Composition, abundance and biomass of zooplankton in Orinoco

Annls
Limnol.
28 (1) 1992 : 3-18
Composition, abundance and bîomass of zooplankton in Orinoco
fioodplain lakes, Venezuela
E. Vâsquez
J. Rey2
1
K e y w o r d s : Z o o p l a n k t o n , species c o m p o s i t i o n , a b u n d a n c e , b i o m a s s , f i o o d p l a i n l a k e s , O r i n o c o R i v e r , V e n e z u e l a .
Z o o p l a n k t o n s a m p l e s w e r e c o l l e c t e d o v e r a 2 1 - m o n t h p e r i o d in five f i o o d p l a i n l a k e s of t h e O r i n o c o R i v e r in o r d e r
t o e s t a b l i s h z o o p l a n k t o n species c o m p o s i t i o n , a b u n d a n c e p a t t e r n s , a n d b i o m a s s a n d t h e i r r e l a t i o n s h i p s with t h e p a t t e r n s
o f i n u n d a t i o n s a n d l a k e m o r p h o m e t r y . 60 r o t i f e r taxa w e r e identified. T h i s g r o u p w a s generally m o r e a b u n d a n t t h a n
c o p e p o d s a n d c l a d o c e r a n s ( m e a n : 7 3 , 4 % of t o t a l z o o p l a n k t o n ) . C o m m o n a n d a b u n d a n t rotifer species i n c l u d e d AT.
americana, K. cochleahs,
B. mirus, B. gessneri, P. vulgaris a n d F. longiseta. C o p e p o d s w e r e d o m i n a t e d by nauplii ( m e a n :
7 3 , 8 °/o o f total c o p e p o d s t a g e s ) . C l a d o c e r a n s w e r e scarce a n d d o m i n a t e d by M. minuta,
C. cornuta a n d D.
spinulosum.
M o s t frequent a n d a b u n d a n t z o o p l a n k t o n species were e u p l a n k t o n i c with a d o m i n a n c e o f filter feeders a n d m i c r o p h a g o u s d e t r i t i v o r e s . M e a n z o o p l a n k t o n d e n s i t y in all lakes r a n g e d f r o m 340 i n d . / l t o 3 4 8 6 i n d . / l . B i o m a s s r a n g e d f r o m
71,1 ^ g . l (dw) t o 4 3 2 , 8 ^ g / l . R o t i f e r s a c c o u n t e d for 64,7 % of t h e t o t a l m e a n z o o p l a n k t o n b i o m a s s in three l a k e s w h i l e
c o p e p o d s a c c o u n t e d for 5 7 , 8 % o f t h e b i o m a s s in t w o l a k e s . B o t h d e n s i t y a n d b i o m a s s w e r e m a r k e d l y s e a s o n a l w i t h
h i g h e s t m e a n v a l u e s at low w a t e r s . M e a n d e n s i t y in the l a k e s w a s 100 t i m e s h i g h e r t h a n in the O r i n o c o main s t e m . L a k e s
with h i g h e s t variabilities in s u r f a c e a r e a a n d w a t e r d e p t h s s h o w e d highest z o o p l a n k t o n d e n s i t i e s . T h e type of c o n n e c t i o n
(direct o r indirect) e s t a b l i s h e d b e t w e e n t h e l a k e s a n d t h e m a j o r s o u r c e o f the w a t e r a l s o seemed i m p o r t a n t t o i n t e r p r e t
the productivity of fioodplain lakes.
Composition, abondance el biomasse du zooplancton dans quelques lacs d'inondation de l'Orénoque (Venezuela)
M o t s clés : Z o o p l a n c t o n , c o m p o s i t i o n s p é c i f i q u e , a b o n d a n c e , b i o m a s s e , lacs d ' i n o n d a t i o n , O r é n o q u e , V e n e z u e l a .
Des é c h a n t i l l o n s o n t été recueillis d u r a n t 21 m o i s d a n s 5 lacs de la p l a i n e d ' i n o n d a t i o n de l ' O r é n o q u e afin d ' é t a b l i r
la c o m p o s i t i o n s p é c i f i q u e , l ' a b o n d a n c e et la b i o m a s s e d u z o o p l a n c t o n et l e u r s c o r r é l a t i o n s a v e c les c a r a c t é r i s t i q u e s d e
l ' i n o n d a t i o n et d e la m o r p h o m é t r i e d e s lacs. 60 t a x a de r o t i f è r e s o n t été i d e n t i f i é s . C e g r o u p e s'est g é n é r a l e m e n t r é v é l é
p l u s a b o n d a n t q u e les c o p é p o d e s et les c l a d o c è r e s (73,4 % en m o y e n n e d u z o o p l a n c t o n total) a v e c , c o m m e e s p è c e s les
p l u s a b o n d a n t e s et les p l u s c o m m u n e s : K. americana,
K. cochlearis,
B. mirus, B. gessneri, P. vulgaris et F.
longiseta.
Les c o p é p o d e s o n t été essentiellement r e p r é s e n t é s p a r des n a u p l i i ( 7 3 , 8 °/o en m o y e n n e d u total des s t a d e s de d é v e l o p p e m e n t ) . L e s c l a d o c è r e s , m o i n s a b o n d a n t s , o n t é t é d o m i n é s p a r M. minuta,
C. cornuta
et D. spinulosum.
