Evaluation of water and sediment quality of urban streams in Santa Cruz do Sul County, RS, Brasil, using ecotoxicological assays

: Aim: This study aimed to assess the quality of water and sediment of urban streams (Lajeado, Preto, Pedras and Lewis-Pedroso) located in Santa Cruz do Sul County, RS, Brazil, using the microcrustacean Ceriodaphnia dubia as test-organism. Methods: Quarterly scientific excursions to the streams were held on August and November 2011, February and May of 2012 in order to collect water and sediment samples, in the upper reaches (P1, P3, P5, P7) and lower reaches (P2, P4, P6, P8), totalizing 8 points. To evaluate the toxicity (acute and chronic), the microcrustacean C. dubia was used. Results and conclusion: The results indicated high toxicity levels detected in samples P2, P6 and P8 (lower reaches), as they caused the mortality of 100% of organisms in the water samples (P6 and P8) and sediment samples (P2 and P8), denoting acute effect. Yet, all upstream sites showed chronic effects in sediment samples, at least for one collection period, with the highest significant toxicity level among all samples (55.2%), which indicates the presence of contamination even in upper areas. These results indicated a strong degradation of the water and sediment quality of urban streams coming from the wastewater and industrial discharges of the urban area, which can cause damage to the biota as well as the public health, due to the multiples uses of water that the local population does, highlighting many of them as inappropriate to the water quality detected, such as the primary contact recreation (balneability).

In the State of Rio Grande do Sul, the Pardinho River is the main source of public water supply for the city of Santa Cruz do Sul, located roughly in the central region of the state. This city exhibits the largest human population as well as industrial activities in the basin; therefore the water quality of the Pardinho River is constantly influenced by human actions, highlighting the discharge of nontreated domestic sewage and industrial effluents, besides the influence of fertilizers and pesticides used in rural areas, particularly tobacco crops, rice, soybean and corn (Hermes et al., 2013).
Environmental monitoring studies in regional freshwater bodies conducted by the University of Santa Cruz do Sul have demonstrated that they already show advanced stages of organic pollution and eutrophication (Lobo and Callegaro, 2000;Lobo et al., 2002Lobo et al., , 2004aLobo et al., , b, c, 2010Oliveira et al., 2001;Hermany et al., 2006;Salomoni et al., 2006). These conditions have been also detected in Pardinho River and their main inflow streams, named Lajeado, Preto, Lewis-Pedroso and Pedras (Schuch et al., 2012). Furthermore, Hermes et al. (2013) claims that there was a significant increase of chemical pollutants in this basin, above environmental standards recommended by the National Council of the Brazilian Environment.
In this context, the aim of this research was to evaluate seasonally the quality of water and sediment of these four main streams running to the Pardinho River in the city of Santa Cruz do Sul, RS, from August 2011 to May 2012, through ecotoxicological assays using the microcrustacean Ceriodaphnia dubia as test-organism.

Methods
Quarterly scientific excursions to the four streams located in the city of Santa Cruz do Sul, RS (Lajeado, Preto, Pedras and Lewis-Pedroso) were held on August and November 2011, February and May of 2012 in order to collect water and sediment samples, in the upper reaches (P1, P3, P5, P7) and lower reaches (P2, P4, P6, P8), totalizing 8 points ( Figure 1). Samples were collected, stored in appropriate containers and maintained in thermal

