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Brazilian Journal of Biology

versão impressa ISSN 1519-6984versão On-line ISSN 1678-4375

Braz. J. Biol. vol.77 no.2 São Carlos abr./jun. 2017  Epub 26-Set-2016

http://dx.doi.org/10.1590/1519-6984.14015 

Original Article

Seasonal variation of limnological features and trophic state index of two oligotrophic reservoirs of southeast Brazil

Variação sazonal das características limnológicas e Índice de Estado Trófico de dois reservatórios oligotróficos do sudeste do Brasil

S. A. Oliveiraa 

C. E. M. Bicudoa  * 

aNúcleo de Pesquisa em Ecologia – NPE, Instituto de Botânica, CP 68041, CEP 04045-972, São Paulo, SP, Brazil

Abstract

Limnological features of two reservoirs were studied in dry (August 2013) and rainy (January 2014) periods to evaluate the water quality that supply the city of Guarulhos, southeast Brazil. Water samples were collected in three depths and the following characteristics were measured: alkalinity, dissolved O2, free and total CO2, HCO3, soluble reactive silica, dissolved and total nitrogen and phosphorus, and chlorophyll-a. Water transparency was also measured and temperature, pH and electric conductivity profiles were obtained. Great seasonal and low spatial variability of the water characteristics occurred in the reservoirs. High values of water transparency, free CO2 availability, and low of pH, soluble reactive silica and total and dissolved nutrients values were recorded at the dry period, and different conditions were found at the rainy season. The two reservoirs were characterized by low nutrients, chlorophyll-a and turbidity, and high transparency, these features being typical of oligotrophic systems. The two reservoirs still remain under low anthropogenic impact conditions, and are presently considered reference systems for the SPMR, São Paulo Metropolitan Region. The need for actions that will reduce the input of nutrients from the neighboring cities and the main tributaries of the hydrographic basin is emphasized to maintain the ecological quality of the reservoirs and their reference conditions among the SPRM reservoirs.

Keywords:  oligotrophy; reference system; reservoir; seasonality

Resumo

Características limnológicas de dois reservatórios foram estudadas durante os períodos seco (Agosto 2013) e de chuvas (Janeiro 2014) para avaliar a qualidade da água que abastece a cidade de Guarulhos, Brasil sudeste. Amostras de água foram coletadas em três profundidades e as características estudadas foram: alcalinidade, O2 dissolvido, CO2 livre e total, HCO3, sílica reativa solúvel e nitrogênio e fósforo dissolvido e total e clorofila-a. Transparência da água foi medida e obtidos os perfis de temperatura, pH e condutividade elétrica. Grande variabilidade sazonal e espacial das características da água ocorreu nos reservatórios. Altos valores de transparência da água, disponibilidade de CO2 livre e menores de pH, sílica reativa solúvel e nutrientes totais e dissolvidos foram documentados durante o período seco; e condições diferentes durante o período de chuvas. Os dois reservatórios foram caracterizados por baixos teores de nutrientes, clorofila-a e turbidez e alta transparência, características estas típicas de sistemas oligotróficos. Os dois reservatórios ainda permanecem sob condição de baixo impacto antropogênico e são atualmente considerados sistemas de referência para a RMSP, Região Metropolitana de São Paulo. A necessidade de ações para diminuir e até frear a entrada de nutrientes provenientes das cidades vizinhas e dos principais afluentes da bacia hidrográfica é enfatizada para manter a qualidade ecológica dos reservatórios e suas condições de referência entre os reservatórios da RMSP.

Palavras-chave:  oligotrofia; sistema de referência; reservatório; sazonalidade

1 Introduction

Maintenance of water resources quality of our Planet is a paramount preoccupation both for the importance of their conservation and their diversified uses. Certain areas in Brazil already have their public supply water reserve partially threatened, especially those of the great urban centers. Among the many reasons that would explain this situation are the inadequate soil use and occupation, the water resources unsustainable management and the great population growth (Tundisi, 2003).

