g e t a t i o n i s r e p r e s e n t e d b y t r o p i c a l s e m i d e c i d u o u s f o r -

e s t a n d z o n e s w i t h i n t e n s e a g r i c u l t u r a l a c t i v i t i e s

SMITH

1 9 8 5) . T h e d r a i n a g e a r e a h a s a b i m o d a l p a t t e r n o f p r e c i p i t a -

t io n w i t h m a x i m u m p e a k s i n A p r i l - M a y a n d O c t o b e r -

N o v e m b e r ( Es PI NO Z A 1 9 9 2) . A v a i l a b l e l i m n o l o g i c a l i n f o r m a -

t i o n s a r e r e s t r i c t e d t o p h y t o p l a n k t o n s p e c i e s c o m p o s i t i o n

( YA C UB SO N 1 9 8 0 ) , a c h e c k l i s t o f r o t i f e r s p e c i e s ( D ~ A z &

C A ST EL LA N O 1 9 8 8 ) , t h e m i c r o b i o l o g i c a l q u a l i t y o f t h e w a t e r s

( R u l z & M O N T IE L D E M O R A L E S 1 9 9 6 ) a n d e p i z o a n s o n c r u s -

t a c e a n z o o p l a n k t o n ( L O PE Z e t a l . 1 9 9 8 ) .

M e t h o d s

Samples were taken at mon thly or a t longer in tervals in two sam pling

periods. During the f irs t period, a to ta l of 9 samplings were per-

forme d from Ma y 1992 to Ap ril 1993. Vertical series of sam ples were

0 0 7 5 - 9 5 1 1 / 0 1 / 3 1 / 0 2 - 1 3 9 1 5 . 00 / 0 1 3 9


2/7

2 0 2 0 w 7 2o 11 0 w 2 0 0 w

T U I 6 t o ~ 6

R e s e r v o i r S am p in g

C h a n n e lr o m S t a t i o n s

S c u y t ~ i

C a c h i ri r, ,, ._ . .t _ ~ f ~ / ~ ) - ~ 1 0 O 0 N

R i ve r / f U v

f C o lo r a d o L a P ro v i n c i a

u t r e ~

Fig . 1 . Locat ion o f Tul6 Reservo i r and sampl ing s ta tions.

taken each 2 m from su rface to bo t tom using a Van Dorn sam pler (21)

only a t Sta t ion A, which i s located near the dam (Fig . 1 ) . At th is s ta-

t i o n , we ma d e t h e fo l l o wi n g me a su re me n t s : t e mp e ra t u re wa s me a -

su re d wi t h a t h e rmo m e t e r i n s i d e th e sa mp l e r ; d i s so l v e d o x y g e n wa s

d e t e rmi n e d b y Wi n k l e r me t h o d ; p H a n d c o n d u c t i v i ty we re me a su re d

wi t h a p o r t a b l e se n so r e q u i p me n t a n d wa t e r t r a n sp a re n c y wi t h a

whi te Secchi d isk , 20 cm in d iameter . Measurements of a lkal in i ty ,

d isso lved s i l ica , to ta l phosphorus, to ta l n i t rogen and ammonium

were performed accord ing to s tandard methods (MACKERETHet al .

1979; A.RH.A. 1992; GOLTERMAN t al . 1971). Orthophosphate, ni-

t r i te and n i t ra te were determined by ion chromatography . Phyto-

p l a n k to n p ro d u c t i v i t y wa s m e a su re d b y d e t e rmi n i n g t h e c h a n g e s i n

oxyg en concent ra t ion in dark and c lear bo t t les , incubated in s i tu ' ,

during 4 hours th roughout water co lumn. Crustacean zooplankton

was co l lec ted wi th a conica l p lankton net (65 jam mesh aperture)

t o we d t h ro u g h t h e wa t e r c o l u m n ( f i l t e re d v o l u m e a b o u t 0 .5 m3 ) . Or -

g a n i sms we re f i x e d wi t h a c i d i f i e d Lu g o l ' s so l u t i o n a n d p re se rv e d

wi t h fo rma l d e h y d e so l u t i o n (4 %). Su b sa mp l e s w e re o b t a i n e d u s i n g a

Folso m subsampler, and count ing was made in Pet r i d i shes. Bentho s

sa mp l e s w e re c o l l e c t e d wi t h a n Ek rn a n d re d g e o f 6 2 5 c m 2 a re a , s a m-

p l e s we re s i e v e d th ro u g h a 0 .1 2 5 m m m e sh a n d we re p re se rv e d wi t h

fo rma l d e h y d e so l u ti o n .

