REGULATED RIVERS: KESEARCII & MANAGEMENT, VOL. 3, 3x1-392 ( l lJX9)

THE ORINOCO RIVER: A REVIEW OF HYDROBIOLOGICAL RESEARCH

ENRIQUE VASQUEZ Fundacidn La Salk de Ciencias Naturales, Apdo. 51, San Fklir, Edo. Bolivar, Venezuela

ABSTRACT

The Orinoco River is one of the world's longest rivers (2060 km) and in terms of average water discharge (36000 m3 s-') it ranks third. Seventy per cent of its basin (1.1 X 106km2) is found in Venezuela and the rest in Colombia. Due to the uneven distribution of the population toward the northern coastal areas (97 per cent of the population inhabits 55 per cent of the country with only 16 per cent of the surface waters) plans have been carried out to develop resources in the Orinoco Basin. So far these programs include mining (mainly iron and bauxite), oil exploration and extraction from the Orinoco Oil Belt, industry, river transport, and hydroelectric exploitation. Development of the latter has involved the construction of two dams on the Caroni River (the most important blackwater tributary of the Orinoco) and two more are planned on this same river. Hydroelectric projects are also being considered for the Orinoco River. Physicochemical and biological characterization has been regarded as of major importance to establish base-line information to detect and quantify possible alterations in this river which is considered to be in a pristine state. Systematic hydrobiological research has recently been carried out in the river, some of its major tributaries, and floodplain lakes, and a fair amount of knowledge has emerged mainly from its lower section.

Phytoplankton studies have revealed the presence of over 400 species of algae. Zooplankton research has identified 116 taxa of rotifers and 58 taxa of cladocerans. In general plankton densities negatively correlate with water level. Diatoms were observed to predominate in the Orinoco River while Cyanophyta predominate in the studied floodplain lakes. Seventeen aquatic macrophyte species have been recorded in the lakes of the Orinoco with high densities of Eichhornia crassipes, Oxycarium cubense, and Puspalurn repens. Rooted emergent and floating-piant cover tends to increase rapidly during high water.

Some 318 species and subspecies of fish have been reported for the Orinoco Basin, even though this number is far from complete. Preliminary data have revealed different species associations among relatively close lakes with biornasses ranging between 30 to 900 kg ha-' displaying considerable variations in diversity and species richness. Fisheries along the Orinoco are mainly of a multispecific nature and their overall potential has been estimated as being in the order of 45000 t yr-I.

In general, management plans have taken into consideration the multispecific potential of the Orinoco Basin but indicate a lack of sufficient knowledge of the physical, biological, and social aspects involved. Furthermore development plans tend to precede the generation of this basic knowledge, thereby increasing the risks of conflict among the various users of the resources involved.

KEY WORDS Tropical rivers Orinoco Development plans Plankton Aquatic vegetation Fisheries

INTRODUCTION

The study of the physical, chemical, and biological characteristics of the Orinoco River waters has been regarded as fundamental to generate basic information for detecting and quantifying possible changes that may arise as a result of development activities in a fluvial system still considered to be in a practically pristine state (Buroz el al., 1983).

For decades, many researchers, both in and out of the scientific community, have indicated that the social and economic future of Venezuela is closely linked to the development of the Orinoco Basin. Although well-founded, it is only in the past few years that integrated planning schemes and development projects have been considered concerning the management and use of this floodplain system. Indeed, until recently, Venezuela had given low priority to the Orinoco, perceiving the river as a limiting factor to

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development rather than as an integral part of it. This neglect becomes evident when examining Venezuelan demography. Ninety-seven per cent of the Venezuelan population is concentrated north of the Orinoco in the Northern Central Coastal Region which comprises 55 per cent of the country’s land mass but only 16 per cent of superficial waterways; only 3 per cent of the population resides in the remaining 45 per cent of the country’s territory, to the south of the river, in which lie 84 per cent of the drained river waters (M.A.R.N.R., 1982). Given this demographic pattern, environmental consequences in the Northern Central Coastal Region are obvious and relate mainly to an ever decreasing quality of life in the overpopulated urban centres of the Northern Coastal areas, an excessive demand for water and a loss of lands of agricultural potential to urban development (M.A.R.N.R., 1982).

With the current recession experienced by Venezuela’s oil-based economy, the need to focus on those resources found in the Orinoco Basin, is clear. Thus far, the development programs that are completed, planned, or being planned for the basin focus primarily on the mining of iron ore and bauxite, the enlargement and establishment of various kinds of industries mainly related to iron and aluminium processing, the exploitation of reserves of heavy oil, river transportation, and the production of hydroelectricity.

Concerning this latter aspect, 76 per cent of the gross national hydroelectrical potential is concentrated in the Guayana and Amazon regions of the country, particularly in the Orinoco River itself and in some of its major tributaries. At the present time, major hydroelectric developments are taking place in the lower section of the Caroni River, one of the most important tributaries of the Orinoco. In this river, the Guri Dam recently started operation at full capacity forming a reservoir of 4250 km2 (normal maximum level). Macagua Dam, located 10 km upstream from the Orinoco, is currently being enlarged and two more dams are planned to be built between Guri and Macagua.

Hydroelectric projects have also been proposed for the Middle and Lower Orinoco. Preliminary environmental impact assessments of the different development projects considered for the basin have indicated that damming of the Orinoco involves the highest environmental risks among other impacts, the reduction in flow variability would involve the flooding of vast portions of land above the dams and a change from periodic to permanent flooding in those areas. Other environmental consequences identified relate to the reduction of habitats available to the aquatic fauna due to permanent flooding of riverine forests, beaches, and floodplain lakes, massive developments of aquatic weeds in the reservoirs and an interruption of the migratory patterns of fishes and other aquatic life forms (Buroz et al . , 1983).

