In South America, there are little changes in the composition of phytoplankton communities at the level of genus or species from temperate to tropical latitudes. Obviously, seasonal succession patterns that are the rule in temperate lakes are absent in tropical lakes. In fact, the phytoplankton communities of shallow tropical lakes appear to remain arrested in a single successional stage, which may explain the simplicity of these communities. As regards zooplankton, several groups, which are well represented in temperate lakes, become scarce as one approaches the equator. Notably, the genus Daphnia (Cladocera) is seldom reported in the tropics, where only small to medium-size species (e.g., D. gessneri and D. leavis) occur at low densities. Similarly the calanoid cope-pods, which are among the largest zooplankton in temperate lakes, are absent or rare in many tropical lakes.
Endemisms are frequent in South America. For example, recent evidence suggests that 14 of 19 species of daphniids reported for South America are endemic. Particularly, three of them (Daphnia inca, Daphniopsis chilensis and D. marcahuasensis) are considered to be Andean endemisms. Many copepod species belong to the family Centropagidae, which is almost entirely restricted to the Southern Hemisphere. As for rotifers, roughly 40% of the species of the genus Brachionus and 40% species of the genus Keratella reported for South America are endemic. Examples include Brachionus ahlstromi, B. gillardi, B. incertus, B. insuetus, B. kultrum, B. mirus, B. trahea, B. voigti, Keratella caudata, K. kostei, K. thomassoni, K. ona and K. yaman.
Freshwater South American fishes belong to the Neotropical ichthyofauna, which contains 5000-8000 different species. The native populations are mostly dominated by five major groups: siluriforms, characi-forms, gymnotiforms, cyprinodontiforms and cichlids. The Amazon Basin is home of the largest fish diversity on Earth, with over a thousand different species. The Orinoco, Parana, and other large, tropical river basins are also rich in fish species. In contrast, fish diversity drops sharply in the watersheds that drain into the Caribbean Sea and the Pacific Ocean. In the temperate region, fish diversity is also lower and the taxonomic composition differs markedly from subtropical and tropical areas.
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