Impact evaluation of the Vila Viçosa small hydroelectric power plant (Portugal) on the water quality and on the dynamics of the benthic macroinvertebrate communities of the Ardena river

241 Impact evaluation of the Vila Viçosa small hydroelectric power plant (Portugal) on the water quality and on the dynamics of the benthic macroinvertebrate communities of the Ardena river T. Jesus1, N. Formigo2, P. Santos2& G. R. Tavares3 1Universidade Fernando Pessoa; Rua da Constituição, 56 5º, 4200-191 Porto; Teleph: + 351 22 5503929, Email: [email protected] ou [email protected] 2Departamento de Zoologia-Antropologia, Faculdade de Ciências da Universidade do Porto – Praça Gomes Teixeira s/n, 4099-002 Porto, Teleph: + 351 22 3101499 Fax: 351 22 3101511 3Escola Secundária de Arouca. Comenda – Rossas, 4540-486 arouca. E-mail: [email protected] ABSTRACT Over a two-year period a study of the Vila Viçosa small hydroelectric development, located on the Ardena river (hydrographical basin of the Douro river, Portugal), was conducted, with the objective of evaluating the impact of its operation on the water quality and on the dynamics of the benthic macroinvertebrate community. A morphological, hydrologic and physico-chemical characterization of the system was done, an analysis of the benthic macroinvertebrate communities looking into their structure and dynamics, as well as ecological and physiological characteristics. With the physical and chemical data, a study of its spatial and temporal variation was performed. With the data related to the macroinvertebrate communities, an analysis of the spatial variation of some indexes and metrics and of the (non) similarity between the samples collected were done, bearing in mind the composition of the communities, through different techniques of multivariate analysis. After the analysis of the data, it is possible to say that the Ardena River is a lothic system, with good water quality, in which the working of the small hydroelectric power plant causes alterations on the macroinvertebrate community, due to the alteration of the “natural” hydrologic regime. The impact on the macroinvertebrate communities is lower upstream of the plant, where the flow is minimal throughout the whole year leading to a change in the structure of the community. Downstream of the plant, where the flow is extremely variable, there is an impoverishment of the communities, leading to the displacement of substrate and organisms. Keywords: environmental impact, small hydroelectric power plant, macroinvertebrates, water quality RESUMEN Durante dos años se ha estudiado la minicentral hidroeléctrica de Vila Viçosa en el río Ardena (cuenca del Duero, Portugal), con el objetivo de evaluar el impacto de su funcionamiento en la calidad del agua y en la dinámica de la comunidad de macroinvertebrados bentónicos, haciendo la caracterización del sistema desde el punto de vista morfológico, hidrológico y físico-químico y estudiando las comunidades de macroinvertebrados bentónicos, atendiendo a aspectos relacionados con su estructura, dinámica y características ecológicas y fisiológicas. Con los datos relativos a los parámetros físico-químicos se ha hecho el estudio de la variación espacio temporal; y con los datos relativos a la comunidad de macroinvertebrados, se ha hecho el estudio de la variación espacial de algunos índices y medidas y de la (dis)similaridad entre las muestras recogidas, teniendo en cuenta su composición, a través de diferentes técnicas de análisis multivariante. Después de analizados los datos, se puede decir que el río Ardena es un sistema lotico con agua de buena calidad y que el funcionamiento de la minicentral hidroeléctrica provoca modificaciones en la comunidad de macroinvertebrados bentónicos, resultado de las alteraciones de caudales. El impacto en las comunidades de macroinvertebrados es menor aguas arriba de las minicentrales, donde el caudal es mínimo todo el año provocando una modificación en la estructura de las comunidades. Aguas abajo de las minicentrales, donde el caudal es muy variable, hay un empobrecimiento de las comunidades, provocando el desplazamiento del substrato y de los organismos. Palabras clave: impacto ambiental, minicentrales hidroeléctricas, macroinvertebrados, calidad del agua Limnetica 23(3-4): 241-256 (2004) © Asociación Española de Limnología, Madrid. Spain. ISSN: 0213-8409 INTRODUCTION In the last years in Portugal, and