During the second period of sampl ing , a to ta l o f 7 sampl ings was

p e r fo rme d f ro m Ju n e t o De c e mb e r 1 9 9 3 . On l y o n e su r fa c e sa mp l e

was co l lec ted a t each sam pl ing date a t a l l s tat ions (A -E ) (Fig . 1 ) .

Samples for he tero t rophic bacteria were co l lec ted in a s ter i le bo t t le

a n d we re c o u n t e d b y t h e Po u r P l a t e M e t h o d (A.R H.A. 1 9 92 ). At e a c h

of the s ta t ions, temperature , pH, w ater t ransparency , conduct iv i ty ,

d isso lved oxyg en , n i t r ite , n it rate , and to ta l phosphoru s w ere me a-

su re d a c c o rd i n g t o t h e me t h o d s p re v i o u s l y d e sc r ib e d .

R e s u l t s a n d D i s c u s s i o n

T e m p e r a t u r e t r a n sp a r e n c y a n d d i s s o l v e d

o x y g e n

W a t e r t e m p e r a t u r e r a n g e d b e t w e e n 2 8 a n d 3 3 C , w i t h a m e a n

v a l u e o f 2 9 . 6 C , t h r o u g h o u t t h e c o l u m n o f w a t e r a t S t a t i o n A .

S o m e m i x i n g p e r i o d s a l t e r n a t in g w i t h p e r i o d s o f t h e r m a l

s t r a t if i c a t io n ( M a y , O c t o b e r 1 9 9 2 a n d F e b r u a r y 1 9 9 3) w e r e

o b s e r v e d ( F i g . 2) . M a x i m u m d i f f e r en c e b e t w e e n t h e s u r f a c e

a n d t h e b o t t o m w a s 3 C i n O c t o b e r 1 9 9 2. T u l 6 R e s e r v o i r

m a y b e c o n s i d e r e d a s a w a r m p o l y m i c t i c w a t e r b o d y . T h i s

t h e r m a l p a t t e r n is c o m m o n i n s h a l l o w n e o t r o p i c a l r e s e r v o i r s

( TU N D I SI e t a l . 1 9 9 3 ; H E N R Y 1 9 9 5 ) . I n T u l 6 R e s e r v o i r f r e -

q u e n t m i x i n g e v e n t s a r e r e l a te d t o t he s t r o n g w i n d s a n d a b -

s e n c e o f h i ll s o r m o u n t a i n s a r o u n d t h e r e s e r v o ir .

W a t e r t r a n s p a r e n c y w a s b e t w e e n 0 . 4 0 a n d 0 . 9 6 m , w i t h a

m e a n v a l u e o f 0 . 51 m a t S t a ti o n A . A t t h i s s t a ti o n , l o w e r v a l -

u e s o c c u r r e d d u r i n g m i x i n g p e r i o d s ( F i g . 2 ) . I n t h e r e m a i n i n g

s t a t io n s ( B - E ) , t r a n s p a r e n c y w a s b e t w e e n 0 . 1 8 a n d 0 . 8 1 m

a n d l o w e r m e a n v a l u e s w e r e f o u n d n e a r t r i b u ta r i e s

( T a b l e 1 ). T r a n s p a r e n c y a p p e a r s t o d e p e n d p r i m a r i l y o f r e s u s -

p e n s i o n o f s e d i m e n t f r o m t h e b o t t o m , a n d c o n t r i b u t io n f r o m

t h e c a t c h m e n t o f s u s p e n d e d o r g a n i c a n d i n o r g a n i c p a r t i c le s ,

p a r t i c u l a r l y d u r i n g t h e r a i n y s e a s o n .