The oil industry and the Ministry of the Environment and Natural Resurces (M.A.R.N.R.) have carried out one of the major efforts aimed at the management of the territory of that section of basin which contains vast reserves of heavy oil. An agreement between these public organizations has led to the elaboration of a program of land planning of the Orinoco Oil Belt (M.A.R.N.R., 1982) and to several other projects geared at providing base-line information regarding the social, physical, and biological characteristics of the basin.

The aim of this paper is to present a summary of the major characteristics of the Orinoco River Basin and the rather recent hydrobiological information thus far generated mainly from its lower section. In addition, certain needs for future research in the area of hydrobiology are briefly outlined.

THE ORINOCO RIVER BASIN

The Orinoco forms a basin (1.1 X lo6 km2) which is shared by Venezeula and Colombia with 70 per cent of it contained within the borders of the former (Figure 1). The Orinoco constitutes one of the world’s largest rivers (2060km) and ranks third in terms of average water discharge (36000m3 s-’). The river transports a suspended load estimated at 200 x lo6 t yr-l. (Meade et al., 1983).

Generally high waters in the Orinoco occur between June and November. The highest water levels are observed in August with average oscillations in Ciudad Bolivar between 29 and 30m a.s.1. The lowest water levels are usually observed in March with average values between 16 and 17m. Therefore mean amplitude is close to 13m (Zinck, 1982). This annual river flooding has given rise to temporary and permanent floodplain lakes of various sizes and shapes.

THE ORINOCO

70 65

7 0 65 60

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60

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Figure 1 . The Orinoco Basin. The Upper Orinico extends from its source to the Guaharibos Rapids ( 1 ) . From here, the Middle Orinoco stretches up to thc Atures Rapids (2) while the Lower Orinoco extends up thc apex of the Delta (3)

To the south, the Orinoco receives waters of rivers draining the Guayana Shield. Generally rivers draining the Shield are characterized by their low electrolyte concentration and often present a dark colour caused by dissolved organic carbon (Lewis et at., 1986). These water courses are usually referred to as blackwater rivers. They tend to present an irregular longitudinal profile characterized by a succession of still waters and sloped sections that lead to the formation of rapids and waterfalls. This group of rivers includes the most important rivers of the basin with regard to length, average annual discharge and hydroelectric potential (Zinck, 1982). Within this group, the Caroni River constitutes the most important blackwater tributary of the Orinoco Basin with an average discharge of 4700 m3 s-' and contributes 12 per cent of the Orinoco discharge to the Atlantic Ocean (C.V.G.-E.D.E.L.C.A.. 1982; Paolini et af., 1983).

To the north, the Orinoco receives the waters of rivers draining the Venezeulan and Colombian Andes as well as the alluvial plains of the Llanos located between the Andes and the Guayana Shield (Figure 2).

384 E. VASQUEZ

72 O 70° 6 8 O 6 6 O 6 4 O 6 Z 0 60° 1 I I I I I

I 1 I I 7Z0 70 6 So 6 6 O 6 4 O 6 2 O 600

Figure 2. Tributaries of the Orinoco drain areas of the Andes, the Llanos, and the Guayana Shield

These rivers tend to have higher amounts of electrolytes and contribute the greatest quantity of sediments transported by the Orinoco, for which they are usually referred to as whitewater rivers (Meade et al., 1983). These water coumes have poorly defined river beds which change in position at high water.

The difference between the two regions can be explained in part as the result of their ages and geological natures. Gibbs and Barron (1983) have written a detailed review of the Guayana Shield and remark that the rocks of the Shield make up one of the world's oldest geological formations (Precambrian and Cambrian). The basement is formed by metamorphic rocks (schist, gneiss) and intrusive igneous rocks (granite, granodiorite, quartz, pegmatite, etc.), all of which are referred to as the Archaic Complex. The Andes and the Llanos belong to the more recent ages of the Tertiary and Quaternary and are made up of schists, gneisses, and intrusive granites covered by sandstones, lutites, slates, and limestones (Vila, 1960; Hernhndez, 1987).

Zinck (1980) states that the valley of the Orinoco River ends at a distance of only 230 km from its source. From there, the river follows an arch-like path with its right bank bounded by the mountains of the Guayana Shield. On its left margin, a peneplain physiography predominates up to the mouth of the Atabapo River. Therefore, the Orinoco passes through the alluvial plains of the Llanos.

Hernhndez (1987) divides the course of the Orinoco into Upper, Middle, Lower, and Delta regions (Figure 1.). The Upper Orinoco extends from its source (1047m of altitude) in the Sierra de Parima (2" 19' 05" N and 63" 21' 42'' W) to the Guaharibos Rapids. As previously noted, along part of this course the

THE ORINOCO 385

Figure 3. Aerial view of the Lower Orinoco and its floodplain

Orinoco passes through a well-defined valley. There is also a pronounced topographic unevenness as well as many falls and rapids, all of which make navigation difficult.

