Forma correcta de citar este articulo: JESÚS E. CONDE 1996. A PROFILE OF LAGUNA DE TACARIGUA, VENEZUELA: A TROPICAL ESTUARINE COASTAL LAGOON. INTERCIENCIA 21(5): 282-292. URL: http://www.interciencia.org.ve

A PROFILE OF LAGUNA DE TACARIGUA, VENEZUELA: A TROPICAL ESTUARINE COASTAL LAGOON

JESÚS E. CONDE

SUMMARY

Estuaries and coastal la goons are among the most productive aquatic habitats. Together with upwelling zones, they can represent high productivity spots in otherwise barren seas. The Caribbean Sea productivity is remarkably low to the point that it can be lower than the waters of temperate regions; thus the importance of estuaries and coastal lagoons as outwelling systems in this region. Along the Venezuelan coast there are several estuaries, including large ones as the Orinoco Delta and its estuarine plume, and the so-called Maracaibo System. On the northeastern littoral of Venezuela, several coastal lagoons conform what has been considered a lacunar complex, given their closeness and common geological origin (Cressa et al., 1993). This system comprises the Laguna de Unare, Laguna de Píritu, and Laguna de Tacarigua. The latter one is the largest, it sustains important fisheries, has appealing scenic values, and its high bird diversity is noteworthy. Although some pioneering studies were carried out in the late 40's (Almeida, 1949; Carbonell et al., 1949; Ginés and Linares, 1949; Martin, 1949; Viso and Schön, 1949; Weibezahn, 1949), it is only since the 60's that this body of water, considered one of the most important coastal lagoons in the southern Caribbean (Díaz and Zelwer, 1985), elicited numerous studies dealing mainly with physicochemical characterization (Gamboa et al., 1971; López-Hernández et al., 1985; Okuda and Benítez-Alvarez, 1985); geological and geomorphological descriptions (Chacartegui and Baldy, 1978; Roa, 1985, 1988); faunal inventories and ecological studies, emphasizing birds, fishes and plankton (Luengo, 1969; Zoppi de Roa, 1974; Bulhosa, 1977; Rodríguez, 1978; Lentino, 1989, 1990; McNeil et al., 1990); and environmental threats and conflicts, and man-made alterations (Salm, 1980; Delgado, 1981a, 1981b; Díaz and Zelwer, 1985; Olivo, 1992; Conde, 1994). Bastardo (1988, 1993) dealt with patterns of mangrove-litter decay in the lagoon. Additional references, including thesis, restricted circulation reports and gray literature on Venezuelan coastal lagoons and estuaries can be found in Rodríguez (1973), Cervigón and Gómez (1986), Pannier (1986), MARNR (1990) and Cressa et al. (1993). In this work I review most of the literature at hand about Laguna de Tacarigua in order to establish the status quo of knowledge, most characteristic processes and research needs for the future.

General Description

Laguna de Tacarigua, located on the eastern-central coast of Venezuela (10º 13' - 10º 20' N latitude and 65º 42' - 65º 56' W longitude; Fig. 1), is a shallow (average depth: 1.2), estuarine coastal lagoon with an approximate area of 72 km². The lagoon, which rests on a coastal plain formed by marine and fluvial sedimentary depositions, next to a broad continental shelf, has a length of 28 km and a maximum width of 5 km. Its long axis is oriented in a southeast-northwest direction (285º). The permanent, continuous, rectangular sandbar that isolates the lagoon from the Caribbean Sea is formed mainly by detrital material; its width ranges from 300 to 1,000 m. In its western extreme, there is a narrow tidal pass that connects this estuary to the open sea. Five embayments or zones, freely communicated by channels, can be delimited (Fig. 1, Table I). The central pan of the lagoon is dotted with red mangrove (Rhizophora mangle L.) islets. There is one important fluvial tributary, the Guapo River, that discharges continuously through the Madre Casañas Canal, and several intermittent, seasonal creeks along the southern shore.

The climate of Laguna de Tacarigua, classified as DS 2A' a', according to the Thornthwaite system, is humid-subhumid and the life zone is considered as Dry Tropical Forest (MARNR, 1986). Rainfall over this area averages 990.0 mm/year. Rains show a marked seasonality with seven to eight wet months. The wet season, when 85% of the precipitation occurs, usually runs from June to December; the maximum is in November with an average of 169.5 mm (MARNR, 1990). The dry season, at the beginning of the year, lasts four to five months. The driest month is March with an average precipitation of only 14.3 mm. Interannual variation in total rainfall is 29.6% (MARNR, 1990). The average annual precipitation at the Guapo River uplands is significantly higher, reaching 2,200 mm (MARNR, 1990).

Evapotranspiration which averages 1,619.2 mm annually, varies from 104.8 mm in February to 154.1 mm in August (MARNR, 1990). From January to September, potential evapotranspiration is higher than precipitation. Mean annual evaporation is 2,003.5 mm, well above precipitation; however, there is a year-round surplus of freshwater, due to the continuous discharge of the Guapo River. The yearly average for the evaporation/rainfall ratio is 2.02.

Mean annual air temperature varies slightly among and within localities in the Venezuelan coast. In the Laguna de Tacarigua annual average temperature is 26.5 ⁰C, with the minimum in January (24.8 ºC) and the maximum in September (27.5 ºC).