D i s s o l v e d o x y g e n f l u c t u a t e d b e t w e e n 4 . 4 a n d 9 . 0 m g / 1 ,

w i t h a m e a n v a l u e o f 6 . 5 m g /1 in w a t e r c o l u m n a t S t a t i o n A .

C h a n g e s i n v e r t i c a l d i s t r i b u t io n ( F i g . 3 ) w e r e r e l a t e d t o t h e r -

1 4 0 L i m n o l o g i c a 3 1 ( 2 0 0 1 ) 2


3/7

Fig. 2 . Temporal ch anges of transparency

(Secchi disk) and ve rtical profiles of tempera-

ture (C) at Station A.

0

1

2

E

r 4

Q . .

5

a

M

J A O J F M A

9 9 2 9 9 3

g

0

1

2

E 3

V

e -

4

(I)

1:1

7.

Fig. 3 . Temporal variations of profiles of dis-

solved oxyg en (rag/l) a t Station A.

M

J A J F M A

9 9 2 9 9 3

m a l s t r a t i f i c a ti o n e v e n t s . O n t h e s u r f a ce , v a l u e s o f d i s s o l v e d

o x y g e n w e r e c o m p a r a b l e b e t w e e n s t a t i o n s ( T a b l e l ) . C a l c u -

l a t e d v a l u e s o f s a t u r a t i o n f o r t h e w a t e r c o l u m n a t S t a t i o n A

m o s t l y s h o w a s u b s a t u r a t i o n w i t h v a l u e s b e t w e e n

5 7 . 9 - 1 1 9 . 7 , t h e m e a n v a l u e w a s 8 8 .6 . O n t h e b o t t o m ,

o v e r s a t u r a t i o n w a s f o u n d o n l y d u r i n g t h e m i x i n g p e r i o d s .

T w o f a c t o r s c a n e x p l a i n th e o x y g e n d e f i c i t o n t h e b o t t o m d u r -

i n g t h e t h e r m a l s t r a t i fi c a t i o n p e r i o d s : h i g h r a t e s o f d e c o m p o -

s i t i o n o f o r g a n i c m a t t e r a n d l o w c o n t r i b u t i o n s o f o x y g e n

f r o m p h o t o s y n t h e t i c a c t i v i t i e s , a s a c o n s e q u e n c e o f l o w t r a n s -

p a r e n c y .

Cond uct iv i ty a lka l in i ty p H and di ssolved

si l ica

C o n d u c t i v i t y w a s b e t w e e n 4 0 . 9 a n d 1 9 1 . 5 p S / c m , w i t h a

m e a n v a l u e o f 1 5 0 .3 ~ S / c m f o r w a t e r c o l u m n a t S t a t i o n A

( T a b l e 1 ) . D u r i n g t h e t h e r m a l s t r a t i f ic a t i o n p e r i o d s , l o w e r

v a l u e s w e r e o b s e r v e d o n t h e b o tt o m .

A l k a l i n i t y r a n g e d b e t w e e n 3 6 a n d 9 4 r a g/ 1 f o r b i c a r b o n -

a t e s , a n d 0 t o 1 6 m g /1 f o r c a r b o n a t e s i n S t a t i o n A , w h i l e t h e

p H v a l u e s w e r e b e t w e e n 6 . 8 a n d 9 . 6 , w i t h a m e a n v a l u e o f 7 . 8

a t t h e s a m e s t a ti o n . L o w e r v a l u e s w e r e f o u n d o n t h e b o t t o m ,

L i m n o l o g i c a 3 1 ( 2 0 0 1 ) 2 1 41


4/7

Ta b l e 1 . M e a n v a l u e s o f so me l i mn o l o g i c a l fe a t u res o f Tu 16 R e se rv o i r. M a x i m u m a n d mi n i mu m v a l u e s i n p a re nt h e s is . In s t a ti o n A , me a n

values were ca lcu la ted from samples co l lec ted th roughout the co lumn of water be tween May 1992 and Apri l 1993 . In the remain ing s ta t ions,

me a n v a l u e s w e re c a l c u l a te d f ro m su r fa c e sa mp l e s c o l l e c t e d b e t w e e n Ju n e a n d D e c e mb e r 1 9 93 .)

