The Piratinim River is located in the northwest of Rio Grande do Sul, and represents an important effluent from the Uruguay River, with streams located far from urbanized areas, in conditions similar to those of environmental integrity, but under the influence of agricultural activities. In this study, we aim at investigating the structure of the aquatic macroinvertebrate community in streams of the Piratinim river basin by observing both spatial and local scales. The sampling was carried out in six streams distributed in three regions (upper, middle and lower) of the basin, thus exploring its upstream, intermediate and downstream stretches, during spring and autumn. Macroinvertebrates were collected using aquatic dipnets and were identified at the family level; trophic-functional categories were established according to the classification adapted to the state of Paraná. The spatial and temporal variations of the groups and of the biotic attributes (density, richness, diversity and equitability) were subjected to Kruskal-Wallis non-parametric tests and a posteriori Dunn’s tests. The abiotic variables were obtained to verify possible influence on the composition of the macroinvertebrate community, evaluated through a Canonical Correspondence Analysis. We sampled 11,564 macroinvertebrate individuals from 72 taxa, and found a predominance of the collector-filter trophic group. Abundance and richness were different between streams; the highest densities were found in the streams located in the upper region of the watershed (Chuní and Itú). The highest taxon richness was found in the lower region of the watershed (Guaracapa stream), and the lowest richness was found in the two streams for the intermediate region (Santana and Ximbocu). Diversity and equitability did not vary; temporal variations were not found. Canonical correspondence analysis explained 31.7% of the data variability. The main environmental variables that influenced macroinvertebrates distribution were temperature, electrical conductivity, dissolved oxygen, altitude and extension of the riparian forest. Seasonality and the longitudinal gradient along the basin represented determining factors for the structure and distribution of the macroinvertebrate community in the tributary streams of the Piratinim River.

environmental descriptors, lotic environments, Piratinim watershed, river ecology, seasonal variations

One of the main goals of community ecology is to identify processes that influence the patterns of community structure along spatial and temporal scales. Patterns observed in communities at a particular scale are often the consequence of complex interactions among several processes occurring at multiplex scales (Dray et al. 2012). Many authors encourage broadening the spatial and temporal scales of studies to recognize the hierarchical organization of the ecological systems and effects of environmental heterogeneity at each scale (Leibold et al. 2004; Vellend 2010).

Rivers and streams are complex ecosystems with patchy environmental heterogeneity at multiple spatial and temporal scales (Ward et al. 2002; Winemiller et al. 2010). Several authors have proposed theoretical frameworks to integrate longitudinal (upstream-downstream), temporal dimensions of river ecosystems, incorporating geomorphological, hydrological, and ecological characteristics (Thorp et al. 2006; Humphries and McCann 2014). Change in community structure along the longitudinal dimension has been attributed to environmental gradients generated as stream order increases along the altitudinal gradient (Vannote et al. 1980) or by geomorphic patches at the landscape scale (Montgomery 1999). Variability in community structure along the temporal dimension generally is related to the seasonal variation in temperature and interannual pattern of river flow regimes (Winemiller et al. 2010).

Benthic macroinvertebrates are an important group of organisms which are found in sediment present beneath the water column and that act as key components in any aquatic ecosystem. Studying them is important because the macrobenthic organisms are known as indicators of anthropogenic stress due to their sedentary habitat. Furthermore, they maintain various levels of interaction between the community and environment (Bieger et al. 2010; Silveira-Manzotti et al. 2016; Brraich and Kaur 2017). The structure of the aquatic macroinvertebrate community provides precise and local information on recent events. They are the first causalities of any environmental change (Salvarrey et al. 2014; Siegloch et al. 2016). However, few investigators have analyzed simultaneously the processes influencing aquatic macroinvertebrates community structure at multiple scales in subtropical rivers and streams (Barbero et al. 2013).

