title: Trophic State Evolution and Nutrient Trapping Capacity in a Transboundary Subtropical Reservoir: A 25-Year Study.
abstract: Artificial reservoirs have been used for drinking water supply, other human activities, flood control and pollution abatement worldwide, providing overall benefits to downstream water quality. Most reservoirs in Brazil were built during the 1970s, but their long-term patterns of trophic status, water chemistry, and nutrient removal are still not very well characterized. We aimed to evaluate water quality time series (1985-2010) data from the riverine and lacustrine zones of the transboundary Itaipu Reservoir (Brazil/Paraguay). We examined total phosphorus and nitrogen, chlorophyll a concentrations, water transparency, and phytoplankton density to look for spatial and temporal trends and correlations with trophic state evolution and nutrient retention. There was significant temporal and spatial water quality variation (P < 0.01, ANCOVA). The results indicated that the water quality and structure of the reservoir were mainly affected by one internal force (hydrodynamics) and one external force (upstream cascading reservoirs). Nutrient and chlorophyll a concentrations tended to be lower in the lacustrine zone and decreased over the 25-year timeframe. Reservoir operational features seemed to be limiting primary production and phytoplankton development, which exhibited a maximum density of 6050  org/mL. The relatively small nutrient concentrations in the riverine zone were probably related to the effect of the cascade reservoirs upstream of Itaipu and led to relatively low removal percentages. Our study suggested that water quality problems may be more pronounced immediately after the filling phase of the artificial reservoirs, associated with the initial decomposition of drowned vegetation at the very beginning of reservoir operation.
authors.affiliation: Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-Carlense, 400, São Carlos, SP, CEP 13566-590, Brazil. davig@sc.usp.br.
authors.forename: Davi Gasparini Fernandes
authors.initials: DG
authors.lastname: Cunha
authors.affiliation: Itaipu Binacional, Avenida Presidente Tancredo Neves, 6731, Foz do Iguaçu, PR, CEP 85866-900, Brazil.
authors.forename: Simone Frederigi
authors.initials: SF
authors.lastname: Benassi
authors.affiliation: Fundação da USP, Polo São Carlos, Avenida João Dagnone, 1100, São Carlos, SP, CEP 13563-120, Brazil.
authors.forename: Patrícia Bortoletto
authors.initials: PB
authors.lastname: de Falco
authors.affiliation: Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-Carlense, 400, São Carlos, SP, CEP 13566-590, Brazil.
authors.forename: Maria do Carmo
authors.initials: Mdo C
authors.lastname: Calijuri
mesh_terms: D001938:Brazil; D002734:Chlorophyll / Q000032:analysis*; D000077194:Chlorophyll A; D005068:Eutrophication; D006801:Humans; D009584:Nitrogen / Q000032:analysis; D010758:Phosphorus / Q000032:analysis; D010839:Phytoplankton; D060753:Water Quality; D014881:Water Supply
chemical_list: D002734:Chlorophyll; D010758:Phosphorus; D009584:Nitrogen; D000077194:Chlorophyll A
keywords: Long-term studies; Nutrient removal; Subtropical aquatic systems; Trophic status; Water resources management