All the Venezuelan coast is influenced by the action of trade winds blowing from east-northeast. In some zones, they can reach high velocities, as for example in the Laguna de Cocinetas and Ciénaga de Los Olivitos in western Venezuela, where wind speeds exceeding 16.7 m/s have been recorded (MARNR, 1986). Similarly, in the Laguna de Tacarigua area the prevailing winds are trade winds coming from E-NE with an average annual speed of 6.3 m/s. Hurricanes are annotated as occasional in this area (MARNR, 1986).

Geology and Geomorphology

Based on water exchange with the coastal ocean and the dominant forcing functions and time scale of hydrological variability, Kjerve (1986) has subdivided coastal lagoons in three geomorphical types. Choked lagoons are characterized by a single narrow entrance channel, long residence times, and dominant wind forcing. Restricted lagoons have two or more entrance channels, well-defined tidal circulation, and vertically-mixed waters. Finally, leaky lagoons are characterized by wide tidal passes, unimpaired water exchange with the ocean, strong tidal currents, and sharp salinity and turbidity fronts. Given its narrow entrance channel and the wind-induced circulation in most of Laguna de Tacarigua, it could be defined as a choked lagoon.

The geological origin of this lacunar complex can be traced to the Flandrian transgression during the Holocene. The sandbar that separates the lagoon from the Caribbean Sea originated during this transgression, but presently

the deposition of sands is due to the waves and littoral currents. These sands come from the nearby mountain range and also from farther zones of eastern Venezuela (Chacartegui and Baldy, 1978).

The sedimentary load entering the lagoon from the river was estimated in 150 m³/year (MARNR, 1990). Font (1978) estimated that every year 3 X 10⁶ m³ of sediments are transported by littoral currents towards the lagoon's inlet. Currently this body of water undergoes a rapid dumping process due to the sedimentary influx of the Guapo River and several seasonal creeks (Chacartegui and Baldy, 1978). This process is most conspicuous at the Madre Casañas Canal mouth, where a delta has been growing briskly at a rate of 0.25 km²/year (Fig. 2). Historical records indicate that depth has decreased at several sites of the lagoon; at the beginning of this century profundities of 7 m were measured at El Guapo enclosure, while currently its maximum depth does not surpass 2 m (Delgado, 1981b). The sedimentation rate has increased during the last years probably because of the multiplication of deforestations in the Guapo River catchment basin.

The sediments draining into the lagoon are dominated by alluvial and marine calcareous materials. The surficial sediments have a detrital origin, although there are neoformations of pyrite and evaporites, and probably there are in situ formations of chlorite, calcite and aragonite (Chacartegui and Baldy, 1978). As a whole, muddy-clay fractions predominate in most of the sediments, although in several zones the gravels fraction is considerable, comprising diverse components: shells and organic debris, and sand of likely marine origin (Chacartegui and Baldy, 1978). The proportion of sands in the sediment has been found to depend mainly on the distance to the lacunar sandbar, resulting in a grain size gradient. The soils next to the lagoon are acid, their pH's range from 3.3 to 6.7 (MARNR, 1990). Clays predominate; they represent an average percent of 85.0 (MARNR, 1990). In the alluvial plains, where mangroves grow, acid soils with sandy textures (tropofibrists), or with high concentrations of organic matter (sulfaquents) predominate. Permanently flooded soils (hydraquents) are found in the islets (MARNR, 1990).

The suspended material in the water varies spatially in a haphazard manner, probably as a consequence of the interplay between the seasonal variations of the Guapo River sedimentary load and the intrusion of marine waters. For instance, in 1978 the material in suspension was high at El Cazote (480 mg/l), Laguna Grande (350 mg/l) and El Placer (328 mg/l); while the lowest values were obtained at El Guapo, probably due to the flocculation that would take place when the river's fresh waters mix with the brackish waters of the lagoon (Chacartegui and Baldy, 1978). However, Gamboa et al. (1971) and Zoppi de Roa (1974) reported the lowest values of transparency at El Guapo embayment (21.5 -58.0 cm) which reflects the high sedimentary load of this river; while the highest transparency was measured at Laguna Grande and El Placer where visibility reached 1.0 m.

The prevailing direction of the surface marine currents off the Venezuelan coast is from east to west. In general, the speed of these current is low, ranging from 0.1 to 0.7 knots (5 -36 cm/sec) (Ginés et al., 1972, 1982). At the central littoral of Venezuela, where the Laguna de Tacarigua lies, tides with daily components, mean variations close to 20 cm, and maximum amplitude of 30 cm predominate (Herrera et al., 1981; MARNR, 1990). Low-energy waves distinguish this coast (MARNR, 1990).

