Karin Furch

Physicochemical Conditions in the Floodplains

The different water colors of Amazonian rivers are documented in the river names e.g., Rio Negro (black river), Rio Branco (white river), Rio Claro (clear river), Rio Verde (green river), and indicate differences in water quality. Sioli (1950) related water color to specific conditions in the catchment areas and recognized three main water types based on water color, load of suspended solids, pH, and load of dissolved minerals, indicated by the specific conductance.

A general review of tropical South American floodplains

High rainfall and its seasonal distribution cause periodic flooding of large areas in tropical South America. Floods result from lateral overflow of streams and rivers, or from sheet-flooding by rains as a consequence of poor drainage. Depending upon the size of the catchment area, flooding can occur with one peak (e.g., in the Amazon River and its large affluents) or in many peaks (e.g., in streams and small rivers).Vegetation cover of floodplains varies from different types of savannas and aquatic macrophyte communities to forests depending upon the hydrologic regime and local rainfall. Large differences exist in primary and secondary production due to large differences in nutrient levels in water and soils.An attempt is made to characterize the floodplains according to their hydrologic regimes, vegetation cover and nutrient status. The areal extent of different types of floodplains is estimated. The human impact is also evaluated.

Seasonal Denitrification in Flooded and Exposed Sediments from the Amazon Floodplain at Lago Camaleao

Denitrification processes were measured by the acetylene-blockage technique under changing flood conditions along the aquatic/terrestrial transition zone on the Amazon floodplain at Lago Camaleão, near Manaus, Brazil. In flooded sediments, denitrification was recorded after the amendment with NO3−(100 μmol liter−1) throughout the whole study period from August 1992 to February 1993. It ranged from 192.3 to 640.7 μmol N m−2 h−1 in the 0- to 5-cm sediment layer. Without substrate amendment, denitrification was detected only during low water in November and December 1992, when it occurred at a rate of up to 12.2 μmol N m−2 h−1 Higher rates of denitrification at an average rate of 73.3 μmol N m−2 h−1 were measured in sediments from the shallow lake basin that were exposed to air at low water. N2O evolution was never detected in flooded sediments, but in exposed sediments, it was detected at an average rate of 28.3 μmol N m−2 h−1 during the low-water period. The results indicate that under natural conditions there is denitrification and hence a loss in nitrogen from the Amazon floodplain to the atmosphere. Rates of denitrification in flooded sediments were one to two orders of magnitude smaller than in temperate regions. However, the nitrogen removal of exposed sediments exceeded that of undisturbed wetland soils of temperate regions, indicating a considerable impact of the flood pulse on the gaseous turnover of nitrogen in the Amazon floodplain.

The Chemical Composition, Food Value, and Decomposition of Herbaceous Plants, Leaves, and Leaf Litter of Floodplain Forests

Structure and function of an ecosystem are determined to a large extent by its trophic dynamics (Lindeman 1942). Organic material produced by primary producers is processed by a suite of consumer and decomposer populations, the species composition and size of which depends upon the amount, quality and availability of the organic material (Boyd and Goodyear 1971). In Amazonian floodplains large amounts of non-woody biomass are available for primary consumers. Herbaceous plant communities produce up to 100 t ha−1 year−1 dry matter in the varzea. The floodplain forest of the varzea provides up to 13.6 t ha−1 year−1 leaf litter and that of the igapo up to 6.7 t ha−1 year−1 (Chaps. 8, 11). However, only a very small fraction is utilized by herbivorous animals.

A simple modelling approach towards hydrochemical seasonality of major cations in a Central Amazonian floodplain lake

Abstract To identify causes of hydrochemical seasonality, dynamic mass balance models are developed. They describe a hypothetical floodplain lake designed according to literature data supplemented by unpublished data on Lago Camaleao, an island channel lake in the lower Solimoes River. The conservative model accounts for advective and atmospheric cation fluxes, the non-conservative model additionally for biogenic cation fluxes due to growth and decomposition of the macrophyte vegetation. During much of the hydroperiod, seasonal evolutions observed for Na, Ca, and Mg, were in accordance with conservative expectation. Increasing concentrations during falling water were not explained by the conservative model. Differences between predictions of the conservative, and the non-conservative model were negligible. Thus, for the evolution of Na, Ca, and Mg, biogenic fluxes are insignificant. We suggest that cation loading during falling water is caused by an abiotic process. Evolution of K was non-conservative during much of the hydroperiod. Deviations from conservative expectation due to K loading during rising, as well as during falling water, were in accordance with predictions of the non-conservative model. Hence, during rising water, macrophyte-derived influxes of K are the key factor for deviations from conservative expectation. During falling water, however, macrophyte decomposition is still poorly understood, and unknown processes causing Na, Ca, and Mg loadings might also procure additional K loading, and thus biogenic K fluxes might not be the sole cause for increasing concentrations.