Metals and limnological variables in an urban reservoir: compartmentalization and identification of potential impacted areas Sheila Cardoso-Silva & Julio Cesar López-Doval & Viviane Moschini-Carlos & Marcelo Pompêo Received: 16 December 2016 /Accepted: 5 December 2017 /Published online: 14 December 2017 # Springer International Publishing AG, part of Springer Nature 2017 Abstract Reservoirs in urban areas are used for differ- ent purposes and are liable to different types of pressures that can cause the loss of chemical and biological qual- ity, hence diminishing their ecological, economic, and cultural benefits. Here, a study of surface water hetero- geneity was undertaken at the Guarapiranga urban res- ervoir (São Paulo, Brazil) in order to improve under- standing of the structure and functioning of these eco- systems. Sampling was performed during the dry and rainy seasons at 33 sites. Limnological variables and total contents of the metals cadmium, nickel, lead, and zinc were analyzed. The risks associated with the metals were evaluated based on the toxicity unit approach. A principal component analysis enabled differentiation of the reservoir into six different areas. Some of the most powerful discriminatory variables (nutrients and metals) showed the existence of anthropogenic impacts on the system. The most strongly affected compartments were located in the following: (1) upstream area, under the influence of the Parelheiros stream, with the highest total phosphorus levels (318 mg L−1) and (2) dam area, with high values for total nitrogen, suspended organic matter, total solids, and pH. The results for the dam compartment were a consequence of substantial urbanization and a longer residence time. Despite high levels of cadmium during the rainy season, no significant potential risk for zooplankton was observed. The data indicated the need to control unauthorized land occupation and to implement adequate sanitation in the Guarapiranga watershed. This research provides information that should assist water resource agencies in the sustainable management of urban reservoirs. Keywords Spatial heterogeneity .Water quality . Urban impact . Cadmium contamination Introduction A majority of the human population is now concen- trated in urban areas, with the proportion likely to increase to 70% by 2050 (ONU 2012a). Latin Amer- ica currently has one of the world’s highest urbani- zation rates (80%), with a value of 90% expected by 2020 (ONU 2012b). The expansion of urban areas and the associated populations requires a greater supply of water of appro- priate quality for economic and social development (Liu et al. 2015). This demand has resulted in the construc- tion of reservoirs to ensure availability (Lehner et al. 2011; Liu et al. 2015). Approximately 850,000 Environ Monit Assess (2018) 190: 19 https://doi.org/10.1007/s10661-017-6387-3 S. Cardoso-Silva (*) :V. Moschini-Carlos Environmental Sciences Program, UNESP, Sorocaba Campus, Av. Três de Março, 511 - Aparecidinha, Sorocaba, São Paulo 18087-180, Brazil e-mail: she.cardosos@gmail.com J. C. López-Doval :M. Pompêo Catan Institute for Water Research (ICRA), Girona, Spain M. Pompêo Ecology Department, Institute of Biosciences, University of São Paulo, São Paulo, São Paulo, Brazil http://crossmark.crossref.org/dialog/?doi=10.1007/s10661-017-6387-3&domain=pdf reservoirs close to urban areas are in operation or will soon be built worldwide (Nilsson et al. 2005; Zarfl et al. 2015). However, population growth and unplanned ur- banization can threaten economic and social sustainabil- ity, especially due to the degradation of natural resources (Chen et al. 2014; Zhou et al. 2015). Water bodies located in urban areas, as in the case of reservoirs, are used for a variety of purposes, including water supply, power generation, and recreation and landscaping (Moggridge et al. 2014). In these regions, they can be liable to various pressures leading to loss of chemical and biological quality, which can restrict their potential uses (such as in the case of integrated use of reservoirs in urban areas) and cause water quality prob- lems. These impacts include the inputs (from point and diffuse sources) of complex mixtures of pollutants (pes- ticides, nutrients, emerging pollutants, metals, and acid compounds) due to runoff from urban paved surfaces such as roads and industrial areas, discharges of treated and untreated sewage, and aerial deposition of materials emitted from industrial or transportation sources (Hall and Ellis 1985; Ellis and Mitchell 2006; de Morais and Guandique 2015). A well-documented phenomenon is the eutrophication of water resources due to nutrient enrichment, which can lead to alterations in reservoir structure and function, with negative consequences