One of the best documented environmental problems that severely affect the aquatic ecosystems at a global scale is the eutrophication (Battarbee et al., 2005). Despite the great number of studies emphasizing its sources, the solution for this problem is yet far from being reached (Sayer and Roberts, 2001); consequently, knowledge of the natural conditions of an aquatic system existing prior to any disturbance is fundamental and absolutely crucial for the implementation of recovery strategies and management of potential impacts (Vilaclara et al., 1997).

Regarding the urban reservoirs in particular, most modifications of their water limnological characteristics derive from the hydrographical basin, that is able to vary rapid and intensively due to the nature and composition of its tributaries (Räsänen, 1986). The difficulty in handling reservoirs water quality comes from the dynamic nature of these systems, since they are important convergence points of the different activities being developed in its hydrographic basin (Tundisi et al., 2008). The multiple human activities and their impact surely will reflect on the water ecological quality of such reservoirs (Rebouças, 1999).

Many SPMR, São Paulo Metropolitan Region reservoirs are being pushed by different types of impacts, inferring that the nationwide socio-environmental situation is directly related to the urban growth, the unplanned soil occupation and the threatening urbanization of the hydrographic basins (Tundisi and Matsumura-Tundisi, 2008). The surrounding area of urban reservoirs and their tributaries are increasingly submitted to the punctual factors resulting from the chaotic urban occupation and the insufficiently managed waste and sanitary sewage production that directly affect the aquatic environment equilibrium (Tomanik et al., 2009) and, therefore, the quality of the population water supply. Tanque Grande and Cabuçu reservoirs are used for public supply and provide water for about 1.3 million people, and are included under the above situation.

For Tanque Grande and Cabuçu reservoirs there are only four papers published: Moutinho et al. (2007), Graça et al. (2007), Lacava et al. (2009) and Piasentin et al. (2009). Considering the importance of knowing the quality of the water of these reservoirs for their preservation, present research aimed at contributing to the seasonal evaluation of the limnological aspects and the ecological quality of Tanque Grande and Cabuçu reservoirs through the evaluation of their physical and chemical characteristics.

2 Material and Methods

2.1 Study area

The two reservoirs are located in the Municipality of Guarulhos, São Paulo State, and southeastern Brazil (Figure 1). Guarulhos is the second largest municipality of the SPMR in terms of population, with c. 1.3 million people. It covers the area of 319 km2, the main city being located northeast of that of São Paulo (23º27’49”S, 46º32’01”W).

Figure 1 Geographic location of Cabuçu (at left) and Tanque Grande (at right) reservoirs and respective sampling sites. 

Tanque Grande reservoir was built in 1958 at the north region of the municipality (23º22’27”S, 46º27’36”W). It has the area of 0.54 km2, storage capacity of 77,401 m3, discharge of 315,000 m3 month-1 and retention time of 7.35 days (Silva et al., 2011; CETESB, 2014). Its hydrographic basin includes seven streams that will flow into the reservoir responsible for 10% of the municipality water supply.

Cabuçu reservoir is located in the Cantareira State Park (CSP), at the southern part of the municipality (22º24’06”S, 46º31’56”W). The reservoir was built in 1904 to water supply the city of São Paulo, being deactivated when the Cantareira system started operating in 2002 (Lacava et al., 2009). The reservoir has a discharge of 300 L s-1, the volume of 1,776 x 106 L distributed in 1.5 km, total area of over 20 ha and the retention time of 68.51 days (Lacava et al., 2009).

2.2 Sampling

Three sampling sites were selected in each reservoir considering the influence of the main tributaries, the water quality heterogeneity and the system depth (Figure 1).

Water samples were collected during the dry (August 2013) and rainy (January 2014) periods, using a van Dorn bottle along a vertical profile (euphotic zone, mean depth and c. 1 m above the sediments) and placed in polyethylene vials for transportation to the laboratory. Temperature, pH and electric conductivity profiles were obtained ‘in situ’ at each 50 cm of the water column using a Horiba U50 multi-parameter probe.

