Print PageThe flood regime is considered the most important force in determining seasonality in river-floodplain systems. Floods usually promote pulses of nutrients and supply floodable areas with minerals, increasing productivity. The population dynamic and life cycle of the biota in the floodplain also depends on the flood pulse; for example, fish migration, spawning and recruitment display a close relationship with the hydrographic attributes. However, dams may alter the hydrological cycle downstream, with impacts on the biota. In the upper Paraná River, there are more than 150 large reservoirs, with only one relevant lotic stretch (230 km). In this stretch there is an extensive floodplain and large tributaries still not dammed. In this paper we evaluated the downstream impacts of dams on the floodplain located immediately below Porto Primavera Dam, to answer the following questions: i) what are the main impacts caused by the cascade of dams upstream the plain? ii) which are the main ecosystem function affected? iii) which is the possible framework to serve as basis for conservation of the area? Data used to approach these questions are been collected since 1986, covering several abiotic (discharge, nutrients, sediments and Secchi depth) and biotic (chlorophyll, phytoplankton, zooplankton, periphyton, benthos, macrophytes and fish) variables, in addition to some social aspects.
Results/Conclusions
Redistribution of the seasonal discharge promoted by dams provoked losses in floodable area and altered connectivity among the plain components. These alterations, along with reduction in nutrients and sediments (increased Secchi depth), impacted all components of the plain, including losses in productivity, proliferation of submerse macrophytes and visual predators, especially non-native species. In addition, alterations in the timing of floods influenced the structure of fish assemblages, particularly the long distance migratory ones. Management actions to minimize impacts of dams in the region are fish passages, fishery control, stocking, and the creation of protected areas. The results of these actions are questionable and with clear failures, or even with additional impacts on the biodiversity. Data obtained during the last decades pointed out that the conservation of the area should restore connectivity by reestablishing the hydrological cycles. Therefore, ecohydrology is a potential theoretical framework to serve as basis for conservation of the area. Proper water quantity during appropriate timing may be the hydrological solution to reestablish ecosystem functions, bringing environmental and socio-economic benefits to the biota and to people who live in the last stretch of the Paraná River with running water.