Les espèces
z o o p l a n c t o n i q u e s les p l u s f r é q u e n t e s et les p l u s a b o n d a n t e s étaient d e s f o r m e s e u p l a n c t o n i q u e s avec u n e d o m i n a n c e d e
filtreurs et de d e t r i t i v o r e s m i c r o p h a g e s . D a n s t o u s les l a c s , la d e n s i t é m o y e n n e d u z o o p l a n c t o n a varié de 3 4 0 i n d . / l
à 3 4 8 6 i n d . / I , et la b i o m a s s e , de 71,1 ^ g / I ( d w ) à 432,8 ^ g / 1 . Les rotifères o n t c o n s t i t u é 6 4 , 7 °7o de la b i o m a s s e m o y e n n e
d u z o o p l a n c t o n d a n s 3 lacs, les c o p é p o d e s , 5 7 , 8 % d e la b i o m a s s e d a n s les 2 a u t r e s lacs. D e n s i t é et b i o m a s s e o n t m o n t r é
les v a l e u r s m o y e n n e s les p l u s h a u t e s à b a s s e s e a u x . L a d e n s i t é m o y e n n e d a n s les lacs s ' e s t révélée e n v i r o n 100 fois p l u s
élevée q u e d a n s le c a n a l p r i n c i p a l d e l ' O r é n o q u e . L e s p l u s fortes d e n s i t é s de z o o p l a n c t o n o n t é t é observées d a n s les lacs
o f f r a n t les plus g r a n d e s v a r i a t i o n s de s u r f a c e et d e p r o f o n d e u r s d ' e a u x . Le t y p e d e c o n n e c t i o n (directe o u i n d i r e c t e )
e n t r e les lacs et la s o u r c e p r i n c i p a l e d e s e a u x s e m b l e ê t r e u n facteur i m p o r t a n t p o u r l ' i n t e r p r é t a t i o n d e la p r o d u c t i v i t é
des lacs d e la p l a i n e d ' i n o n d a t i o n .
1. Fundaciôn La Salle de Ciencas Naturales, Estaciôn Hidrobiolôgica de Guyana, Apartado 51, San Félix, E d o , Bolivar,
Venezuela.
2. Laboratoire d'Hydrobiologie, Université Paul Sabatier, U A
695 du CNRS, 118, route de Narbonne, 31062 Toulouse Cédex,
France.
Article available at http://www.limnology-journal.org
or http://dx.doi.org/10.1051/limn/1992004
4
1.
E. VÂSQUEZ, J. REY
Introduction
Wetzel (1990) discussed the aspects linked to the
role played by land-water interfaces and their high
productivity. In his examination of the n u m b e r of
lakes in the world in relation t o lake area and mean
water d e p t h , he found an overwhelming global pre­
d o m i n a n c e of small shallow lakes.
In fluvial floodplain systems, wetlands and litto­
ral c o m p o n e n t s highly contribute to low ratios of
pelagic/wetland + littoral. Junk et al. (1989) divide
t h e river floodplain system into permanent lotie
h a b i t a t s , permanent lentic habitats, and the aqua­
tic/terrestrial transition zone.
Despite the fact that floodplain lakes p r o b a b l y
best represent lowland tropical lakes (Tundisi et al
1984) a n d considering their importance in the meta­
bolic processes associated to floodplain river
systems, information o n this type of waterbodies is
rather scarce compared to other more classical lakes.
In S o u t h America, floodplain areas have been
estimated in - 300 000 k m (Welcomme 1985). O n
t h e O r i n o c o River floodplain (— 7 000 k m ) some
2 300 p e r m a n e n t lakes have been identified (Hamil­
t o n & Lewis, 1990a). Morphometric features of
s o m e of these lakes were investigated by Vâsquez
(1988, 1989). These waterbodies show a dynamics
in their m o r p h o m e t r i c characteristics under the
influence of the hydrological regime of the river.
Physical, chemical, a n d biological variables in these
lakes have been mainly explained by the seasona­
lity of river discharge and basin morphology (Vas­
q u e z & Sanche2 1984, H a m i l t o n & Lewis 1987,
1990 b ) .
1
!
Considering that contribution of lake biomass to
rivers m a y be high during inundations (Junk 1984),
relatively few studies have been done in O r i n o c o
lakes t o evaluate the dynamics a n d production of
their p l a n k t o n communities. In a large lake, Vas­
q u e z a n d Sanchez (1984) found highest p l a n k t o n
density at low waters. T w o m b l y & Lewis (1987)
observed in a small floodplain lake that total zoop l a n k t o n population sizes might in fact be larger at
high waters due to large changes in lake volumes.
Z o o p l a n k t o n densities were also found to be affec­
t e d b y r e t e n t i o n of o r g a n i s m s by floating
m a c r o p h y t e beds ( H a m i l t o n et al. 1990).
T h e p u r p o s e of this paper is to present informa­
t i o n o n z o o p l a n k t o n density, seasonality a n d
(2)
biomass from a set of five Orinoco floodplain lakes.
The research was aimed at establishing the ranges
of variations of the previous aspects of zooplank­
ton communities in relation to the seasonality of
inundation a n d lake basin morphology. This is the
first long-term study of zooplankton communities
developed in several lakes on the Orinoco River
floodplain. T h e results of this study may also prove
valuable to improve current classification schemes
of freshwater bodies in the world (Higler & Statzner 1988) mainly based on data from the temperate
region.
2 . Research sites
Zooplankton samples were collected in five per­
manent floodplain lakes : L. Playa Blanca, L. Orsinera, L. Jobera, L. Lagoven, and L. Rio Claro (Fig.
1). Based o n Drago's classification of floodplain
lakes (Drago 1976), Vâsquez (1988, 1989) classified
them (except L. Rio Claro) as lateral levee lakes
(type 13). This is a very frequent type of lake in the
area of study. L. Rio Claro was classified as a
confluence lake (type 11) formed by the regressive
inundations of the River Upata (intermittent water
course). L. Rio Claro shows an indirect connection
with the Orinoco River (Connection type 2.3, in the
sense of Drago 1981) while the remaining lakes show
a direct connection with the Orinoco River (connec­
tion type 1.1). A detailed morphological description
of the lakes is given by Vâsquez (1989). Table 1
shows some morphometric features of these waterbodies at low and high waters. From the m o r p h o ­
metric data for both periods, highest percentage of
variation corresponds to water depth followed by
surface, perimeter, maximum breadth, and the shore
development factor.
In the area of study, the Orinoco River hydrograph usually shows an amplitude ranging from
10 m t o 12 m (Fig. 2). In a 24-month period (Octo­
ber 1983 - September 1985), lake water depths ran­
ged from 0,1 m (L. Rio Claro) to 5,8 m (L. Orsinera) with a general mean value of 1,7 ± 0,6 m
(Table 1). Based of depth data, Vâsquez (1989) dis­
tinguished four hydrological phases in these water
bodies (Fig. 3) : low water phase (December-June),
rising water phase (July-August), high water phase
(August-September) and falling water p h a s e
(October-November). Figure 4 shows estimates of
lake volumes. These were m a d e using the formula
C O M P O S I T I O N , ABUNDANCE A N D BIOMASS O F Z O O P L A N K T O N IN O R I N O C O FLOODPLAIN LAKES
Fig. 1. Location of the study area. Approximate lake surfaces correspond to low water boundaries.