The following water characteristics were measured: transparency (Secchi disk), alkalinity (Golterman and Clymo, 1969), dissolved oxygen (Golterman et al., 1978), free CO2, total CO2 and HCO3 (Mackereth et al., 1978), ammonium (Solorzano, 1969), nitrate and nitrite (Mackereth et al., 1978), soluble reactive phosphorus, total dissolved phosphorus and soluble reactive silica (Strickland and Parsons, 1965), and total nitrogen and total phosphorus (Valderrama, 1981). Chlorophyll-a (corrected by the phaeophytin) was extracted using ethanol 90% as the organic solvent (Sartory and Grobbelaar, 1984).

2.3 Data analysis

Data were evaluated using Principal Components Analysis (PCA) to ordain the sampling units and the environmental information according to the climatic periods, and were previously transformed by ranging [(x - xmin)/(xmax - xmin)]. Randomization test (999 permutations) was employed to understand the PCA dimension (p < 0.05). Statistical program used was PC-ORD 6.0 (McCune and Mefford, 2011).

The Trophic State Index (TSI) was calculated according to Lamparelli (2004). Considering the surface total phosphorus and chlorophyll-a, TSI can classify the aquatic environment into six trophy categories: TSI 47 (ultraoligotrophic), 48-52 (oligotrophic), 53-59 (mesotrophic), 60-63 (eutrophic), 64-67 (supereutrophic) and > 67 (hypereutrophic).

3 Results

Air temperature and the average monthly precipitation were greater in January (respectively 26.8°C, 137.9 mm) than in August (18.7°C, 8.2 mm). The water surface temperature followed the same pattern between the two climatic periods, with their average values higher in January (Tanque Grande 26°C, Cabuçu 22.6°C) and lower in August (Tanque Grande 18.2°C, Cabuçu 16°C).

Secchi disk depth was higher during August at all sampling sites of both reservoirs, showing little variation among sampling sites. During August, the lowest values were measured at the in-between sites and during January, the highest values were measured near the dam (Tables 1, 2).

Table 1 Limnological variables of Tanque Grande (TG) and Cabuçu (CB) reservoirs in August.  

Local Temp Cond pH Secchi DO N-NH4 N-NH2 N-NH3 TN P-PO4 TDP TP SRS CO2 HCO3 Chlo-a
(°C) µS cm-1 (m) mg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 mg L-1 mg L-1 mg L-1 µg L-1
TG1S 19.9 52 6.86 1.52 7.35 16.6 < 5.0 13.9 79.2 5.8 < 4.0 6.50 4.75 7.13 29.87 6.6
TG1B 16.6 53 6.68 --- 7.00 11.1 < 5.0 12.4 75.5 5.7 < 4.0 10.1 4.80 11.99 30.24 6.6
TG2S 21.5 49 6.34 1.72 7.48 11.1 < 5.0 < 8.0 76.8 4.9 < 4.0 8.4 4.64 23.47 29.67 10.4
TG2B 17.9 51 6.08 --- 8.14 11.1 < 5.0 < 8.0 168.1 5.4 < 4.0 6.8 4.55 44.15 30.68 9.9
TG3S 17.6 53 6.24 1.66 7.55 11.1 < 5.0 < 8.0 231.3 7.1 < 4.0 15.4 4.68 30.58 30.71 10.4
TG3B 15.4 64 5.75 --- 8.26 11.1 < 5.0 11.5 224.1 5.8 < 4.0 12.7 4.74 103.62 30.71 9.3
CB1S 18.2 39 6.31 3.10 7.74 33.8 < 5.0 85.6 229.7 < 4.0 < 4.0 10.5 5.05 16.20 19.12 3.3
CB1M 15.6 39 6.06 --- 7.35 37.0 < 5.0 85,6 237.8 < 4.0 < 4.0 14.0 5.86 32.10 19.43 4.4
CB1B 13.6 40 6.05 --- 6.27 61.1 < 5.0 87.2 187.8 < 4.0 4.7 14.4 5.12 33.98 20.10 4.4
CB2S 20.0 36 6.37 2.70 7.96 19.0 < 5.0 91.4 171.9 < 4.0 4.2 9.8 5.90 14.45 19.58 3.7
CB2M 14.4 40 5.90 --- 4.63 85.2 < 5.0 101.2 244.0 4.9 5.0 11.9 5.30 45.96 19.24 5.0
CB2B 13.0 40 5.39 --- 2.29 164.8 < 5.0 100.0 392.7 4.5 4.1 25.2 5.24 160.77 20.80 3.1
CB3S 22.1 35 6.45 3.10 7.87 21.3 < 5.0 87.8 510.7 4.2 < 4.0 12.7 4.78 11.59 18.87 3.3
CB3M 14.2 39 6.23 --- 3.28 74.5 < 5.0 120.7 643.9 4.2 5.4 11.2 4.91 21.67 19.40 7.3
CB3B 12.7 42 6.10 --- 2.04 150.9 < 5.0 98.4 606.1 4.4 < 4.0 12.7 5.13 31.21 20.71 4.0