since the publication of the law decree 189/88 which created incentives to the production of hydroelectric energy in small hydroelectric developments (installed power less then 10MVA) (APMH, 1997), there was a big increase in the building of this kind of enterprises, mainly in the water courses north of the Mondego river (Costa, 1997). Although in comparison with other forms of the electrical energy production (nuclear or thermal centrals and large barrages) this kind of exploitation may be considered as an environment-friendly one (Costa, 1997; Monteiro, 1996), what really happens is that the operation of the hydroelectric power plant causes some problems in the water courses. These problems are primarily related to the alteration of the “natural” hydrological regime and the alteration of the abiotic conditions of the systems, with the consequent changing of the structure and dynamics of the biotic communities. Therefore, it is becoming increasingly important to proceed to the characterization and quantification of these problems (Herrmann & Sahlén, 1999; Kefford & Lake, 1999). In this work, that is part the doctoral thesis “Small hydroelectric developments: impact on the benthic macro invertebrate communities dynamics and on the water quality variability (small hydroelectric power plant of Vila Viçosa and “Alva waterfall”)” (Jesús, 2001), a study of the Vila Viçosa small hydroelectric power plant was done. The location of this study is on the last 4 Km of the Ardena river (Fig. 1), an affluent of the right margin of the Paiva river (hydrographic basin of the Duero river). The objectives of this study were to: - Evaluate the impact of the operation of one small hydroelectric power plant on the physico-chemical and biological water quality; - Evaluate the influence of the flow variations produced by the operation of the small hydroelectric power plant in the composition and structure of the benthic macro invertebrate communities; - Verify the existence of a relation between the water physical-chemical characteristics and the benthic macro invertebrate communities’ structure. 242 Jesus et al. Figure 1. Sample points location on Ardena River. Localización de las estaciones de muestreo en el río Ardena. MATERIALS AND METHODS During this study, and over a two-year period (February 1998 to January 2000), some abiotic parameters were measured monthly (Table 1) and the benthic macroinvertebrate communities were sampled every three months, along the last 6 km of the Ardena river. The selection of the sampling points was made keeping in mind the localization of each element of the small hydroelectric power plant, and six sampling points were chosen as follows (Fig. 1): –3 reference points, situated upstream of the lagoon, where the torrent regime was as closest to “natural” as possible: point A, about 6 km from the mouth of the Ardena river; point B, situated above the first dam, upstream of the lagoon, and point C, immediately upstream of the lagoon; in conjunction with point B. This point allowed us to verify whether the many dams that exist throughout the river caused alterations in the composition and structure of the benthic macroinvertebrate community. –1 point between the dam and the hydroelectric plant: point D, where the river flow is generally low throughout the whole year. –2 points downstream of the hydroelectric plant discharge: point E and point F (F/X). The samples of the benthic macroinvertebrate community were collected with a surber Impact evaluation of a small hydroelectric power plant in Portugal 243 Table 1.Abiotic parameters analysed on the sampling points of the Ardena River, their methods, units and bibliography references. Parámetros abióticos analizados en las estaciones de muestreo del río Ardena, metodologías, unidades y referencias bibliográficas. Parameter Method Units Bibliographic references Width and Depth Metric m (JESUS, 2001) Water velocity Distance covered by time unit m/s (JESUS, 2001) Flow Calculated through the with, depth m3/s (PLATTS et al., 1983) and water velocity data Canopy Visual observation of area of the river % (JESUS, 2001) with shadow Substrate Visual observation of the proportion of each % (JESUS, 2001) substrate component: mud, silt, gravel, pebbles, stones and macrophits Air and Water temperature With a thermometer ºC APHA, 1992 Conductivity Electrometric, using an portable apparatus µS/cm APHA, 1992 l HI 933000 