Fig. 1. Localisation de la région étudiée. L'évaluation des surfaces des lacs correspond aux périodes de basses eaux.
6
(4)
E. VÂSQUEZ, J. REY
Table 1. M o r p h o m e t r y data of the Orinoco lakes (Hw : high waters ; Lw : low waters), (1 : lenght ; b : breadth ; L : shore line ;
A : area ; Dl : development of shore line ; Z m a x . : maximum depth).
Tableau 1. Caractéristiques morphométriques de 5 lacs d'inondation de l'Orénoque (H.w : hautes eaux ; Lw : basses eaux),
(I : longueur ; b : largeur ; L : périmètre ; A : surface ; Dl : développement de la ligne de côte ; Zmax. : profondeur maximum).
Hw
P. Blanca
Orsinera
Jobera
Lagoven
R. Clara
I (m)
Lw
690
640
670
1 1 20
520
450
590
560
820
260
Hw
210
300
470
660
200
b (m)
Lw
Hw
200 1900
280 1500
360 2300
580 4400
140 1200
L (m)
Lw
1500
1400
1 600
2400
600
Hw
A (ha)
Lw
9.6
12.1
14.7
40.9
5.0
O
Hw
5.1
11.5
10.1
29.3
1.4
D
F
Dl
1 .73
1 .22
1.69
1.94
1 .51
A
J
A
l:b
Lw
Hw
1.87
1.17
1.42
1.25
1 .44
3.29
2.13
1.43
1.70
2.60
198 3-19 85
0
D
F
A
Lw
2.25
2.1 1
1.56
1.41
1.86
1
Zmax.(m)
Hw
Lw
4.2
5.8
4.8
4.5
2.4
0.3
1.2
0.6
0.9
0.1
A
0 I
Fig. 3. Water depth variations in the Orinoco lakes.
Fig. 3. Variation de la hauteur d'eau dans 5 lacs d'inondation
de l'Orénoque.
(5)
COMPOSITION, ABUNDANCE AND BIOMASS O F ZOOPLANKTON IN ORINOCO FLOODPLAIN LAKES
7
3
3
Volume (m j X 10
600 •
••-
P. Blanca
0 - Orsinera
••- Jobera
Lagoven
R. Claro
Fig. 4. Estimates of water volumes in the Orinoco lakes (mean monthly values of the 1983-1985 period).
4. Estimation des volumes d'eau dans 5 lacs d'inondation de l'Orénoque. (Valeurs moyennes mensuelles pour la
période 1983-1985).
of a cone. For monthly changes in lake surfaces, we
used planimetric data from high and low waters and
assumed, using these two extremes, similar decre­
ments in lake surfaces. Depth values were used in
the estimates obtaining the monthly mean of a
24-month period. Given the limitations of these esti­
mates they are used only to give an idea of lake
volumes.
Data on water depth, Secchi disc transparency,
conductivity, dissolved oxygen, and water tempera­
ture are given in Table 2. Mean transparency in
lakes was 32.8 cm ± 7.5 ; mean conductivity was
106.8 ± 107.9 j t S / c m . L. Rio Claro showed higher
conductivity values (mean : 295.7 ^ S / c m ) compa­
red to a mean of 59.6 ^ S / c m for the remaining
lakes. Seasonally, lakes transparency was highest
during inundation while conductivity generally sho­
wed high values at low waters. Low transparency
and high conductivity at low waters are the result
of b o t t o m sediments resuspension when lakes are
shallowest. Vâsquez (1992) classified these lakes
as continuous w a r m polymictic. Dissolved oxygen
shows a highly significant negative relationship with
water depth. The lakes are generally below dissol­
ved oxygen saturation.
3. Materials and m e t h o d s
Monthly zooplankton collection were m a d e over
a 21-month period (Junuary 1984 - September 1985)
at sites over the deepest part of the lakes in areas
free of macrophyte beds. A variable volume of sub­
surface water (20-40 1) was filtered through a net of
45
mesh. For counting, 5 ml subsamples were
examined. This volume generally allowed the iden­
tification of at least 100 of the most common spe­
cies or stages. When density was low, the entire sam­
ple was usually examided. Rose bengal was added
to the samples to facilitate separation of organisms
from suspended matter.
Counting of rotifers and cladocerans was done to
the species level. Copepods were counted as deve­
lopmental stages (adult cyclopoids, adult calanoids,
copepodites, and nauplii). Rotifers and cladocerans
(6)
E. VÂSQUEZ, J . REY
8
Table 2. Ranges, mean and standard deviations of depth, transparency, conductivity (at 2S°C), dissolved oxygen and
temperature in the Orinoco lakes.
Tableau 2. Valeur minimale, maximale, moyenne et déviation standard de la profondeur, de la transparence, de la
conductivité (à 2 5 ° Q , de l'oxygène dissous et de la température dans 5 lacs d'inondation de l'Orénoque.