The hydrological patterns in Laguna de Tacarigua follow a characteristic annual course modulated by the variable discharge of the tributaries. According to Chacartegui and Baldy, (1978), the circulation patterns are a consequence of the interplay of trade winds coming from the E-NE and the permanent discharge of the main tributary to the lagoon, the Guapo River, which provides a year-round surplus of freshwater, and of several intermittent streams. The prevailing direction of the strongest winds opposes the direction of the principal riverine inflow; thus the mixing of waters is constrained, bringing about differences and gradients in the horizontal distribution of salinity and nutrients (Okuda and Benítez-Alvarez, 1985). Also, the presence of mangrove islets in the central part hinders water circulation (Okuda and Benítez-Alvarez, 1985). The effects of tidal movements are probably damped down in the embayments at the eastern end of the lagoon, as suggested by the salinity distributional patterns. The entrance channel acts as a filter that eliminates the effects of littoral currents and subdues the influence of tidal fluctuations. During the rainy season there is a net water flow from the lagoon to the open sea due to the increase in the lagoon's water level, resulting in an outwelling current (MARNR, 1990). When the inlet is open and during the dry season, the movements produced by the tides originated in the Caribbean Sea provoke an oscillation of 2 cm in the lacunar water level (MARNR, 1990). In most of the coastal lagoons of Venezuela fluvial inflow is low or absent. The Tacarigua Lagoon, which receives the fluvial subsidy of the Guapo River and of several seasonal creeks, is an exception; however, the volume and rate of freshwater entering the lagoon have not been determined.

There are thermal differences between the five embayments. In the period from 1967 to 1969 and in 1982, the lowest water temperatures (26.4 and 26.6 ºC, and 25 ºC, respectively) were recorded at El Guapo; while the highest values (33.6 and 33.8 ºC) were measured at Laguna Arena in 1967 and 1968, respectively, and at El Placer and Laguna Grande in 1969 with 31 ºC (Gamboa et al., 1971; Pagavino, 1983). Higher temperatures, however, were recorded in Laguna Arena (36 ºC) and at El Placer and Laguna Grande (35 ºC) by Chacartegui and Baldy (1978). The widest range in water temperature was detected in Laguna Arena in 1967 and 1968, and at El Guapo in 1969 (Gamboa et al., 1971), while the most stable temperatures were reported for El Placer in 1967, El Guapo in 1968 and Laguna Grande in 1969.

As a consequence of the scant depth of the lagoon, water temperature fluctuates periodically and daily. During their three-year study, Gamboa et al. (1971) recorded a clearly seasonal pattern of water temperature variations for the five embayments of the lagoon. Maxima were observed in August and September, when temperature reached 33.8 ºC, while minima were registered in February and April (26.4 ºC).These changes follow more or less closely the variation of air temperatures in this zone. Shorter ranges were registered from 1980 to 1983, when the maximum temperature was 30.6 ºC, while the minimum was 26.3 ºC (Bonilla and Cedeño 1989). As a whole, the temperature mean values did not vary greatly among years during that period, oscillating between 30.0 and 30.1 ºC (Gamboa et al., 1971); however, from 1980 to 1983 lower mean values were recorded: 28.3 to 29.8 ºC (Bonilla and Cedeño, 1989). The reasons for these differences have not been elucidated, but probably arise from the obstruction episodes of the tidal pass.

Vertical temperature inversions have been registered sporadically in El Guapo and Laguna Grande (Gamboa et al., 1971). At El Guapo, during June and October (1967), April, June, October and November (1968), and from April to July and in October 1969 vertical inversions of temperature were observed. In Laguna Grande, during April 1968 and October, 1969, temperature inversions were also observed, probably as a consequence of freshwater inflow coming from several tributary creeks in this zone. Gamboa et al. (1971) ascribed this phenomenon to the invasion of seawater.

Since the 60's, several authors have reported surface salinity values in Laguna de Tacarigua, so various data sets are available (Gamboa et al., 1971; Chacartegui and Baldy, 1978; Pagavino, 1983; Okuda and Benítez-Alvarez, 1985; Rodríguez and Conde, 1989; Olivo, 1992). Although this body of water can be considered euhaline-mixohaline as a whole, salinity can vary locally and temporarily from hypersaline levels (54.0‰) at the eastern and central zones, where the freshwater influx is sporadic, to limnetic records close to the Guapo River discharge area. Salinity regimes depend on several factors: freshwater input, rainfall, intrusion of seawater, and evaporation.

Salinity is higher in the central part of the lagoon and during the dry season. The greatest fluctuations in salinity occur at El Guapo and El Placer embayments, close to the riverine freshwater influx and seawater intrusion, respectively. The lowest range is observed at El Cazote in the eastern extreme of the lagoon and at Laguna Arena, a wide channel that runs parallel to the sandbar in the central-eastern part of the lagoon (Fig. 1). From April, 1967 to December, 1969, Gamboa et al. (1971) recorded values ranging from 11.0 to 38.7‰ in Laguna Arena and from limnetic levels (0.4‰) to 34.9‰ at El Guapo. Also, the lowest values of salinity were recorded in this embayment (Gamboa et al., 1971). Rodríguez and Conde (1989) reported higher values of salinity, 46.0‰, close to the tidal pass. Salinity also experiences large variations in a time scale, fluctuating synchronously in the five zones of the lagoon. Minima take place at the end or beginning of the year, coinciding with the wet season, while maxima appear during the dry season (Gamboa et al., 1971; Okuda and Benítez-Alvarez, 1985).

