Chironomus larvae (Chironomidae: Diptera) as water quality indicators along an environmental gradient in a neotropical urban stream

Anthropogenic interference in urban lotic systems is a factor affecting the biota of waterbodies. Aquatic macro invertebrates are an important food source for fish and are valuable indicators of water quality. The objective of this work was to study Chironomus larvae (Chironomidae: Diptera) distribution along an environmental gradient in Barbado Stream, Cuiabá, MT, Brazil. No individual Chironomus was found in the springs of Barbado Stream, which may indicate preservation of the area. During the study period, we found 40.3 and 94.4 individuals/m 2 at points 3 and 4 (low course), respectively. There is eutrophication in these sites due to domestic sewage discharges, indicating low quality water. The Barbado Stream needs restoration projects that include an awareness of the residents of their neighborhood’s environmental importance, and investments in the sanitation sector to prioritize the collection and treatment of wastewater and solid waste collection.


INTRODUCTION
Population growth causes an increase in consumption, space, and housing. Urban centers, which grow very quickly, are a great example, not taking into account the environmental pollution they cause. Aquatic ecosystems are one of the most affected by these changes (Braga et al., 2001). In recent decades, aquatic ecosystems have been altered to different degrees as a negative consequence of anthropogenic activities (e.g., mining, plumbing, dam construction, eutrophication, etc.). Rivers are impacted by everything that happens in the surrounding areas, to include land use and human occupation. Thus, the environmental characteristics of rivers, especially of their resident biological communities, provide information about the consequences of human action (Callisto et al., 2001).
The occupation of the river banks, considered as Permanent Preservation Areas (PPA) due to the expansion of urban areas, causes the removal of vegetative cover and soil sealing (Pellegrino et al., 2006). In addition, urban rivers and streams are subject to successive hydraulic engineering, drainage systems, altering their characteristics and transforming them into systems of underground drainage and wastewater receptacles (Galdino and Andrade, 2008). This occurs due to a lack of compliance with environmental criteria that seek the balance of the environment. Veyret (2001) argues that the notion of environment encompasses not only nature, much less merely fauna and flora alone, but also involves the interdependencies that exist among humans, societies and physical, chemical and biotic components, and also integrates its economic, social and cultural aspects.
Currently, at least a million people live in PPA areas in Brazil. Most of these are low-income people who have no legal access to affordable housing in urban areas with adequate infrastructure (Marandola and Hogan, 2005). As a result, the banks of rivers and streams and permanent preservation areas are occupied by low-income populations (unauthorized occupations) and also by middle-class condominiums. Coelho (2001) emphasizes that the upper class has large areas that allow vegetation and soil preservation, while the poorer class is concentrated in densely populated areas, which alters the carrying capacity of the soil.
Cuiabá, capital of Mato Grosso state, fits this scenario, due to rapid and unplanned urban growth. The Barbado Stream, one of the main tributaries of Cuiabá River which runs through Cuiabá, is environmentally degraded due to the sewage discharge of several homes and shopping areas (Bordest, 2003). Much of its stretch is already channeled, and another part doesn't have riparian vegetation, causing soil problems such as erosion, waterproofing, siltation, etc. (Colet and Soares, 2013;Ventura, 2011). It is degraded in the middle and lower courses, and the mouth while it is well preserved in the springs located in the State Park Massairo Okamura (Colet and Soares, 2013;Kreischer et al., 2012;Ventura, 2011).
Anthropogenic interference in lotic waters near urban areas can be an important factor affecting the biota of the river. These factors can impact aquatic fauna directly, through specific changes in habitats, or through temporary reduction in food availability and changes in other environmental variables (Callisto et al., 2001). There are several ways to assess water quality in waterbodies such as methods that focus on physical and chemical properties (i.e. dissolved oxygen, mercury, and water clarity), and biological measures (i.e. species as biological indicators) (Kenney et al., 2009). Biological indicators are based on the premise that biotic communities respond to changes in habitat and water quality resulting from anthropogenic disturbance (Karr, 1999;Machado et al., 2011).
In order to assess water quality on the basis of ecosystem health, it is best to study the response of the entire aquatic community to stress (Metcalfe, 1989). As this is obviously impractical, most studies have focused on a particular sector of the ecosystem, such as periphyton, plankton, macrobenthos or fish. While fish and algal assemblages have particular advantages in bioassessments (Barbour et al., 1999), macroinvertebrates provide a more localized assessment of their response to stream conditions due to the simple equipment used to collect them, and their lower mobility than fish (Kenney et al., 2009).
Macroinvertebrates, an important food source for fish, are valuable indicators of environmental degradation, as well as influence on nutrient cycling, primary productivity and decomposition (Wallace and Webster, 1996). These organisms inhabit the bottom substrate (sediment, debris, logs, macrophytes, filamentous algae, etc.) of freshwater habitat in at least one phase of their life cycle (Loyola, 1994). Some of them survive in extreme environments, and are able to live in a total absence of oxygen. Substrates directly influence their existence, and may thus be used as bioindicators (Melo and Froelich, 2001).
Among the groups of macroinvertebrates, the chironomid group is widely found in an environment impacted by sewage (Fagundes and Shimizu, 1997). This group belongs to the order Diptera, which can be found in all parts of the world, because they are considered extremely resistant; they are able to live in a complete absence of oxygen for several hours, and are considered scavengers, eating only organic matter (Pinder, 1986;Callisto and Esteves, 1995). Chironomid larvae are opportunistically omnivorous, ingesting a wide variety of food items (Cummins and Klug, 1979). In general, these larvae ingest five kinds of food: algae, detritus and associated microorganisms, macrophytes, wood debris, and invertebrates (Berg, 1995).
Considering the environmental gradient from the springs to the mouth of Barbado Stream, our objective was to study Chironomus larvae distribution as indicators of water quality in a neotropical urban stream.