Min
Max
Mean
sd
P. B l a n c a
Depth (m)
Transparency (cm)
C o n d u c t i v i t y (tiS/cm)
DO (mg/l)
T e m p e r a t u r e (°C)
0.3
5.0
22.0
1.8
27.0
4.2
120.0
83,P
fà
33.5
1.3
42.2
43.7
5.3
29.7
0.96
31.76
14.56
1.73
1.85
Orsinera
Depth (m)
Transparency (cm)
Conductivity (u.S/cm)
DO (mg/l)
T e m p e r a t u r e (°C)
1.2
5.0
21.0
3.1
26.0
5.8
120.0
56.6
7.6
31.5
2.5
36.4
35.1
5.8
29.0
1.16
29.10
8.80
1.09
1.67
Jobera
Depth (m)
Transparency (cm)
C o n d u c t i v i t y (u.S/cm)
DO (mg/l)
T e m p e r a t u r e (°C)
0.6
5.0
28.0
3.9
26.5
4.8
75.0
196.0
7.8
31.0
1.7
27.4
67.3
5.8
29.2
1.18
20.45
38.81
1.11
1.23
Lagoven
Depth (m)
Transparency (cm)
C o n d u c t i v i t y (u.S/cm)
DO (mg/l)
T e m p e r a t u r e (°C)
0.9
10.0
24.0
4.0
26.0
4.5
93.0
261.0
7.8
31.5
1.9
35.0
92.3
6.1
29.1
1-07
25.36
64.40
0.89
1.34
R. C l a r o
Depth (m)
Transparency (cm)
C o n d u c t i v i t y (nS/cm)
DO (mg/l)
T e m p e r a t u r e (°C)
0.1
5.0
93.0
4.6
21.5
2.4
53.0
658.0
9.8
35.0
1.0
23.2
295.7
7.0
29.5
0.58
15.44
155.01
1.41
2.55
(7)
C O M P O S I T I O N , ABUNDANCE AND BIOMASS O F ZOOPLANKTON IN ORINOCO FLOODPLAIN LAKES
demanding detailed taxonomic analysis were mounted on polyvinil alcohol and glicerine alcohol
respectively.
4.
9
Ps = 43,9 % , high waters) (Table 5). As suggested
by Paggi & José de Paggi (i 980), a high similarity
in species composition should be expected in proximate lakes from a same hydrographie basin, with
similar geological characteristics and highly influenced by a common source of water during floods.
Results
4.1. Species composition and relative abundance
Qualitative zooplankton analysis revealed the presence of 60 rotifer taxa. Brachionus and Keratella
together accounted for 30,5 °7o of the rotifers followed by species of Trichocerca (18,6 °7o) and
Lecane(l3,6 °7o). In spite of the high number of rotifera taxa, analysis of the specific frequency revealed that constant species (C > 50 %) were few and
mostly truly planktonic (Table 3). The number of
constant species was 7 in all iakes except in L. Jobera
which only showed 3 constant species. Dominant
rotifer species included members of the Brachionidae plus a reduced group of species from the
Synchaetidae a n d Filinidae.
In terms of mean abundance, rotifers were always
more abundant than copepods and cladocerans
(Fig. 5). Mean abundance of rotifers in all lakes
accounted for 73,4 % of total zooplankton (L. Rio
Claro : 95,5 97o ; L. Jobera : 78,9 % ; L. Playa
Blanca : 73,4 Wo ; L. Orsinera : 63,2 <7o ; and
L. Lagoven : 56,2 °7o). C o m m o n abundant species
included mainly euplanktonic species such as Keretetia americana, K. cochlearis, Brachionus
mirus,
B. gessneri, Polyarthra vulgaris a n d Filinia longiseta (Tables 6-10).
Table 3. Frequency of constant rotifer species (C > 50 % ) in
In all lakes, rotifer species richness showed a mean
value of 10.6 species (range : 9.3 species in L. Playa
Blanca - 11.6 species in L. Orsinera). The ratio of
the number of species found at high water per lake
was similar (Table 4). Richness per month and per
lake, however, was generally higher during floods
the Orinoco lakes.
Tableau 3. Frequence des espèces constantes des rotifères
(C > 50 %) dans 5 lacs d'inondation de l'Orénoque.
et.5
The percentage of similitarity of the rotifer communities in all lakes was estimated by means of the
index employed by Green (1972), Ps = 100-0.5 £
I a-b I, where « a » is the relative a b u n d a n c e in
the inventory A a n d « b » is the relative abundance
of the same species in the inventory B, both of them
expressed in terms of the total percentage. Mean
similarity among lakes was very similar for both high
and low water periods (Ps = 43,7 % , low waters ;
76 0
K cochleerls
P. vulgaris
B. onguloris
B1.5
71.0
B6.5
92.5
75.0
Bt.5
82.5
1 00,0
P. lyber
T. stylol
39.5
Table 4. Mean species richness of rotifers at low waters (Lw), high waters (Hw), ratio Lw :Hw, and means for the period
study.
Tableau 4. Richesse spécifique moyenne des rotifères à basses eaux (Lw) et hautes eaux (Hw) et valeurs moyennes pour
la période d'étude.
P. Blanca
Orsinera
Jobera
Lagoven
R-Claro
10.0
12.1
8.7
10.6
11.7
8.1
11.0
12.4
10.9
10.0
1.2
1.1
0.7
1.0
1.2
9.3
11.6
10.1
10.7
11.1
4.4
5.2
4.6
3.8
5.8
10
A b u n d a n c e (%)
E. VASQUEZ, J. REY
(8)
Biomass (%)
P.Blanca
Fig. 5. Variations in the relative abundance and biomass of ihe major zooplankton groups in the Orinoco lakes. White : rotifers ;
grey : copepods ; black : cladocerans.
Fig 5. Valeurs relatives de l'abondance et de la biomasse des principaux groupes de zooplancton dans 5 lacs d'inondation de
l'Orénoque. Blanc : rotifères, gris : copépodes, noir : clacocères.
(9)
C O M P O S I T I O N , ABUNDANCE A N D BIOMASS O F Z O O P L A N K T O N IN O R I N O C O F L O O D P L A I N LAKES
11
Table 5. Percentage similarity of rotifers at low waters (a) and at high waters (b> in the Orinoco lakes.
Tableau 5. Pourcentage de similitude des rotifères à basses eaux (a) et à hautes eaux (b) dans 5 lacs d'inondation de l'Orénoque.
PB
OR
je
RC
PB
OR
JB
RC
OR
JS
RC
LG
35.6
3 4.5
39.3
33.9
35.6
57.9
55.1
55.6
49.0
40.0
OR
JS
RC
LG
47.6
45.5
47.0
26.1
33.1
57.9
56.5
S7.7
22.1
43.0
a
b
Table 6. Abundance of the common zooplankton species or developmental stages and biomass values of zooplankton groups in
L. Playa Blanca.
Tableau 6. Abondance des espèces communes ou des stades de développement des espèces du zooplancton, et valeurs de biomasse des
groupes zooplanctonique dans le lac Playa Blanca.
P.