da HANNA instruments pH Electrometry, using na portable apparatus Sorensen scale APHA, 1992 pH Meter HI 9025 da HANNA instruments Alkalinity Colorimetric method mg CaCO3/l WELCH, 1948 Hardness With a titrimetric method using mg CaCO3/l APHA, 1992 a MERK-Aquamerk 8047 apparatus Dissolved Oxygen (O2) Winkler method mg O2/l APHA, 1992 Biochemical Dissolved oxygen determination by Winkler mg O2/l APHA, 1992 Oxygen Demand (BOD5) method, before and after 5 days of incubation (20ºC ± 1ºC without light) Ammonium (NH4+); Colorimetric reaction mg NH4+/l APHA, 1992 Nitrites (NO2-); Colorimetric reaction mg /l STRICKLAND Nitrates (NO3-); & PARSONS, 1972 Phosphates (PO43-) Total Suspended Solids Filtration with a membrane of 0.4 mm, mg /l APHA, 1992 (T.S.S.) drying a 105ºC and weight (0,09 m2area) (Campaioli et al., 1994) and as many samples as the represented mesohabitats were collected at every sampling point (Armitage et al., 1995; Armitage & Pardo, 1995). This way, 7 sub-samples were collected in each point, representing the several mesohabitats present: mud, sand, gross sand, gravel, rolled pebbles, blocks and macrophytes. In point F, two sets of sub-samples were collected, designated as F and X. Due to its great hydro-morphological variety there was a zone which was constantly flooded (F) and an area that was only covered by water during the discharge periods (X) of the small hydroelectric power plant, respectively. After being collected, the samples were conserved in formaldehyde and transported to the laboratory for a posterior washing, triage and identification of the individuals, whenever possible to the genus (Karr, 1999). With the data relating to physico-chemical parameters a Principal Components Analysis (PCA) was made to verify the existence of some pattern of space or time variation, followed by a discriminating analysis with the samples grouped by season of the year, in order to test the existence of a seasonal distribution of the samples. Both analysis were made with the help of the STATISTICA 6.0 software, after the standardization of the data (x=(x-x)/sdx), in order to reduce the variability between the variables imposed by the different units in which they were determined (Clarcke & Warwick, 1994). With the data related to the benthic macroinvertebrate communities, a study of the spatial variation of Shannon-Weaver diversity and the Pielou equitability indexes was done (Ludwig & Reynolds, 1988). Two biological water quality indexes were used (BMWP’- modified biological monitoring working party score system of Alba-Tercedor & Sánchez-Ortega, 1988 and IBB - Belgian Biotic Index of De Pauw & Vanhooren, 1983). Some metrics were also estimated (community: density, taxonomic group number, % Ephemeroptera, Plecoptera and Tichoptera (EPT), % of scrappers, % of fixed organisms and % of Hydropsychidae), as well as a sample expressed in taxonomical terms (Jesus, 2001). In order to determine how the operation of the small hydroelectric power plant altered the composition and structure of the benthic macroinvertebrate communities, a non-Multidimensional Scaling Analysis (n-MDS) was carried out. This analysis was aimed to put the samples in order, bearing in mind its taxonomic composition and the season of the year, using the Bray-Curtis similarity coefficient applied to the transformed data matrix (x=log (x+1)). A similarity analysis (ANOSIM) and an analysis of the specific composition (SIMPER) were applied to the groups formed in the n-MDS, in order to verify its validity and to determine the main taxa responsible for the differentiation of the groups. For this analyses the PRIMER 5.2.2 software was used (Field et al., 1982; Clarcke & 244 Jesus et al. Figure 2. Temporal discharge variation in each sample point. Variación temporal de los caudales en cada estación de muestreo. Green, 1988; Clarcke, 1993; Clarcke et al., 1993; Clarcke & Warwick, 1994). The evaluation of the habitat quality was made, in situ, through the calculation of two indexes: index of quality of the riparian vegetation (QBR) (Munné et al., 1998), and visual evaluation of the habitat (AVH) (EPA, 1999), as well as the determination in percentage of the composition of the substrate in each sampling point (Jesus, 2001). Characterization of the Ardena River and sampling points The Ardena river is a small water course of about 19 Km in length, which springs on the Montemuro mountain as a small brook