Pa ra me t e r A * B * * C * * D * * E* *

Depth 6 .8 2 .1 1 .9 3 .3 4 .0

m) 6 -7 .2 ) 0 .6 -4 ) 0 .5 -3 .5 ) 2 -5 .2 ) 2 .5 -5 .1 )

Transparen cy 0 .51 0 .33 0 .42 0 .62 0 .67

m) 0 .4 0 -0 .9 6 ) 0 .2 1 -0 .4 7 ) 0 .1 4 -0 .5 2 ) 0 .4 5 -0 .6 1 ) 0 .4 6 -0 .8 1 )

Diss o lved oxyg en 6 .5 6 .9 6 .5 6 .7 6 .3

mg/1) 4 .4-9 .4) 3 -7 .6) 4 .4-9 .7 ) 6 -7 .2) 6 .1-7 .2)

C o n d u c t iv i ty 150.3 . . . .

p S / c m) 4 0 .9 -1 9 1 .5 )

T ota l a lk a lin ity 7 0 . . . .

mg CaCO3/1) 36-94 )

pH 7.8 8.1 8.3 8.2 8.4

6 .8-9 .8) 7 .7-9 .8) 7 .8-8 .9) 7 .8-8 .9) 7 .7-8 .4)

N = number o f samples. *Depth and transparency N = 9). Mean values of remaining features were calculated from vertical samples N = 44). ** N -- 7.

Ta b l e 2 . M e a n v a l u e s o f n u tr i e n ts a n d h e t e ro t ro p h i c b ac t e r ia . M a x i mu m a n d m i n i mu m v a l u e s in p a re n th e s i s. In s t at i o n A , e x c e p t i n g h e t ero -

t ro p h ic b a c t e r ia , m e a n v a l u e s w e re c a l c u l a te d f ro m sa mp l e s c o l l e c t e d th ro u g h o u t t h e c o l u mn o f w a t e r b e t w e e n M a y 1 9 92 a n d A p r i l 1 9 93 .

In re ma i n i n g s t a t io n s , me a n v a l u e s w e re c a l c u l a t e d f ro m su r fa c e sa mp l e s c o l l e c t e d b e t w e e n Ju n e a n d D e c e m b e r 1 9 93 .)

Pa ra me t e r A * B * * C * * D * * E* *

D iss o lv e d s i l ica 5.2 . . . .

rag / l ) 3 .3-10 .6)

Nitr i te 35.7 30.8 39.8 30.0 32.7

p g /1 ) 0 -3 0 0 ) 8 -6 0 ) 1 4 -7 2 ) 1 2 -4 0 ) 9 -6 6 )

Nitra te 45.0 28.2 21.5 33.1 18.1

~ag/1) 2-15 4) 2-50) 4-49 ) 2-61 ) 2-38)

A m m o n iu m 28 0 . . . .

pg /1) 43-1030)

T ota l n it ro g e n 67 0 . . . .

,ug/1) 0-1680)

Orthophosph ate 27 108 338 8 9

g g /1 ) 4 .0 -1 5 0 ) 1 0 -3 0 8 ) 1 0 -6 0 9 ) 1 -2 0 ) 1 -2 0 )

T ota l ph o sp ho ru s 120 . . . .

pg /1) 30-381)

Hetero t ro phic bacteria 2870 .42 4365 .00 3688 .75 1676 .67 1651 .67

C o l o n y - fo rmi n g u n i t s /ml ) 9 9 8 -9 1 0 0 ) 8 1 0 -8 2 8 9 ) 6 7 8 -8 3 8 7 ) 1 1 0 0 -2 8 9 0 ) 5 6 7 -7 4 6 4 )

N = number fo samples. * Mean values of heterotrophic bacteria w ere caIculated from surface samples N = 7). In remaining features N = 44). ** N = 7.

d u r i n g t h e t h e r m a l s t r a t i f i c a ti o n p e r i o d s . O n t h e s u r f a c e s a m -

p l e s , p H w a s a b o u t 8 . 1 a n d 8 . 4 T a b l e 1 ) .