BLANCA
DENSITY(org./l)
Z O O P L A N K T O N : total
R O T I F E R A : total
B. g e s s n e r i
K. americana
K. cochlearïs
P. vulgaris
COPEPODA
Adults ciclopoids
Adults calanoids
Copepodites
Nauplii
CLADOCERA
B. tubicen
B. deitersi
D. spinulosum
M . minuta
BIOMASS(u.g/1)
Rotifera
Copepoda
Cladocera
Lw
Hw
640.0
487.3
11.5
137.3
176.8
103.9
1 4 4 . .3
7 7 . .1
21 .6
8 .9
12 .0
2 .5
4 . .4
6 .3
0..5
1 5 .4
1 4 .7
4 1 .6
451.1
331.1
15.4
88.8
114.0
65.2
4.0
2.0
0.0
0.0
0.0
0.0
2 2 5 9 .0
2 0 8 4 .0
1 3 3 . .0
9 1 7 . .0
1 6 1 9 .0
4 7 8 .. 0
16.0
0.0
25.6
77.3
5..4
0 .4
8 .8
4 2 .6
3 .0
11.9
2 .9
1 .8
19.2
73.6
0.0
0.0
0.0
0.0
1 0 0 .. 0
2 .0
1 7 2 .. 0
225 .0
0.4
1.2
4.2
12.6
0
0
0
0
1. .3
1 .3
4 2 .0
1 4 .0
0.3
2.6
8.2
0.0
0.0
0.0
0.0
2
8
33
143
56.1
16 3
3 4 .2
1 .2
40.9
62.4
0.1
0.0
7.1
o.o
2 1 2 .9
445 .8
9 2 .5
79.8
10.8
Lw:Hw
.3
.9
.1
.9
Mean
Min
0.1
3 .4
2 .3
9 .0
1.1
Max
.0
.0
.0
.0
12
(10)
E. VÂSQUEZ. J. REY
Table 7. Abundance of the common zooplankton species or developmental stages and biomass values of zooplankton groups in L. Orsinera.
Tableau 7. Abondance des espèces communes ou des stades de développement des espèces du zooplancton, et valeurs de biomasse des
groupes zooplanctoniques dans te lac Orsinera.
ORSINERA
DENSITY(org./l)
Z O O P L A N K T O N : total
R O T I F E R A : total
B. gessneri
B. mirus
Conochilus
F. tongiseta
H. intermedia
K. americana
K. cochlearis
L. proiecta
P. vulgaris
T. similis
COPEPODA
Adults ciclopoids
Adults calanoids
Copepodites
Nauplii
CLADOCERA
B. tubicen
B. deitersi
C.cornuta
D. spinulosum
M. minuta
B I O M A S S (u.g/1)
Rotiîera
Copepoda
Cladocera
Lw
Hw
6 9 5 .0
3 7 7 .0
1 0 3 .0
9 2 . .0
0..0
1 3 .0
1 .0
8 . .0
0..0
30. 0
1 3 . .0
1 . .0
126.0
35.0
0.0
2.0
4.0
2.0
5.0
2.0
3.0
1.0
8.0
1.0
1 2 .0
0..1
3 9 . .0
2 6 5 .0
1.0
Mean
Min
Max
30.0
1.6
1.0
340 0
215.1
38.8
41.0
0.9
3.2
1.6
9.5
0.8
34.1
14.3
0.3
16.0
2.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1 569.0
1 147.0
283.0
239.0
10.0
48.0
21.0
134.0
9.0
498.0
80.0
5.0
12.0
0.1
4.9
3.6
3.1
0.2
13.5
102.4
0.0
0.0
0.0
12.0
360
2.0
106.0
693.0
8.0
0.2
0.1
0.2
4.0
0.4
0.5
0.2
0.3
0.1
4.5
0.0
0.0
0.0
0.0
0.0
5.0
2.0
3.0
2.0
35.0
8.0
18.0
6.0
11.9
6.1
0.3
52.9
35.4
3.3
0.2
2.0
0.0
295.6
298.0
23.7
1.0
8.0
74.0
1.0
0. 2
0..1
0..2
1 . .0
3 . .0
9 5 . .0
1 1 0 .0
2 .0
Lw:Hw
0.7
1.0
0.3
C o p e p o d species included the cyclopoids Mesocyclops s p . , Oithona
amazonica,
Thermocyclops
minutus, a n d T. decipiens and the calanoids Diaptomus negrensis, Notodiaptomus
amazonicus,
N.
caerensis, a n d Rhacodiaptomus
calatus. All -devel o p m e n t a l c o p e p o d stages were observed, nauplii,
h o w e w e r , comprised the highest p r o p o r t i o n a m o n g
c o p e p o d s . M e a n a b u n d a n c e of nauplii with respect
t o t h e other c o p e p o d stages was : L. Orsinera :
85.9 % ; L. Jobera : 75.6 <% ; L. Lagoven : 72.6 <7o ;
L . P . Blanca : 70.2 °/o and L.R. Claro : 64.5 Vo
(Fig. 5). Highest mean abundance of adult copepods
5.5
10.8
46.0
6.5
0.2
4.0
.
was due to cyclopoids except in L . R. Claro where
calanoids were more abundant (Tables 6-10).
Cladocerans from these lakes and other lentic and
lotie water bodies were investigated by Rey & Wâsquez (1986). These authors identified 7 pelagic and
25 littoral species (mainly Chydoridae) from floodplain lakes of the Orinoco. In the present study, cladocerans were dominated by pianktonic species
(mainly Moina minuta, Ceriodaphnia cornuta and
Diaphanosoma
spinulosum
(Tables 6-10). M.
minuta accounted for almost 60 9?o of the mean
(11)
C O M P O S I T I O N , A B U N D A N C E A N D BIOMASS OF Z O O P L A N K T O N IN O R I N O C O FLOODPLAIN LAKES
13
Table 8. Abundance of the common zooplankton species or developmental stages and biomass values of zooplankton groups in L. Jobera.
Tableau 8. Abondance des espèces ommunes ou des stades de développement des espèces du zooplancton, et valeurs de biomasse des
groupes zooplanctoniques dans le lac Jobera.
JOBERA
DENSITY(org./l)
Z O O P L A N K T O N : total
R O T I F E R A : total
A navicula
A. saltans
B. gessneri
B. mirus
Conochilus sp.
