Orinoco Floodplain Research Articles

Multi‐annual contemporary flood event overbank sedimentation within the vegetated lower Orinoco floodplain, Venezuela

AbstractMulti‐annual contemporary flood event overbank sedimentation rates were quantified on the World's third largest river in terms of discharge, the tropical lower Orinoco. We discuss the role of variables at the basin and reach scales that contributed to the complexity of spatio‐temporal overbank sediment deposition patterns. Monitored in situ plots were characterized by distance to the main channel, hydroperiod, different geomorphological units, and vegetation cover. Flood event sedimentation rates showed a high spatial variability ranging from the absence of sediment deposition up to 225.46 kg m‐2 yr‐1. Banks and levees received relatively high amounts of sediment (39.6 kg m‐2 yr‐1), whereas observed mean sedimentation rates on the more distant floodplain and backswamps tended to be lower (17.7 kg m‐2 yr‐1). Significant differences in sedimentation rates were observed in two major vegetation types: dense herbaceous and shrubby vegetation (42.2 kg m‐2 yr‐1) and floodplain forest (12.7 kg m‐2 yr‐1). However, overbank sedimentation patterns also reflected imbricated hydrosedimentary and biogeomorphological vegetation feedbacks that co‐construct fluvial landforms. The incidence of an El Niño–Southern Oscillation–La Niña episode during the study period on sediment availability and floodplain sedimentation suggests that within whitewater rivers, where suspended sediment concentrations are naturally high, hydrological connectivity seems to be more important for floodplain sedimentation than variations in suspended sediment concentrations. These results may provide a good basis for future biogeomorphological investigation projects using complementary methodologies, in order to better anticipate global change and fluctuations in the occurrence, strength or duration of El Niño–La Niña episodes in the tropical zone and their consequences for flood discharge and sediment dynamics during channel–floodplain exchanges.

Habitat specifity, endemism and the neotropical distribution of Amazonian white‐water floodplain trees

.The Amazon basin is covered by the most species‐rich forests in the world and is considered to house many endemic tree species. Yet, most Amazonian ecosystems lack reliable estimates of their degree of endemism, and causes of tree diversity and endemism are intense matters of debate. We reviewed the spatial distribution of 658 of the most important flood‐tolerant Amazonian white‐water (várzea) tree species across the entire Neotropics by using data from herbaria, floras, inventories and checklists. Our results show that 90% of the várzea tree species are partially or widely distributed across neotropical macro‐regions and biomes. Chi‐square analyses indicated that várzea species richness in non‐várzea macro‐regions was dependent on the flooding gradient and the longitudinal position. Cluster analysis combined with association tests indicated four significant patterns of várzea species distributions depending on species flood‐tolerance (low vs high) and spatial distribution (restricted vs widespread). We predict that the predominance of Andean substrates is the most important factor that determines the distribution of várzea tree species within and beyond the Amazon basin and explains the high floristic similarity to the Orinoco floodplains. Distribution patterns in other extra‐Amazonian macro‐regions are more likely linked to climatic factors, with rainforest climates housing more várzea species than savanna climates. 130 tree species were restricted to South‐American freshwater floodplains, and 68 (> 10%) were endemic to Amazonian várzea. We detected two centers of endemism, one in the western Amazon characterized by low and brief floods, and one in the central Amazon, characterized by high and prolonged floods. Differences in taxonomic composition of endemic centers in the western and central Amazon are the result of different abiotic factors (i.e. flood regimes), as well as the regional species pools from where the species are recruited from.We hypothesize that numerous morphological, physiological and biochemical adaptations permit survival of trees in flooded environments. Furthermore, these adaptations are independently derived across many taxa and result in a highly specialized flora. We attribute higher than expected levels of endemism to the great spatial extent and age of floodplain ecosystems in the Amazon basin, and highlight the role of Amazonian várzea as an potential driver in speciation and diversification processes.

<strong>Notes on Venezuelan Ricciaceae (Marchantiophyta)</strong>

New data on Ricciocarpos natans and on five species in the genus Riccia are given, including four Riccia species new to Venezuela, all from the Orinoco flood plains. The number of Ricciaceae species known from Venezuela is presently eight.

Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data

AbstractStable isotope data and indirect methods for estimating consumer production were used in a foodweb analysis for the Orinoco River floodplain, Venezuela. The ratio of annual production to mean annual biomass (Pa/B) was estimated from mass at maturity for all major categories of consumers. Field data on B then were used to estimate Pa for each major category. Carbon sources for all categories of consumers were identified through δ13C analysis, and trophic shifts in δ15N were used in assigning trophic levels to consumers. The ultimate C source for both invertebrates and fish was algae (phytoplankton and periphyton), even though macrophytes and litterfall from the floodplain forest composed 98% of potentially available C. Production of invertebrate consumers (benthos, rhizofauna, and zooplankton), which occurred almost entirely through 1st-level consumption, was estimated as 14 g C m−2 y−1. Fish, which produced 11 g C m−2 y−1, showed a mean δ15N trophic shift of 1.8 units above the algal C source, s...

Limnologia comparada de lagos e lagoas de planícies de inundação da América do Sul

The primary purpose of this study was to compare different lagoons from eight floodplains in South America (Amazon, the Upper Parana, the Middle Parana, Mogi- Guacu, Araguaia, Pantanal, Sao Francisco and Orinoco floodplains). Secondly, the effect of water level upon selected limnological variables (important and frequently used variables in limnological studies) was evaluated. Data were obtained from published articles and dissertations. Some unpublished data from Upper Parana, Amazon and Araguaia floodplains were also used. Despite the general differences among the floodplains such as climate, geology and limnological characteristics of the main river, certain constant patterns were found. Oxygen concentration was always higher during low water phase and, in general, the lagoons have an acid water independently of the period. By comparing the Amazon, Parana and Araguaia floodplains, whose data are more abundant, high variability in chemical factors was found during the low water phase when measured by the variation coefficient. This result supports the hypothesis that the flood pulse acts as a regional process that increases similarity among the lagoons.