D i s s o l v e d s i l i c a w a s p r e s e n t i n t h e r a n g e o f 3 . 3 - 1 0 . 6 m g / 1,

w i t h a m e a n v a l u e o f 5 . 2 m g /1 a t S t a t i o n A T a b l e 2 ) .

hosphorus and nitrogen

O r t h o p h o s p h a t e w a s d e t e c t e d i n M a y , J u l y a n d A u g u s t , 1 9 9 2

a n d J a n u ar y , 1 9 9 3, w h e n t h e c o n c e n t r a t i o n w a s b e t w e e n

4 . 0 a n d 1 5 0 ~ g / l t h r o u g h o u t t h e w a t e r c o l u m n a t S t a t io n A .

T h e m e a n v a l u e w a s 2 7 p g /1 T a b l e 2 ) . O n s u r f a c e s a m p l e s

f r o m o t h e r s t a t i o n s , m e a n v a l u e s w e r e f o u n d b e t w e e n 9 a n d

3 3 8 ~ tg /1 , h i g h e r v a l u e s w e r e f o u n d n e a r t o t r i b u t a r i e s a t

s t a t io n s B a n d C T a b l e 2) . T o t a l p h o s p h o r u s w a s b e t w e e n 3 0

a n d 3 8 1 p g / 1 , w i t h a m e a n v a l u e o f 1 2 0 p g/ 1 i n S t a t io n A

T a b l e 2 ) .

N i t r a t e c o n c e n t r a t i o n f l u c t u a t e d b e t w e e n 2 a n d 1 5 4 p g / 1 ,

w i t h a m e a n v a l u e o f 4 5 p g/ 1 t h r o u g h o u t t h e w a t e r c o l u m n a t

1 4 2 L i m n o l o g i c a 3 1 2 0 0 1 ) 2


5/7

3 8 0 0 0

~ 6 0 0 0

4 0 0 0

'~

2 0 0 0

[ ~ A

IE~I B

~ c [ ] D

t

S

1 9 9 3

350

N

D

Fig. 4. T emporal fluctuations of heterotrophic bacteria (Colony-

forming units/ml) in sam pled stations.

300

5 0

2 5 0

e l

E 20o

150

E

100

1 0 0

[ ] G r o s s P r o d u c t i v it y

[ ] N e t P r o d u c t i v it y

M J J A J

1 9 9 2

g M

1 9 9 3

Fig.

5

Temporal variations o f phytoplankton productivity (mg C/m3h)

at Station A.

m a

L 80

I1)

L

6 0

..Q

E 4 0

z

2 0

S t a t io n A ( T a b l e 2) . M a x i m u m v a l u e s w e r e f o u n d i n t h e p e r i-

o d o f th e r m a l s t r at i fi c a ti o n . I n s u r f a c e s a m p l e s , m e a n v a l u e s

were in the range o f 18 .1 -33 .1 ~g /1 (Tab le 2 ) . N i t r i t e was be -

t w e e n u n d e t e c t e d v a l u e s a n d 3 0 0 p g / 1 , w i t h a m e a n v a l u e o f

35 .7 ~ag /1 n S ta t ion A. In the o the r s t a t ions m ean va lues w ere

i n th e r a n g e o f 3 0 .0 - 3 9 .8 p g/ 1 (T a b l e 2) . A m m o n i u m w a s b e -

twee n 43 an d 1030 ~g /1, wi th a mea n va lue o f 280 lag /1 in

S ta t ion A (Tab le 2 ) . A t the same s ta t ion , to ta l n i t rogen was

be tw een 3 .0 and 16 80 gg /1 , wi th a me an va lu e o f 670 ~ tg /1

(Tab le 2 ) . Thes e l eve l s o f n i t rogen nu t r i en t s a re h igher in

c o m p a r i s o n t o t h e v a l u e s r e p o r t e d f r o m o t h e r n e o t r o p i c a l

rese rvo irs (CRESSA SENIOR 1990; TUND~S~ et al. 1993) a nd

s h o w p r i m a r i l y t h e e f f e c t s o f i n f l o w s o f n u t r i e n t- r i c h

h y p o l i m n e t i c w a t e r s f r o m S o c u y R e s e r v o i r i n t o T u l 6 R e s e r -

vo i r th roug h the d i scharge c hanne l (SoTO e t a l. 1994) and the

e f fec t s o f ag r icu l tu ra l ac t iv i t i e s in the ca tch men t a rea .