Okuda (1981) and Díaz and Zelwer (1985) postulated a salinization process of the lagoon's waters. According to the information gathered by Okuda (1981), during 1967-1969 salinities over 40‰ were never registered, and the mean values ranged from 19.0 to 24.0; however, in 1978 salinities were much higher, ranging from 45.0 to 51.0‰, depending on the zone of of the lagoon. The results presented by Rodríguez and Conde (1989) support Okuda's contention; from January to July 1974, they registered mean salinity values in excess of 40.0‰, with a maximum of 46‰ during the latter month. Nevertheless, the information in Olivo (1992), which includes salinity records from 1976 to 1986, apparently disclaims the salinization hypothesis. During this period the average salinity ranged from 13.4 to 27.7‰, with a maximum of 40.1‰. These magnitudes are confirmed by the data in Pagavino (1983), who reports salinities from 6.0 to 40.0‰ for the period from February 1982 to January 1983. Furthermore, the average salinity during the span from 1967 to 1969 (21.0‰), although slightly lower, does not differ significantly (t. = 0.92; 0.2 < P < 0.4) from the mean value for 1980-1983 (23.9‰). In an updated review, Okuda and Benítez-Alvarez (1985) reappraised the information at hand and concluded that the salinity at Laguna de Tacarigua did not undergo major shifts in response to the freshwater diversion and catchment basin alterations. Thus, more than a sustained trend in time, the salinity patterns in the lagoon probably spring from the complex interaction of the Guapo River discharges, tidal cycles, and the impairment of the entrance channel, an episode that has occurred repeatedly as a consequence of the hydrological modifications and the sediment load of the Guapo River. These obstruction episodes may last long periods, even reaching a whole year (Humberto Díaz, personal communication)

Depending on the zone of the lagoon and time of the year, dissolved oxygen can vary substantially, ranging from totally anoxic conditions to 20.5 ml/l (Gamboa et al., 1971; Chacartegui and Baldy, 1978; Pagavino, 1983; Bonilla and Cedeño, 1989). However, with the exception of 1978, when values were abnormally high, yearly mean values do not seem to vary greatly, ranging from 3.53 to 4.62 ml/l (Table II). In general, the highest annual averages have been registered at the central eastern sector of the lagoon (Table II). The widest oxygen oscillations have been observed in the channel known as Laguna Arena, where the lowest average values (2.94 at the water surface and 2.73 ml/l at the bottom) were registered during 1969 (Gamboa et al., 1971). From 1967 to 1969, variations were lower a the Guapo River embayment (1.54 - 8.54 ml/l), while fluctuations were greater in the central eastern enclosures; yet, from June to October of 1969 low values of oxygen were observed at the central zone (Gamboa et al., 1971). Dissolved oxygen also fluctuates through time in a haphazard fashion with no manifest periodic patterns. For instance, Pagavino (1983) observed oxygen peaking in January (12.2 ml/l) and July (8.4 ml/l), while Gamboa et al. (1971) registered several maxima distributed randomly throughout their three-year observation period. Gamboa et al. (1971) ascribed toe stability of oxygen in the western zone to the larger mixing of water in that area, and the reduced values of oxygen to pulses in the decomposition of organic matter.

The main chemical characteristics of the surficial sediments are recapitulated in Table III. Total phosphorus, with an average of 0.056%, and nitrogenous organic matter in the sediments are low (Bonilla and Cedeño, 1989). The C/N ratio is high (26.5), which indicates a prevalence of decomposition processes with a large organic contribution of terrigenous influence. Also, the N/P quotient is steep, which suggests quick degradation and little capacity for phosphorus retention by the sediments (Bonilla and Cedeño, 1989). However, López-Hernandez et al. (1985) found that the sediments in this lagoon have a high capacity for phosphorus retention, acting as a sink for this element and thus restraining eutrophication during short periods. The average water content of the sediments is 50.5% with extreme values of 30.2 and 74.6% (Bonilla and Cedeño, 1989). Organic carbon in the sediments of Laguna de Tacarigua aver aged 6.7%; a higher value than those reported for the nearby Laguna de Unare and Laguna de Píritu (Bonilla and Cedeño, 1989) and also larger than the values considered normal for oceanic waters (Turekian, 1976).

The vegetation found in the lagoon's area has been described in some detail in several works (Ginés and Linares, 1949; Huber and Alarcón, 1988; MARNR, 1990) and reviewed by Conde and Alarcón (1993). The most conspicuous vegetation in the lagoon's surroundings are mangrove forests, which cover approximately 4,000 ha (Conde and Alarcón, 1993) (Fig. 1). In spite of deforestations and extractions, from 1964 to 1975 mangrove forests have increased, mainly in the islets, at a rate of 35 ha/ year (Delgado, 1981b). Overall, the dominant species is the red mangrove, Rhizophora mangle L., which accounts for 71% of the coverage. Avicennia germinans L. (black mangrove) and Laguncularia racemosa Gaerth (white mangrove) stretch out through only 6 and 2% of the area, respectively. The rest of the coverage is made up of Conocarpus erectus L. (buttonwood mangrove) and mixed forests of the other three species (MARNR, 1990).