Some studies indicate that aquatic macroinvertebrate communities show gradual changes in species richness and composition along the longitudinal dimension as not only the stream order and composition but also habitat complexity increase, giving rise to nested patterns composition or variations in macroinvertebrate functional groups (Pereira and De Luca 2003; Milesi et al. 2009; Baptista et al. 2014). In rivers with large longitudinal variation in climate, geology, substrate and relief, community assemblage generally shows strong discontinuities with high species turnover between adjacent sites (Bagatini et al. 2012; Barbero et al. 2013; Mc Conigley et al. 2017). In this sense we hypothesized that the structure of the aquatic macroinvertebrate community may vary both at the basin level and within the river. Seasonal variations of aquatic macroinvertebrate assemblage have been observed in both rivers and streams in works conducted at temperate, tropical and subtropical latitudes (Marshall et al. 2006; Johnson et al. 2012; Brraich and Kaur 2017; Martins et al. 2018). In this regard, we also expect differences in the structure of the community between autumn and spring seasons.

The present study was conducted in the Piratinim River basin, an important river in southern Brazil and the Pampa biome. The pampa biome occupies a large portion of South American territory, including part of Argentina, Uruguay, and the state of Rio Grande do Sul, Brazil (Capítulo et al. 2001). Typical native fields of this biome have been losing ground to cultivated crops, further altering land uses (Overbeck et al. 2007; De Oliveira et al. 2017), which reflect changes in watercourses. Specifically, the Piratinim River sub-basin is part of the Middle Uruguay River region and is little known in terms of scientific literature, especially with regard to the streams that make up its drainage network. In this scenario, the Piratinim River basin offers the possibility of understanding the structure of the aquatic macroinvertebrate community in environments far from urbanized areas, in conditions similar to those of environmental integrity, but under the influence of agricultural activities.

The samplings considered some of the main tributaries of Piratinim in its upper, middle and lower sections in two seasons of the year. The objective of our study was to apply a multiscale approach to identify the main factors leading the spatial and temporal changes in the structure of aquatic macroinvertebrates of Piratinim river basin, investigating: 1) How the structure and distribution of the macroinvertebrate community varies between basin regions, between streams and in different stretches of streams and 2) The relative importance of climatic seasonality.

Studies in neotropical aquatic environments are mostly focused on areas affected by urbanism, mainly because these studies seek to investigate the themes of environmental degradation and public health through disease vectors, or because those environments are often located near the research institution (Cruz et al. 2016). In contrast, few studies have been carried out in more preserved environmental conditions, or environments that are influenced by other human activities, such as agriculture and livestock (Petesse and Petrere 2012).

In this study we covered a drainage basin with considerable variation in geomorphological conditions and landscape structure (Cordeiro and Hasenack 2009; Sema 2009). We find that there is a high diversity of macroinvertebrates, with significant variation in the community structure, that is, species’ richness, abundance and composition. In addition, the results indicate that environmental (such as altitude, riparian forest and limnological characteristics) and seasonal factors (seasons) mostly explained the variation in organism distribution. The data obtained support the prediction that specific environmental conditions select different assemblies, but indicate that temporal aspects also play an important role.

We found a highly diverse macroinvertebrate fauna in the tributaries of the Piratinim River. Other studies carried out in southern Brazil indicated a lower taxa representativeness (Bueno et al. 2003; Pereira and De Luca 2003; Ayres-Peres et al. 2006; Strieder et al. 2006a; Buckup et al. 2007; Milesi et al. 2009 ; Biasi et al. 2010; Sobczack et al. 2013; Baptista et al. 2014; Salvarrey et al. 2014). The study conducted the closest to our study area was carried out in the Ibicuí River basin, west of Rio Grande do Sul, also within the Pampa biome (Baptista et al. 2014). These authors sampled more than 9,000 individuals, distributed in 26 aquatic insect families, in the evaluated streams. The richness found in the Piratinim River basin was higher, comprising 57 insect families.

