The ecological dynamics of Barra Bonita (Tietê River, SP, Brazil) reservoir: implications for its biodiversity.

Barra Bonita reservoir is located in the Tietê River Basin - São Paulo state - 22 degrees 29' to 22 degrees 44' S and 48 degrees 10 degrees W and it is the first of a series of six large reservoirs in this river. Built up in 1963 with the aim to produce hydroelectricity this reservoir is utilized for several activities such as fish production, irrigation, navigation, tourism and recreation, besides hydroelectricity production. The seasonal cycle of events in this reservoir is driven by the hydrological features of the basin with consequences on the retention time and on the limnological functions of this artificial ecosystem. The reservoir is polymitic with short periods of stability. Hydrology of the basin, retention time of the reservoir and cold fronts have an impact in the vertical and horizontal structure of the system promoting rapid changes in the planktonic community and in the succession of species. Blooms of Microcystis sp. are common during periods of stability. Superimposed to the climatological and hydrological forcing functions the human activities in the watershed produce considerable impact such as the discharge of untreated wastewater, the high suspended material contributions and fertilizers from the sugar cane plantations. The fish fauna of the reservoir has been changed extent due to the introduction of exotic fish species that exploit the pelagic zone of the reservoir. Changes in the primary productivity of phytoplankton in this reservoir, in the zooplankton community in the diversity and organization of trophic structure are a consequence of eutrophication and its increase during the last 20 years. Control of eutrophication by treating wastewater from urban sources, adequate agricultural practices in order to diminish the suspended particulate matter contribution, revegetation of the watershed and riparian forests along the tributaries are some possible restoration measures. Another action that can be effective is the protection of wetlands in the main tributaries as an effort to control eutrophication and particulate material load. Hydrology, climatic forcing and retention time are major forcing functions that promote the circulation (vertical and horizontal) in the reservoir and probably have a strong effect on dissolved and particulate material distribution. The 114 tributaries are systems that enhance spatial heterogeneity promoting diversity throughout ecological niches. Switches of control systems of this artificial ecosystem seems to be related with physical - physical forces; physical - biological forces during short periods of time, and biological - biological interactions at varying degrees during the seasonal cycle.