in terms of both the ecosystem and the economy (Smith et al. 1999; Moschini-Carlos et al. 2009; Azevedo et al. 2015a; Azevedo et al. 2015b). Although there is increasing interest in the study of urbanization impacts and the effects of urbanized land- scapes on water bodies, there are still challenges in- volved in monitoring, maintaining, and improving the quality of these ecosystems. In the case of Brazil, a country with an emerging economy and rapid economic and industrial growth, there are many examples of such challenges, including in the Metropolitan Region of São Paulo (RMSP), the country’s most important economic and industrial center. The metropolitan region has expe- rienced rapid growth over the past 50 years and now includes the city of São Paulo and a further 39 munic- ipalities, with a total population of 21,000,000 inhabi- tants and an area of 8000 km2, making it the largest metropolitan area in South America. The main land uses are urban and industrial (Braga et al. 2006; Drucrot et al. 2005; Formiga-Johnsson and Kemper 2005; SEADE 2015). The RMSP is supplied by 23 reservoirs (Tundisi and Matsumura 2008), some of which (such as the Guarapiranga and Billings reservoirs) are located in the metropolitan region itself. The water quality in these reservoirs is degraded due to inputs of untreated sewage containing high levels of organic and inorganic pollutants, resulting in eutrophication and loss of bio- logical quality (Braga et al. 2006; CETESB 2013; Fontana et al. 2014). The Guarapiranga reservoir is the second largest source of water for the RMSP, providing water to 3.7 million people (Whately and Cunha 2006). The reser- voir basin has a high rate of urbanization, with anthropic uses corresponding to 42% of the area (Whately and Cunha 2006). Approximately 10% of the population lives in squatter slum settlements (Santoro et al. 2008). The water of this reservoir has, since the 1960s, shown a historic loss of chemical and biological quality, due to increased loads of organic material and nutrients (ANA 2005; CETESB 2014). The presence in sediment of metals (such as cadmium and copper) at concentrations above background has been confirmed in various re- ports and studies (CETESB 2014; Pompêo et al. 2013). Conservation of the biological and chemical quality of reservoirs is essential in order to ensure the continu- ing economic and cultural benefits of these systems (Turner and Daily 2008; Molozzi et al. 2012). As point- ed out by Pinto-Coelho et al. (2010), the detection of spatial patterns is a fundamental step in establishing the real causes of declines in the ecological health of trop- ical reservoirs. Therefore, the objective of this study was to identify possible spatial and temporal patterns in the Guarapiranga reservoir, based on the water quality (con- sidering nutrients and metals) determined at various sampling points distributed throughout the water body. Since metals are among the contaminants that have been detected in this reservoir, the potential risk to biota was also evaluated. This work aims to provide water re- source managers with information helpful for managing reservoirs in a more sustainable way. Materials and methods Study area The Guarapiranga subwatershed covers a total area of 630 km2 and is located in the Alto Tietê watershed in the municipalities of São Paulo, Embu, Embu-Guaçu, and Itapecerica da Serra, as well as small portions of the territories of Cotia, São Lourenço da Serra, and Juquitiba (SABESP 2016). The Guarapiranga reservoir, 19 Page 2 of 13 Environ Monit Assess (2018) 190: 19 the second most important source of water supplied to the RMSP, is considered polymictic (Maier 1985) and has a maximum volume of 194 × 106 m3, an area of 34 km2 (Melchor et al. 1975), a mean water residence time of between 110 and 143 days (Beyruth 1996), a flow of 14 m3 s−1 (SABESP 2016), equivalent to 1.2 billion liters of water per day, and a maximum depth of 13 m (Maier and Takino 1985). The main tributaries of the reservoir, in terms of water volume, are in the downstream direction, with the Embu-Mirim and Embu rivers on the left bank and the Parelheiros stream on the right bank (Melchor et al. 1975) (Fig. 1). The Guarapiranga river basin exhibits dendritic drainage, with crystalline and sedimentary soils (Ab Saber 1957). The region has a mean annual temperature of 17.5 °C and a mean annual rainfall of 1400 mm. According to the Köppen classification, the climate is humid subtropical (class C, type C + b) (Melchor et al. 1975). Sampling, laboratory procedures, and data analysis Two sampling campaigns were performed, one in the dry season (September 2006) and one in the rainy sea- son (April 2007), at 33 locations along the Guarapiranga reservoir (Fig. 2). Surface water samples