Accepted: 16/7/014 by Ecohydrology & Hydrobiology (Elsevier)

Abrehet et al. 2014

Accepted: 16/7/014 by Ecohydrology & Hydrobiology (Elsevier)

Spatial and seasonal variation in the macro-invertebrates and physico-chemical parameters of the Enfranz River, Lake Tana Sub-Basin (Ethiopia)

Abrehet Kahsay Mehari1, Ayalew Wondie2, Minwyelet Mingist1 and Jacobus Vijverberg3*

1Fisheries, Wetlands and Wildlife Management Program, Bahir Dar University, PO Box 79, Bahir Dar, Ethiopia

2Biology Program, Bahir Dar University,PO Box 79, Bahir Dar, Ethiopia

3Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB Wageningen, The Netherlands

*) Corresponding author: e-mail:

Abstract

The main objective of the study was to assess the water quality of the Enfranz River by studying the distribution of macro-invertebrate taxa onthe longitudinal gradient of the river. The macro-invertebrate community of the Enfranz River, located northwest of Bahir Dar city in the southern part of Lake Tana watershed, was studied to family taxonomic level in wet and dry seasons from August 2010 to May 2011. The river was sampled along its whole length at four sites from headwaters until its outflow in Lake Tana. A total of 15,286 macro-invertebrate individuals belonging to 35 families and 2 higher taxa were collected. The Shannon-Wiener diversity Index,the Hilsenhoff family-level biotic index, and three macro-invertebrate metrics, were measured and related to five physico-chemical parameters. Macro-invertebrate diversity and biotic indices, and community metrics differed significantly among sampling sites (p < 0.05), diversity being higher at the headwaters. Spearman’s correlation coefficients showed that dissolved oxygen was significantly correlated with the macro-invertebrate diversity and biotic indices, and all three macro-invertebrate metrics (p < 0.05). Diversity index, percent Ephemeroptera and percent Trichoptera were positively correlated to dissolved oxygen, whereas the biotic index and percent dipterans showed negative correlations. Furthermore, percent Ephemeroptera was negatively correlated with conductivity (p<0.05)and diversity was negatively related to total dissolved solids and conductivity. We conclude that downstream, the river is severely affected by land use of the people living along the river.

Key words: Benthic macroinvertebrates, Biodiversity, Biomonitoring, Bioindicators, Effects of land use, Water quality, Water quality management.

Running headline: Spatial variation in macro-invertebrates

1

1. Introduction

Freshwater ecosystems have been altered by human disturbances such as agriculture, urban development, impoundment, channelization, mining, forest fire suppression, road construction and species introductions (LaBonte et al., 2001).All of these have led to severe degradation and loss of biodiversity (Vinson and Hawking, 1998) and as a result these ecosystems have become the most endangered ecosystems on the planet (Dudgeon et al., 2006).While many taxa contribute to biodiversity in freshwater ecosystems, aquatic macro-invertebrates play a central ecological role in many running water ecosystems (Boulton, 2003) and are among the most ubiquitous and diverse organisms in freshwaters (Strayer, 2006). Aquatic macro-invertebrates form an important component of the trophic structure of freshwater ecosystems since they play an important role in the food webs(e.g., Grubh and Mitsch 2004) and stimulate nutrient cycling by reducing the size of organic particles (Callisto et al., 2001).

Macro-invertebrates are often used as biomonitoring tools (Dallas andMosepele, 2007). Biomonitoring is based on the principle that organisms are the ultimate indicators of the health of the environment they are within (USEPA, 2002). Biomonitoring has the advantage that it can detect cumulative physical, chemical and biological impacts of adverse activities to an aquatic system. Aquatic macro-invertebrates are often preferred for biomonitoring because of the following three reasons: firstly,they are not very mobile and therefore they are representative of the area from which they are collected, secondlythey have relatively short life cycles and therefore can reflect environmental changes quickly through changes in their community composition and finally they respond to pollutants in both water column and sediments (Reece andRichardson, 2000).

In Africa thereisanincreasingtrendtousebenthic macro-invertebratecommunities in rivers and streams asindicators ofenvironmentalquality(e.g., Shivoga, 2001; Dickens and Graham, 2002; Ndaruga et al., 2004; Kibichii et al., 2007; Kasangaki et al., 2008; Masese et al., 2009b; Minaya et al., 2013). However, to best characterize ecological conditions of rivers and streams, the development of a single index from biological and environmental variables is preferred (Masese et al., 2013). This approach involves integration of a number of structural and functional attributes of the macro-benthic community, termed ‘metrics’, into a composite index withtheratingofeachmetricbasedon quantitativeexpectations (based on comparisons with reference conditions)ofwhat represents highbioticintegrity. The employment of the multimetric Index of Biotic Integrity (IBI) is rapid and cost effective (Masese et al., 2009a). Besides EuropeandtheUnitedStates,theuseof an Index of Biotic Integrity (IBI)tomonitorstreamsandrivers to assessthedegreeofecosystemdegradationhasnot been much used (Masese et al., 2013). To date, there are only a few African studies available, most studies are from SouthAfricawhere they developed a fastscoring system(e.g., Dickens and Graham2002) and a few studies from NE Africa (Sitotaw, 2006,Kobingietal.,2009;Maseseetal.2009a;Raburuetal.2009a, b; Aura et al., 2010).