The Middle Orinoco stretches from the Guaharibos Rapids to the Atures Rapids. In this section, after receiving the waters of the Ventuari River, the Orinoco follows its course westwards and receives the waters of, among others, the Atabapo and Guaviare Rivers. In San Fernando de Atabapo, the Orinoco then changes its course northwards and, subsequently, receives the waters of the Llanos rivers Vichada and Tomo. Within this section, the river has a substrate of boulders with rapids, and part of its water flows, via the Casiquiare River, to the Rio Negro-a tributary of the Amazon.

The Lower Orinoco stretches from the Atures Rapids to Piacoa (the apex of the Delta). In this section are located major development activities and population centres. The slope of the river is low falling to approximately 4-5m per 100krn on the 490km reach between Caicara and Ciudad Bolivar (M.A.R.N.R., 1979); water velocity is 1-2m s-' and discharge is high. Navigation by smaller vessels in this section of the river is frequent up to Puerto Ayacucho and, with the aid of dredging, ocean-going vessels can reach Puerto Ordaz and its industrial area (Figure 3).

With respect to the Delta of the Orinoco, van Andel (1967) wrote a thorough review. The Delta is situated between the northern coastal range of Venezuela and the Guayana Shield margin to the South, with an area of some 20000 km2. At the apex of the delta (Piacoa) the Orinoco presents, at high waters, a width of approximately 20 km.

HYDROBIOLOGICAL ASPECTS OF THE ORINOCO FLOODPLAIN SYSTEM

Several limnological research programs are currently being carried out on the Orinoco River, and in some of its floodplain lakes and other seasonally-flooded forested areas. In particular, studies of sediments and water chemistry constitute lines of active research by various terms from the Universidad Simdn Bolivar,

386 E. VASOUEZ

the University of Colorado, the Instituto Venezolano de Investigaciones Cientificas, and the U.S. Geological Survey.

Weibezahn (1985) published a report on the hydrochemistry of the Upper and Middle Orinoco. Other recent studies on this aspect as well as on sediment and carbon transport in the Lower Orinoco and its Delta, and the Caroni and Caura rivers include those of NCmeth et al. (1982), Meade et al. (1983), Paolini et al. (1983), Stallard (1985), Lewis (1986), Lewis etal. (1986). Sanchez and Vhsquez (1986a), and Lewis et al. (1987). Previous works include those of Gessner (1960, 1965), Thornes (1969), Edwards and Thornes (1970). Bonazzi eral. (1972), Eisma etal. (1978), and Lewis and Weibezahn (1981). Even though detailed discussion of their research is beyond the scope of this paper we can however summarize that, based on their findings, the Orinoco waters are slightly acidic. Water is poor in electrolytes and conductivity correlates negatively with water discharge.

The most common order in cation concentration seems to be Ca++> Na+> Mg++> K+, while HC03 is considered the principal component of anions (Siinchez and Vasquez, 1986a). In 1981 Paolini el al. (1983) established that total carbon transport of the Orinoco amounted to 8.8 X 10t yr-'.

In general, the whitewaters of the Orinoco differ from the blackwaters of the Caroni River in their higher pH, conductivity, and nutrient concentrations. Additionally, a lower sediment load as well as higher values in both dissolved and particulate carbon have been reported for the Caroni River (Paolini et al., 1983). The latter authors believe that the characteristics of the Caroni are, in general, typical of blackwater rivers draining the Guayana Shield.

Concerning the hydrochemistry of the floodplain lakes of the Orinoco, there only exist the results of Hamilton and Lewis (1987) who surveyed, over an annual cycle, the water chemistry of a lake (Lake Tineo) in relation to the river. They concluded that the water chemistry of the lake was largely determined by influent river water during the filling and flow-through phases (high water) and that macrophytes and phytoplankton played an important role in affecting lake water chemistry mainly at low water.

Several floodplain lakes of the Lower Orinoco have recently been surveyed by our research team in a comprehensive temporal and spatial study of hydrochemistry and biological aspects such as phytoplankton, zooplankton, benthos, and invertebrates associated with aquatic plants. Processing of samples and data are currently being undertaken. Presently, researchers from the University of Colorado are conducting similar investigations in various lakes of the floodplain.

Phy toplan kton Varela and Varela (1983), Varela et al. (1983), and Blanco and Sanchez (1984, 1986) have made

extensive taxonomic surveys of phytoplankton samples collected in the Caroni River, Lower Orinoco, Delta, and several floodplain lakes. Over 400 species have thus far been identified. In a seasonal and longitudinal study of phytoplankton carried out in a 253 km section of the Lower Orinoco, Siinchez and Vasquez (1986a) found Bacillariophyceae to be the major component of phytoplankton followed, in a lesser proportion, by Chlorophyceae and Cyanophyceae. Major representatives of the Bacillariophyceae were Melosira granulata and M . granulata var. angustissima together with species of the genera Gyrosigma, Eunotia, Gomphonema, and Surirella. In general, diatoms have also been reported as one of the major algae groups in African and South American rivers (Rz6ska, 1974; Shiaffino, 1977; Bonetto et al. 1979; Uherkovich, 1984; Iltis, 1982; Garcia de Emiliani, 1985).

Vasquez and Sanchez (1984) and Sanchez and Vhsquez (1986a) reported that phytoplankton abundance in the Orinoco tended to correlate negatively with water level. Highest observed abundance was found to be 68 145 organisms I-' at low water while at high water abundance declined to a lowest value of 65 organisms I - ' .

