Biodiversity in the high Paraná River floodplain

Abstract

Despite being the most regulated river in the world, the high Parana River in Brazil supports a great wealth of biological diversity. An extensive region of floodplain habitat remains within the high Parana River between the Porto Primavera and Itaipu reservoirs, but it now experiences a modified flood regime due to the influence of the upstream reservoirs. Annual flooding and drying of the floodplain occurs, and this regime is greatly influenced by tributary rivers, especially the Ivinheima River which enters from the west. Most groups of aquatic organisms show high species diversity and strong patterns of ecological dynamics in response to habitat heterogeneity and water level fluctuations. Physicochemical attributes and primary production show high spatiotemporal variation in both river and floodplain habitats. In general, the floodpulse reduces phytoplankton densities, promotes periphyton and aquatic invertebrate production in the floodplain which supports juvenile fish feeding, growth, and development. Most fishes of the region are stimulated to spawn during the early stages of the annual flood pulse but some fishes reproduce over extended periods. The abundance of most species is positively associated with the timing and duration of the flood, and this is especially the case for migratory species with high fecundity. Several important iliophagous, herbivorous, and omnivorous fishes are among those showing evidence of enhanced recruitment during very high flood years. Some brood-guarding and piscivorous fishes show patterns of increased abundance in association with low flood years. We demonstrate how the integrity of the nver-floodplain ecosystem of the high Parant is dependent on the maintenance of some degree of annual flooding. Several conservation areas have been established to protect the biological diversity supported by the floodplain region that remains in a relatively natural condition.