K. americana
K. cochlearis
K. n h a m u n d a
P. vulgaris
COPEPODA
Adults cicîopoids
Adults calanoids
Copepodites
Nauplii
CLADOCERA
B. tubicen
B. deitersi
C. cornuta
M. minuta
B I O M A S S (ng/l)
Rotifera
Copepoda
Cladocera
Lw
Hw
Lw:Hw
417.7
343.1
0.0
0.0
3.7
10.9
0.0
269.4
1.3
0.0
2.1
276.8
195.9
65.3
4.0
17.8
0.0
16.1
5.6
4.3
21.4
1.8
3.8
0.9
10.9
47.0
Min
Mean
1.2
364.0
287.0
24.9
1.5
9.0
6.8
6.1
168.9
2.4
8.1
2.0
38 .0
5 .0
0 .0
0 .0
0 .0
0..0
0 .0
0 .0
0 .0
0 .0
0..0
1 2 8 6 .0
1 2 2 2 .0
4 9 9 . .0
3 2 . .0
5 5 . .0
5 2 , .0
1 2 9 . .0
1 054 .0
3 3 .. 0
1 2 1 . .0
1 9 . .0
4.8
2.7
10.8
59.3
0.8
0.3
1.0
0.8
4.2
1.6
10.9
51.7
0 .0
0 .0
0 .0
1 .0
16.. 0
17..0
4 7 .. 0
198 .0
0.4
0.1
5.4
5.9
0.0
2.0
0.1
1.5
0.05
54.0
3.9
0.2
0.8
3.4
4.2
0 .0
0 .0
0 .0
0 .0
2.. 0
6 .0
7 0 .. 0
37 .0
36.4
29.5
7.0
29.3
37.0
1.7
1.2
0.8
4.1
33.7
32.4
5.0
0 .5
0 .3
0..0
1 1 1 .. 3
1 1 3 .. 7
51 . 2
cladoceran a b u n d a n c e in all lakes. In general, species richness a n d m e a n a b u n d a n c e of cladocerans
was relatively low as previously reported by Rey and
Vâsquez (1986).
4.2. Patterns of abundance and biomass
M e a n z o o p l a n k t o n density in the lakes showed the
following sequence : R: C l a r o > P. Blanca >
J o b e r a > L a g o v e n > Orsinera. Excluding L. Rio
C l a r o , m e a n z o o p l a n k t o n density was 375.4 i n d . / l .
L a k e Rio Claro s h o w e d almost a 10-fold increase
in its m e a n z o o p l a n k t o n density c o m p a r e d t o the
other lakes.
1.5
1.8
Max
0.2
48.1
0.3
-
Analysis of the m e a n z o o p l a n k t o n density at low
and high waters revealed highest mean density values
during the low water period for most lakes. T h e ratio
of low water m e a n density to high water m e a n d e n sity r a n g e d between 1.0 a n d 43.7 and s h o w e d the
following sequence : Rio C l a r o > Orsinera > P.
Blanca > J o b e r a . L. Lagoven showed a r a t i o close
t o 1 indicating similar m e a n density values for b o t h
low a n d high water periods.
Estimates of z o o p l a n k t o n population sizes f r o m
densities a n d lake volumes showed highest values
d u r i n g i n u n d a t i o n in lakes Lagoven, Orsinera, a n d
J o b e r a . L a k e s P . Blanca and R. Claro s h o w e d ,
14
(12)
E. VÂSQUEZ, J. REY
Table 9. Abundance of the common zooplankton species or developmental stages and biomass values of zooplankton groups in L. Lagoven..
Tableau 9. Abondance des espèces communes ou des stades de développement des espèces du zooplancton, et valeurs de biomasse des
groupes zooplanctoniques dans le lac Lagoven.
LAGOVEN
DENSITY(org./l)
Z O O P L A N K T O N : total
R O T I F E R A : total
B. mirus
B. zahniseri
F. longiseta
K. americana
K. cochlearis
P. vulgaris
COPEPODA
Adults cicîopoids
Adults calanoids
Copepodites
Nauplii
CLADOCERA
B. tubicen
B. deitersi
C . cornuta
D. spinulosum
M. minuta
B I O M A S S (ng/l)
Rotifera
Copepoda
Cladocera
Lw
Hw
340.5
215.2
4.9
0.0
0.0
75.5
33.7
13.6
356.2
161.7
5.3
2.6
3.5
42.5
32.3
9.0
1.0
1.3
0.9
7.9
0.4
29.0
82.7
Lw:Hw
Mean
Max
Min
1.8
1.0
1.5
346.5
194.8
5.1
1.0
1.3
62.9
33.1
11.9
9.0
8.0
0.0
0.0
0.0
0.0
0.0
0.0
985.0
603.0
57.0
11.0
19.00
355.0
247
114.0
7.3
3.3
28.8
130.4
1.1
0.1
1.0
0.6
7.6
1.5
28.9
100.9
0.0
0.0
0.0
0.0
25.0
19
80.0
253.0
1.5
0.5
0.0
0.1
3.4
0.4
7.0
5.4
4.1
8.1
3.8
0.07
0.02
0.4
1.0
3.0
2.1
1.6
5.2
0.0
0.0
0.0
0.0
0.0
11.0
18.0
26.0
29.0
17.0
40.3
58.9
3.1
29.8
68.3
12.5
1.4
0.9
0.3
36.3
62.3
6.7
0.9
2.8
0.0
92.1
170.2
33.6
h o w e v e r , highest p o p u l a t i o n densities at low waters
( T a b l e 2). C o m p a r i s o n of m o r p h o m e t r i c d a t a from
b o t h high a n d low waters showed that lakes P .
Blanca a n d R. C l a r o presented the highest variability in lake surfaces a n d depths. T h e ratio of high
water t o low w a t e r surface was almost 2 for L. P .
B l a n c a a n d 3.6 for L. R. Claro. T h e ratio of water
d e p t h was 14 for L. P . Blanca and 24 for L. R.
C l a r o . These drastic changes in surface a n d water
d e p t h coupled with high z o o p l a n k t o n densities may
explain t h e highest m e a n p o p u l a t i o n sizes recorded
at low water in the these two lakes as a consequence
of high c o n c e n t r a t i o n of z o o p l a n k t o n organisms.