In his study of primary production carried out in the Orinico and in the major tributaries Apure (whitewater), Caura and Caroni (blackwater), Lewis (1988) recorded the highest annual mean gross primary production for the Apure river (26 mg C m-' d-') compared to the blackwater tributaries, with 13 mg C m-' d-' for the Caura river and 4mg C m-' d-' for the Caroni river. Mean productivity in the Orinoco ranged from 19 to 43 mg C m-' d-'. In all cases highest production per unit volume and per unit

THE ORINOCO 387

area was observed at low water. Using a mass-balance method this author estimated that non-floodplain sources (stagnant or slow-flowing water, for example) accounted for most of the yield of phytoplankton biomass transported by the Orinoco River.

In one of the largest floodplain lakes of the Orinoco (Lake Mamo) the phytoplankton abundance pattern was similar to that observed in the river but with higher density values (range between 450000 organisms I - ' and 429 organisms I - ' ) . In this particular lake the Cyanophyceae (primarily Anabaena spp.) were responsible for the highest abundance values observed at low water (VBsquez and Sanchez, 1984). Nutrient concentration (N, P) in the Orinoco seems to be sufficient to allow an increase in biomass of autotrophic organisms (Lewis and Weibezahn, 1981). Current velocity and turbidity, however, seem to be major limiting factors for phytoplankton. In lakes, conditions would be more suitable for algae growth, turbidity, however, is still a limiting factor. In the study of Vasquez and Sanchez (1984) highest water transparency in the river was observed at low water together with highest phytoplankton density, transparency, however, never exceeded 80cm. In Lake Mamo, transparency reached a highest value of only IlOcm.

Zooplankton To date studies of zooplankton have focussed on surveying many types of water body at different times

of the year, together with in-depth investigations. The following information has been summarized from Vasquez and Sanchez (1984); Vasquez (1984a, 1984b); and Rey and Vgsquez (1986a, 1986b, 1986c, 1986d).

The list of rotifers identified in water samples from the Orinoco, Caroni, and several floodplain lakes has revealed the presence of 116 taxa of which 36 were new identifications for the country. Biogeographically this study enabled us to extend the distribution of some species previously known to exist only in the Amazon region. Specific composition consisted of an assembly typical of tropical areas with predominance of species of Brachionus and Lecane.

Concerning cladocerans, the examination of pelagic and littoral samples from several lakes and rivers revealed 58 taxa of which 24 were new to the Venezeulan fauna and two, Alona ovata and Bosminopsis macaguensis, were new to science. In general, 35 per cent of the fauna was represented by neotropical forms of which eight were endemic to South America. Highest pelagic and littoral species richness was observed in the Orinoco lakes. Faunistic pelagic composition showed a high similarity with the floodplain lakes of the Amazon. Additionally, while the whitewaters of the Orinoco River and its lakes were characterized by pelagic associations of Moina minuta, Ceriodaphnia cornuta, Bosmina tubicen, and Bosminopsis deitersi, the blackwaters (Caroni River and Macagua Hydroelectric Reservoir) were characterized mainly by the presence of Bosminidae (Bosmina hagmanni, Bosminopsis macaguensis, B. deitersi, and Ceriodaphnia cornuta).

As observed in the phytoplankton, zooplankton in the Orinoco and in a floodplain lake (Mamo) was found to relate to hydrological events. Fluctuations in seasonal zooplankton abundance were mainly due to rotifers from such genera as Keratella, Lecane and Brachionus. In their study of zooplankton from the Apure River, Saunders and Lewis (1987) reported that nine rotifer species made up over 90 per cent of the observed total mean density. These authors consider that primary source areas of zooplankton appear to be secondary branches of the braided channel of the Apure River. Some species, however, were also capable of reproducing in the main channel at low water. This latter observation agrees with our own on preliminary analysis of zooplankton samples collected along the Orinoco River from Puerto Ayacucho to the Delta.

In the Caroni River and in the Macagua Reservoir, observed zooplankton abundance was comparatively lower than that observed in the Orinoco. Littoral and benthic rotifers were more abundant than other zooplankton organisms. In another recent study carried out by Saunders and Lewis (1988) in the blackwaters of the Caura River originating in the Guayana Shield, the authors pointed out that density of organisms was regulated by the flow regime rather than by biological mechanisms. Similarly, they estimated that the annual export of zooplankton biomass from the watershed was 39270 kg C.

E. VASQUEZ

I , 3 , 5

388

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0.4

- 0.8 E &J 1.2

'c; 1.6 n

2.0

r

L 3 , 5 , mg I-' , L 3

Figure 4. Temperature (T) and dissolved oxygen (DO) profiles of vegetated (A-B) and open water sections of a floodplain lake in the Lower Orinoco (Lake Lagoven, November, 1985)

Aquatic Macrophytes Aquatic macrophyte communities in tropical floodplain systems are very heterogeneous, highly

productive, and play an important role in nutrient recycling (Welcomme, 1979; Junk and Howard-Williams, 1984). They are also important as shelter and foraging places for animals associated with them (PCrez, 1984; Sazima and Zamprogno, 1985). Neiff (1981) considers that microclimates are established in vegetated areas which differ from those of open waters. This spatial heterogenity of lakes corresponds equally to a heterogenity concerning water chemistry and the quantitative and qualitative composition of animal and vegetation communities found in them. Figure 4 illustrates the spatial heterogenity of temperature and dissolved oxygen profiles measured at high waters in open water and vegetated sections of a floodplain lake (Lake Lagoven) of the Orinoco.

