Land use influence on eutrophication-related water variables: case study of tropical rivers with different degrees of anthropogenic interference Influência do uso e ocupação do solo sobre as variáveis da água relacionadas à eutrofização: estudo de caso de rios tropicais com diferentes níveis de interferência antrópica

AIM: This study reports an investigation of limnological characteristics and aquatic macrophyte occurrence in a neotropical reservoir in order to assess the spatio-temporal variation of water and sediment variables and their influence on plant distribution. METHODS: Macrophytes, water and sediment samples were collected from a Brazilian reservoir in different seasons from four main arms of the reservoir. In total sixteen water-sediment variables were analyzed including N:P ratio and Trophic State Index. The plants were collected using a quadrat sampling procedure and the dry weight per sample was measured. MANOVA was performed to evaluate spatial and temporal variation of environmental variables as well as seasonal biomass differences. To assess the relationship among environmental variables and macrophytes an ordination analysis (using Canonical Correspondence Analysis: CCA) was carried out. RESULTS: The spatial and temporal variation of limnological variables generated a heterogeneous system which supports the presence of different species of macrophyte. pH, dissolved oxygen and sediment composition were important predictors of Polygonum lapathifolium occurrence while nutrients were associated with Eichhornia crassipes and Pistia stratiotes. Inorganic substances were related to biomass variation of Eichhornia azurea and Myriophyllum aquaticum. CONCLUSIONS: The spatial variation of the environmental variables has caused heterogeneity in the reservoir and it may support the occurrence of different species of macrophyte. Limnological variables highlighted in CCA are important to predict the species occurrence and their control in the study area.

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Aim: This study aimed to evaluate the water quality of Canha River micro watershed (Ribeira de Iguape Valley, São Paulo State, Brazil) and to calibrate the QUAL2K model with field data; Methods: Five variables of water quality were analyzed (water temperature, dissolved oxygen, pH, electric conductivity and turbidity), as well as flow and velocity, in eight sampling stations from September, 2006 to July, 2007. It was performed a correlation test between observed data and simulated data; Results: The dissolved oxygen concentration was high (over than 5.0 mg.L -1 ) during sampling period. The headwater presented the lowest concentrations of this variable. The electric conductivity did not exceed 80 µS.cm -1 and the pH did not range (0.5-1). The hydraulic variables were in agreement with River Continuum Concept. Although correlation test was not successful for some variables, the simulated data followed observed data; Conclusions: The rainfall linked to land use was important for obtained results. The watershed characteristics, as high slope, for example, were essential mainly to dissolved oxygen concentrations. Even though QUAL2K has some limitations its use is recommended for water resources management and future purposes.Keywords: modeling, QUAL2K, water quality, land use, brazilian watershed.Resumo: Objetivo: O presente estudo teve por objetivo avaliar a qualidade da água da microbacia do rio Canha (Baixo Ribeira de Iguape, São Paulo, Brasil) e calibrar o modelo QUAL2K com os dados obtidos em campo; Métodos: Foram analisadas cinco variáveis de qualidade da água (temperatura da água, oxigênio dissolvido, pH, condutividade elétrica e turbidez), bem como vazão e velocidade de escoamento, em oito pontos amostrais durante setembro de 2006 a julho de 2007. A comparação entre os dados simulados e observados foi feita através de um teste de correlação simples; Resultados: As concentrações de oxigênio dissolvido foram altas (acima de 5,0 mg.L -1 ) durante todo o período de amostragem, sendo que na nascente observaram-se as menores concentrações. A condutividade elétrica não ultrapassou 80 µS.cm -1 e o pH teve uma pequena faixa de variação (0,5 a 1). As variáveis hidráulicas seguiram a teoria do Continuum fluvial. Os dados simulados seguiram a mesma tendência dos dados observados, embora o teste de correlação não tenha sido satisfatório para todas as variáveis; Conclusões: A precipitação pluviométrica aliada à modificação do tipo de uso e ocupação do solo foram os principais responsáveis pelos resultados obtidos. As características da bacia, como alta declividade, por exemplo, também foram importantes, principalmente para as concentrações de oxigênio. Embora o modelo QUAL2K tenha algumas limitações, seu uso para gerenciamento dos recursos hídricos e propostas de cenários futuros é recomendado.Palavras-chave: modelagem, QUAL2K, qualidade da água, uso e ocupação do solo, micro bacia brasileira.