Abstract
The interannual variation of phytoplankton communities in the three main rivers of the Upper Paraná River floodplain is evaluated in relation to changes in the hydrosedimentological regime. These changes are a result of climatic variability and the formation of Porto Primavera Reservoir, located at the upper Paraná River. Phytoplankton species richness and density were investigated in rivers during a prior period (1993-1994) and eight years after reservoir impoundment (2000-2007). Multiple analyses were conducted to test the differences between these time periods in order to find predictor variables for phytoplankton attributes. A total of 454 phytoplanktonic taxa were found. The regression analysis revealed significant differences between periods. In the years following construction of the Porto Primavera dam, species richness was lower in the Paraná River and density was higher in the three rivers. In general, the algal density decreased from 2005 to 2007. Diatoms and cyanobacteria contributed significantly to the total density during the period from March 1993 to February 1994. The years 2000-2007 presented the lowest diatom contribution to species richness and the highest cyanobacteria contribution. From 2000 on, cryptomonads and cyanobacteria dominated. The interannual variability of phytoplankton was probably influenced by changes in hydrosedimentological regime due to climatic variations (La Niña and El Niño-Southern Oscillation events--ENSO) and the operational procedures associated with an upstream reservoirs. Studies on climatic variability and its effects on hydrosedimentological regimes of the Paraná, Baía and Ivinhema rivers and the biota therein are necessary to obtain subsidies for management, including decisions related to the operation of dams upstream and downstream of the study area, with the purpose of minimizing risks to the Environmental Protection Area.
Highlights
River segment characteristics reflect interactions among multiple factors acting on different spatial and temporal scales, which can induce changes in aquatic communities (Lair and Reys-Marchant, 1997)
This study aimed to evaluate the interannual variability of phytoplankton in the Paraná, Baía and Ivinhema rivers after the formation of the Porto Primavera Reservoir
We evaluated the influence of environmental variables on the phytoplankton community, hypothesizing that the lowest fluviometric and river discharge result in decreased species richness, due to a reduced connectivity between the river-floodplain system
Summary
River segment characteristics reflect interactions among multiple factors acting on different spatial and temporal scales (e.g., geomorphology, climatic and natural disturbances), which can induce changes in aquatic communities (Lair and Reys-Marchant, 1997). Changes in the hydrosedimentological regime of floodplain rivers (sensu Neiff, 1996) have significant effects on these ecosystems (Ward et al, 1999), and those promoted by operational procedures associated with upstream dams are among the principal anthropogenic alterations of floodplains systems (Agostinho et al, 2008; Souza Filho, 2009). The effects of impoundments on the upper Paraná River increased after the filling of the Porto Primavera Reservoir (Souza Filho et al, 2004), leading to a remarkable increase of water transparency, reduction of phosphorus concentrations and decrease of Paraná River discharge. Certain relationships among climatic factors have been identified (Train and Rodrigues, 1998; Train and Rodrigues, 2004; Bovo-Scomparin and Train, 2008), such as the dominance of cyanobacteria in La Niña periods, characterized by lower precipitation and reduced Paraná River fluviometric levels, as well as the dominance of diatoms and cryptomonadas in El Niño periods, when higher values of precipitation, higher fluviometric levels and greater Paraná River discharge were observed
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