In t e r m s of b i o m a s s , mean dry mass in all lakes
was 1 6 2 . 2 ^ 1 ranging from 71.1 ^ g / 1 t o 4 3 2 . 8 ^ 1 .
-
-
T h e following sequence was observed : R. Claro >
P . Blanca > Lagoven > Orsinera > J o b e r a . In
lakes Orsinera, Jobera, and R. Claro, rotifers accounted for 64.7 % of the total mean zooplankton biomass.
In lakes P. Blanca and Lagoven, however, copepods
showed highest mean biomass values (57.8 % ) .
Seasonally, the ratio of mean low water zooplankton biomass to mean high water biomass revealed
highest mean biomass values at low waters. T h e
mean overall ratio was 6.5 ranging from 0.9 to 18.4.
These figures indicate that seasonality of biomass
is a conspicuous characteristic of the lakes (Tables
6-10, Fig. 5). The ratio showed the following
sequence : R . Claro > Orsinera > P. Blanca >
J o b e r a > Lagoven.
(13)
C O M P O S I T I O N , A B U N D A N C E A N D BIOMASS O F Z O O P L A N K T O N IN O R I N O C O F L O O D P L A I N LAKES
15
Table 10. Abundance of (he common zooplankton species or developmental stages and biomass values of zooplankton groups in
L. R. Claro.
Tableau 10. Abondance des espèces ommunes ou des stades de développement des espèces du zooplancton, et valeurs de biomasse des
groupes zooplanctoniques dans le lac Rio Claro.
RIO
CLARO
Lw
DENSITY(orgJI)
Z O O P L A N K T O N : total
R O T I F E R A : total
B. angularis
B. caudatus
B. havanaensis
F. longiseta
K. americana
K. tropica
L. proiecta
P. vulgaris
P. libera
COPEPODA
Adults ciclopoids
Adults calanoids
Copepodïtes
Nauplii
CLADOCERA
B. tubicen
B. deitersi
D. spinulosum
M. minuta
1. spiniîer
B I O M A S S (u.g/1)
Rotifera
Copepoda
Cladocera
Hw
Lw:Hw
Mean
43.7
55.9
2.2
5 553.0
5 316.5
24.1
19.0
13.2
9.9
4 438.2
183.7
111.7
19.8
0.1
1 270.0
95.1
11.0
0.0
0.2
1.5
1.6
9.6
0.1
4.3
0.4
82.7
6.6
2 722.8
19.1
859.2
4.7
0.1
7.9
14.3
62.2
135.3
0.1
0.0
0.4
30.4
155.5
4.5
0.02
0.2
8.2
8.4
0.5
0.0
0.04
0.1
0.1
0.3
595.5
72.5
1.9
15.8
5.7
0.4
3 486.1
3 327.4
19.1
11.8
8.3
6.7
2 748.1
117.4
69.2
13.9
0.2
-
Min
Max
7.5 4 9 4 3 0 . 0
0.4 4 9 0 7 8 . 0
313.0
0.0
125.0
0.0
96.0
0.0
123.0
0.0
0.0 45 378.0
2 388.0
0.0
0.0
1 445.0
0.0
135.0
0.0
3.0
4.9
8.9
38.6
95.3
0.0
0.0
0.0
0.0
48.0
163.0
316.0
692.0
3.8
63.4
64.8
1.8
0.01
0.1
5.2
5.3
0.4
0.0
0.0
0.0
0.0
0.0
0.2
1.0
100.0
105.0
3.0
37.6
12.7
4.9
384.5
47.0
1.3
0.1
0.0
0.0
5 331.4
149.6
11.1
60.8
-
-
6
Table 11. Estimates of zooplankton population sizes (ind. x 10 ) in the Orinoco lakes.
Tableau 11. Estimation de la taille des populations zooplanctoniques (ind. X 10 ) dans 5 lacs d'inondation de l'Orénoque.
6
Lakes
P. B l a n c a
Orsinera
Jobera
Lagoven
R. Claro
Hw
Lw
Hw:Lw
Mean
6868
12707
0.54
1 0483
32642
27199
1.20
29532
Min
Max
337
87021
3278
146257
31186
15927
1.96
21 7 4 0
960
86007
111736
45799
2.44
70918
1047
188850
19675
0.12
13071
67
87876
2339
16
5.
E. VÂSQUEZ, J. REY
Discussion
T h e z o o p l a n k t o n of Orinoco fioodplain lakes fre­
q u e n t l y included a m e a n of 7 constant rotifer spe­
cies, a n d o n e or t w o species of Moina,
Ceriodaphnia a n d Diaphanosoma.
The n u m b e r of species b y
w a t e r b o d y for a given moment was from 1 t o 24
rotifers, a n d 0 to 5 cladocerans. Including the overw­
helming a b u n d a n c e of nauplii stages over o t h e r
c o p e p o d stages, most z o o p l a n k t o n from t h e lakes
were euplanktonic with a predominance of filter fee­
ders a n d m i c r o p h a g o u s detritivores. This general
p i c t u r e of z o o p l a n k t o n composition and m a i n fee­
ding regimes of z o o p l a n k t o n of O r i n o c o lakes
resembles that o b s e r v e d in lakes of other tropical
a n d subtropical fioodplain systems, such as t h e
Niger in Africa ( D u m o n t 1986) ; the A m a z o n a n d
P a r a n a in South America (Robertson & H a r d y 1984,
Paggi & José de Paggi 1990) a n d the Murray-Darling
in A u s t r a l i a (Shiel 1986).
C o m p a r e d t o the O r i n o c o River m a i n stem (Vâsq u e z & Rey 1989), both the river and the lakes show
a numerically rich rotiferan fauna with a g r o u p o f
few, a b u n d a n t a n d frequent species, relative scar­
city of cladocerans a n d dominance of nauplii a m o n g
c o p e p o d stages. In the lakes the association of domi­
n a n t rotifer species is largely composed of e u p l a n k ­
t o n i c f o r m s while, as expected, the river shows a
d o m i n a n c e of e u p l a n k t o n i c a n d t y c h o p l a n k t o n i c
f o r m s . T h e most typical cladoceran association in
the lakes was formed b y M. Minuta, C. cornuta a n d
D. spinulosum.
In the river d o m i n a n t species inclu­
ded B. deitersi, B. tubicen, M. minuta a n d M.
reticulata.