Regarding the most abundant taxa, Hydropsychidae and Philopotamidae were the most representative (32.54% of the total sampled individuals). These taxa are grouped in the order Trichoptera, which is considered one of the most diverse insect Orders in lotic environments, with families widely distributed in temperate and tropical latitudes (Wallace and Merrit 1980; Dudgeon 1997). In this study, seven (7) families were sampled; Breda et al. (2018) found ten (10) families in streams in the northern region of Rio Grande do Sul, while Spies et al. (2006) recorded nine (09) families for the central region. Simuliidae (Diptera) was also representative for the Piratinim watershed. In general, this family is representative of well-oxygenated waters (Callisto et al. 2001), but some species have a wide tolerance range to environmental pollutants (Strieder et al. 2006b).

In the sampled tributaries, some insect families, such as Baetidae (Ephemeroptera) and Chironomidae (Diptera), were present in all collection locations. A similar result was obtained by Biasi et al. (2010), when studying streams in the northern region of Rio Grande do Sul for eight years, having observed the presence of the families Baetidae, Chironomidae and Hydropsychidae in all sampled locations.

Usually, Ephemeroptera are used as bioindicators of good water quality; however, Baetidae has a wide range of pollution tolerance, which can occur in areas under intermediate impact. Likewise, Chironomidae is an indicator of disrupted environments; however, some genera have higher tolerance to organic contamination than others (Hilsenhoff 1988). Callisto et al. (2001) reiterate that the use of macroinvertebrates as indicators of environmental quality should not be restricted to its presence in a given watercourse, but mainly to their abundance within the community. The biotic attributes (diversity, density, richness and equitability) structuring the aquatic macroinvertebrate community did not show temporal variation between the sampled seasons, similarly to other studies carried out in the country (Melo and Froelich 2001; Costa et al. 2006; Buckup et al 2007; Crisci-Bispo et al. 2007; König et al. 2008; Bieger et al. 2010). In general, the values found for these attributes were high; Chagas et al. (2017) reported similar results associated with streams of good water quality.

Density varied significantly among the investigated tributaries, and the streams in the upper region of the basin had the highest average density of individuals per m². The sampling points from this region, even though containing the most considerable portion of riparian vegetation among the investigated streams, show a physiognomy of the surroundings conditioned to disturbances in agricultural and livestock practices close to the banks, with direct access by cattle. In part, such results may be associated with the level of disturbance around these water courses, which favor the predominance of tolerant or resistant taxa (Strieder et al. 2006b; Milesi et al. 2009).

For the richness attribute, the highest average was recorded in the Guaracapa stream, located in the lower region of the basin. This stream has a more closed riparian forest, which is difficult to access, when compared to other tributaries analyzed in this study. In streams in the middle region of the basin, the surrounding vegetation is present, but it is more conditioned to disturbances, and in these places the lowest average values of richness were found. Therefore, in the evaluated streams, the extent of riparian vegetation, solely, does not represent an indicator of environmental integrity. It is necessary to assess how the surrounding environment interacts with these areas, enabling sources of non-specific impacts on water courses.

Concerning the trophic-functional groups, besides the fact that predators showed a high taxonomic richness, the abundance was relatively low when compared with gathering-collectors and filtering-collectors. These two groups were predominant in the total sample, representing more than 70% of all sampled individuals. This is mainly due to the fact that the most abundant families (Hydropsychidae, Philopotamidae and Simuliidae) are mostly collector-filters. However, it is important to highlight that some families or groups may have a generalist habit, adapting to food resources availability (Silveira-Manzotti et al. 2016). De Castro et al. (2016) found that changes in land use, such as the cultivation of sugar cane and livestock, can lead aquatic macroinvertebrate assemblages to exhibit more generalist feeding habits.

According to Cummins et al. (2005), the occurrence rates of the functional groups are indicative of the streams ecosystem attributes. In this case, a higher abundance of filtering-collectors and gathering-collectors indicate availability of particulate organic matter in suspension and deposited in the bottom. Callisto et al. (2001) observed a dominance of the collecting groups in headwater streams in the Midwest of Brazil, with characteristics of narrow beds, rocky bottoms and preserved riparian vegetation, with riparian vegetation being the most influential substrate in the richness. Such environmental descriptors are similar to those found in the streams analyzed in Piratinim, which in turn are all located in rural areas. Dovciak and Perry (2002) sampled hydrographic basins inserted in agro-regions of the USA, and found that filtering-collectors were dominant in areas of good environmental integrity. With a larger availability of food resources, this group is favored due to the morphological adaptations of many of its representatives, which require stable substrates to filter fine particulate organic matter. As the restrictions on microhabitats increased, the abundance of this group was reduced, while gathering-collectors became predominant.