were collected in polyethylene bottles and stored in thermal bags until analyzed in the laboratory. The following parameters were measured in situ in the surface water: pH, electrical conductivity, and water temperature (YSI 63 multipa- rameter probe); dissolved oxygen (Hanna HI 9142 probe); and turbidity (Secchi disk depth). In the labora- tory, measurements were made of total suspended solids by a gravimetric method (Wetzel and Likens 1991), total suspended particulate matter and the corresponding or- ganic and inorganic fractions (Wetzel and Likens 1991), chlorophyll a and pheophytin (Lorenzen 1967), and total nitrogen and total phosphorus (Valderrama 1981). All reagents were of analytical grade (Synth) and were used as received without further purification. After collection, the samples used for analyses of metals (cadmium, nickel, zinc, and lead) were acidified to pH 2 with HNO3 and then stored in polyethylene bottles at 4 °C until analyzed. Samples were digested (in replicate) in 10-mL test tubes, with addition of 0.5 mL of analytical grade HNO3 (Merck), at a temperature of 105 °C until the volumewas reduced to about 2 mL. The samples were then filtered through 125-mm Whatman 41 filters and stored at 4 °C prior to analysis using inductively coupled plasma atomic emission spectros- copy (ICP-AES, Spectro spectrometer) (APHA 1998). The data were analyzed using basic descriptive sta- tistics and principal component analysis (PCA) based on a correlation matrix (Legendre and Legendre 1998). In the PCA, the centroid was calculated as the mean values of the scores of axes 1 and 2 corresponding to each group, observed using cluster analysis employing Eu- clidian distances and Ward’s method, as described by Cardoso-Silva et al. (2016). Data analysis was per- formed using the PAST software package (Hammer et al. 2001). The risks associated with the total metal concentra- tions were evaluated using the toxicity unit (TUs) ap- proach, described by Sprague (1971). The TUs were calculated for each metal at each sampling point, ac- cording to Eq. 1: TUi ¼ Ci=LC50i ð1Þ where Ci is the concentration of the metal imeasured in the environment (mg/L) at the sampling point, and LC50i is the concentration (mg/L) of the metal lethal to 50% of a population of the Cladoceran Daphnia magna. As- suming that the toxicity of the mixture of metals was additive for each point, the total TU value was calculat- ed based on the sum of the individual TU values for each metal (Eq. 2). TUtotal ¼ ΣTUiþ…þ TUn ð2Þ The existence of toxicological risk was assumed when TU ≥ 1, while TU < 1 indicated an absence of risk. The Cladoceran D. magna was selected as a represen- tative zooplankton organism because its community was dominant in the study area. The LC50 values were obtained from the database available at: www. systemecology.eu. Results The existence of horizontal spatial heterogeneity in the surface water of the Guarapiranga reservoir was evident in both sampling periods, with the formation of a gradi- ent along the reservoir. The use of PCA enabled the identification of six different compartments, some of which varied in terms of extent and location. Environ Monit Assess (2018) 190: 19 Page 3 of 13 19 http://www.systemecology.eu http://www.systemecology.eu Compartments I and II In order to improve interpretation of the PCA results, some of the sampling points were removed. In both campaigns, points 1 and 2 showed the highest solids, total nitrogen, total phosphorus, and electrical conduc- tivity values, and the lowest Secchi disk values (Table 1), differentiating these points from the rest of the sampling network. The data suggested that these areas formed a separate compartment in the reservoir, which was denoted compartment I (Fig. 2). Other points removed from the PCA were point 5 (first campaign) and points 5 and 6 (second campaign). These locations showed the highest SIM and total zinc (dry season) values, together with the lowest values for electrical conductivity, solids, TSM, and total 1) 2) Fig. 1 Sampling sites at 33 locations in the Guarapiranga reservoir. 1 Dry season (September 2006). 