From the Amazon, we have a good working knowledge of aquatic vegetation with respect to species composition, biomass, growth rates, chemical composition, etc. (Junk and Howard-Williams, 1984). In the Lower Orinoco, Hamilton and Lewis (1987) analysed the role of aquatic grasses in a lake (Lake Tineo) in relation to nutrient cycling. These authors observed that after flooding, lake macrophytes decomposed and released nutrients. Increases in potassium and chloride in the lake coincided with macrophyte decomposition. In another study of four lakes, Sanchez and Vasquez (1986b) identified 17 aquatic plant species with Eichhornia crassipes, Oxycarium cubense, and Paspalum repens as common species. Rooted emergent and floating plants were observed to increase rapidly during high water while at low water a progressive reduction in plant cover was observed. Species such as Paspalurn repens, Ludwigia helminthorrira, and L. seisioides survived during this period. Species similarity among the lakes tended to increase during high water, changing at low water, to a more heterogeneous situation.

THE ORINOCO 389

An analysis of periphytic communities associated with aquatic plants in lakes was conducted together with the previous inventory of macrophytes. We have also designed projects to evaluate the potential use of aquatic macrophytes as fish food.

Fisheries Some 600 species constitute the known freshwater fish fauna of Venezeula out of which 318 species and

subspecies have been reported for the Orinoco Basin. Naturally, this inventory is far from complete (Mago-Leccia, 1970,1978). Between 1978 and 1979 important fish collections were taken from the Lower Orinoco by research teams from Universidad Central de Venezeula and Duke University. Samples collected in the main river channel were characterized by the presence of a very diverse and productive fauna with dominance of electric fishes (Gymnotiformes) and catfishes (Siluriformes) with several apparently undescribed species (L6pez-Rojas et af., 1984). So far, results of this reseach program are included in Lundberg and Stager (1985), Lopez-Rojas et af. (1984), Mago-Leccia et af. (1985), and Lundberg et af. (1987).

The existing knowledge on fisheries of the Orinoco has mainly been the product of research carried out by Novoa and Ramos (1978), Novoa (Ed.) (1982), Novoa et af. (1984), and Novoa (1986). In general, these fisheries are characterized by their multispecific nature, i.e. the simultaneous exploitation of various species by means of different fishing gear and corresponding methods (Novoa, 1986). From observations carried out between Caicara and Barrancas, Novoa (1982) has estimated a current yield of 43.8 kg ha-' yr- ' . Similarly, Novoa estimated the fisheries potential from Puerto Ayacucho to Barrancas to be between 10000 and 12500 which added to his estimations of 30000 to 35000 of the Delta region indicate a global potential of some 45 OOO t yr-'.

The Orinoco fisheries are in an expansion phase. However, some species of high economic demand such as the large catfishes as well as cachamas (Cofossorna macropornurn) and morocotos (Piaracfus brachipomurn) are showing signs of some reduction in population densities in certain stretches of the Lower Orinoco. Future exploitation strategies then will have to be based on the political and socioeconomic objectives which could be established for the development of the Orinoco fisheries. Novoa (1986) considers that if the intention is to keep the integrity of all species, restrictive measurements will have to be established aimed at keeping or even reducing the current exploitation level. Conversely, if a considerably higher substained yield is desirable, a tendency toward an increase in fishing pressure should be favoured accepting the loss of certain species.

In conclusion it can be stated that existing information on the Orinoco fisheries is relatively good concerning the inventory of commercial species, current catches, and exploitation tendencies. However, there exist gaps of information mainly related to: biomass and population density, structure of fish associations found in floodplain lakes (diversity, richness, etc.), and population dynamics.

To generate information on some of the previously-mentioned aspects, our team has been working in small waterbodies on the floodplain of the Lower Orinoco. PCrez (1983) obtained preliminary results for low waters which show the existence of very different fish species associations in geographically proximate lakes. In his results, biomass ranged between 30 to 900 kg ha-'. parameters such as diversity and species richness showed large variations. Similarly, while in some of these small lakes there was a net dominance of primary consumers (mainly mud-eaters) in some other lakes tertiary consumers were more important in terms of biomass. Current work includes collections at different times of the year in a larger number of waterbodies with the aim of integrating physical, chemical, and morphometric data. Future analysis of the results will seek to find environmental factors which determine the conformation of established fish associations in each of the studied water bodies.

CONCLUSIONS

In comparison to other major tropical floodplain systems and particularly to the Amazon, current hydrobiological knowledge of the Orinoco River and its floodplain is still limited. However, recent investigations and current plans for research conducted by national and foreign teams have added, and

390 E. VASQUEZ

will continue to add, valuable information to the ecological knowledge of this floodplain system. To a great extent, advances thus far obtained have been supported by the national oil industry and several other government agencies who have shown interest in this information in order to elaborate development plans for the basin which will be helpful to prevent or minimize adverse environmental impact. At the present time the major reason for the researches is to obtain and- supply information that can be incorporated within the decision-making process of the planned development programs.

There is a fairly good understanding of the river system concerning hydrochemistry and sediment transport in the Upper, Middle, and Lower Orinoco. Similarly, valuable information exists on plankton and fisheries on the Lower river section. Shortly results from in-depth investigations in floodplain lakes will be available concerning benthic and other invertebrates communities associates to aquatic vegetation. Major gaps include long term studies identifying the role played by floodplain areas within the whole ecosystem. Similarly, there is an important need to establish continuous monitoring programs of the Orinoco waters in the'industrial area of Peurto Ordaz and in those major urban centres (Peurto Ordaz and Ciudad Bolivar) located at the margins of the river.