Nitrogen fixation and growth of non-crop legume species in diverse environments

Of the 643 legume genera, few have been exploited in agriculture and 40% have not even been evaluated for their ability to nodulate and fix nitrogen. Most of these are in tropical/subtropical regions, with habitats ranging from extremely dry to flooded. Recent work in some of these areas shows that plants can nodulate under conditions previously thought to be disadvantageous. The accepted dogma that nitrogen fixing legumes have a high demand for P is challenged and examples of how legumes can extract P from soils with low available P described. Species tolerant to shading and high soil Al are cited, although the mechanisms of adaptation are not yet clear. Some tropical soils have high nitrate levels and, contrary to perceived wisdom, there are legumes which can nodulate under such conditions. Many tropical tree legumes prefer ammonium to nitrate and are able to fix nitrogen and assimilate ammonium at the same time. In all these cases, there are genotypic differences, both within and among species. Large areas of tropical fresh water, such as the Brazilian Pantanal and the Orinoco floodplain have nodulated legumes predominant in their flora. The ecological potential of these has not been evaluated. One of the sites of nodule evolution is likely to have been in such areas. Modes of infection of legumes by rhizobia vary with taxonomic tribe and may represent evolution for survival in different environments. As more legumes, from more ecosystems are studied, a wider range of adaptations is likely to be found. Work is urgently needed to study these, especially in areas being cleared for agriculture or by logging.

Regulation and stability in fish assemblages of neotropical floodplain lakes.

Neotropical floodplain lakes provide an excellent opportunity to examine the regulation and stability of fish assemblages. At low water, when lakes are separated, fish are concentrated in the lakes and are presumably subject to strong interspecific interactions that can shape assemblage structure. At high water, when the lakes and river channels become broadly interconnected, ample potential exists for alteration of assemblage structure because eggs, larvae, and older fish may undergo spatial reshuffling among waterbodies. Twenty lakes of the Orinoco River floodplain in Venezuela were surveyed in the early and late dry seasons of two consecutive years. Marked differences in assemblage structure among lakes were established in the early dry season. Changes in assemblage structure during the dry season were large and detectable even at the ordinal level; they reflected a strong reduction of visually oriented fish relative to fish with adaptations to low light. Changes were similar in the two years and were apparently due to species-specific differences in mortality, which was high over the dry season. The annual flood is a strong natural fluctuation that greatly modifies assemblage properties during the wet season. Nevertheless, the potential for alteration of assemblage structure by reshuffling was not realized: assemblage properties early and late in the dry season were similar in the two years, indicating a regular and predictable annual cycle of change in assemblage structure. This regularity can be explained by mechanisms of regulation which appear to be linked to piscivory and the optical environment. In contrast with the prevailing views on neotropical fish assemblages, there seems to be a strong deterministic component to assemblage structure and dynamics in Orinoco floodplain lakes.

Zooplankton abundance and evidence for its reduction by macrophyte mats in two Orinoco floodplain lakes

Zooplankton abundance and evidence for its reduction by macrophyte mats in two Orinoco floodplain lakes

Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain, Venezuela1

Lake Tinco, which lies on the Orinoco floodplain, passes through four seasonal phases (filling, through‐flow, drainage, and isolation), during which the lake volume changes 20‐fold. During filling and through‐flow, when the lake is inundated by river water, floating grasses (Paspalum repens) and phytoplankton grow rapidly, causing depletion of inorganic N and P between inflow and outflow, although phytoplankton biomass does not accumulate because of flushing. Inundation of the surrounding forest has no quantifiable effects on concentrations of major ions or macronutrients. Following through‐flow, phytoplankton biomass accumulates, and high abundances of blue‐green algae increase N in the lake by fixation (15 kg ha‐1). Quantities of some elements released by decomposition of floating grasses over a 4‐week period are very high in relation to lake‐water inventories (K, 126%; Cl, 66%; organic C, 170%; N, 74%; and P, 89%). Mass balance for Lake Tinco shows, on an annual basis, net retention of N (27%)— partly because most nitrogen fixation is limited to the isolation phase—and P (39%), and net yield of inorganic C (24%). There is neither detectable net retention nor yield of organic C, major cations, sulfate, or chloride. The major chemical transformation of river water during passage through the lake is conversion of inorganic N and P to organic forms.

Stochasticity and Assemblage Organization in an Indiana Stream Fish Assemblage

Site d'etude: zone situee sous le barrage de Markles; problemes d'echantillonnage; nombre d'especes incluses dans l'analyse. Inclusion des jeunes d'un an dans cette analyse; interpretation et nouvelle analyse des modeles alternatifs

Comment upon the Economic Potential of Fish Utilization in Riverine Environments and Potential Archaeological Biases

New data from the South American lowland tropics are used in support of a recent argument extolling the potential of fish utilization in major floodplains. The discussion will cover five points: (1) major floodplains in general, and the Amazon and Orinoco floodplains in particular, have similar characteristics that make them biologically productive regions; (2) high fish productivity and the use of mass-fishing techniques in floodplain regions are characteristics of lowland South America; (3) the ecological dynamics of the seasonally inundated savanna are particularly productive and propitious for seasonal exploitation using mass-fishing techniques; (4) differences in species composition and fish size may have implications for seasonal and spatial variations in fish exploitation; and (5) substantial biases are apparent against the retrieval of small fish remains using traditional archaeological recovery techniques.