2 Rainy season (April 2007). Sampling stations were georeferenced according to the UTM coordinates system (datum SAD69 and central meridian 45°) Fig. 2 Compartments of the Guarapiranga reservoir identified using PCA based on limnological parameters measured in the surface water. Sampling campaigns performed in 1 the dry season (September 2006) and 2 the rainy season (April 2007) 19 Page 4 of 13 Environ Monit Assess (2018) 190: 19 Table 1 Means and standard deviations (X ± SD) for the follow- ing variables: depth (Z, m); Secchi disk depth (SD, m); tempera- ture (T, °C); pH; electrical conductivity (EC, μS cm−1); total solids (TS, mg L−1); suspended particulate matter: total (TSM, mg L−1), organic (SOM, %), and inorganic (SIM, %); total phosphorus (TP, μg L−1); total nitrogen (TN, μg L−1); chlorophyll a (Chl, μg L−1); pheophytin (Phe, μg L−1); total Zn (mg L−1); total Cd (mg L−1); and total Ni (mg L−1). The designation of compartments (C) was based on the PCA results. For compartments I, II, and VI, only the raw values are presented (P sampling point) Campaign 1 CI CII CIII CIV CVa CVb X P5 X ± SD X ± SD X ± SD X ± SD Z 1.8 5.0 4.1 ± 1.2 5.8 ± 2.3 8.8 ± 0.6 6.4 ± 2.1 SD 0.6 1.2 11 ± 0.2 1.3 ± 0.1 1.4 ± 0.1 1.9 ± 2.7 T 17.9 18.5 18.6 ± 0.3 19.1 ± 0.3 19.3 ± 0.3 16.5 ± 6.3 pH 7.2 7.6 7.5 ± 0.04 7.7 ± 0.2 8.0 ± 0.3 6.9 ± 2.4 EC 227.2 99.7 1286 ± 3.2 128.4 ± 2.3 127.8 ± 0.9 111.2 ± 44.1 TS 143.3 68.0 84.4 ± 1.0 82.5 ± 2.5 88.5 ± 16 75.2 ± 29.1 TSM 14.1 3.3 5.0 ± 0.6 4.0 ± 1.32 4.6 ± 0.6 5.3 ± 4.7 SOM 53.0 45.0 59.4 ± 4.7 91.2 ± 11.1 97.7 ± 3 76.0 ± 31.5 SIM 47.1 55.0 40.6 ± 4.7 8.8 ± 11.1 2.3 ± 3 17.9 ± 28.7 TN 794.1 399.0 406.9 ± 26.4 393.6 ± 101.4 368.6 ± 136.5 373.8 ± 174.9 TP 116.7 12.7 24.7 ± 4.2 21.5 ± 6.0 26.4 ± 3.8 22.6 ± 7.8 Chl 2.9 0.9 * 11.2 ± 8.3 25.7 ± 11.8 36.4 ± 300.8 Phe 23.0 5.3 28.4 ± 7.3 36.3 ± 15 19.9 ± 9.6 26.9 ± 15.0 Zn * 0.3 * 0.02 ± 0.03 0.02 ± 0.03 0.1 ± 0.1 Cd * * * * * 0.001 ± 0.001 Campaign 2 CI CII CIII CIV CV CVI X X X ± SD X ± SD X ± SD P23 Z 3.8 6.7 6.5 ± 2.2 6.9 ± 2.4 8.3 ± 2.8 9.3 SD 0.9 1.6 1.3 ± 0.2 1.4 ± 0.1 1.4 ± 0.1 1.4 DO 5.0 5.8 5.0 ± 0.3 5.7 ± 0.5 6.6 ± 0.8 6.2 T 22.9 24.1 24.0 ± 0.2 24.4 ± 0.1 24.9 ± 0.2 25.1 pH 6.6 6.4 6.5 ± 0.1 6.4 ± 0.1 6.6 ± 0.2 6.3 EC 143.4 80.3 106.2 ± 3.1 109.1 ± 2.0 110.8 ± 2.1 112.4 TS 83.8 63.8 78.8 ± 7.0 79.7 ± 4.1 75.3 ± 9.2 88.0 TSM 8.7 2.4 4.5 ± 0.8 6.1 ± 2.8 6.0 ± 3.3 9.2 SOM 78.3 64.6 70.4 ± 8.4 91.0 ± 5.8 95.4 ± 4.9 100.0 SIM 21.7 35.4 29.6 ± 8.4 9.0 ± 5.8 4.6 ± 4.9 5.0 TN 834.2 349.3 438.0 ± 158.3 582.5 ± 80.9 649.2 ± 82.5 609.3 TP 194.7 14.2 23.7 ± 3.8 27.0 ± 2.7 28.2 ± 2.2 35.4 Chl 54.3 6.2 15.7 ± 3.7 11.2 ± 7.6 8.8 ± 11.8 16.0 Phe 13.5 6.0 12.9 ± 6.1 15.7 ± 7.5 22.1 ± 10.2 13.6 Cd * 0.04 0.04 ± 0.05 0.03 ± 0.01 0.01 ± 0.03 0.1 Zn 0.004 0.03 0.002 ± 0.001 0.004 ± 0.001 0.005 ± 0.001 0.01 Ni 0.001 0.01 * 0.002 ± 0.001 0.002 ± 0.002 0.01 *Values below the method detection limit Environ Monit Assess (2018) 190: 19 Page 5 of 13 19 phosphorus (Table 1). These points were located in a more remote region of the central reservoir body (Figs. 1 and 2), under the influence of the Embu river, and suggested the formation of another compartment in the reservoir, denoted compartment II. Compartments III, IV, V, and VI The data analysis presented below suggests that the extents of the observed compartments varied according to the season (dry or rainy). Sampling campaign 1 The results of the PCA showed that most of the data variability (50.02%) could be explained by the first two axes (Fig. 3). The most influential variables were chlo- rophyll a (0.85) and pH (0.79) in axis 1, and nitrogen (0.72) and pheophytin (− 0.54) in axis 2. The SIM variable influenced the points located in the upstream area of the reservoir, suggesting the formation of a third compartment (Figs. 2 and 3). The pheophytin variable was responsible for the arrangement of the points located after the eucalyptus island (in the upstream-downstream direction) in an area under the influence of the Embu Mirim river, which was desig- nated compartment IV. Another observed compartment (Va) showed posi- tioning influencedmainly by the parameters chlorophyll a, pH, and Secchi disk depth (Fig. 2). Points located in the dam area showed the influence of the variables Cd, Zn, total solids, and total nitrogen, indicating the exis- tence of a compartment Vb (Fig. 3). Sampling campaign 2 In the PCA results for the second sampling campaign, axis 1 explained 26.82% of the variability and together with axis 2 explained 42.26% of the variability (Fig. 4). Inclusion of the third axis resulted in explanation of most of the data variability (56.91%). The most influen- tial variables in axis 1 were SIM (− 0.85) and total nitrogen (0.69), while cadmium (0.59) and pH (− 0.59) were most important in axis 2. The results for the second sampling campaign showed a compartmentalization pattern in the reservoir that was similar to that for the first campaign, in terms of compartments III and IV (Fig. 2). The variables Zn, total solids, and chlorophyll a influenced the identification of compartment IV. The existence of compartment V was indicated by grouping of the var iables Cd, phosphorus, electrical conductivity, depth, and pheophytin (Fig. 4). Point 23, located on the left bank of the reservoir in a dendritic area under the influence of the Guaravituva and Itupu streams, remained isolated in the scores plot, suggesting the existence of a sixth