Impact assessment of an effluent discharge on benthic macroinvertebrates in a subtropical river using indexes

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Industrial effluent discharge comprises one of the most significant pollution sources in fresh waters because the release of chemicals in rivers is often done without previous treatment (Rajaram & Das, 2008). In between the discharge and the deposition into river sediments, many compounds can bind with other particles and undergo chemical changes that may, in turn, enhance or decrease their toxicity. Numerous chemicals are also incorporated into the aquatic food webs via benthic organisms (Araujo, Zagatto, & Bertoletti, 2006).

Benthic macroinvertebrates are organisms that offer useful information for water quality assessments, providing data on past and present environmental conditions (Pratt & Coler, 1976). This is possible because impacts can change invertebrate community structure and composition, and also modify their vital functions. The evaluation of the environmental quality of an ecosystem requires information on abiotic and biotic attributes related to ecosystem functioning (Silva, Galvíncio, & Almeida, 2010).

Aquatic insect larvae are the most abundant and diverse benthic macroinvertebrates in running waters (Hynes, 1970; Lake, 1990). Their distribution is related to habitat physical, chemical and structural characteristics, as well as to food resource availability and to the habits of each species (Resh & Rosenberg, 1984; Boltovskoy, Tell, & Dadon, 1995; Merrit & Cummins, 1996). Aquatic insects are used to detect environmental disturbance for presenting low mobility and high abundance, besides being relatively easy to sample and identify, features that make them good candidates to compose biotic indices for water quality assessments (Roque & Trivinho-Strixino, 2000; Queiroz, Trivinho-Strixino, & Nascimento, 2000).

Currently, aquatic environments are undergoing continuous change due to natural disasters, abusive degradation, and overexploitation by human activities. As a result, researchers and organizations have been pursuing strategies for their recovery (Lopes & Pacagnan, 2014). Thus, this study and identification of bentonic macroinvertebrates in the Coutinho River will make possible to ascertain the environmental quality of this environment and to identify possible environmental problems caused by industrial effluent discharges, since the river may be used in the future district (Secretaria Estadual de Meio Ambiente [Sema], 2008).

The application of habitat assessment protocols evidenced that the three Coutinho River reaches (P1 = 45 points; P2 = 50 points; P3 = 46 points) were altered by anthropogenic actions that led to habitat disruption along the river, such as deforestation by agricultural enterprises. Although the three reaches were classified accordingly, the P2 reach received the highest score (50 points) and evidenced local degradation. According to Dudgeon (1996), the suppression of the riparian forest decreases the reproductive success of aquatic and semi-aquatic species that need appropriate sites for mating and laying eggs.

The water quality results (Table 1) revealed that the dissolved oxygen, pH, and turbidity values were the lowest at the P2 reach, which is located near the effluent discharge site of the industrial area of Guarapuava. The lowest phosphate concentration (0.1 mg L^-1) was found in P3. Temperature varied little among sampling points, and turbidity was the highest at P1 (Table 1).

All pH values registered were below six, evidencing an acidified environment. According to Siqueira et al. (2012), pH can influence species distribution and biological interactions by hindering the performance of the physiology of aquatic organisms.

The National Council of Environment (Conselho Nacional do Meio Ambiente [Conama], 2005) establishes minimum Dissolved Oxygen (DO) values of 5 mg L^-1 for Class II water bodies. However, only the P1 reach was within this limit (Table 1), being the P2 (1.85 mg L^-1) and the P3 (3.35 mg L^-1) below minimum quality. Sperling (2005) states that DO concentration can indicate pollution effects of organic wastes in freshwaters: when DO values range between 4 and 5 mg L^-1, sensitive fish species die out; when DO is 2 mg L^-1, nearly all fish species die out, and when DO drops to 0 mg L^-1, anaerobic conditions take place.

Phosphate values of P2 and P3 were within the range pre-established by Conama (2005) (i.e, ≤ 0.1 mg L^-1). Values of P1 were above this limit (0.16 mg L^-1), likely due to agricultural runoff from adjacent areas, which may include phosphate and organophosphate fertilizers.

Sediment evaluation revealed that the P2 presented, on average, the highest percentage of sand (85.1%), followed by P3 (83.7%) and P1 (81.9%). Finer sediment as clay and silt were less representative, being the highest average value registered at P1 (18.1%) (Figure 2). The sediment granulometry can be indicative of anthropic actions that cause siltation and change the physical nature of sediments in lotic ecosystems, consequently influencing the benthic fauna (Bryce, Lomnicky, & Kaufmann, 2010).

Fifteen taxa were identified (Table 2), being Oligochaeta and Chironomidae the most abundant. These two taxa are considered resistant and tolerant to environmental impacts, likely because they present morphological adaptions and generalist habits that allow their survival in oxygen-poor environments (Merrit & Cummins, 1996). Oligochaetes represented 100% of the benthic community assemblage of P3 (September/2013 and January/2014). The presence of Oligochaeta indicates the existence of organic matter in the environment and poor water quality (Bruno et al., 2012; Kamada, De-Lucca, & De-Lucca, 2012).

Table 1.
Average values (± standard deviation) of physical and chemical parameters registered in the three sampling points of the Coutinho River.

