Biodiversidad

20 Biodiversity in Aquatic Systems of the Tropical Andes
Mabel Maldonado, Javier A. Maldonado-Ocampo, Hernán Ortega, Andrea C. Encalada, Fernando M. Carvajal-Vallejos, Juan Francisco Rivadeneira, Francisca Acosta, Dean Jacobsen, Álvaro Crespo, and Carlos A. Rivera-Rondón

Aquatic systems and their biodiversity are probably the environments that have received the least research attention in thetropical Andes. Current knowledge is limited and derived from studies that are relatively narrow in scope and restricted to specific systems and taxonomic groups of aquatic organisms. Thus, the information available to describe and interpret aquatic diversity patterns in the Andes is far from complete, and emerging patterns are therefore preliminary and still lack causal explanations. The presentchapter reviews the current state of knowledge about Andean aquatic biodiversity with the primary goal of identifying present knowledge gaps.

Aquatic Systems of the Tropical Andes
Andean aquatic systems include a wide range of environments, including small to large lakes of different origins and with different characteristics, headwater streams that constitute part of South America’s largestriver basins, extensive associated wetlands, and extreme environments such as salt lakes and geothermal springs. The greatest variety of aquatic environments is found in the high-Andean zone above 3000 m (Izurieta 2007) in the terrestrial ecoregions known as páramo and puna. The most representative wetlands in the páramos of Colombia, Venezuela, Ecuador, and northern Peru are glacial lakes andSphagnum peat bogs (Flachier 2005, Rubio et al. 2005), whereas lakes and peat bogs without Sphagnum are characteristic of the puna of Peru, Bolivia, Argentina, and Chile (Arévalo 2005, Castro 2007, Navarro and Maldonado 2002). These environments can be characterized as follows: • Lakes: Lakes are very numerous and primarily of glacial or tectonic origin, although lakes of volcanic and fluvial origin,among others, also exist. The largest lakes are found in the Altiplano of Peru and Bolivia. Many are deep, warm monomictic lakes, whose waters mix once a year from top to bottom during austral winter, but are thermally

276 

stratified throughout the rest of the year. The mesotrophic Lake Titicaca is characterized by intermediate levels of primary productivity (Iltis et al. 1991, Richersonet al. 1991) and support important fisheries. Farther south lies the shallow, highly mineralized, and eutrophic Lake Poopó with its high primary productivity, as well as the large salt lakes of Uyuni and Coipasa, which lack aquatic biota but support extensive wetlands on their shores (Navarro and Maldonado 2002). These three lakes cover a combined area of about 25,000 km2 (Montes de Oca 2004).Throughout the high Andes, lakes with small surface areas are so numerous that, despite their limited individual size, cumulatively they account for large areas. In Ecuador, for example, all lakes > 25 ha cover a combined area of almost 500 km2, whereas the salt lakes in the watershed of the southern Bolivian Altiplano add up to a total of about 2,000 km2 (Montes de Oca 2004). Lakes of glacial originare abundant in the páramo and humid puna. They are relatively deep, oligothermal (cold-adapted), and cold polymictic (their waters can mix from top to bottom throughout the ice-free period) (Aguilera et al. 2006). In the dry puna, shallow salt lakes with an extreme seasonality are more common (Castro 2007, Navarro and Maldonado 2002). • River systems: The Andean hydrographic network includesopen exorheic systems whose water constantly flows out of the system under almost all climatic circumstances and that drain to the Pacific or the Atlantic oceans. These include the major headwaters of the Amazon system. There are also closed, endorheic systems, the largest of which is the internal rivers and lakes of the Altiplano basin. Characteristic river environments include headwater streams,...