Legend: S (surface); M (intermediate depth); and B (1 m from the bottom); (smaller than the method detection limit).

Table 2 Limnological variables of Tanque Grande (TG) and Cabuçu (CB) reservoirs in January.  

Local Temp Cond pH Secchi DO N-NH4 N-NH2 N-NH3 TN P-PO4 TDP TP SRS CO2 HCO3 Chlo-a
C µS cm-1 m mg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 µg L-1 mg L-1 mg L-1 mg L-1 µg L-1
TG1S 27.39 63 7.33 0.78 6.35 < 10.0 < 5.0 < 8.0 283.1 9.0 5.5 23.0 6.13 2.65 35.08 4.9
TG1B 24.84 67 6.80 --- 6.63 < 10.0 < 5.0 < 8.0 225.3 9.3 6.8 12.9 6.09 9.04 35.32 3.8
TG2S 27.34 63 6.89 0.69 6.54 < 10.0 < 5.0 < 8.0 192.3 10.2 6.4 11.5 6.03 7.58 36.42 5.5
TG2B 25.45 63 6.79 --- 7.18 < 10.0 < 5.0 < 8.0 157.5 13.4 8.4 11.1 6.03 9.41 35.94 4.9
TG3S 26.54 62 6.54 1.15 9.49 < 10.0 < 5.0 < 8.0 181.9 8.6 8.0 12.9 5.92 22.49 48.29 3.3
TG3B 24.87 65 6.76 --- 3.54 < 10.0 < 5.0 < 8.0 159.8 9.2 7.5 10.6 6.03 10.20 36.33 4.4
CB1S 25.79 38 8.48 1.45 8.64 < 10.0 < 5.0 < 8.0 303.0 7.0 13.9 19.4 2.87 0.10 18.36 11.0
CB1M 25.67 37 7.36 --- 7.14 12.0 < 5.0 12.9 384.0 7.0 13.6 26.1 2.93 1.36 19.25 18.7
CB1B 21.32 44 6.30 --- 3.50 46.2 9.3 82.0 543.4 8.2 16.4 23.8 3.12 17.00 19.61 13.2
CB2S 26.19 38 7.37 1.97 8.63 < 10.0 < 5.0 < 8.0 243.8 7.3 13.5 16.2 2.76 1.36 19.74 8.3
CB2M 23.30 38 6.47 --- 1.74 97.7 6.3 24.8 430.9 8.0 11.7 21.0 3.11 11.13 20.34 26.4
CB2B 17.05 101 6.59 --- 0.78 1019.3 18.7 < 8.0 1910.6 8.2 14.1 9.7 4.07 17.16 35.19 55.2
CB3S 26.72 38 7.29 2.15 7.60 < 10.0 < 5.0 < 8.0 239.3 9.9 13.9 17.4 2.75 1.64 19.81 5.7
CB3M 21.36 40 6.32 --- 1.60 39.1 5.2 < 8.0 367.0 7.9 11.4 25.2 3.19 17.70 21.38 48.3
CB3B 16.34 103 6.50 --- 0.59 830.6 16.1 < 8.0 1552.8 8.3 12.9 9.7 4.14 20.56 34.27 49.6

Legend: S (surface); M (intermediate depth); and B (1 m from the bottom); smaller than the method detection limit.