I n l a k e s a n d p o n d s , r a t i o N : P h a s b e e n u s e d s u c e s s f u l l y t o

es tab l i sh l im i t ing nu t r i en t . VOLLENWEIDER 1968) p ro pos ed a

r a t io o f 9 : 1 ( N : T P ) f o r p h y t o p l a n k t o n . W a t e r b o d i e s w i t h a

ra t io h igher than 9 :1 a re po ten t ia l ly l imi ted fo r phosphorus .

W h e n r a t io i s l o w e r t h a n 9 : 1 , t h e w a t e r b o d y i s li m i t e d f o r n i -

t rogen .

I n T u l 6 R e s e r v o ir , m e a n r a t i o N : P i s 1 2 .8 :1 , a c c o r d i n g t o

va lues s ugges ted by VOLLENWEIDER 1968) , p hosp horu s can

be the l imi t ing nu t r i en t in th i s sys tem. S imi la r s i tua t ions have

been repor ted fo r o the r neo t rop ica l r e se rvo i r s (GIANESELLA-

GALVAO 1985 ; CRESSA SENIORS 1990 ; TUNDISIet al . 1993;

M A R IN e t a l. 1 9 9 9 ). H o w e v e r , s o m e p r o b l e m s c a n e m e r g e ,

w h e n m o d e l s r e l a t i n g d i r e c t l y p h y t o p l a n k t o n p r o d u c t i v i t y

wi th nu t r i en t s ava i lab i l i ty a re app l ied to rese rvo i r s . Accord-

ing to LrND e t a l. (1993) , such a b io t i c f ac to r s a s tu rb id i ty an d

s h o r t w a t e r -r e s i d e n c e t i m e c a n b e o f g re a t e r i m p o r t a n c e f o r

a lga l p roduc t ion in rese rvo i r s than the av a i lab i l iy o f nu t r i-

en t s . Pa r t i cu la r ly , in sha l low and po lymic t ic r ese rvo i r s ,

w h e r e u s u a l l y t h e m i x i n g d e p t h e x c e e d s t h e d e p t h o f t h e e u -

p h o t i c z o n e , l o w l i g h t a v a il a b i li t y l i m i ts p h y t o p l a n k t o n p r o -

d u c t i o n ( K I M M E L GROEGER 1984 ; KIMMELet al . 1990).

Hetero trophic bacteria

M e a n v a l u e s o f h e te r o t r o p h i c b a c t e r i a w e r e f o u n d t o b e b e -

t w e e n 1 6 5 1. 6 7 a n d 4 3 65 . 0 0 C o l o n y - f o r m i n g u n i t s / m l . T h e

h i g h e s t v a l u e s w e r e f o u n d i n s t a ti o n s B a n d C , w h i c h a r e t h e

neares t to t r ibu ta r ies (Tab le 2 ). Max im um va lue s were

o b s e r v e d d u r i n g t h e r a i n y s e a s o n ( F i g . 4 ) . T h i s p a t t e rn o f s p a -

t i a l and temp ora l ch ange s a l so show s the e f fec t s o f the con t r i -

b u t i o n s o f o r g a n i c m a t t e r f r o m t h e t r i bu t a r ie s a n d r u n - o f f

wa te r s .

M J J A O J F M A

1 9 9 2 1 9 9 3

Fig. 6. Temporal fluctuations of crustacean zooplankton at Station A.