compartment. Although the existence of compartments in the Guarapiranga reservoir was evident in both sampling periods, there were differences in the variables that influenced the arrangements. For example, in the first sampling period, chlorophyll a influenced the arrange- ment of the points located further downstream, while in the second period the effect of chlorophyll a was further upstream (Table 1, Figs. 3 and 4). The metal analyses showed that the Pb concentra- tions remained below the method detection limit (0.12 mg L−1), throughout the reservoir and in both sampling periods. Ni was only detected at some points in the second period, with the highest values in com- partment V (0.001 ± 0.002 mg L−1). In the first period, the concentrations of Cd were below the method detec- tion limit (0.0001 mg L−1) at most sampling points. However, in the second period, higher mean concentra- tions were measured in the compartments located near the dam (Table 1). Toxicological risk Calculation of the toxicological risk, following the TU approach (Sprague, 1971), revealed no risk to zooplank- ton at any of the sampling points, with values ranging from 0 to 0.125 for the first sampling campaign and from 0.014 to 0.27 for the second campaign. In the first sampling campaign, the highest value was obtained in compartment II, with Zn being responsible for the total toxicity, while in the second campaign, the highest value was obtained in compartment III, with Cd providing the main contribution to the toxicity. However, the TU values obtained were low, and any toxicological risk could be discounted, according to the criterion of the TU method. Discussion Based on the water quality at the different sampling points, the reservoir could be differentiated into six 19 Page 6 of 13 Environ Monit Assess (2018) 190: 19 Fig. 3 PCA of variables measured in the Guarapiranga reservoir surface water. Points 1, 2, and 5 were removed. The scores were related to the variables depth (Z), temperature (T), pH, electrical conductivity (EC), Secchi disk depth (SD), total solids (TS), suspended inorganic particulate material (SIM), chlorophyll a (Chl), pheophytin (Phe), total nitrogen (TN), total phosphorus (TP), Zn, and Cd. Sampling performed in September 2006 Fig. 4 PCA of variables measured in the Guarapiranga reservoir surface water. Points 1, 2, 5, and 6 were removed. The scores were related to the variables depth (Z), temperature (T), pH, electrical conductivity (EC), Secchi disk depth (SD), total solids (TS), suspended inorganic particulate material (SIM), chlorophyll a (Chl), pheophytin (Phe), total nitrogen (TN), total phosphorus (TP), Zn, and Cd. Sampling performed in April 2007 Environ Monit Assess (2018) 190: 19 Page 7 of 13 19 different areas, showing the existence of spatial hetero- geneity. The zoning was seasonally dependent, with some of the most powerful discriminatory variables (nutrients and metals) revealing anthropogenic impacts on the system. However, considering the metal concen- trations in the water and the TU values, there was no significant potential risk to zooplankton. Compartments I and II The formation of compartment I, located near the Parelheiros stream, was associated with characteristics typical of riverine areas of reservoirs, as well as anthro- pogenic impacts. The former included increased amounts of particulate matter that restricted light pene- tration in the water body, hence limiting primary pro- ductivity (Henry 2004). This area presented the most critical water quality conditions, even though previous research has suggested improvement of the quality of the water entering the reservoir, due to the presence of the Parelheiros wetland (Andrade 2005). The Parelheiros region receives a high amount of sew- age and occasionally the water pumped from Billings res- ervoir,between2.0and4.0m3s−1 (Piresetal.2015),alsoan important but impacted reservoir with high levels of nutri- ents. Therefore, the poor water quality that enters the Parelheiros area is responsible for thecharacteristicsof this compartment, such as highECandnutrient concentrations (Nishimura et al. 2014). According to the local environ- mental agency, CETESB, who monitors the reservoir in theParelheirosandinthedamareas,since2006,highlevels of phosphorus have been recorded in Parelheiros region exceeding the Brazilian Standards limit value (Resolution 357–CONAMA 2005) (CETESB 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2017). The environ- mental local agency classified the area as eutrophic be- tween2006and2010 andas supereutrophic between2011 and 2016, according to the Trophic State Index of Carlson (1977) adapted to tropical ecosystems, as proposed by Lamparelli (2004) (CETESB 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2017). The