T h e d o m i n a n c e of a core g r o u p of e u p l a n k t o n i c
z o o p l a n k t o n species plus some other m o r p h o l o g i ­
cal a n d chemical features of Orinoco lakes (high
w a t e r level fluctuations in relation t o lake v o l u m e ,
d i u r n a l a n d a n n u a l variations in dissolved oxygen
c o n t e n t ) represent c o m m o n features t o the shallow
lakes category of stagnant waters proposed by Higler
& S t a t z n e r (1988). O r i n o c o lakes, however, m a y
reach w a t e r d e p t h values similar to t h e category o f
very shallow lakes.
C o m p a r e d to m e a n density of z o o p l a n k t o n in the
O r i n o c o m a i n stem (Saunders & Lewis 1989, Vâsq u e z & Rey 1989), m e a n density in t h e lakes was —
100 times higher t h a n in the river. A similar value
was r e p o r t e d by H a m i l t o n et al. (1990) from a n o ­
ther O r i n o c o fioodplain l a k e . Considering t h e
(14)
extension of the fioodplain and the high abundance
of zooplankton found in these areas, the contribu­
tion of zooplankton biomass from fioodplain areas
t o the river is very small (Saunders & Lewis 1989).
Hamilton et al. (1990) ascribed the absence of a
significant export of z o o p l a n k t o n as a consequence
of retention of planktonic organisms by aquatic
macrophytes. Reduced hydraulic washout due to
reduced flow in fioodplain areas as a consequence
of flow atenuation in forested areas may also be
important factors reducing the likelihood of zoo­
plankton export. For exemple, Neiff et al. (in press)
found a reduction of flow in the lower Paraguay
during inundation form 1.7 m / s in the main stem
to 0.09 m / s in forested areas. W a t e r movement
inside the fioodplain would be reduced by the « bio­
logical rugosity » mainly imposed by the density and
height of herbaceous vegetation which would disipate energy (Neiff et al. in press). T h e sheet floo­
ding of the lakes would also lead t o serial connec­
tions a m o n g the lakes in the path of flow (Lewis et
al. 1990). These connections plus the retention of
zooplankton organisms by macrophyte mats would
most probably explain the reduced advective trans­
port of zooplankton from fioodplain areas t o the
river. Prédation o n z o o p l a n k t o n may also be an
important factor involved in removal of organisms
in transit. Preliminary d a t a o n s t o m a c h contents of
several fish species from O r i n o c o lakes (V. Ponte,
pers. com.) revealed a dominance of cladocerans and
copepods in the stomachs of 21 fish species. In sta­
gnant waters of the Apure River fioodplain a simi­
lar observation has been m a d e ( E . Vâsquez, u n p u ­
blished). Twombly & Lewis (1989) also suggested
prédation by fish a n d Chaoborus as the major fac­
tor controlling cladoceran abundance in an Orinoco
lake.
We may summarize that reducted export of zoo­
plankton biomass from fioodplain waterbodies to
the river may be directly or indirectly controlled by
physical (serial connections a m o n g lakes, high
hydraulic residence times) and biological factors
(retention of zooplankton by vegetation, prédation).
Both in terms of a b u n d a n c e per unit volume of
water and in terms of population sizes, zooplank­
ton of Orinoco lakes was markedly seasonal. Mean
annual density and biomass values allowed us to
rank the lakes in the following order : density : RC
> P B > J O B > L A G > O R S > ; biomass :
(15)
COMPOSITION, ABUNDANCE AND BIOMASS OF ZOOPLANKTON IN O R I N O C O FLOODPLAIN LAKES
R C > P B > L A G > O R S > J O B . Excluding L.
Rio Claro, mean zooplankton abundance and bio­
mass values in the remaining four lakes were very
similar (mean density : 375.4 ± 51.5 i n d . / l ; mean
biomass : 94.6 ± 17.6 ^g/1). The peculiarities of L.
R. Claro which may help interpret its higher zoo­
plankton density ( ~ 10 times higher) may be due
to the fact this lake is seasonally inundated by waters
from the Orinoco and Upata rivers. The latter intermitent river drains agricultural areas and we could
expect a high organic load in its waters. The pecu­
liarities of L. Rio Claro basin also include very night
conductivity values with respect t o the other lakes
( — 5 times higher), and a much higher phytoplankton density (L. Sanchez, pers. com).
Analysis of the ratios of high water over low water
lake surfaces a n d lake water depths revealed the
same sequence for both parameters (RC > PB >
J O B > LAG > ORS) which corresponds to the
sequence of zooplankton density. In other words,
in lakes with highest variabilities in surface and
water depths, zooplankton abundance was higher.
Increased variations in surfaces and water depths
imply higher water-land interactions in the eulittoral zones of the lakes. Breen et al. (1978) consider
water depth, water residence time and the extension
of inundated areas, of particular importance to the
biological productivity of floodplain lakes. These
factors would determine inputs of organic and inor­
ganic material from the margins into the lakes lea­
ding to increased levels of food available for
zooplankton.
From the previous information, we may summa­
rize that in terms of zooplankton species composi­
tion, Orinoco lakes show a high degree of simila­
rity among themselves. The observed differences in
terms of density and biomass seem to indicate,
however, that m o r p h o m e t r y of each lake basin is
important to explain differences in zooplankton pro­
ductivity. Lakes with highest variability both in lake
surfaces and water depths seem t o support highest
zooplankton a b u n d a n c e a n d biomass.
When an indirect connection is established with
the major source of waters, the particular charac­
teristics of the connector also seem important to
interpret productivity. This may be the case of L.
Rfo Claro which is seasonally connected to the Ori­
noco river by the regressive inundations of the Upata
River rich in electrolytes and organic load.
17
Acknowledgements
We are grateful to Dr. C. H . Fernando (Waterloo, Ontario,
Canada) for his collaboration in the identification of copepods.
We are also deeply grateful to Ms. M-L. Medina and Mr. J. C.
Nasser for their support in the analytical work.
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