At the Tanque Grande reservoir, pH (Tables 1, 2) was slightly acidic during the two climatic periods, getting close to neutral mainly during January. At the Cabuçu, however, pH varied from slightly acidic to alkaline, its lowest values recorded in August. Highest electric conductivity was observed at the Tanque Grande reservoir, and the values varied between 49 and 64 µS cm–1 in August and between 62 and 67 µS cm–1 in January. At the Cabuçu reservoir, however, its variation was slightly lesser in August and the values were homogeneous along the water column during the entire period, whereas in January their values increased towards the bottom of reservoir (Tables 1, 2).

Tanque Grande free CO2 concentrations were higher in August than in January. At the Cabuçu the highest values were measured in January. Considering the vertical distribution, highest values were registered at the surface and sometimes at the intermediate layers, but some other times mostly at the bottom of reservoir (Tables 1, 2). Dissolved oxygen showed a homogeneous distribution at the Tanque Grande water column, but slightly greater in August (Table 1). Dissolved O2 in Cabuçu reservoir showed a tendency towards its reduction to the bottom of reservoir. In January, however, the tendency was more stressed at all sampling sites, the bottom concentrations almost reaching anoxia (Table 2).

The N series (NH4, NO3 and TN) exhibited in Tanque Grande reservoir low and similar values in the two climatic periods sampled (Tables 1, 2, Figure 2). At the Cabuçu reservoir, the highest NH4 values were recorded in January, whereas those of NO3 were in August. Highest values of TN were found in January, and an increasing gradient was observed towards the bottom of reservoir (Tables 1, 2, Figure 2a, b and c).

Figure 2 Ammonium (a), Nitrate (b), Total nitrogen (c), Orthophosphate (d), Total dissolved phosphorus (e) and Total phosphorus (f) concentrations (average ± SD) of Tanque Grande and Cabuçu reservoirs during the two climatic periods. 

Phosphorus total and dissolved forms showed low values, in some cases below the method detection limit as was the case of dissolved TP of Tanque Grande reservoir in August. At the Cabuçu reservoir, P dissolved fractions were low in August. Highest TP values were detected in January, and the greatest ones at the reservoir surface (Table 2, Figure 2f). Regarding the spatial scale, TP highest values were obtained at the sampling sites located near to the tributaries entrance.

Tanque Grande surface chlorophyll-a concentrations were higher in August, whereas at the Cabuçu reservoir the opposite occurred, i.e. the highest values being obtained in January (Tables 1, 2, Figure 3).

Figure 3 Chlorophyll-a concentration (µg L-1) at the surface of Tanque Grande and Cabuçu reservoirs during the two climatic periods studied. 

Principal Components Analysis considering 12 limnological variables resumed 64.8% of data total variability (Table 3, Figure 4). August sampling units gathered at the negative side of axis 1 were mainly associated with the water transparency (Secchi disk) and NO3 (r > 0.8) greatest values as well as the lowest ones of temperature and dissolved TP. At the positive side of this axis were assembled the January sampling units associated to the highest temperature, PO4 and pH (r > 0.8) values and the smallest ones of water transparency and NO3. At the positive side of axis 2, were the Tanque Grande sampling units and some surface sampling units of the Cabuçu reservoir intermediate region, associated to the DO and SRS highest values (r > 0.7). At the negative side of this axis, Cabuçu sampling units were positioned associated to the highest TN and NH4 values (r > 0.6) and the lowest ones of DO and SRS. Thus, axis 1 ordained the sampling units in regard to seasonality, whereas axis 2 did it in relation to the reservoir’s compartments differences.

Table 3 Correlation of the limnological variables with PCA axes 1 and 2 (between parentheses: acronyms). 