Particularly interesting is the Orinoco Delta Region where, except for some information concerning its fisheries, there is an almost complete lack of limnological information in spite of its important fish resources and the close dependence of the indigenous and creole population on the use of the river waters and fisheries.

Research carried out on the Amazon limnology has been of foremost importance for the advancement of our knowledge of major tropical floodplain river systems and has been of immense value for past and current research on the Orinoco. Similarly, development experiences in the Amazon region, not always suited for that region, should represent a valuable framework to evaluate development plans for the Orinoco.

ACKNOWLEDGEMENTS

I sincerely than Luis Eduardo PCrez for all his information concerning the Orinoco fisheries and for his reading and commentaries made to the manuscript. I also thank Werner Wilbert and Amalia PeAa who helped me in writing and improving the English version and Ana Solis and Cenaida de Reyes who typed the manuscript.

REFERENCES

van Andel, T. H. 1967. 'The Orinoco Delta', Journal of Sedimentary Petrology, 37. 297-310. Blanco, L. and Sbnchez, L. 1984. 'Contribucion al estudio taxondmico de las diatomeas del Orinoco Medio, Bajo Caroni y algunas

lagunas de inundacfon (Venezeula)', Memoria de la sociedad de Ciencias Naturales La Salle, 44, 35-94. Blanco, L. and Sbnchez, L. 1986. 'Contribucion al estudio taxondmico de las Euglenophyta. Cyanophyta, Chlorophyta y

Chromophyta de 10s rios Orinoco, Caroni y Uracoa y algunas lagunas de inundadon', Memoria de la Sociedad de Ciencias Naturales La Salle, 45. 7-48.

Bonazzi, A., Ouero, M., and Martinez, S. J. M. 1972. 'Estudios preliminares sobre mecanismos catamdrficos y migratorios que afectan a 10s suspensoides fluviales. I. La desembocadura del rfo Orinoco', Revista de la Sociedad Venezolana de Quimica, 8. 55-105.

Bonetto, C. A., Zalocar, Y., Caro, M., and Vallejos, E. R. 1979. 'Producci6n primaria del fitoplancton del rio Paranb en el brea de su confluencia con el no Paraguay', Ecosur, 6. 207-227.

Buroz, E., Ravinovich, J., and Gonzalo, D. 1983. Taller de Trabajo sobre el Proyecto Orinocc+Apure y sus lmpactos Ambientales, Inforrne de resultados presentado al Ministerio del Ambiente y de 10s Recursos Naturales Renovables, Caracas, 98 pp!

Corporacidn Venezolana de Guayana-Electrificacidn del Caronl (C.V.G.-E.D.E.L.C.A.) 1982. Desarroflo del Potencial Hidroelictrico del Rio Caroni, Caracas, 35 pp.

Edwards, A. M. C. and Thornes, J . B. 1970. 'Observation on the dissolved solids of the Casiquiare and Upper Orinoco, April-June, 1968', Amaroniana, 2,245-256.

Eisma, D., Gaast, J. Van der, Martin, J. M., and Thomas, A. J. 1987. 'Suspended matter and bottom sediments and bottom deposits of the Orinoco Delta: turbidity, mineralogy and elementary composition', Netherlandr Journal of Seu Research, 12, 224-25 1.

Garcia de Emiliani, M. 0. 1985. 'Fitoplancton de 10s principales cauces y tributarios del valle aluvial del rio Paranb: Tramo Goya-Diarnante, 111'. Revista de la Asociaci6n de Ciencias Naturales del Litoral, 16, 95-112.

Gessner. F. 1960. 'Ensayo de una comparacidn qulrnica entre el rio Amazonas, el rio Negro y el Orinoco'. Acta Cientiljka Venezolana, 1, 63-44.

TllE ORINOCO 39 1

Gessncr, F. 1965. 'Zur limnologie des untcrcn Orionco', Internationale Revue der gesamten Hidrobiologie, 50, 305-333. Gihhs, A. K. and Barron. C. 1983. 'The Guiana Shield reviewed', Episodes, 1983, 7-14. Hamilton, S. and Lewis, W. M. Jr. 1987. 'Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain,

Hernhndez. R. 1987. Geografia del Eslado Bolivar. Academia Nacional de la Historia, Caracas, 260 pp. Iltis. A. 1982. 'Peuplcments algaux dcs rivitrcs de Cote d'lvoire. 11. Variations saisonnieres des biovolumcs dc la compositions et de

Junk, W. J . and Howard-Williams, C. 1984. 'Ecology of aquatic macrophytes in Amazonia', in Sioli, H. (Ed.), The Amazon.

Lewis, W. M. Jr. 1986. 'Nitrogen and phosphorus runoff losses from a nutrient-poor tropical moist forest', Ecology, 67, 127.5-1282. Lcwis, W. M. Jr. 1988. 'Primary production in the flowing waters of the Orinoco River in relation to its tributaries and floodplain',

Lewis. W. M. Jr. and Wcihczahn, F. H. 1981. 'The chemistry and phytoplankton of the Orinoco and Caroni rivers, Vcnczeula',

Lewis. W. M. Jr . , Hamilton, S. K., Joncs. S. L., and Runnels, D. A. 1987. 'Major element chemistry, weathering and element

Lewis, W. M. Jr., Saunders, J . F. 111, Lcvines, S. A., and Weihczahn, F. 1986. 'Organic carbon in the Caura River, Venezeula',

L6pez-Rojas. H., Lundherg, J . G., and Marsh, E . 1984. 'Design and operation of a small trawling apparatus for use with dugout