Three mangrove landscapes can be recognized. The lagoon is dotted with small islets of R. mangle (Fig. 1). On the alluvial plain, mangrove forests, with a width of 500-1000 m, border the southern perimeter of the lagoon; a belt of Avicennia-Laguncularia is adjacent to the xerophilous deciduous forest (MARNR, 1986, 1990). On the coastal plain sited on the sandbar which separates the lagoon from the sea, all the four mangrove species are present. These mangrove trees are well-developed; their average height is 15 m, although in some zones they can reach 25 m. The diameter at breast height ranges from 25 to 30 cm in R. mangle, 10 to 40 cm in A. germinans, and 12 to 13 cm in L. racemosa (MARNR, 1990).

Besides arid and semiarid littoral formations, where the xerophilous scrubs and the psammophilous or halophilous meadows dominate, there are also tropophilous basimontane deciduous forests (Huber and Alarcón, 1988), also known as dry tropical forest (MARNR, 1990). These forests, with one or two strata and a very dense understory, reach heights that go from low to middle (10-15 m), with emergents up to 20 m. Among the more abundant species are Bauhinia megalandra, Bourreria cumanensis, Calliandra caracasana, Cassia marginata, Cecropia spp., Erythrina poeppigiana, Hura crepitans, Inga punctata, Ritterocereus griseus, Tabebuia billbergii and T. rosea. There is also hydrophilous vegetation in the areas by the mangroves, where the slope is less that 1%, and soils are poorly drained (MARNR, 1990). Roystonea regia, some highly developed herbs, grasses, forbs, and the musaceae Heliconia latispatha are common in these areas (MARNR, 1990).

Numerous censuses and taxonomic surveys of the bird fauna of Laguna de Tacarigua have been undertaken since the early 40's (reviews in Lentino 1989, 1990; McNeil et al., 1990). Even for the lay observer, the diversity and densities of birds in and around this body of water are noteworthy. A total of 136 aquatic species have been reported (Lentino, 1990), which ranks this estuary as the third wetland in Venezuela in number of species. Eighty of these species are residents, including 58 which nest in this area. Most of the residents prefer or are confined to the mangrove grounds (Lentino, 1989). Common birds, including various indexed as vulnerable or endangered, are listed in Table IV.

The presence and abundance of the American flamingo (Phoenicopterus ruber), an endangered species with a few nesting sites in the Caribbean, have been monitored quite accurately since the early 80's, although interspersed observations go back to the 40's (Lentino, 1989). Flamingo densities fluctuate greatly; the highest abundance was registered in May, 1984, when a likely breeding population of P. ruber, estimated in more than 2,500 individuals, was observed. However, since 1984 the quantity of birds has not surpassed 200 specimens and their breeding status is considered uncertain (Lentino and Goodwin, 1988; Lentino, 1989). Although Lentino (1989), and the authors therein referred, did not observe any reproductive activity recently, fishermen, who are able to describe accurately the flamingo's nests, claim to have sighted nests during recent years. The exploitation patterns of the lagoon by P. ruber have shifted in time, apparently as a consequence of the depth changes in the lagoon. They used to harvest their food at El Cazote in the eastern extreme of the lagoon, but now they are sighted mainly in El Guapo embayment in the west.

The brown pelican (Pelecanus occidentalis), a species that is disappearing in much of its range in the world, is also abundant in Laguna de Tacarigua, with one of the few reproductive colonies remaining in Venezuela (Lentino, 1989). During the 70's, a breeding population of Eudocimus ruber, the scarlet ibis, was witnessed in Laguna de Tacarigua (Spaans, 1975), but presently the approximately 500 individuals left do not reproduce in this area (Lentino, 1989).

Laguna de Tacarigua is also an important destiny in the migrations of Neartic Limicolae or as a stepping stone on their way to their hibernation quarters. Twenty-seven species have been reported, ranking as the second wetland in Venezuela in number of species (Lentino, 1990). Some of those Limicolae spend up to 70% of their life-span on the Venezuelan coast.

In general, in Laguna de Tacarigua plankton biodiversity is very low. A few species, typical of brackish waters, are continuously present in high numbers. Zoppi de Roa (1974) found that from October, 1969 to January, 1970, only three species accounted for an average of 84.0 % of the zooplankton abundance in this estuary. The bulk of the zooplankton was composed by the copepod Oithona hebes, the tintinnid Favella panamensis, and the rotifer Brachionus plicatilis. In Laguna Arena, this latter species peaked at 108,236 individuals/ml during the rainy season of 1969 (Zoppi de Roa, 1974). B. plicatilis was also the dominant species in an ensuing study (Rodríguez, 1978). The algae Ceratium fusus v. seta was also found in great numbers by Zoppi de Roa (1974), although its abundance was not quantified. In the proximities of the lagoon's inlet, the number of species was higher due to the intrusion of marine organisms; Acartia lilljeborghi and Paracalanus parvus predominated in this area, and Sagitta tenuis and Lucifer fexoni were also present (Zoppi de Roa, 1974). Another species, Paracalanus crassirostris, has been reported for the lagoon (Rodríguez, 1978). With the exception of the algae Ceratium fusus v. seta all species annotated above have also been accounted for in the nearby Laguna de Unare, and some of them have a wider distribution ranging down to Cananeia, Brazil (Teixeira et al., 1965).