Variable Axis 1 Axis 2 Variable Axis 1 Axis 2
Temperature (Temp) 0.868 0.255 Total nitrogen (TN) -0.027 -0.731
pH 0.772 0.022 Total phosphorus (TP) 0.278 -0.446
Secchi depth -0.815 -0.359 Silicate (SSR) -0.373 0.696
Dissolved oxygen (DO) 0.184 0.724 Orthophosphate (PO4) 0.824 0.030
Ammonium (NH4) -0.042 -0.621 Free CO2 (CO2) -0.535 -0.052
Nitrate (NO3) -0.838 -0.168 Total dissolved phosphorus (TDP) 0.675 -0.682

Figure 4 Principal Components Analyses (PCA) based on 12 limnological variables of the sampling sites of Tanque Grande (TG) and Cabuçu (CB) reservoirs during August and January of present study. Legend: S (surface); M (intermediate depth); and F (1 m from the bottom). 

The Trophic State Index (Figure 5) classified oligotrophic the Tanque Grande reservoir in August, except for the pelagic zone that was considered mesotrophic. In January the opposite occurred, i.e. the reservoir was classified mesotrophic and its sampling site TG3 oligotrophic. The Cabuçu reservoir was classified oligotrophic in August and mesotrophic in January. Considering the TSI annual average, however, both reservoirs were classified oligotrophic, except for its upstream sampling site that was classified mesotrophic (Table 4).

Figure 5 Trophic State Index of Tanque Grande and Cabuçu reservoirs during August and January. 

Table 4 Trophic State Index for chlorophyll-a (TSI Chlo-a) and total phosphorus (TSI TP), average index (TSI average) and yearly classification Meso (mesotrophic), Oligo (oligotrophic). Aug (August), Jan (January). 

Reservoir Site TSI Chlo-a TSI TP TSI average Classification
Aug Jan Aug Jan Aug Jan Aug Jan Annual
TG1 56.0 54.6 45.8 53.5 50.9 54.0 Oligo Meso Oligo
Tanque Grande TG2 58.2 55.1 47.3 49.3 52.8 52.2 Oligo Meso Oligo
TG3 58.2 52.6 51.0 50.0 54.6 51.3 Meso Oligo Oligo
CB1 52.6 58.5 48.7 52.4 50.7 55.5 Oligo Meso Meso
Cabuçu CB2 53.2 57.1 48.3 51.3 50.7 54.2 Oligo Meso Oligo
CB3 52.6 55.3 49.9 51.8 51.2 53.5 Oligo Meso Oligo

4 Discussion

The greatest limnological variation currently observed for the Guarulhos Producing System occurred according to the seasonal scale (dry and rainy periods), an already known characteristic of tropical and subtropical reservoirs (Naselli-Flores, 1999; Ferrareze et al., 2005; Sartori et al., 2009). Differences among reservoirs were also detected.

Temperature values followed the pattern defined for the tropical region, i.e. mild during August and higher in January. Surface water temperature is affected by several factors including depth and the presence of surrounding vegetation (Percebon et al., 2005). It was presently observed that milder temperatures were registered at the Cabuçu reservoir due to its well preserved surroundings, differently than in Tanque Grande when 4ºC and 2ºC were recorded in January and August, respectively.

A joint evaluation of abiotic data indicated that both reservoirs were similar in August mainly because of the greatest water transparency and DO values and of CO2, SRS and NO3 availability, and the smallest values of temperature, pH, PO4 and dissolved TP. In January, the reverse occurred, i.e. lower values of water transparency and DO, of CO2, SRS and NO3 availability, and greater values of temperature, pH, PO4 and dissolved TP.

Water transparency was greater in August than in January, and was related to the lower amount of precipitation that led to a lesser input of particulate material during the period, as was registered by Wengrat and Bicudo (2011) for the Billings Complex.

