The Orinoco River Delta, Venezuela
63
with sandy soils and central depressions, where peat, grasses and sedges
cover clay and silt deposits (Colonnello 1996). The delta has a slope of
less than 1 %. The Lower Delta ( -1 to 1m above sea level) is under the
influence of semidiurnal tides (T=12.4 h; Herrera et al. 1981; Herrera and
Masciangoli 1984), while the Middle (1-2.5m) and Upper (2.5-7m) Delta
are seasonally flooded during longer and shorter periods, respectively.
Owing to well-defined periods of precipitation in the basin, the freshwater discharge through the Orinoco River, with a mean annual flow of
36,000m 3 s- 1 (1.1 x 106 m 3 year- 1 ), is highly seasonal. During the rainy
periods, low salinity waters ( <5) extend throughout the Gulf of Paria and
surround Trinidad (Rodriguez 1974). Brackish surface waters may disperse all the way to Puerto Rico some 1,200km away (300,000km 2 ;
Mi.iller-Karger et al. 1989), owing to NW flow of the Guyana Current
and trade wind-induced NW and W drift currents (mean annual surface
velocity 20cm s- 1 ; Herrera et al. 1981; Herrera and Masciangoli 1984). In
contrast, the dry season with reduced runoff limits brackish waters to a
narrow strip along the delta coast and western coast of the Gulf of Paria.
The effect of tides on water levels (ll60 em) is felt as far as 250 km upstream (Colonnello 1995) and marine diatoms occur in several tributaries
(Varela and Varela 1983).
Although temperature, pH and dissolved organic carbon concentrations
vary little in the course of the year, conductivity decreases with discharge
and average particulate organic carbon concentrations increase from the
dry (0.84 mg 1- 1 ) to the wet season (4.02 mg 1- 1 ; Paolini et al. 1983). Physicochemical parameters and nutrient concentrations also differ significantly between channels in the delta. Conductivity values tend to be higher
(32.5-78.1 JlS) in the Manamo channel (Macareo 19.5-20.9f.1S), but mixohaline conditions (8) only occur at the mouth. As a consequence of freshwater flow controlled by a cofferdam, suspension loads in the Manamo
channel (11.5-45.8 mg 1- 1 ) are lower than those in the Macareo channel
(66.6-149.5mg 1- 1 ; Paolini et al. 1983). The NIP values (Manamo 7.1,
Macareo 6.8) suggest, apart from plankton, contribution of re-suspended
sediments and terrestrial material. Carbon and nitrogen levels in the
sediments are significantly higher in the Manamo ( 11% C, 1.58% N) than
the Macareo channel (2.4% C, 0.36% N) or even in the Orinoco River (Paolini 1995).
63
with sandy soils and central depressions, where peat, grasses and sedges
cover clay and silt deposits (Colonnello 1996). The delta has a slope of
less than 1 %. The Lower Delta ( -1 to 1m above sea level) is under the
influence of semidiurnal tides (T=12.4 h; Herrera et al. 1981; Herrera and
Masciangoli 1984), while the Middle (1-2.5m) and Upper (2.5-7m) Delta
are seasonally flooded during longer and shorter periods, respectively.
Owing to well-defined periods of precipitation in the basin, the freshwater discharge through the Orinoco River, with a mean annual flow of
36,000m 3 s- 1 (1.1 x 106 m 3 year- 1 ), is highly seasonal. During the rainy
periods, low salinity waters ( <5) extend throughout the Gulf of Paria and
surround Trinidad (Rodriguez 1974). Brackish surface waters may disperse all the way to Puerto Rico some 1,200km away (300,000km 2 ;
Mi.iller-Karger et al. 1989), owing to NW flow of the Guyana Current
and trade wind-induced NW and W drift currents (mean annual surface
velocity 20cm s- 1 ; Herrera et al. 1981; Herrera and Masciangoli 1984). In
contrast, the dry season with reduced runoff limits brackish waters to a
narrow strip along the delta coast and western coast of the Gulf of Paria.
The effect of tides on water levels (ll60 em) is felt as far as 250 km upstream (Colonnello 1995) and marine diatoms occur in several tributaries
(Varela and Varela 1983).
Although temperature, pH and dissolved organic carbon concentrations
vary little in the course of the year, conductivity decreases with discharge
and average particulate organic carbon concentrations increase from the
dry (0.84 mg 1- 1 ) to the wet season (4.02 mg 1- 1 ; Paolini et al. 1983). Physicochemical parameters and nutrient concentrations also differ significantly between channels in the delta. Conductivity values tend to be higher
(32.5-78.1 JlS) in the Manamo channel (Macareo 19.5-20.9f.1S), but mixohaline conditions (8) only occur at the mouth. As a consequence of freshwater flow controlled by a cofferdam, suspension loads in the Manamo
channel (11.5-45.8 mg 1- 1 ) are lower than those in the Macareo channel
(66.6-149.5mg 1- 1 ; Paolini et al. 1983). The NIP values (Manamo 7.1,
Macareo 6.8) suggest, apart from plankton, contribution of re-suspended
sediments and terrestrial material. Carbon and nitrogen levels in the
sediments are significantly higher in the Manamo ( 11% C, 1.58% N) than
the Macareo channel (2.4% C, 0.36% N) or even in the Orinoco River (Paolini 1995).