Species composition can change -dramatically in time and space. In February 1973, in the same stations where they were sampled before, the proportions of Oithona hebes, Brachionus plicatilis, and Favella panamensis were much lower, with an average of 16.1% and ranging from 0.0 to 36.4% (Zoppi de Roa, 1974). The lowest abundance of zooplankters was registered in the inlet where the Guapo River drains through the Madre Casañas Canal and which is also close to the lagoon's mouth. Variations in zooplankton spatial distributions were detected between zones, seasons, depths and time of day (Rodríguez, 1978). In a short span that comprised salinities from 3 to 8‰, Rodríguez (1978) found a high correlation between salinity and the abundance of Oithona hebes. During the wet season, the vertical distribution of zooplankton's mean density experienced great changes. However, during the dry season the vertical distribution of zooplankton was uniform throughout the water column (Rodríguez, 1978).

Plankton primary productivity at Laguna de Tacarigua is higher than the productivity estimated at other offshore and inshore localities on the Venezuelan coast, which average less than 10 mgC/m³/h (Ginés et al., 1972; Gonzalez, 1989; Rodríguez and Conde, 1989). The values displayed in Rodríguez and Conde (1989) are four-fold those registered in nearby coastal waters. Productivity is markedly seasonal, averaging 36.1 mgC/ m³/h (standard error = 14.8); the maximum values coincided with rain peaks, during the middle and the end of the year, when the primary production reached 187.76 mgC/m³/h (Rodríguez and Conde, 1989). In spite of its scant depth, this lagoon exhibits considerable vertical differences in productivity. Crossover incubations showed that the surficial sample cultured in the surface rendered 41.4 mgC/m³/h, while the sample cultured at the bottom yielded only 2.6 mgC/m³/h. Similarly, the bottom sample productivity was 44.0 mgC/m³/h in the surficial water, whereas it only produced 2.1 mgC/m³/h at the bottom. These differences probably stem from the dramatic luminosity decay which occurs at a 50 cm depth, as a consequence of the increased turbidity values.

As far as we know, inventories of Crustacea have not been undertaken in Laguna de Tacarigua. Nonetheless, there are several isolated ecological studies dealing with this group. One of the species studied is the mangrove crab Aratus pisonii. In the Laguna de Tacarigua several features of the life history and population dynamics of A. pisonii are related to salinity fluctuations and rainfall patterns (Conde and Díaz, 1989a). The first, probably a reflection of tidal cycles, is associated to the apparent abundance of this species. Rainfall correlates with the time-course of the reproductive effort. Although ovigerous females were continuously present, fluctuating from 23.2 to 82.7%, the largest values were registered during the rainy season at the end of the year, when increments of nutrients and primary productivity in the lagoon have been recorded. Recruitment of this crab is also seasonal; the largest percentages of recruits were observed during March or April of each year (Conde and Díaz, 1989a). To determine if these patterns are present in other species that live in the lagoon would be of great practical interest. Also, consequences of those patterns on the trophic chains culminating in species exploited commercially, could be useful in a sustainable management scheme.

Also, Conde and Díaz (1989b, 1992a, 1992b) and Conde et al. (1989) have found that there is a close relation among some of the biometric and life history traits of Aratus pisonii and a mangrove productivity gradient. The largest crabs, including ovigerous females, and the highest percentages of ovigerous females have been found in estuarine (Laguna de Tacarigua) and riverine arboreal mangroves, while the smallest crabs and the lowest reproductive effort, on the contrary, have been observed in arbustive and scrub mangroves (Table V). Similarly, the land crab Cardisoma guanhumi, a commercially exploited species, attains larger sizes and greatest fecundities in Laguna de Tacarigua than in a marine coastal zone in western Venezuela (Moreno, 1980). These differences are ascribed by Moreno (1980) to the higher productivity of the estuary.

There used to be a large population of the American crocodile (Crocodylus acutus) in Laguna de Tacarigua. In 1949 the sightings of crocodiles were common (Almeida, 1949); however, from 1940 to 1960, this species was relentlessly exploited by French and English companies which exported hides to Europe (Boede, 1982). Presently a small population of this reptile is left (Figueroa and Seijas, 1986). Another common reptile that lives among the mangrove trees is the arboreal snake Corallus hortulanus.

Among the several species of terrestrial mammals that have been observed within the mangrove forests and among the vegetation that surrounds the lagoon are: the crab-eating raccoon (Procyon cancrivorus), the prehensil-tailed porcupine (Coendou prehensilis), the ocelot (Felis pardalis), the capuchin or ring-tail monkey (Cebus olivaceus), the paca (Agouti paca), the agouti (Dasyprocla aguti), the brocket deer (Mazama americana), the capybara (Hydrochaeris hydrochaeris), and bats of the genus Noctilio and Saccopteryx (Boede, 1982).

On a global scale, estuarine fisheries are considered insignificant (Barnes, 1974). However, more than sheer catches, the importance of estuaries ensues from their role as nursery grounds for marine fish stock. Many commercially-important species spend part of their life cycles in estuaries and coastal lagoons, where large predators are not common and, hence, the predation pressure over juveniles, including those of species whose adults live in deeper waters, is subdued. Besides, these fisheries can have a considerable regional and local importance.