The analysis showed that seasonality represented the most determining factor regulating the distribution of macroinvertebrate taxa (CCA1). During the spring, the electrical conductivity parameter showed the highest values, most clearly associated with streams in the upper region of the basin. Among the families of which the spatial distribution was associated with such condition, are the dipterans Chironomidae and Simuliidae. During autumn, we recorded the highest levels of dissolved oxygen, being more associated with the mouth of the basin. For this season, several families were correlated, such as the hemiptera Veliidae and Gerridae.

Electrical conductivity can be considered one of the factors that significantly influence fauna distribution (Braun et al. 2014; Salvarrey et al. 2014), however, this condition occurs only in extreme values or when associated with organic pollution (Bispo et al. 2006), as well as in areas of agricultural crops. The pH can also influence the values of electrical conductivity (Esteves 1988), but this parameter was not significant in our study. Factors such as local geology and climate also influence these parameters (Pedrosa and Rezende 1999).

On the axis corresponding to CCA2, segregation of the upper and lower regions of the basin were once again observed. The streams located in the upper basin, of higher altitude and extension of riparian forest, were correlated with the distribution of the families Hydrobiidae and Baetidae. However, the streams located in the lower basin, of lower altitude, exhibited a reduction in the average extension of the riparian forest, in relation to the other streams.

In fact, among the environmental variables evaluated, the width of the riparian forest, given its presence and preservation, was the one that demonstrated crucial influence in determining the distribution of the macroinvertebrate community of the streams analyzed. Moraes et al. (2014) verified the impact of the removal of riparian vegetation on streams in Rio Grande do Sul. These authors verified that the riparian zone should not be less than 15 m wide, on both banks, so that the composition of the macroinvertebrate community remains similar to that found in reference preservation sites.

Degradation and removal of riparian vegetation is considered one of the factors that may lead to an increase in water temperature of streams (Caisie 2006; Fantin-Cruz et al. 2010; Ferreira et al. 2014; Hepp et al. 2016), practices that are relatively common in agricultural areas (Delong and Brusven 1998; Hepp et al. 2010). In addition to reducing the entry of sediments from soil erosion, riparian forest is an efficient controller of water temperature and dissolved oxygen rates, contributing to the balance of benthic populations (Percebon et al. 2005; Siegloch et al. 2016).

However, despite the variations and the influence of riparian forest in taxa distribution, it is important to highlight that the average stream width in the upper basin was slightly higher than in the tributaries from the low region, considering that such variables may be proportional. According to Chaves et al. (2005), the size or width of a stream is among the best predictors of aquatic macroinvertebrate richness, yielding increases in diversity as the flows become larger. However, Lenat (1988) found that small streams do not show a considerable reduction in taxa richness under adequate rainfall conditions.

When high values of richness and diversity are verified in a study, it is assumed that the composition and structure of a community are similar to those found in areas of environmental integrity. The results found for the Piratinim River streams revealed a richness of aquatic macroinvertebrates similar or higher to those found in other studies conducted in southern Brazil, evidently influenced by seasonal and spatial aspects. However, these studies also demonstrate the influence of non-specific sources of impact affecting these organisms. The Piratinim River drainage basin is located in an agricultural region, where the reduction of natural vegetation coverage, especially native grasslands, contributes to landscape homogenization, also affecting aquatic biodiversity.

In this sense, fauna surveys and the investigation of spatial distribution patterns and community structure of aquatic macroinvertebrates contribute to fill the current knowledge gap concerning these taxa in the Pampa biome, and to expand the patterns already described for the south of Brazil. Therefore, awareness-raising and management activities are necessary to promote the maintenance of environmental quality along the evaluated water courses.

The authors are thankful to the Federal University of Fronteira Sul – Cerro Largo campus, for logistical support during the performance of this study.