high levels of nutrients in the area have been observed by several other authors as Nishimura et al. (2014), Pires et al. (2015), Machado et al. (2016), and López-Doval et al. (2017). The critical water quality in Parelheiros area was also evidenced by biological indexes applied by CETESB (2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2017), such as the zooplankton community index (ZCI) and the aquatic life index (ALI). The ZCIwas classified as bad and the ALI presented values oscillating between the regularandbadstate.Thesefindingssuggest that thepublic policies adopted in Guarapiranga basin have not been effective once the water quality is getting worst and indi- cate the need to control unauthorized occupation and to invest in sewagecollectionand treatment inareas currently not served. The formation of compartment II was associ- ated with the fact that the region has a distinct residence time (Occhipinti 1973;Beyruth 1996), since it is located in adendritic area that is relatively isolated from the influence ofthecentral flow.Dendriticregionsinreservoirsgenerally increase theheterogeneityof thesystem(Henryetal. 1998; Nogueira 2001), with characteristics different to those of the other regions of the water body, as observed in the present case. Compartments III and IV Compartment III showed characteristics typical of inter- mediate areas of reservoirs, with increasing light pene- tration, decreasing concentration of suspended solids, and progressive decrease of SIM, relative to the up- stream region (Fig. 2). In addition to these characteris- tics, these reservoir zones typically exhibit longer mean residence times and higher sedimentation rates (Kimmel et al. 1990), making them more favorable for primary productivity, associated with higher contents of chloro- phyll a and nutrients (Pagiori et al. 2005). In the present case, however, this was only observed for chlorophyll a in the second sampling period. This pattern of higher productivity in the intermediate zone occurs because less turbulence leads to a smaller amount of suspended material, so light availability is no longer a limiting factor in productivity (Kimmel et al. 1990). Compartment IV consisted of a region with a water residence time distinct from the other reservoir areas (Occhipinti 1973; Beyruth 1996). This compartment was mainly characterized by the presence of the highest pheophytin levels, which could be correlated with the senescence of phytoplankton derived from the waters of the Embu-Mirim river. Critical water quality conditions were found by Santos et al. (2015) in a sampling per- formed in compartment IV, in 2010 during dry season in a 48-h period (sampling every 3 h). The authors observed in superficial water, high levels of total nitrogen 1912.4 ± 198.8 μg L−1, and total phosphorus 42.5 ± 8.7 μg L−1 as well as high levels of chlorophyll a 32.4 ± 5.1 μg L−1. These data suggested an increase in anthropic impacts in anthropic impacts as observed in Parelheiros area, once in 19 Page 8 of 13 Environ Monit Assess (2018) 190: 19 this research mean values for phosphorus, nitrogen and chlorophyll a where lower than these. Compartments V and VI In the first sampling period, compartment V could be divided into two subcompartments, with the results indicating the influence of temporary factors. Compart- ments Va and Vb (first period) and V (second period) were in the downstream region of the reservoir (Fig. 2), with the longest mean residence time (Occhipinti 1973; Beyruth 1996). In compartment Vb, located closer to the dam in the area generally considered as the lentic zone of the reservoir, it was expected to observe low levels of nutrients and TSM, but the opposite was found. This probably reflected the presence of a large urban settle- ment in the region, together with the accumulation of these species due to a longer water residence time. In compartment Vb, the highest average values for pH could be explained by increased CO2 consumption during the photosynthesis process. This was corroborated by high values for chlorophyll a, as well as a statistically significant correlation between pH and chlorophyll a. An increase in pH is usually associated with algal blooms, and conse- quently with high concentrations of chlorophyll a (Buzelli and Cunha-Santino 2013; Tracann et al. 2014). In the second sampling campaign, the characteristics of compartment V were similar to those of compart- ments Va and Vb, although the values of some of the variables that influenced these groupings varied be- tween sampling periods. These differences were proba- bly due to seasonal factors. The higher nutrient levels found during the rainy season (second sampling period) were expected, since at this time there is an increase in the supply of