named Noninha, which takes the name of Bustelo brook at about 3 km from the spring. Approximately 5 km from the spring there is a dam where most of it deviates into a watering channel – “the drain of the ox” – which ends on the Paiva River. It is from this place that the Ardena River is formed by the non-deviated water and by the junction of waters from superficial spilling and from small lateral water lines. Throughout its course, the Ardena river runs through a valley of a considerable slope and presents an alternation of rapids and dead waters, caused either by the natural features of the ground or by the several little dams built for the deviation of the waters to watering channels. The Vila Viçosa small hydroelectric plant is situated in the last 4 Km of the river since 1993 (Fig. 1) (Jesus, 2001). In the lagoon of the small hydroelectric plant, situated about 4,5 Km from the mouth of the Impact evaluation of a small hydroelectric power plant in Portugal 245 Table 2. Main characteristics of each sampling point of the Ardena River. Principales características de cada estación de muestreo del río Ardena. Characteristics Point Point Point Point Point Point A B C D E F/X Length (m) 5 5 5 7.5 5 5 Hydrologic regime “Natural”“Natural”“Natural”Artificial Artificial Artificial Mean discharge Drought season(1) 0.20 0.43 0.53 0.04 0.07 0.12 (m3/s) Rainy season(2) 1.01 1.65 1.77 0.16 3.20 2.60 With Minimal 7.20 12.60 5.50 4.10 6.20 3.50 (m) Maximal 9.50 12.60 9.10 6.30 13.20 9.60 Maximal depth Lower discharge 0.35 0.35 0.65 0.55 0.35 0.30 (m) Higher discharge 0.65 0.60 1.00 0.80 0.80 0.65 Canopy (%) 95 0 15. 50 30 80 AVH values 161 161 144 153 132 143 QBR Values 90 65 60 100 55 85 Quality class Good Accetable Accetable Natural Accetable Good quality quality quality stade quality quality AVH – index of visual habitat evaluation; QBR – index of riparian vegetation quality (1) On the sample points E and F/X this value was calculated with data obtained during the time when the small electric power plant was stopped Ardena river, the water is retained, and is afterwards deviated through a derivation channel that runs parallel to the river into the hydroelectric plant. There, after being turbinated, it returns to the original stream, about 1 Km from the mouth. This deviation and posterior return of the water is a source of great daily fluctuations on the hydrologic regime (Fig. 2 and Table 2). At the three reference points (A, B and C), as at the other sampling points, the substrate is composed by rocky materials of different dimensions, but without predominance of any type of materials. The aquatic vegetation is relatively scarce and limited to the riverbanks. These three points present a hydrologic regime that, according with the rainfall intensity, can be considered natural with seasonal fluctuations (Fig. 3). In the last three sampling points (D, E, F), there is a predominance of rougher materials on the river substrate that can be explained by the artificial character of the hydrologic regime. In point D, the flow is reduced almost all the time to avoid the transportation of medium size materials. In points E and F, there are periods with very strong discharges that cause the dragging of light materials. In point E the the density of the aquatic vegetation increases because there is a great percentage of the riverbed that remains dry for long periods a time and is colonized by vegetation from the margins (Fig. 3). Every sampling point presents riparian vegetation of acceptable quality and a habitat with favourable characteristics for the development of aquatic life. This may be because the river runs through a very confined valley where human activities are still reduced, predominating the practice of subsistence agriculture and Eucalyptus forestry exploitations (Table 2). RESULTS AND DISCUSSION Analysis of the physical-chemical parameters The analysis of the samples’distribution in the space defined by the two first factors of the principal components analysis (38,8% of explained variance) shows that the presence of the small hydroelectric plant doesn’t significantly influence the physical and chemical quality of the water. However, the existence of a distribution of the samples collected throughout the first factor (20,9% of the variance) was verified, indicating a seasonal character in the variation of these parameters. This, in turn, seems explained by the natural