Introduction
To improve the quality of water is a major challenge for man in this century. Currently, the pollution of water resources is increasing rapidly, since rivers and others aquatic systems are been often polluted by the indiscriminate disposal of sewage, industrial waste and a multitude of human activities (Oliveira-Filho et al., 2008).
Due to a historical concern with standards of microbiological quality, where the goal is to ensure public health, the group of coliforms bacteria has been used as an indicator of water quality since 1904. These bacteria, however, can not survive in the presence of certain chemicals; therefore the absence of these bacteria can not be related with a good water quality (Doyle and Erickson, 2006). In addition, some methodologies based on instantaneous measurements of physical and chemical variables are used. In this case, however, the environmental monitoring only permits snapshot measurements, therefore restricting the knowledge of water conditions to the period when the measurements were taken. These limitations become somewhat more drastic when the object of study is a lotic system where the current promotes the continuous renewal of water in each location (Lobo and Callegaro, 2000).
In this sense, the aquatic ecotoxicology is an effective tool for monitoring, prevention and control of water quality especially in environments with contaminants difficult to identify, by measuring the impact of chemicals or complex mixtures on living organisms, using ecotoxicological assays. Studies assessing the quality of water bodies have undergone substantial changes, including the assessment of sediment quality, a compartment that was until recently considered only as an accumulator of nutrients and a variety of contaminants. Sediments are potential sources of contaminants in aquatic systems; therefore the analysis of pollutants in sediment is a tool of great importance for the study of anthropogenic pollution (Nascimento, 2008).
Microcrustaceans have long been used in toxicity tests to estimate the toxic potential of substances, industrial effluents and environmental samples (water and sediment), where test organisms are exposed to different sample concentrations and toxic effects on them are observed and quantified (Costa et al., 2008). Among them, organisms belonging to the genus Ceriodaphnia have been selected as test organisms, considering some criterions such as availability and abundance, official protocols for cultivation in the laboratory Acta Limnologica Brasiliensia calculated using the statistical Trimmed Sperman-Karber method (Hamilton et al., 1979), with α = 5%.
C. dubia's neonates between 6 and 24 hours were exposed to water samples and sediment elutriate samples for a minimum period of seven days, using 10 replicates for each sample and 10 replicates for the negative control. The bioassay was completed when 60% of adult organisms that survived in the negative control had produced at least 15 neonates, not exceeding the eighth day. For the test validity, the control should not present mortality exceeding 20% of test organisms as well as have at least an average of 15 neonates produced by each female.
Sediment elutriate samples were prepared based on ABNT (2007). The samples were mixed in a 1:4 (v/v) ratio of sediment to distilled/deionized water. They were vigorously homogenized, remaining on boxes for transportation, with temperatures below 10 °C.
The chronic toxicity tests were performed based on the Brazilian Standard NBR 13373 (ABNT, 2005), using Cerodaphnia dubia as testorganism. The stock cultures were grown in medium established by ABNT (2005), which has a total alkalinity of 40.0 to 48.0 mg L -1 as CaCO 3 , and a pH between 7.5 and 7.6.
Culturing and toxicity testing were conducted at a temperature of 25 ± 2 °C and a photoperiod of 16h/08h light/dark. Culture medium was renewed once a week and females were fed with an algal suspension of Pseudokirchneriella subcapitata. Also, the food was complemented with a solution of Artemia salina. To validate the bioassays, acute sensitivity tests were performed with potassium dichromate (K 2 Cr 2 O 7 ). The EC(I)50 24h was In general, water samples showed acute toxicity along the four sampling times, in all streams, but only in downstream sites (P2, P4, P6 and P8) as shown in Figure 2, highlighting site P8 in November 2011 and May 2012, and site P6 in May 2012 because they showed the highest toxicity levels as they reached 100% of acute mortality.
Regarding to chronic effects, water samples showed toxicity in all seasons except February 2012 (Figure 3), with significant differences (p<0.05) in sites P2, P4 and P8 (lower reaches) and sites P1 and P7 (upper reaches), indicating sub-lethal effects.
Concerning the sediment samples, acute toxicity was verified along the four sampling times, but only in downstream sites (P2, P6 and P8), except P4 (Preto stream), as shown in Figure 4 In a general way, it should be noted the high toxicity levels detected in samples P2, P6 and P8 desk during 24 h for decantation. After that, the aqueous fraction (elutriate sample) was poured off and stored at 4 °C. Then, the test proceeded as for water samples.
The results for reproduction rate were analyzed using the statistical program GraphPad 5.0, applying the nonparametric statistical test of Kruskal-Wallis with Dunns post-test for multiple comparison between columns, considering a significance levels α = 5%. To interpretate the results obtained, three toxicity classes were considered (ABNT, 2005; CETESB, 2012): (a) No toxic effect, if no significant differences were found in the survival and reproduction rates of organisms, compared with the negative control; (b) Toxic effect, if significant differences were found in the reproduction rates of organisms, compared with the negative control; (c) Acute effect, if significant differences were found within 48h in the survival rates of organisms, compared with the negative control.

Results and Discussion
Regarding the sensitivity tests, the average EC(I)50 24h for C. dubia exposed to potassium dichromate (K 2 Cr 2 O 7 ) was 0.212 ± 0.046 mg L -1 (CV = 22.0%) thus validating the experimental Figure 2. Survival rate (%) observed during the tests for water samples. The dashed line represents the minimum survival rate that must be achieved to express non acute effects.  that although the sites are located in the upper part of the streams, nowadays they are receiving a significant pollution load, probably originating from local domestic and agricultural activities.
In a similar way, Lucheta et al. (2010) working with bioassays using D. magna in sediment samples from Gravataí river, RS, a polluted water body by industrial and domestic wastes, verified a high chronic toxicity reaching 88% of all samples. Working in Gravataí River, this condition was also ratified by Salomoni et al. (2011), since a pollution gradient (organic and eutrophication) was detected from the headwaters to the mouth, mainly by considering a significant decrease in the concentration of dissolved oxygen and turbidity, as well as a significant increase in the biochemical oxygen demand, total nitrogen, ortho-phosphate and thermotolerant coliforms.
Although mortality rates between water and sediment samples did not show significant differences (p>0.05), it is remarkable the chronic effects observed in sediment samples (Figure 3 and 5). As Burton et al. (2001) pointed out, the sediment serves as a deposit of organic and inorganic materials and accumulates chemical substances of anthropogenic origin, whose contamination can cause harmful effects to the aquatic ecosystem, (lower reaches), as they caused the mortality of 100% of organisms in the water samples (P6 and P8) and sediment samples (P2 and P8), denoting acute effect, which indicates discharges of toxic wastes along the course of streams. A similar condition was verified in the environmental monitoring conducted in sediment samples of Sapucaia stream, RS (Portela et al., 2006), where the results of the toxicity essays using Hyalella azteca and C. dubia as test-organisms, indicate an increase in environmental deterioration along the sampling locations, from upstream to downstream, mainly by domestic, industrial and agricultural discharges.
It is important to emphasize that all upstream sites showed chronic effects in sediment samples, at least for one collection period, with the highest significant toxicity level among all samples (55.2%), which indicates the presence of contamination even in upper areas. This condition was also verified by Schuch et al. (2012) in the study about the evaluation of water quality of these urban streams in Santa Cruz do Sul. The results indicated that the streams are polluted by organic contamination and advanced stages of eutrophication, highlighting that, in spite of the higher pollution levels were detected in the lower reaches, those located in the upper reaches were also contaminated, suggesting (1) Without evaluation of chronic toxicity, because it showed 100% of acute mortality.