64
J.E. Conde
4.3 Biological Components
4.3.1 Phytoplankton Community
The phytoplankton community in the lower Orinoco River and Delta is
composed of Bacillariophyceae (94), Dinophyceae (6), Cyanophyceae (29),
Euglenophyceae (5), Chrysophyceae (1), Xanthophyceae (1), Euchlorophyceae (24) and Zygophyceae (38) (Varela et al. 1983). While marine diatoms,
Melosira granulata being the most abundant and widely distributed species, and dinoflagellates are more common at the mouth of channels and
along the coast, five species of marine diatoms have also been reported for
the lower Orinoco River. The species of most other groups prevail under
limnic conditions in the delta. In general, the diversity of the plankton
community is higher in tributary channels of the delta than in the Orinoco
River (Varela and Varela 1983). Seasonal biomass differences are a function
of variations in phytoplankton biomass (Lewis 1988); however, since
primary production rates are low, autotrophy does not contribute significantly to nutrient concentrations and carbon balance.
4.3.2 Vegetation
About 100 aquatic vascular plant species (e. g., Cabomba aquatica, Utricularia spp., Eichhornia crassipes, Pistia stratiotes, Limnobium laevigatum,
Nymphaea rudgeana, Ludwigia helminthorhiza) and many grasses and
sedges (e. g., Leersia hexandra, Oxicaryum cubense) have been recorded
from the delta, though species that form rooted meadows (Ludwigia octovalvis, Polygonum acuminatum, Oxicaryum cubense) prevail. Hymenachne
amplexicaulis and Mimosa pigra are restricted to the shores (Colonnello
1996). Although eight sea grass species have been reported for the coast of
Venezuela (Vera 1992), none has been reported for the delta. The absence
of sea grasses and benthic algae might be a consequence of high suspension loads and lack of suitable substrates.
The dominant terrestrial vegetation of the delta is mangroves, marsh
forests and sand dune vegetation (Pannier 1979). More than 73%
( 183,500 ha) of Venezuela's mangrove forests are located between the Gulf
of Paria and the Orinoco Delta (Conde and Alarcon 1993) and about
100,000 ha (MARNR 1991) fringe the coast, tributaries and rivers of the
delta, occasionally more than 80km upriver (Heinen et al. 1995). The
dominant species are the red mangrove Rhizophora mangle, the black
mangroves Avicennia germinans and Laguncularia racemosa, though R.
J.E. Conde
4.3 Biological Components
4.3.1 Phytoplankton Community
The phytoplankton community in the lower Orinoco River and Delta is
composed of Bacillariophyceae (94), Dinophyceae (6), Cyanophyceae (29),
Euglenophyceae (5), Chrysophyceae (1), Xanthophyceae (1), Euchlorophyceae (24) and Zygophyceae (38) (Varela et al. 1983). While marine diatoms,
Melosira granulata being the most abundant and widely distributed species, and dinoflagellates are more common at the mouth of channels and
along the coast, five species of marine diatoms have also been reported for
the lower Orinoco River. The species of most other groups prevail under
limnic conditions in the delta. In general, the diversity of the plankton
community is higher in tributary channels of the delta than in the Orinoco
River (Varela and Varela 1983). Seasonal biomass differences are a function
of variations in phytoplankton biomass (Lewis 1988); however, since
primary production rates are low, autotrophy does not contribute significantly to nutrient concentrations and carbon balance.
4.3.2 Vegetation
About 100 aquatic vascular plant species (e. g., Cabomba aquatica, Utricularia spp., Eichhornia crassipes, Pistia stratiotes, Limnobium laevigatum,
Nymphaea rudgeana, Ludwigia helminthorhiza) and many grasses and
sedges (e. g., Leersia hexandra, Oxicaryum cubense) have been recorded
from the delta, though species that form rooted meadows (Ludwigia octovalvis, Polygonum acuminatum, Oxicaryum cubense) prevail. Hymenachne
amplexicaulis and Mimosa pigra are restricted to the shores (Colonnello
1996). Although eight sea grass species have been reported for the coast of
Venezuela (Vera 1992), none has been reported for the delta. The absence
of sea grasses and benthic algae might be a consequence of high suspension loads and lack of suitable substrates.
The dominant terrestrial vegetation of the delta is mangroves, marsh
forests and sand dune vegetation (Pannier 1979). More than 73%
( 183,500 ha) of Venezuela's mangrove forests are located between the Gulf
of Paria and the Orinoco Delta (Conde and Alarcon 1993) and about
100,000 ha (MARNR 1991) fringe the coast, tributaries and rivers of the
delta, occasionally more than 80km upriver (Heinen et al. 1995). The
dominant species are the red mangrove Rhizophora mangle, the black
mangroves Avicennia germinans and Laguncularia racemosa, though R.