Lundherg, J . G. and Stager, J. C. 1985. 'Microgeographic diversity in the neotropical knife-fish Eigenmanniu macrops

Lundhcrg, J . G . , Lewis, W. M. Jr., Saunders, J. F. 111, and Mago-Leccia, F. 1987. 'A major food web component in the Orinoco

Mago-Leccia, F. 1970. Listu de los Peces de Venezeula, lncluyendo un Estudio Preliminar sobre la Icliogeografla del Pais, Ministerio

Mago-Leccia, F. 1978. Lost Peces de Agua Dulce de Venezeula, Cuadernos Lagoven, Cromotip, Caracas, 35 pp. Mago-Lcccia, F., Lundbcrg, J. G., and Basking, J. N. 1985. 'Systematics of the South American freshwater fish genus

Adontosternarchus (Gimnotiformcs, Apteronotidae), Contributions in Science of the Natural Hktory Museum of the Los Angeles County, 358, I-IY.

Meade, R. H.. Nordin, C. F., PCrez-Hernandez, D., Mcjia, A., and Perez-Godoy, J. M. 1983. 'Sediment and water discharge in Rio Orinoco, Venezeula and Colombia', in Proceedings of the Second International Symposium on River Sedimenlution. Water Resources and Electric Power Press, Beijing, China, 1134-1 144.

Ministcrio dcl Amhientc y dc 10s Recursos Naturales Renovables (M.A.R.N.R.) 1979. Atlas de Venezeula, Direccidn de Cartografia Nacional. Caracas, 331 pp.

Ministerio dcl Ambientc y de 10s Recursos Naturales Rcnovables (M.A.R.N.R.) 1982. Proyecto Orinocc+Apure, Bases Conceptuales y Opcrativas, Serie de Informcs Tecnicos, DGSPOA/IT, Caracas, 22 pp.

Neiff, J . J. 1981. 'Panorama ccoldgico dc 10s cuerpos de agua del nordeste argentino', Simposio V I Jornadas Argentinas de Zoologia, Argentina, 11.%151.

NCmeth, A, , Paolini, J . , and Hcrrera, R. 1982. 'Carbon transport in the Orinoco river: preliminary results', Mitteilungen aus dem Geologisch-Pulaontologi.~chen Institute der Universitul Hamburg. SCOPEIUNEP Sonderbond. 55, 357-364.

Novoa. D. (Ed.) 1982. Los Recursos Pesqueros del Orinoco y su Explotacion, Corporaci6n Venezolana dc Guayana, Editorial Arte, Caracas, 368 pp.

Novoa, D. 1982. 'Analisis historicio dc las pcsqucrias del rio Orinoco', in Novoa, D. (Ed.), Los Recursos Pesqueros del Rio Orinoco y su Explotacicin. Corporacicin Vcnczolana de Guayana, Editorial Artc, Caracas, 21-50.

Novoa. D. 1986. 'Una revisi6n dc la situacion actual de las pesquerias multiespecificas dcl rio Orinoco y una propucsta dc ordcnamicnto pcsqucro'. Memoria de la Sociedad de Ciencias Naturales La Salle, 46, 167-191.

Novoa. D. and Ramos, F. 1978. Las Pesquerh Comerciales del Rio Orinoco, Corporaci6n Vcnczolana de Guayana, Editorial Vcncgrafica. I68 pp.

Novoa. D.. Ramos. F., and Cartaya, E. 1984. 'Las Pesqucrias artcsanalcs dcl rio Orinoco, sector Caicara-Cabruta. Parte I', Memoriu de La Sociedad de Ciencias Naturales La Salle, 44. 163-215.

Paolini. J . . Hcrrcra, R., and NCmeth. A. 19x3. 'Hydrochemistry of the Orinoco and Caroni Rivers', Mitteilungen aus dem Geo/ogiscii-Palaiintrilogi,schen Insritut der Universitat Hamburg, SCOPEIUNEP, Sonderband, 55, 223-236.

PCrez. L. E. 1983. 'Comparacion dc las comunidadcs dc peces de algunos cuerpos dc aguas dcl area de inundacidn del rio Orinoco Mcdio', Aria Cient(fica Venezolana, 34, (Supplement), 494.

PCrcz, L. E. 1084. 'Uso dcl hibitat por la comunidad de peccs dc un rio tropical asociado a un bosquc', Memoria de fu Sociedad de Ciencias Naturales Le Salle. 44. 14.3- 162.

Rcy. J . and Visqucz. E. 1986a. 'CladocCrcs de quclqucs corps d'eaux du bassin moyen de I' Ortnoque (VtnezuCla)', Annales de

Rcy, J . and VBsqucz. E. 1986b. 'Contribution a la connaissancc des cladoctres ntotropicaux; redescription dc Leydigiopsis ornala

Rcy, J. and Visqucz, E. 19x6~. 'Note taxonomiquc sur Camptocercus dadayi Stingelin, 1913. comb. nov. (Crustacea, Cladocera)'.

Rcy, J . and Vasqucz. E. 1986d. Rosminopsis rnacaquensis n. sp. ct Alona ovata n. sp. (Crustacca, Cladocera), Cladoctres nouveaux

Rzdska, J . 1974. 'The Uppcr Nilc Swamps, a tropical wetland study', Freshwater Biology, 4, 1-30. Srinchcz, L. and Vrisqucz, E. 1986a. 'Estudio cstacional y longitudinal de la hidroquimica y fitoplancton cn una secci6n del Bajo

Venezcula', Limnology and Oceanography, 32. 1277-1290.

la diversite spkcifique', Revue d'Hydrobiologie Tropicale, 15, 241-251.