Heterogeneous pattern of dissolved oxygen distribution along the water column was found in August, with a concentration reduction with depth. O2 concentration reduction at the bottom of reservoir shall be related to the organic matter decomposition at the bottom of reservoir, which led to a high CO2 production and, consequently, to a high O2 consumption (Hill et al., 1993).

pH variation range in the two reservoirs studied is characteristic of unpolluted continental waters (Kalff, 2002). Both reservoirs exhibited similar pH values along the water column and also among the sampling sites, however slightly smaller in August. In relation to the several factors affecting the pH, interpretation in nature may become somewhat complex (Esteves, 2011). According to Nascimento (2012), smaller pH favors an increase of CO2 concentrations depending on the equilibrium between pH and the carbon inorganic forms, similar to what was observed in the present study. Furthermore, higher CO2 values may be associated with the increase of respiration and the organic matter decomposition processes (Branco, 1966).

Soluble reactive silica availability seems to be related to the stratification and circulation periods. Circulation allows a greater SRS availability along the water column (Wetzel, 2001). Nevertheless, during the present study the smallest SRS concentrations were measured. A small variation of temperature in relation to the increase in depth leads to a homogeneous SRS distribution along the water column and the reduction of Si concentration at the reservoir surface (Esteves, 2011).

Both reservoirs were characterized by a low nutrient concentration in total and dissolved forms that are emblematic of oligotrophic systems (Arcifa et al., 1981; Bicudo et al., 2002; Lopes et al., 2005; Tundisi et al., 2006). However, integration of the limnological data allowed also differences between the two reservoirs. Thus, Cabuçu showed higher TP and TN and a higher HCO3 availability when compared to Tanque Grande reservoir. Considering that the latter reservoir is inserted in a preservation area (“Reserva Florestal do Núcleo Cabuçu”) it is much probable that its enrichment is a natural process.

Longitudinal gradient in reservoirs represented by the zones under influence of the system regimen flux differ from each other in terms of their local physical, chemical and biological characteristics (Thorton et al., 1990). Variation in the nutrient contents along the longitudinal gradient was already observed in several reservoirs, as in Segredo (Thomaz et al., 1997) and Salto Grande (Zanata and Espíndola, 2002). In the present study, similarity among the sampling sites of each reservoir was observed. This fact should be associated to that of the reservoirs having about the same depth along their entire extensions.

Both Tanque Grande and Cabuçu reservoirs were classified oligotrophic, except for the Cabuçu upstream sampling site classified mesotrophic (TSI = 53.1), however, very close to the oligotrophy zone upper limit (TSI = 47-52). Downstream sampling site was classified mesotrophic mainly because of the influence of its greater August chlorophyll-a values. However, such results compared to the eutrophic reservoirs like the Guarapiranga at its intake site (57.74 µg L-1) and the Billings main body (65.22 µg L-1) (CETESB, 2014) should be considered somewhat low. Present values coincide with those measured for oligotrophic to mesotrophic reservoirs like Jaguari-Jacareí (Nascimento, 2012: < 8.3 µg L-1), Pedro Beicht (Silva, 2012: 9.5 µg L-1), Ribeirão Perová (< 0.01 µg L-1) and Ribeirão dos Cristais (0.76 µg L-1) mentioned in CETESB (2014).

5 Conclusion

The Guarulhos Producing System showed low dissolved nutrient’s concentrations, besides low values of total phosphorus and chlorophyll-a, that are characteristic of the reservoir’s oligo-mesotrophic classification. Also were coincident due to slightly acidic pH and good oxygen availability. These characteristics indicate that Tanque Grande and Cabuçu reservoirs are still subjected to small anthropogenic impact and/or some satisfactory level of alteration.

Acknowledgements

This work was supported by funds provided by FAPESP, Fundação de Amparo à Pesquisa do Estado de São Paulo (Project nº 2009/53898-9). SAO and CEMB thank CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico for the scholarship granted to the first author and to research grant to the second one (CEMB). Authors are also grateful to Prof. William de Queiróz (Universidade de Guarulhos, Geoprocessing Laboratory) for providing the illustration of the study area.

(With 5 figures)

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Received: September 08, 2015; Accepted: February 24, 2016

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