Figure 2
Average sediment granulometry in the three reaches of the Coutinho River during the study period.

Table 2
Relative abundance values (%) of benthic macroinvertebrates sampled with a Petersen dredge in three points (P1, P2, and P3) of the Coutinho River during the study period.

Chironomidae are generally found in sediments with decaying organic matter (Amorim & Castillo, 2009) and therefore have the characteristic of being tolerant to variations in the aquatic environment. The Chironomus midge represented 74% of the macroinvertebrate fauna of P1 in January/2014, 76% of the P2 fauna in January/2014, and 71% in March/2014. Chironomus is related to water degradation, as it is abundant in eutrophic places, besides supporting low values of dissolved oxygen in the environment (Trivinho-Strixino & Strixino, 1995; Ceretti & Nocentini, 1996; Marques, Ferreira, & Barbosa, 1999).

The P1 reach presented the highest taxonomic richness (10 taxa in September/2013) and the orders Ephemeroptera (3%) and Trichoptera (3%) were exclusive to P1. According to Rosenberg and Resh (1993), insects of these orders are ecologically important for environmental assessments in lotic ecosystems for their sensitivity to disturbances and occurrence in good-quality waters. In this context, their occurrence in P1 was likely associated with the higher levels of dissolved oxygen observed therein.

Hirudinea and Bivalvia also presented high relative abundance. The highest relative abundance of bivalve mollusks (80%) was found in P1 in March/2014, and that of Hirudinea (60%) was found in P2 in September and November/2013. The presence of these organisms is favored in environments with high levels of organic pollution (Barnes, 1984; Amorim & Castillo, 2009; Rodrigues, 2013; Bruno et al., 2012).

The high density of the few taxa observed in the Coutinho River (Figure 3), along with low overall richness, indicate degradation. In P1, the Chironomus presented the highest density observed, with 161 ind. m^-2 (January/2013), followed by Oligochaeta, with 89 ind. m^-2 (November/2013) and Bivalvia, with 50 ind. m^-2(September/2013). In P2, Chironomus (128 ind. m^-2 in March/2014) and Hirudinea (39 ind. m^-2 in March/2014) were the most abundant, whereas in P3 the highest density registered was that of Oligochaeta, with 27 ind. m^-2 in March/2014.

The P1 reach presented the highest diversity (H' = 0.78), equitability (J = 0.70) and richness (S = 13 taxa), while the P3 presented the lowest values for these metrics. The equitability values differed in all sampling points, indicating taxa dominance. Moreover, these metrics tended to decrease along the river continuum, from P1 to P3 (Table 3), as observed for water physical and chemical parameters as well, indicating increased environmental degradation from P2 onwards.

Based on the FBI scores, the three sites of the Coutinho River were classified as presenting poor water quality and significant organic pollution. The BMWP and BMWP-ASP indices showed differences among reaches, being the P1 classified as polluted (BMWP) or with moderate pollution (BMWP-ASPT), and the P2 and P3 classified as strongly polluted (BMWP), or with severe pollution (BMWP-ASPT). Ruaro, Agustini, and Orssatto (2010) obtained similar values for the Clarito River, in the state of Paraná, which also receives pollution from industrial sources along its course. In São Paulo State, the Pequerê River of Cubatão city also underwent degradation and riparian forest removal, being classified as presenting very poor water quality (Amorim & Castillo, 2009). On the other hand, the preserved riparian cover of the Correntoso River (Aquidauana, State Mato Grosso do Sul) ensured all sampling points presented good water quality results of biotic indices (FBI, BMWP, and BMWP-ASPT) (Silva, Favero, Sabino, & Garnés, 2011).

The CCA between the physicochemical parameters of the water and the macroinvertebrates present in the Coutinho River (Figure 4, axis 1 - 79.8 and axis 2 - 20.2%) at three sampling points showed a possible association of Oligochaeta with increased turbidity levels. This group usually lives in environments with high electrical conductivity and turbidity (Alves, Marchese, & Escarpinati, 2006; Gorni & Alves, 2015).

Table 3.
Shannon-Wiener diversity, Pielou equitability, and species richness registered in three points of the Coutinho River (P1, P2 and P3).

Figure 3.
Density values (ind. m-2) of benthic macroinvertebrates sampled at three points of the Coutinho River in (a) September/2013, (b) November/2013, (c) January/2014, and (d) March/2014.

Figure 4.
Sorting axes produced by CCA between the physicochemical parameters present in water (temperature, pH, dissolved oxygen, phosphates and turbidity) and the taxa sampled in the Coutinho River at three sampling points (P1, P2 and P3).

The use of benthic macroinvertebrates for the Coutinho River environmental quality assessment was efficient and enabled the diagnostic of local limnological variations. Our results indicate that the sampled reaches are impacted by both agricultural runoff from adjacent areas and industrial effluent discharges. Evidence includes the higher taxonomic richness found upstream from the industrial area together with the high density of tolerant taxa (as Oligochaeta and Chironomus) downstream from it.

All biotic indices (FBI, BMWP, and BMWP-ASPT) point to moderate-towards-severe pollution along the Coutinho River. Thus, our results should serve as an environmental degradation warning and a call for continuous monitoring of the Coutinho River water quality and impact mitigation actions.