nutrients from the drainage basin to the reservoir (Akinyemi and Nwankwo 2007; Smith et al. 2014). The solids concentrations (TSM and SOM) were generally higher in the second period, possibly for the same reason given for the nutrients. In the case of chlorophyll a, the observed differences in the distribution patterns between the two sampling periods could be attributed to two main factors: (1) the residence time and (2) the application of algicides to control algal blooms. In the Guarapiranga reservoir, the residence time of the water tends to increase from up- stream to downstream. The mean values range from 11 to 83 days in the dam area, and from 0 to 27 days in the central reservoir region (Occhinpinti 1973; Beyruth 1996). Therefore, greater nutrient accumulation can occur in the dam area, favoring primary productivity, as indicated by the higher chlorophyll a concentrations downstream which exceeded the Brazilian Standards limit value (Resolution 357–CONAMA 2005). During the rainy season, despite a greater input of nutrients, higher flow rates lead to a shorter water residence time, so that nutrients are continuously transferred to the downstream system (Leite et al. 2004). This hindered observation of a clear pattern of higher chlorophyll concentrations upstream, opposite to the pattern ob- served by Olds et al. (2011) in Harlan County Reservoir (Nebraska). The application of algicide to control phytoplankton in the reservoir could also have influenced the spatial distri- bution pattern of chlorophyll a. The application of algi- cides (copper sulfate and hydrogen peroxide) was higher in 2006 than in the previous year, with values between 12 and 62 t (CETESB 2007). Applications tend to be higher during the dry season (Szajubok 2000), which could explain the lower chlorophyll a concentrations observed upstream during the first sampling period. The use of copper sulfate was responsible for the observed high concentrations of this metal. According to data published by CETESB (2007, 2008), the dis- solved copper concentrations measured in 2006 and 2007 were above the limit of 0.009 mg L−1 established in current legislation (CONAMA regulation 357/05). Although, at least in the short term, copper does not cause severe harm to humans (Gárcia-Villada et al. 2004), this metal can disrupt the balance of the ecosys- tem. Treatment with copper sulfate leads to the release of large quantities of nutrients, kills algae, reduces com- petition, and promotes the formation of anoxic sedi- ments, with the subsequent resuspension of nutrients favoring opportunistic species (Beyruth 2000). The use of copper sulfate as an algicide is a contro- versial palliative practice. This practice has resulted in concentrations of copper in sediments up to 46-fold (Pompêo et al. 2013) background, and up more than tenfold the PEL (probable effect level—137 mg kg−1) value for copper, which suggest toxicity is likely to occur in sediments (Leal et al. 2017). Leal et al. (2017) evaluated copper concentrations along Guarapiranga sediments and through a geostatistical approach estimated the sediment copper stock in the reservoir. The stock was estimated in a value of 1158.85 t(copper), corresponding to 11 years of copper sulfate application. Extrapolation to a period of 43 years of constant copper application gave a value of 4530.05 t of copper applied to the Guarapiranga Environ Monit Assess (2018) 190: 19 Page 9 of 13 19 reservoir. The authors concluded that the Guarapiranga water management policy is mainly structured considering short-term financial costs, without adoptingmiddle-term or long-term strategies as the sewage treatment. For metals, the highest concentrations found in sur- face water in the dam area could be explained by the tendency of this material to accumulate here due to the longer residence time. This finding was in agreement with Pompêo et al. (2013), who reported high levels of Cr, Ni, Cu, and Cd in superficial sediments from the Guarapiranga dam area. The concentrations of total cadmium measured in the second sampling period also exceeded the current limit established in CONAMA regulation 357/05, but were within the limit of 0.005 mg L−1 established by the World Health Organization. However, attention to cadmium con- centrations is required since the metal is toxic towards humans and aquatic organisms (Barbier et al. 2005; Garcia-Santos et al. 2005). Cadmium iswidely recognized as one of themost toxic pollutants in the environment, due to its neurotoxic effects and its capacity to cause injury in various organs and tissues, following acute or chronic exposure (Méndez-Armenta and Ríos 2007). In fact, in the present work, the elements Cd and Zn showed the highest TU values. The presence of cadmium in the reser- voir