Study area
The basin of Barbado Stream is located in an urban area in the central-eastern portion of Cuiabá, Mato Grosso State (Brazil), and it flows to Cuiabá River (Figure 1).
The land cover is different throughout the course of the stream. In spring and upper course, there is Cerrado vegetation; in mouth, medium and lower courses, there is virtually no vegetation, because it was removed by human occupancy (Kondo et al., 2010). Points 1 and 2 were located in the springs (preserved areas located at Parque Estadual Massairo Okamura) and points 3 and 4 were located on the lower course (disturbed areas) (Colet and Soares, 2013;Kreischer et al., 2012;Ventura, 2011). According to the Köppen classification, the regional climate is Aw, which represents a hot and wet climate with rainfall in the summer and drought in the winter (Machado et al., 2014). The annual average air temperature ranges from 24 to 26°C, with two distinct seasons, dry (autumn-winter) and wet (spring-summer); annual rainfall ranges from 1250 to 1500 mm (Maitelli, 1994).

Data collection
Aquatic macroinvertebrates were sampled in 04 sites (Table 1) by Surber Sampler (mesh 200 mm and sample area of 30 cm x 30 cm). The collected material was upturned, washed and placed into plastic bags containing 70% alcohol. The material was sorted and identified to the lowest possible taxonomic level. Samples were collected in the even months from 2010 to 2012, totaling 15 samples. We measured width and depth using graduated tape along the stream channel. The substrate composition in each site was determined from surveys with a stick in equidistant points along the stream channel, as information on the channel depth was gathered. The overall substrate composition of each stream was characterized by the frequency of occurrence (%) of each type of substrate per site. The substrate type was classified into the following categories: root, trunk, leaves, sand, clay, silt, rock, slime, and plastic. Rainfall data were obtained from Instituto Nacional de Meteorologia (INMET).

