Amazonian floodplains: a limnological perspective

Summary

Amazonian floodplains are characterized as land- water transition areas which oscillate between pronounced aquatic and terrestrial phases. The Aquatic Terrestrial Transition Zone (ATTZ) is described as the area with the highest diversity of habitats, plant and animal species, and biological and biogeochemical processes in river floodplain systems. Floodplain lakes differ from classical lakes by the fact that most of their water is annually exchanged and that this exchange is related to large changes in lake volume and lake area. The driving variable in the floodplain is the flood pulse.

Flooding influences nutrient cycles by determining the amount of dissolved and suspended solids which are im- and exported during the hydrological cycle and by the periodical availability of the sediments of the lake basin for plant growth during the terrestrial phase. Herbaceous plants and the floodplain forest play an important role in primary production, nutrient storage, and the nutrient transfer between sediments and water. The ratio of the ATTZ to the Maximum Lake Surface Area (ATTZ/MLSA ratio) is an important variable to describe the limnological conditions in floodplain lakes as shown for the consumption of oxygen and the inputs of K, Ca, R and N to a hypothetical floodplain lake.

Oxygen demand in floodplain lakes is highly dependent on the amount and the decomposition rate of organic material produced in the ATTZ. The main source of oxygen is diffusion from the air. In lakes of the Amazon River floodplain, the contribution of K, N, and P of the ATTZ by transfer from the sediments via organic material of the floodplain forest and herbaceous plants is much larger than the dissolved riverine input, even when the ATTZ is relatively small. For Ca, on the contrary, the dissolved riverine input is much larger than that of the ATTZ.