Phytoplankton productivity

T h e m e a n v a l u e o f g ro s s p r o d u c t i v i t y t h r o u g h o u t w a t e r c o l -

u m n a t S t a t io n A w a s 1 3 1 .6 6 m g C / m 3 h a n d n e t p r o d u c t i v i -

t y w a s 3 8 .1 5 m g C / m 3 h . H i g h e r v a l u e s w e r e f o u n d n e a r t h e

sur face and decreased i r regu la r ly to the bo t tom. Seasona l ly , a

L i m n o l o g i c a 3 1 ( 2 0 0 1 ) 2 1 43


6/7

m a x i m u m p e a k i n g r o s s p r o d u c ti v i t y w a s o b s e r v e d i n M a y

1 9 92 a n d t w o m i n o r p e a k s i n O c t o b e r , 1 9 9 2 a n d M a r c h , 1 9 93

( F i g . 5) . T h e p e a k i n M a y 1 9 9 2 c o r r e s p o n d e d t o t h e h i g h e s t

v a l u e s o f w a t e r t r a n s p a r e n c y ( F i g . 2 ) . P r o d u c t i v i t y v a l u e s o f

p h y t o p l a n k t o n a t S t a t i o n A a r e l o w e r w i t h r e s p e c t t o o t h e r

n e o t r o p i c a l r e s e r v o i r s ( S o Pd A N O C R E SS A 1 9 8 9 ; T U N DI SI e t

a l . 1 9 93 ) . T h i s s i t u a t io n m a y b e e x p l a i n e d b y t h e l o w c o n c e n -

t r a t io n s o f o r t h o p h o s p h a t e d u r i n g t h e b i g g e r p a r t o f t h e s a m -

p l i n g p e r i o d a n d t h e f r e q u e n t m i x i n g a n d r e s u s p e n s i o n o f

s e d i m e n t s . A l s o , m i x i n g a n d r e s u s p e n s i o n o f s e d i m e n t s c a n

e x p l a i n t h e s i g n i f i c a n t d i f f e r e n c e b e t w e e n v a l u e s o f g r o s s a n d

n e t p h y t o p l a n k t o n p r o d u c t i v i t y i n T u l 6 R e s e r v o i r . I n t u r b i d

a n d w e l l m i x e d r e s e r v o i r s , p l a n k t o n i c a l g a e a r e e x p o s e d n o t

o n l y t o r a p i d l y f l u c t u a t i n g l i g h t b u t i n t e r m i t t e n t l y t o a p h o t i c

con d i t io ns (KIMMEL e t a l . 1990) .

Zoop lankton and benthos

T h e c r u s t a c e a n z o o p l a n k t o n c o m m u n i t y a t S t a t io n A w a s

c o m p o s e d o f o n e c a l a n o i d c o p e p o d Notodiaptomus rnara-

caibensis

K I E FE R ), tW O c y c l o p o i d c o p e p o d s

T hermocyc l ops

decipiens KIEFER, Mesocyclops brasi l ianus K I E FE R ) a n d f o u r

c l a d o c e r a n s p e c i e s Cer i odaphni a cornu t a S A R S ; B osmi na

tubicen BREHM; M o i n a sp . ; D i a p h a n o s o m a s p p . ). A p a r t f r o m

M o i n a s p ., w h i c h w a s e x t r e m e l y r a r e, a l l c r u s t a c e a n t a x a

w e r e p r e s e n t t h r o u g h o u t t h e s a m p l i n g p e r i o d . N a u p l i i a n d

c y c l o p o i d c o p e p o d s w e r e d o m i n a n t t a x a , e x c e p t f o r t he p e r i -

o d M a r c h - A p r i l 1 9 9 3, w h e n

B. tubicen

s h o w e d m a x i m u m

a b u n d a n c e a n d w a s t h e d o m i n a n t t a x o n ( F i g . 6 ) . D o m i n a n c e

o f c y c l o p o i d c o p e p o d s , e s p e c i a l l y T . decipiens, a n d s m a l l

c l a d o c e r a n s p e c i e s , i s a t y p i c a l f e a t u r e o f c r u s t a c e a n z o o -

p l a n k t o n c o m m u n i t i e s i n e u t r o p h i c r e s e r v o i r s i n S o u t h

A m er ica (ARCIFA 1984 ; ARCIFA e t a l . 1992 ; L6PEZ BELLO

1993) .