The main Venezuelan fisheries rest on the activity of trawl boats in marine areas. Compared to these highly capitalized and state of the art fisheries, the catches in coastal lagoons, where fishermen use casting nets and small boats with outboard engines, seem insignificant. Yet, fishing in these lagoons can have a paramount local significance for the villagers, whose income can depend, almost totally, on this activity. In Laguna de Tacarigua, fisheries transcend the subsistence status, reaching a marginally-commercial level. From 1975 to 1982 yearly catches fluctuated from 195,746 to 534,561 kg, with an average of 383,282 kg (Pagavino, 1983). Higher yields, however, have been reported: 1,015,000 Kg of fish were landed by 300 fishermen in 1966 (Gamboa et at., 1971). In 1982 the registered fishing fleet comprised 117 small boats, whose length did not surpass 6 m and were propelled by outboard engines with a maximum power of 9 HP (Pagavino 1983). Most of the fishing efforts take place at El Placer embayment, followed by El Guapo enclosure (Pagavino, 1983).

Although many fish species can be found in tropical and subtropical inshore lagoons, a small number of species concentrate the majority of catches. The most common species caught in coastal lagoons of Venezuela are presented in Table VI. The piscatorial activities at Laguna de Tacarigua rest strongly on the exploitation of Mugil liza, which represents 56% of captures. Other commercially-important species are Centropomus ensiferus, C pectinatus and C. undecimalis (15%), M. curema (12%), and Eugerres plumieri (11%). The gray snapper (Lutjanus griseus), Tarpon atlanticus, Arius spixii, and Selenaspis herzbergii are also caught, but their volumes lack any market importance. Until a few years ago, shrimps (mainly P. schmitti) were also a main staple in the lagoon villagers budgets; however, the low volumes currently caught hint that this resource, highly-priced, has been overexploited. Offshore of the Laguna de Tacarigua, besides P. schmitti, other species of shrimps have been collected during exploratory fishing campaigns: P. duorarum, P. brasiliensis, P. aztecus, and Xiphopenaeus kroyeri (Larez and Khandker, 1972). These species presumably spend part of their cycles in the lagoon.

In spite of the fact that most of the species referred are extracted throughout the year, captures are distinctly seasonal. During a one-year lapse, Pagavino (1983) found that the major catches were harvested during two periods: April-June and September-January, with a monthly average of 12,951 Kg, while lower volumes were cropped up during the other months (average: 6,378 Kg). Mugil liza dominated the catches for most of the year, with the exception of the period from March to May. M. curema catches prevailed during March and April, while Centropomus spp. dominated in May (Pagavino, 1983).

The advent of coastal lagoons and mangrove forests expansion, probably unchained seminal social changes among nomadic human groups, by prompting the communities of gatherers, fishermen, and hunters to settle (Sanoja, 1992). In eastern Venezuela, 6,000 or 5,000 years ago, human populations coming from the inner lands took up residence in coastal areas by the mouths of the rivers that descend from the Paria Sierra. Coastal wetlands provided a spectrum of resources for diverse forms of extraction and spurred some inchoate cultivation of edible plants. Particularly, extraction of mangrove oysters could have induced seminomadic settlements, in a fashion similar to the use of soil in slash-and-burn farming (Veloz Maggiolo and Pantel, 1976; cited in Sanoja, 1992). The migration of human populations to the coastal areas is testified by the presence of archaeological preceramic sites of shellfish pickers and fishers in the coastal area of Tucacas (State of Falcón) (Cruxent and Rouse, 1958-1959).

Traditionally, the main activities of inhabitants of zones next to coastal lagoons in Venezuela have been small-scale fisheries, subsistence farming and livestock raising. The explosive development of tourism, that propelled the construction of marinas and resorts, however, has introduced changes in these occupational patterns during the last three decades. Nowadays, many fishermen -characterized by low per capita incomes and illiteracy- are engaged in tourism-related activities. This occupational shift has been studied in Agua Salobre, a small village located in western Venezuela. Until 1965 the traditional labor patterns were centered in fisheries, livestock raising and farming; however, when tourism boomed in the area it thrusted people to occupations related to tourism (Scorza, 1978). It seems that similar patterns have arisen in Laguna de Tacarigua (MARNR, 1990). In the 70's, 36.7% of the population was engaged in the primary sector of the economy: farming, cattle raising, hunting and fishing, with a heavy emphasis on the latter activity. In 1971, the number of persons engaged in these occupations in the counties that surround Laguna de Tacarigua was estimated at 2,865. However, the increase in the population density in this area, from 10.3 inh/km² in 1950 to 19.8 inh/km² in 1981, has been ascribed to a wider offer of jobs in the touristic and recreational sector (MARNR 1990).

The income profile of coastal dwellers in Venezuela is low. In the 70's the monthly per capita income of the inhabitants of Laguna de Tacarigua County averaged US$ 87; a great majority (78.7%) earned US$ 175 or less (MARNR, 1980). Since 1983 the average income in Venezuela has dropped dramatically, so it is not hard to guess that in Laguna de Tacarigua the income has dwindled too. Occasionally, fishermen get high monetary returns, due to the high prices of some fishes and shrimps; however the economic levels of fishermen are very low, because they do not own their boats, outboard engines, neither their piscatorial apparels. Their profits end up in the pawnbrokers hands and fisheries companies, which control the top links of the commercialization chains (Suárez and Bermúdez, 1988).