Limnology und Landscape Ecology of a Mighty Tropical River and its Basin, W. Junk Publisher, Dordrecht, 269-293.

Ecology, 69. 679492.

Archiv fur Hydrohiologie, 91. 521-528.

yields for the Caura river drainage, Venczcula', Riogeochemistry, 4, 159-181.

Limnology and Oceanography, 31, 65-3-656.

canoes', North American Journal of Fisheries Management, 4, 33 1-334.

(Gymnotiformcs, Sternopygidal), Environmental Biology of Fishes, 13, 173-181.

river channel: evidence from planktivorous electric fishes', Science, 237, 81-83.

dc Agricultura y Cria, Oficina Nacional dc Pesca. Caracas, 283 pp.

LiMnOkJgie. 22. 137-168.

Daday. 1905. Crustacca. Cladoccra)'. Annales de Limnologie. 22, 169-176.

Annales de Limnolrigie. 22, 177-180.

du VfnCzuCla', Annales de Limndogie, 22, 219-229.

Orinoco. (Vcnczeula)', Memoria de la Sociedad de Ciencias Naturales La Salle, 46, 69-93.

392 E. VASQUEZ

SBnchez, L. and VBsquez, E. 1986b. 'Notas sobre las macrofitas aculticas en la seccidn baja del rio Orinoco'. Memoria de la

Saunders, J . F. 111 and Lewis, W. M. Jr. 1987a. 'Zooplankton abundance and transport in a tropical whitewater river'.

Saunders, J. F. 111 and Lewis, W. M. Jr. 1987b. 'Zooplankton abundance in the Caura River, Venezeula'. Biotropica, 20,206-214. Sazima, I. and Zamprogno, C. 1985. 'Use of water hyacinths as shelter, foraging place, and transport by young piranhas',

Shiaffino, M. 1977. 'Fitoplancton del do ParanB. I. Sus variaciones en relaci6n al ciclo hidroldgico en cauces secundarios de la

Stallard, R. 1985. 'River chemistry, geology, geomorphology, and soils in the Amazon and Orinoco basins', in Drever. J. (Ed.), The

Thornes, J . B. 1969. 'Variability in specific conductance and pH in Casiquiare-Upper Orinoco', Nature, 221, 4 6 1 4 2 . Uherkovick, G. 1984. 'Phytoplankton'. in Sioli, H. (Ed.), The Amaton, Limnology and Landscape Ecdogy of a Mighty Tropical

River and its Basin, W. Junk Publishers, Dordrecht, 295-310. Varela, M. and Varela, R. 1983. 'Microalgas del Bajo Orinoco y Delta Amacuro, Venezcula. 2. Bacillariophyceae, Dinophyceae',

Memoria de la Sociedad de Ciencias Naturales La Salle, 43, 89-1 11. Varela, R., Varela, M., and Farifia, A. C. 1983. 'Microalgas del Bajo Orinoco y Delta Arnacuro, Venezeula. 1. Cyanophyceae,

Euglenophyceae, Chrysophyceae, Xanthophyceae. Euchlorophyceae, Zigophyceae', Memoria de la Sociedad de Ciencias Naturales La Salle, 43, 59-88.

Vasquez, E. 1984a. 'Estudio de las comunidades de rotiferos del Orinoco Medio. Bajo caroni y algunas lagunas de inundacidn (Venezuela)', Memoria de la Sociedad de Ciencias Naturales La Salle, 44, 95-108.

Vlsquez, E. 1984b. 'El Zooplancton de la seccidn baja de un fio de aguas negras (Rfo Caroni) y de un embalse hidroelectrico (Macagua I). Venezeula', Mempria de la Sociedad de Ciencias Naturales La Salle, 44. 109-129.

VBsquez, E. and SBnchez, L. 1984. 'Variacidn estacional del plancton en dos sectores del rio Orinoco y una laguna de inundacidn adyacente', Memoria de la Sociedad de Ciencias Naturales La Salle, 44, 11-31.

Vila, P. 1960. Geografia de Venezeula. 1. El Territorio Nacional y su Ambienre Flsico, Ministerio de Educacidn, Departmento de Publicaciones, Direcci6n de Cultura y Bellas Artes. 454 pp.

Weibezahn, F. 1985. Concentraciones de especies qulmicas disueltas y transporte de sdlidos swpendidos en el Alto y Medico Orinoco y sw variaciones estacionales (Febrero 1984Febrero 1985), Informe de Resultados, Convenio Ministerio del Arnbiente y de 10s Recursos Naturales Renovables y petr6leos de Venezeula, S. A., 235 pp.

Welcornme, R. 1979. Fkheries Ecology of Floodplain Rivers, Longman, London, 317 pp. Zinck, A. 1980. Valles de Venezeula, Cuadernos Lagoven, Caracas, 150 pp. Zinck, A. 1982. Rios de Venezeula, Cuadernos Lagoven, Caracas, 63 pp.

Sociedad de Ciencias Naturales La Salle. 46, 107-125.

Hydrobiologia, 162, 147-155.

Serrasalmur spilopleura', Environmental Biology of Fishes, 12, 237-240.

llanura aluvial'. Physis, 36, 115-125.

Chembtry of Weathering, D. Reidel Pub. Co., Dordrecht, Holland, 293-316.