could be attributed to the existence of industrial activ- ities in the northern and northwest reservoir regions, with possible sources of contamination including pigments and plastics manufacturing facilities (Pompêo et al. 2013). The highest metal concentrations were observed during the rainy season, probably associated with the drainage of water from the basin to the reservoir (Gaur et al. 2005; Mastoi et al. 2008). For the metals, no ecotoxicological riskwas indicated using the TU criteria. Since the risk level was calculated based on the total concentrations of the dissolved metals (as a worst case scenario), rather than dissolved metal concentrations, as recommended (EU 2011; Schmidt et al. 2010), there was no evidence of any risk posed by these metals to D. magna or, by extrapola- tion, to other zooplankton organisms. As observed in compartment I, critical conditions for the water quality in compartment V persist and high nutrients and chlorophyll a concentrations, exceeding the Brazilian Standards limit value (Resolution 357– CONAMA 2005), have been recorded (CETESB 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2017; Machado et al. 2016; López-Doval et al. 2017). However, for cadmium, besides Pompêo et al. (2013), no other study pointed the high levels for this element. Conclusions and final considerations The six compartments observed along the Guarapiranga reservoir were the result of the operational regime and the dendritic structure of the water body, as well as the human impacts affecting the system. Anthropogenic effects were evident in the compartment in the Parelheiros region, associated with high levels of nutri- ents, and in the compartment near the dam, where high levels of nutrients and metals were recorded. Remedial measures are required in order to reduce nutrient inputs into this ecosystem, by means of the control of urban settlements in the region, implementation of appropriate basic sanitation, and effective sewage collection and treatment. Only by taking protective and restorative measures will it be possible to maintain the sustainabil- ity of this important watershed. This research provides decision makers with infor- mation to assist in the management of urban ecosys- tems. Given the increasing number of water bodies potentially affected by urbanization processes, we be- lieve that the findings of this work should be useful in other geographical contexts. Acknowledgements We are grateful to the Ecology Department at the Biosciences Institute of the University of São Paulo for technical support. We thank Prof. Dr. Elisabeth de Oliveira and the Fundamental Chemistry Department at the Chemistry Institute of the University of São Paulo for assistance with the ICP-AES analyses. 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Environ Monit Assess (2018) 190: 19 Page 13 of 13 19 https://doi.org/10.1023/A:1011946708757 https://doi.org/10.1080/07438141.2011.601401 https://doi.org/10.1080/07438141.2011.601401 https://doi.org/10.1016/j.limno.2009.11.011 https://doi.org/10.1590/S2179-975X4914 https://doi.org/10.5327/Z0102-9800201300020003 https://doi.org/10.5327/Z0102-9800201300020003 http://www.sabesp.com.br https://doi.org/10.1590/1519-6984.05313 https://doi.org/10.1590/1519-6984.05313 https://doi.org/10.1002/etc.302 https://doi.org/10.1002/etc.302 http://produtos.seade.gov.br/produtos/msp/index.php?tip=met4&opt=s&tema=SNE&subtema=2 http://produtos.seade.gov.br/produtos/msp/index.php?tip=met4&opt=s&tema=SNE&subtema=2 http://produtos.seade.gov.br/produtos/msp/index.php?tip=met4&opt=s&tema=SNE&subtema=2 https://doi.org/10.1016/S0269-7491(99)00091-3 https://doi.org/10.1016/S0269-7491(99)00091-3 https://doi.org/10.1590/S2179-975X2014000100009 https://doi.org/10.1016/0043-1354(71)90171-0 https://doi.org/10.1016/0043-1354(71)90171-0 https://doi.org/10.1590/S2179-975X2014000400005 https://doi.org/10.1590/S2179-975X2014000400005 https://doi.org/10.1007/s10640-007-9176-6 https://doi.org/10.1007/s10640-007-9176-6 https://doi.org/10.1016/0304-4203(81)90027-X https://doi.org/10.1016/0304-4203(81)90027-X https://doi.org/10.1007/978-1-4757-4098-1 https://doi.org/10.1007/978-1-4757-4098-1 https://doi.org/10.1007/s00027-014-0377-0 https://doi.org/10.1007/s00027-014-0377-0 https://doi.org/10.1016/j.habitatint.2015.05.020 Metals and limnological variables in an urban reservoir: compartmentalization and identification of potential impacted areas Abstract Introduction Materials and methods Study area Sampling, laboratory procedures, and data analysis Results Compartments I and II Compartments III, IV, V, and VI Sampling campaign 1 Sampling campaign 2 Toxicological risk Discussion Compartments I and II Compartments III and IV Compartments V and VI Conclusions and final considerations References