variations of the water and air temperatures, as well as by the water flow, which reverberates in factors such as pH, hardness, conductivity, alkalinity, and dissolved oxygen concentration (Figs. 4 and 5) (Gasith & Resh, 1999). The pH, conductivity, alkalinity and hardness, present higher values during the hottest months, which can be related to the fact that in these months the flow is at its lowest, provoking an accumulation of sediments and an increase in the primary productivity (Peckarsky et al., 1990). Dissolved oxygen concentration is inversely related to temperature and directly related to the water flow, so that, the oxygen solubility 246 Jesus et al. Figure 3. Substrate composition in each sample point. Composición del substrato en cada estación de muestreo. decreases with the increase in temperature. The same happens with the decrease of the flow, which reduces the turbulence of the water. In addition to these factors, we also found an increase in the metabolic rate of the aquatic organisms, which leads to an increase in oxygen consumption (Giller & Malmqvist, 1998). The discriminating analysis grouped the samples collected in the summer (July, August, and Impact evaluation of a small hydroelectric power plant in Portugal 247 Figure 4. Sample ordination for the main physic-chemical parameters analysis in all sample points in the space formed by the two first factors of the principal components analysis (PCA) (explained variance: factor 1 – 20.9%; factor 2 – 17.9%). Ordenación de las muestras en todas las estaciones de muestreo según el análisis de los principales parámetros físico-químicos, en el espacio formado por los dos primeros factores del Análisis de Componentes Principales (PCA) (varianza explicada: factor 1 – 20.9 %; factor 2 – 17.9 %). Figure 5. Representation of the correlation circle of the physical-chemical parameters determined with the two first axis of PCA. Representación del círculo de correlación de los parámetros físico-químicos determinado con los dos primeros ejes del PCA. Figure 6. Graphic representation of the discriminant analysis showing the distribution of the collected samples grouped by seasons on the plane of the two principals axis (root 1 and root 2). Discriminant function: y = -0.69 air temperature – 0.60 NO3-- 0.45 air temperature + 0.29 O2+ 0.27 pH + 0.26 conductivity + 0.22 NO2-. Representación gráfica del análisis discriminante mostrando la distribución de las muestras recogidas agrupadas por estaciones del año en el plano de los dos ejes principales (root 1 y root 2). Función discriminante: y = –0.69 temperatura aire - 0.60 NO3-- 0.45 temperatura aire + 0.29 O2+ 0.27 pH + 0.26 conductividad + 0.22 NO2-. September) and in the winter (January, February, and March) in an opposite situation, occupying the autumn and spring samples an intermediate situation (Fig. 6). The analysis of the discriminating function makes it clear that those factors such as air, water temperature, and nitrates are the main responsible factors for the variations in the values obtained for the physical and chemical parameters. The physical-chemical parameters analysed in this fragment of the Ardena river present values within a range, which is proper of a good water quality, with weak mineralization and low productivity. These characteristics favour the normal development of the aquatic life and even present the right conditions for the development of recreational activities (Table 3) (Nisbet & Verneaux, 1970; Giller & Malmqvist, 1998). 248 Jesus et al. Table 3. Average, minimum, maximum, and standard deviation of the physical-chemical parameters of each sampling point. Media, mínimo, máximo y desviación estándar de los parámetros físico-químicos de cada estación de muestreo. Point A Max 33.0 18.0 1.31 76.1 7.2 11.0 9.0 11.5 4.1 0.04 0.01 4.37 0.05 0.02 Aver 16.2 11.9 0.36 44.5 6.6 6.3 5.3 10.3 1.1 0.01 0.01 2.75 0.01 0.01 Min 3.0 5.0 0.06 22.8 6.0 3.0 1.0 8.9 0.1 0.00 0.00 1.44 0.00 0.00 Sdx 8.22 3.69 0.361 14.36 0.36 2.74 1.97 0.80 0.96 0.014 0.003 0.821 0.013 0.008 Point B Max 35.0 19.0 2.24 89.9 7.2 14.0 13.0 12.4 3.3 0.06 0.07 5.13 0.30 0.05 Aver 17.1 14.5 0.75 55.5 6.6 7.2 6.3 10.2 1.2 0.01 0.01 3.35 0.02 0.01 Min 3.0 6.0 0.15 30.8 5.9 3.0 2.0 8.5 0.1 0.00 0.00 1.59 0.00 0.00 Sdx 8.89 3.95 0.563 17.74 0.36 2.55 2.57 0.87 0.91 0.017 0.014 0.862 0.060 0.01 Point C Max 35.0 19.0 2.66 87.1 7.3 12.0 11.0 12.5 3.4 0.03 0.05 5.13 0.19 0.06 Aver 