One of the most thoroughly documented cases of ecological impacts and threats on coastal habitats in Venezuela is the Laguna de Tacarigua (Salm, 1980; Delgado, 1981a, 1981b; Díaz and Zelwer, 1985; Conde 1994). Within the past 40 years a considerable number of environmental threats and alterations has been borne on the lagoon and its surroundings. Delgado (1981a) catalogued the impacts on this lagoon until 1980. This list includes massive deforestations; accumulation of tannins due to changes in the hydrologic regime; progradation due to sedimentation; dragging operations with alteration of the natural opening and closing of the lagoon's inlet, with a consequent change of tidal rhythms; overexploitation of commercial species, as crocodiles and turtles; overfishing and fishing with banned apparels; and urban invasion.

Early environmental impacts were the massive deforestations of mangroves during 1920, 1927-1931, and 1953-1957 (Delgado, 1981a). Already in the 40's, fishermen were strongly opposed to the dredging of the lagoon's inlet, arguing that conventional ore suggested that fishes would escape toward open waters (Viso and Schön, 1949). During the late 50's, as a consequence of the construction of a highway, the Cúpira River, that flowed directly into the lagoon, was diverted into the Caribbean Sea, and several creeks were obstructed. A very important impact was the deviaton of the original course of the Guapo River, by means of the Madre Casañas Canal in 1963 (Díaz and Zelwer, 1985). These hydraulic works have brought about important changes in the hydrodynamic regime of the western portion of the lagoon, including an increase in the erosion and sediment transport towards the lagoon, provoking a progradation of around 225 ha (Fig. 2) and, as a consequence an alleged decrease in fish catches (Salm, 1980).

Recently, Díaz and Zelwer (1985) compiled a detailed chronology of current impacts and the alleged leniency of several governmental organisms to stop the interventions that arose from building a resort. The exclusion of the western bar from the legal protection regime; the probable use of defoliants of phenoaxiacetic type; and, according to Díaz and Zelwer (1985), the scarce interest of the Ministry of the Environment and Renewable Natural Resources (MARNR) to fulfill its functions, epitomize the threats and man-made alterations that can affect coastal lagoons in Venezuela. Presently, threats should be subdued, because of the Laguna de Tacarigua current National Park status; however, since the early 80's, many resorts have been built in the neighboring areas.

Concluding remarks

Although numerous studies have been carried out in Laguna de Tacarigua, they are disjointed. Their emphasis has been descriptive and their span very restricted. Additionally, aliasing, that is the introduction of variability into data sets because of infrequent sampling, might conceal the real magnitude of changes in the physicochemical parameters and biotic dynamics. Accordingly, the knowledge about this lagoon is highly fragmented, which makes variations and long-term trends difficult to assess.

Many questions remain to be answered to understand natural patterns and processes that unfold in Laguna de Tacarigua. Among other features, attention should be directed to the water replacement times and circulation patterns as influenced by the complex interaction of winds, tides and river discharge. The repercussions of the impairment of water exchange between the lagoon and the open sea have not been kept tack of, and the treatment of this phenomenon, as critical as it is, is mostly anecdotic. With a few exceptions, trophic relations, so important when dealing with fisheries or managing a conservation unit, are also unknown, and so is the role of this body of water as nursery grounds for commercial species. The concatenation of events that are triggered by the rainfall pulses, and culminate in a raise of primary and secondary productivity, is also a black box. Although thorough inventories of birds and fishes have been conducted, their life history traits and population dynamics are not known. In spite of the fact that an increment in the use of pesticides and fertilizers in the drainage basin of the Guapo River has been recognized, the concentrations and consequences of pollutants and their bearing on the lagoon's biota have not been evaluated.

Furthermore, the way in which data in most of these studies have been analyzed is, to say the less, candid. To overcome these deficiencies, in a first stage factors should be correlated and regressed. Afterwards, models should be implemented, probably beginning with an empirical-logical approach. Finally, as a base for long-range monitoring of hydrology, nutrients, primary and secondary productivity, physicochemical parameters and other features, a network of stations should be deployed. Only through such a systematic approach, and through the standardization of the sampling methods, will it be possible to improve our knowledge base, and to understand major processes to obtain an integrated view of Laguna de Tacarigua and enhance our ability to wisely manage this valuable wetland.

ACKNOWLEDGEMENTS

Special thanks to Percy Zanders, Gilberto Rodríguez and Clara Alarcón (IVIC) for critically reading the manuscript. Bibliographic material, important precisions, and enlightening discussions were provided by Humberto Díaz, Milagro Rinaldi, Jorge Paolini, Juan García Miragaya and Eddie Laboy (IVIC), and Angela Martino (UNEFM). Berta Sánchez's (IVIC) contributions to the manuscript were also most valuable. My gratitude is extended to Irama Goa and Henry Ramos. CONICIT and UNEFM, through projects SI-0766 and CTI-88-01, respectively, provided funds for some of the studies on crustacea whose results are summarized herein

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