RESULTS AND DISCUSSION
The annual rainfall was 1596.50 mm in 2010, 1673.00 mm in 2011, and 1599.87 mm in 2012. The driest months were from May to September, and the wettest months were from January to March (Figure 2). The distribution of monthly rainfall from 2010 to 2012 showed a pattern similar to the normal climate of 1961-1990 for Cuiabá, with a wet season from October to March and dry season from April to September. The annual mean of accumulated precipitation (1623.12 mm) was higher than the annual average ranging from 1250 to 1500 mm (Maitelli, 1994). Points 1P (pool), 1R (rifle), 2P (pool) and 2R (rifle) were mainly composed of rocks, leaves, and roots ( Figure 3). On the other hand, points 3P and 4R were composed of sand and stone. However, plastic and slime were found only in points 3P and 4R, indicating environmental degradation. Chironomus (Chironomidae: Diptera) was only found at Barbado Stream. None was found at points 1P, 1R, 2P and 2R (Figure 4), which may indicate that this area is preserved. We found 40.3 and 94.4 individuals/m 2 at points 3P and 4R (low course), respectively. No Chironomus was found at points 3P and 4R in October and December of 2011 and 2012. May-10 Oct-10 Mar-11 Aug-11 Jan-12 Jun-12 Nov-12 Accumulated precipitation (mm) Oliveira et al. (2010) confirmed Chironomus as indicators of organic pollution. They found Chironomus mean density varying from 23999.69 to 30253.96 individuals/m 2 in polluted sites while 24.13 individuals/m 2 in the best water quality site. According to Sibley et al (1997), the numerical abundance of this genus is greatly influenced by food availability independently of the size of the substrate particles. The genus Chironomus is tolerant to organic and industrial pollutants, which means its occurrence and dominance are an effective biological indicator of stream pollution . The aquatic macroinvertebrates include representatives of crustaceans, gastropods, bivalves, oligochaetes and many insect orders (Allan, 1995;Merritt et al., 2008;Thorp and Covich, 2001). However, insects are often the dominant group of freshwater benthic macroinvertebrates in both absolute numbers and species diversity, which is not surprising given that the juvenile stages of many terrestrial insects are typically aquatic (Merritt et al., 2008). The benthic macroinvertebrate community of freshwater organisms is more than 0.5 mm in size, and is therefore visible to the naked eye (Pérez, 1996).
Chironomus Meigen (1803) is a genus of non-biting midges in the subfamily Chironominae (Chironomidae: Diptera). According to Simpson and Bode (1980), this genus is ecologically versatile, with various species living in standing or flowing waters as well as in polluted or clean waters. Chironomus is commonly associated with the presence of decomposing organic matter and aquatic macrophytes (Sanseverino and Nessimian, 2001). Resende and Takeda (2007) and Fusari (2006) recorded the genus in areas strongly impacted by anthropogenic actions. The increase in the density of larvae of the Chironomus in environments with eutrophic features has been registered in several types of ecosystems (Frank, 1963;Learner and Edwards, 1966;Devái, 1990;Tate and Heiny, 1995;Botts, 1997).
They contribute to many important ecological functions, such as decomposition and nutrient cycling, and also serve an important role in aquatic food webs as both consumers and prey (Covich et al., 1999;Moore, 2006;Vanni, 2002;Wallace and Webster, 1996). Thus, they play an important role in nutrient dynamics, transforming organic matter into energy (Callisto et al., 2001). Dead organic matter, i.e., the proportion of substrates, is the main pathway in most carbon ecosystems (Wallace et al., 1997).
Habitat quality is one of the most important factors in the success of colonization and the establishment of biological communities in lentic and lotic environments (Marques et al., 1999). Rolling sediment and fragmentation of litter from riparian vegetation are examples of processes under the responsibility of the benthic community, resulting in the release of nutrients to the water and aeration of the sediments (Cummins et al, 1989;Devái, 1990). The health and the quality of a waterbody depend on these processes (Marques et al., 1999).