A b u n d a n c e o f c r u s t a c e a n z o o p l a n k t o n a t s t a t i o n A f l u c t u a t -

e d b e t w e e n 2 4 . 9 a n d 1 0 8 .5 i nd /1 . M a x i m u m v a l u e s w e r e f o u n d

b e t w e e n M a y a n d J u l y d u r i n g t h e r a i n y s e a s o n ( F i g . 6) . R a n g e

o f b o d y s i z e i n c r u st a c e a n z o o p l a n k t o n w a s b e t w e e n 0 . 3 5 - 1 . 2

m m . A c c o r d i n g t o A R C I FA 1 9 8 4 ) a n d A R C~ FAet a l . (1992) , th is

r a n g e o f b o d y s i z e is s m a l l i n c o m p a r i s o n w i t h v a l u e s r e p o r t e d

t o t e m p e r a t e l a k e s . T w o e p i z o a n t a x a , Synedra s p . ( a p e n n a t e

d i a t o m ) a n d Epistylis s p . ( a c i l i a t e p r o t o z o a ) , w e r e f o u n d o n

e x o s k e l e t o n o f c y c l o p o i d c o p e p o d s a n d c l a d o c e r a n s p e c i e s .

M a x i m u m p r e v a l e n c e o f t h e s e e p i b i o n t s o n c r u s t a c e a n s w a s

f o u n d i n t h e e a r l y d r y s e a s o n ( L6 P EZ e t a l . 1 9 9 8 ) .

T h e b e n t h i c c o m m u n i t y w a s v e r y p o o r , o n l y

Chaoborus

s p e c i e s l a r v a e w e r e c o l l e c t e d a t S t a t i o n A . I n c o n t r a s t w i t h

S o c u y R e s e r v o i r (L O P EZ C R E SS A 1 9 9 6 ) , w h i c h is c o n n e c t -

e d w i t h T u l 6 R e s e r v o i r t h r o u g h t h e d i s c h a r g e c h a n n e l , i n T u l 6

R e s e r v o i r Chaoborus s p e c i e s l a r v a e s h o w e d l o w p o p u l a t i o n

d e n s i t i e s a n d w e r e f o u n d o n l y i n t h e b e n t h o s .

S p e c i e s c o m p o s i t i o n a n d a b u n d a n c e o f f i s h c o m m u n i t i t e s

a r e p o o r l y s t u d i e d . H o w e v e r t h e p r e s e n c e o f p l a n k t i v o r o u s

f i s h e s s u c h A s t yanax s p p . a n d Xenomelanir i s s p p . i s k n o w n

i n t h i s r e s e r v o i r .

Ack now ledgem ent : We express our thanks to s ta f f o f Labora to -

rio de Qufmica Amb iental in Facultad Experimental de Ciencias o f

Unive rsidad del Zulia , and to Insti tuto p ara la Conservaci6n de Lag o

de M aracaibo for some chem ical analyses, and J.J . EWALD or revi-

sion of English grammar. S pecial thanks to Consejo de Desa rrollo

Cienfffico y Human/stico de la U niversidad del Zulia for f inancial

support. Corrections and suggestions from anonymous referees im-

proved g reatly the manuscript.

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Received: February 16 , 2000

A c c e p t e d : A u g u s t 1 7 , 2 00 0

Correspondence address Prof. CARLOS LOPEZ, D e p a r t a me n t o d e

B i o l o g i a , Fa c u l ta d E x p e r i me n t a l d e C i e n c i a s , U n i v e rs i d a d d e l Zu l i a ,

A p d o . 1 5 29 9 , Zo n a P o s t a l 4 0 0 5 -A , La s D e l i c i a s , M a ra c a i b o , V e n e -

z u e l a ; e -ma i l : c a r l u z @l u z .v e

L i m n o l o g i c a 3 1 ( 2 0 0 1 ) 2 1 45

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