South America

Much of our current knowledge about forested floodplains has been derived from extensive studies performed in the sub-basins of the Amazon River. In particular, our understanding of floodplain biogeochemistry, NPP, vegetation dynamics, geomorphology, and faunal relationships has been greatly influenced by Amazonian research.

In comparison to river basins in other parts of the world, the water balance of Amazonia lowlands is roughly evenly divided between evapotranspiration and runoff. This contrasts with systems in Asia where runoff dominates due to generally steeper terrain and many African systems where broad floodplains and high potential evapotranspiration result in low runoff. Floodplain forests in South America are typically composed of a small number of fast-growing, early-successional species capable of surviving periodic floods and large amounts of sediment deposition (e.g., Salix and Inga spp.).

The 'flood pulse' concept was originally conceptualized in relation to the Amazon and similar floodplains and can be applied worldwide. The major river flood-plains of South America such as the Amazon, Orinoco, and the Parana display singular, river-borne flood pulses of large amplitude and duration. In contrast, inundation on floodplains situated within large depressions such as the Pantanal is generally rainfed (as opposed to overbank flow from rivers), and also displays a singular periodicity but with lower amplitude. Finally, multiple flood pulses that are less predictable in terms of occurrence and amplitude are characteristic of floodplains associated with smaller order streams.

Some of the classic research that defined global variation among floodplains took place in Amazonia and was associated with contrasts between blackwater versus brownwater or whitewater rivers. Similar types of flood-plain systems occur in many parts of the world. The color of the river waters is reflective of the geomorphology of particular systems and is a strong indicator of flood-plain biogeochemistry, vegetation dynamics, and NPP. Whitewater rivers in the Amazon Basin derive their color from white clay sediments that originate in the Andes. The suspended clays contain higher levels of nutrients (particularly base cations) which, when deposited, often create fertile floodplains labeled varzea.

In contrast, blackwaters are stained by fulvic acids and other organic compounds and are more acidic than whitewater counterparts (pH <5.0 vs. >6.0 for blackwater and whitewater, respectively). Due to the low sediment loads, floodplains associated with blackwater streams are often nutrient poor and are referred to as igapo. Consequently, forest litterfall production on varzea floodplains is often considerably higher than that of the igapo (approximately 10 vs. 5tha~ yr~ for the respective system types). Also, the standing crop of fine roots is much higher in igapo soils compared to varzea, a reflection of greater belowground allocation of biomass as would be expected in resource-poor soils. Such adaptations increase the likelihood of nutrient capture from decomposing igapo litter. The distinctions in hydrology and biogeochemistry between the igapo and varzea also drive major differences in vegetation species occurrence, root, shoot, and reproductive phenology, and community structure.

Distinctions between floodplains types are also important in regard to animal populations. This is particularly true for fish which depend on interactions with inundated floodplains for resource acquisition, reproductive habitats, and other factors. As an example, the lower NPP on igapo floodplains may translate to lower food resources for fish. While the amount of plant detritus exported from varzea floodplains is higher, phytoplankton production also depends on settling of the clay sediments so that sufficient light can penetrate the waters. Although more difficult to document in riverine systems, fish catches are generally much lower in igapo lakes compared to varzea counterparts.

As is the case in much of the world, South American floodplain ecosystems are under pressure from an array of human activities. As an example, the lower reaches of the Parana' River have undergone changes in hydrology due to construction of dams and upstream expansion of agriculture. The altered hydrology, along with increased concentrations of sediment and other contaminants have resulted in heavy impacts to fish populations and concomitant economic declines in local fishing communities. Although there is a growing voice for conservation and protection of natural resources, it is unclear to what extent anthropogenic impacts may be curtailed.

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