17.3 12.5 0.84 56.3 6.6 7.4 6.0 10.5 1.2 0.01 0.01 3.43 0.02 0.01 Min 3.0 6.0 0.17 33.8 5.7 3.0 3.0 8.9 0.2 0.0 0.00 1.69 0.00 0.00 Sdx 9.27 3.99 0.695 16.44 0.39 2.51 1.87 0.91 0.90 0.008 0.011 0.855 0.038 0.013 Point D Max 32.0 19.5 0.20 99.2 7.3 16.0 13.0 11.8 3.9 0.05 0.03 5.22 0.109 0.06 Aver 17.0 12.8 0.07 72.7 6.6 9.3 9.3 10.5 1.1 0.01 0.01 3.41 0.03 0.01 Min 4.0 4.0 0.01 39.2 5.7 4.0 5.0 8.9 0.0 0.00 0.0 2.27 0.00 0.00 Sdx 8.14 4.52 0.055 16.18 0.41 3.06 2.20 0.96 1.06 0.012 0.007 0.843 0.032 0.014 Point E Max 34 20.0 5.00 97.6 7.1 15.0 12.0 12.8 2.8 0.04 0.01 5.21 0.07 0.02 Aver 17.2 12.7 0.73 65.4 6.6 8.8 8.7 10.4 0.9 0.01 0.00 3.45 0.02 0.01 Min 4.0 4.0 0.010 30.8 5.9 5.0 5.0 8.4 0.1 0.00 0.00 2.19 0.00 0.00 Sdx 7.80 4.50 1.369 20.35 0.32 2.67 2.18 1.24 0.68 0.011 0.002 0.816 0.019 0.005 Point F Max 36.0 20.0 4.50 98.3 7.5 16.0 14.0 13.0 2.9 0.06 0.03 4.63 0.04 0.03 Aver 18.3 12.8 0.63 62.7 6.6 8.54 8.2 10.5 1.2 0.01 0.00 3.44 0.01 0.01 Min 4.00 3.0 0.02 29.2 6.0 4.0 4.0 8.9 0.1 0.00 0.00 2.40 0.00 0.00 Sdx 8.77 4.67 1.173 20.26 0.41 2.84 2.36 1.17 0.88 0.015 0.007 0.626 0.014 0.011 BOD – Biochemical Oxygen Demand; TSS – total suspended solids; Max – maximal; Min – minimal; Aver – average; Sdx – standard deviation Air temperature (ºC) Water temperature (ºC) Flood (m3/s) Conductivity (µS/cm) pH Alkalinity (mg CaCO3/l) Hardness (mg CaCO3/l) Dissolved Oxygen (mg O2/l) BOD5(mg O2/l) Ammonium (mg NH4+/l) Nitrites (mg NO2-/l) Nitrates (mg NO3-/l) Phosphates (mg PO43-/l) TSS (mg/l) Analysis of the benthic macroinvertebrate communities Composition and structure The macroinvertebrate communities found in the Ardena River are formed by a great variety of taxa where the organisms most sensitive to the abiotic factors, such as the Ephemeroptera and Trichoptera, are the most numerous (Fig. 7). They present high diversity and equitability, which may mean that the populations are more or less stable and adapted to the fluctuations of the abiotic factors. These facts can be explained by the hydromorphological characteristics of the sampling points, that present a diversified and relatively stable substrate throughout the whole water course, with several interstitial spaces which are proportionately important refuges for the process of re-colonization (Seddel et al., 1990). The relative abundance of scrapers and of fixed individuals is uniform in every sampling point, which is due to the similar structure of its substrate and to the fact that they are individuals that are hardly dragged by a sudden increase of the flow (Hauer & Lamberti, 1996). Spatial structural variability Despite this high richness and taxonomic diversity, there is a clear difference between the points situated upstream of the Vila Viçosa small hydroelectric power plant (points A, B, and C), and the points D, E, F, and X, situated downstream of the same development (Fig. 8). The points D, E, F, and X present a density and specific richness, which are quite lower than the other sampling points. They are characterised by a decrease in the percentage of the more sensitive organisms (Ephemeroptera, Trichoptera and Plecoptera), and an increase in the percentage of Hydropsychidae, which can explain the decrease of the biotic index values. Attending to the results obtained for the abiotic parameters, this impoverishment of the macro invertebrate communities seems to be explained by the variability of the river flow imposed by the different elements of the hydroelectric power plant (Poff & Ward, 1989; Palmer & Poff, 1997; Puckridge et al., 1998). In point D the river flow is reduced and constant throughout almost the whole year, which allows for the homogenisation of habitat conditions and the instauration of characteristics common to lenthic systems. These can promote the replacement of individuals with more demands, in environmental terms, and adapted to waters with a higher flow speed (Plecoptera, Ephemeroptera and Trichoptera), by individuals that are characteristic of places with waters that flow slower (Heteroptera, Coleoptera and Diptera). The reduction of the density and of the species’richness can be related to the reduction Impact evaluation of a small hydroelectric power plant in Portugal 249 Figure 7. Average of taxonomic composition of the macro invertebrate community in each sample point. Media de la composición taxonómica de la comunidad de macroinvertebrados en cada estación de muestreo.