Abstract
Satellite images are an effective tool for the detection of phytoplankton blooms, since they cause striking changes in water color. Bloom intensity can be expressed in terms of chlorophyll-a concentration. Previous studies suggest the use of Landsat TM4/TM3 reflectance ratio to retrieve surface chlorophyll-a concentration from aquatic systems. In this study we assumed that a remote sensing trophic state index can be applied to investigate how changes in HRT along the hydrologic year affect the spatial distribution of the phytoplankton blooms at Ibitinga's reservoir surface. For that, we formulated two objectives: (1) apply a semi-empirical model which uses this reflectance ratio to map chlorophyll-a concentration at Ibitinga reservoir along the 2005 hydrologic year and (2) assess how changes in hydraulic residence time (HRT) affect the spatial distribution of phytoplankton blooms at Ibitinga Reservoir. The study site was chosen because previous studies reported seasonal changes in the reservoir limnology which might be related to the reservoir seasonality and hydrodynamics. Six Landsat/TM images were acquired over Ibitinga reservoir during 2005 and water flow measurements provided by the Brazilian Electric System National Operator - ONS were used to compute the reservoir´s residence time, which varied from 5.37 to 52.39 days during 2005. The HRT in the date of image acquisition was then compared to the distribution of chlorophyll-a in the reservoir. The results showed that the HRT increasing implies the increasing of the reservoir surface occupied by phytoplankton blooms.
Highlights
The assessment of water quality has become critical in São Paulo State as water demand has intensified in the last decades (Tundisi, 2003; Tundisi et al, 1991, 2008; São Paulo, 2011)
In spite of the fact that the LUT was developed for data acquired in October/2005, the results show that it captured the variability of the Ibitinga reservoir chlorophyll-a distribution in time and space
The results show that in September and October, when HTR is larger than 30 days (Bennion et al, 2005) the reservoir surface is covered by continuous and dense phytoplankton blooms
Summary
The assessment of water quality has become critical in São Paulo State as water demand has intensified in the last decades (Tundisi, 2003; Tundisi et al, 1991, 2008; São Paulo, 2011). In hydroelectric reservoirs the residence time is manipulated to balance the need of water storage during the dry season and water renewal for maintaining quality requirements. According to Straškraba (1999) the HRT is a determinant factor in stratification processes and nutrient availability, affecting phytoplankton development. It might explain the time and space changes in the chlorophyll concentration at reservoir surface (Straškraba, 1999)
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