A classification of the major habitats of Amazonian black-water river floodplains and a comparison with their white-water counterparts

The Amazon River and its large tributaries are bordered by floodplains covering tens of thousands of square kilometers. Studies on the structure, function, and species composition have allowed a classification of the macrohabitats of Amazonian white-water floodplains, rich in suspended matter and nutrients and of neutral pH (várzea). Here we describe the use of a similar approach to classify the macrohabitats of the black-water floodplains, rich in humic substances, poor in nutrients and acidic (igapó) of the Negro River and its black-water tributaries. With 12 subclasses and 25 macrohabitats, the igapó is less complex than the várzea. Although white-water and black-water rivers are subjected to similar flood regimes, the low sediment load and shallower declivity of the Negro River lead to reduced sedimentation and erosion processes. Differences in nutrient levels between both ecosystems influence species composition, richness, and growth rates of higher plant communities. Species richness is lower in igapó than in várzea, and wood increment and litter production of igapó trees is about half that reported for várzea trees. In addition, igapó lacks highly productive herbaceous plant communities that are common in várzea. The classification of igapó macrohabitats provides a valuable tool for the elaboration of sustainable management strategies and conservation. While many várzea macrohabitats are suitable for small-scale agriculture, animal husbandry, forestry and commercial fisheries, the carrying capacity of igapó is limited and allows only for subsistence-level fishery and agriculture, the capture of ornamental fishes, and ecotourism. We argue that the biota of most igapó macrohabitats is highly sensitive to changes in hydrological cycles as caused by river damming and/or by climate change.