Substrates as physically complex as leaves, wood, weeds and mosses generally have greater biodiversity than simpler substrates such as sand and rock bed (Vinson and Hawkins, 1998). Compared with other substrates, the accumulations of litter in streams of forested areas appear to be preferentially occupied by chironomid larvae (Sanseverino and Nessimian, 2001). Accumulations of litter release energy, matter and nutrients into the water. The benthic macroinvertebrate community can have its structure strongly influenced by substrate composition and micro habitats within riverbeds. In this perspective, each type of substrate supports a particular community of macroinvertebrates, which are not randomly distributed (Melo and Froelich, 2001).
Chironomid larvae are usually found where there is organic matter, an almost total absence of oxygen, and in many lentic and lotic environments. Climatic factors also influence the amount of Chironomidae larval found. Tropical regions are considered favorable for the growth of the larvae, which usually takes around 15 days, ranging from a temperature from 0° to 32°C (Callisto et al., 2001). With the enabling factors it can be predicted that a high number of generations occur each year. Kreischer et al. (2012) observed that the values of dissolved oxygen and conductivity indicate dumping of domestic sewage into points located on the lower course of Barbado Stream. Colet and Soares (2013) showed low environmental quality at the top, middle and lower courses and at the mouth of Barbado Stream, while the springs showed better quality. The monitoring of water quality of the sub-basins of the Cuiabá River revealed increased turbidity, a decrease in dissolved oxygen and a high amount of total coliforms and Escherichia coli in Barbado downstream, due to the implementation of sanitary sewers (Mato Grosso, 2006). Oliveira and Silva (2013) showed the presence of iron and lead in the water of Barbado Stream the sources of which include improper disposal of untreated sewage, solid waste and urban runoff, which makes the water unfit for public supply without prior treatment, and which may compromise the aquatic flora and fauna of the stream basin.
The CONAMA resolution 357/2005 (CONAMA, 2005) establishes that uncategorized rivers must comply with the limits for water bodies of type 2 waters which are those for: (a) supply for human consumption, after conventional treatment; (b) protection of aquatic communities; (c) primary contact recreation such as swimming, water skiing and scuba diving; (d) Irrigation of vegetables, fruit trees and parks, gardens, sports fields and leisure, with which the public may come into direct contact; and (e) aquaculture and fishing activity. Therefore, most of the Barbado Stream (high, medium and low courses and mouth) does not fit the water type 2 established by the CONAMA resolution.
The quality of surface waters is a very sensitive issue because anthropogenic actions degrade surface waters and impair their use for drinking, industrial, agricultural, recreation or other purposes (Carpenter et al., 1998;Jarvie and Neal, 1998). The concern that fresh water will be a scarce resource in the future (Pesce and Wunderlin, 2000) has forced countries into the evaluation of river water qualities in recent years (Kannel et al., 2007). A comprehensive river water quality monitoring program is becoming a necessity in order to safeguard public health and to protect the valuable fresh water resources (Kannel et al., 2007).
In this context, macroinvertebrate community composition changes along a gradient of stream habitat and water quality (Resh et al., 1995), and stream health can be assessed in relation to reference conditions (Barbour and Gerritsen, 2006). Thus, the use of benthic macroinvertebrate indicators greatly enhances states' ability to identify and subsequently improve impaired water (Kenney et al., 2009).

CONCLUSION
The Chironomus larvae was the only recorded group of macroinvertebrates at Barbado Stream during the study period, probably due to the presence of organic matter from sewage in its downstream course, indicating eutrophication. Chironomus larvae can be used as an indicator of water quality, because they mostly appear in sites where there is sewage input. The Barbado Stream needs restoration projects which include an awareness on the part of the residents of their neighborhood's environmental importance, and investment in the sanitation sector to prioritize the collection and treatment of wastewater and solid waste collection, avoiding sewage discharge.

ACKNOWLEDGEMENTS
We are grateful to Instituto Federal de Mato Grosso (IFMT) for financial and logistical support.