In Ethiopia studies on benthic macroinvertebrates in streams and rivers are sparse. Hynes et al. (1989) and Hailu and Legesse (1997) were the first to study macro-invertebrates and to use them to assessthe pollution status of Ethiopian streams. Harrison and Hynes (1988) described in detail the community composition of benthic macro-invertebrates of mountain streams belonging to 7 different rivers systems. Their interest was mainly on the biogeography of Afrotropical mountain stream fauna. Sitotaw (2006) was the first in Ethiopia to assess a Benthic Index of Biotic Integrity in his study on nine Ethiopian rivers. He concluded that extensive agricultural activities and industrial and urban land-use were the most important threatening factors to river ecosystems in Ethiopia. Beyene et al. (2009) assessed the relative performance of diatoms and macro-invertebrates to measure municipal and industrial impacts on the biological integrity of three major rivers flowing through Addis Ababa. Ambelu et al. (2010) collected data on macro-invertebrates and physico–chemical characteristics in the Gilgel Gibe River basin in South-Western Ethiopia to developed models predicting macro-invertebrate metrics, which could be used for river management.

The present study is the first study on macro-invertebratesin one of the ca. 47 rivers flowing into Lake Tana, Ethiopia’s largest lake. The purpose of this study on Enfranz River was three fold: (1) to assess the spatial and seasonal variation of physico-chemical parameters and macro-invertebrate diversity and other community metrics over a river continuumfrom headwaters until its outflow into Lake Tana, (2) to assess the spatial and seasonal variation in water quality over this river continuum, and (3) to develop a monitoring system on which guidelinesforconservationandmanagement could be based.

2. Materials and methods

2.1.Study area and land-use

The Enfranz River is situated northwest of Bahir Dar city and the upper stream of the river is rich with a number of springs. These head springs are the source of drinking water for the city. It drains to southwest of Lake Tana and its catchment area is ca. 198 km2 (Kidan 2010).The main water source of the Enfranz River, besides surface water, is groundwater. This water is directly pumped to the people in the city from wells drilled near the river (Kassahun, 2008).The total population of Bahir Dar was in 2007 ca. 220,000 inhabitants and has a population growth rate of 6.6 % per year (CSA, 2007), which is more than twice as high as the average population growth rate in Ethiopia.

The climate of the Enfranz River area is characterized by rainy season during July-September, dry season during December–April, pre-rainy season during May–June and post-rainy season during October–November. The wetlands in the Enfranz River watershed occupy an area of ca. 2500 ha and are inhabited by ca. 24000 people. The wetlands are used for extensive grazing and agriculture by subsistence farmers mainly during the dry season (December-April). Most of the land (ca. 80%) is used for grazing of cattle and ca. 10% is used for extensive agriculture mainly chat (Cathi edulis) and the culture of flowers (Fig. 1). The remaining land is occupied by shrubs (ca. 10%) which are dominated by Aloe spp. and human settlements (< 5%).The riparian vegetation is diverse and consists of about 27 species of shrubs of which three species are endemic to Ethiopia (Erythrina brucei, Mellitia ferruguina and Acanthus senni) (Kidan, 2010). Trees are sparse along the stream.Most trees, predominantly Scysigiumguineense (dok’ma), Ficusspp. and Euphorbia spp., are present around station E3.

2.2. Sampling

This study was conducted in wet and dry seasons of 2010-2011. Samples for both physico-chemical parameters and macro-invertebrates were collected in August and October 2010 and January and May 2011. Four sampling sitesalong the longitudinal river gradient were selected and the sites were designated as E1 to E4. In total 16 pooled samples were taken. Sampling sites ranged from headwaters (E1) to the mouth of the river (E4), where the water flows into Lake Tana. The detailed description of the sampling sites is presented in Fig. 1 and Table I.

Physico-chemical parameters

Samples for physico-chemical parameters were taken at the same location and almost simultaneously with the samples for macro-invertebrates. Water temperature, dissolved oxygen (DO), pH, total dissolved solids (TDS) and conductivity were measuredin situ using electronic measure equipment. Conductivity, pH, TDS and temperature were measured with a SyberScan PC300 (Eutech Instruments), whereas dissolved oxygenwas measured with a SyberScan DO300(Eutech Instruments).

Macro-invertebrates

Quantitative sampling was carried out based on the rapid bio-assessment protocols that are used for rivers and wadeable streams (Barbour et al. 1999). Samples were taken using dip net with mesh size of 500µm (mouth 50 x 30 cm, 60 cm deep, handle 132 cm).In the field, the collected material was sieved through 500 µm and 250 µm mesh sieves and put into collection bottles. The sampling effort at each site was 30 minutes.Within a site two riffles and two pools were sampled, but macro-invertebrates were pooled so as to obtain a single sample from each site. All samples were preserved with 70% ethanol until laboratory analyses and counting. All the organisms in the sample were counted and identified to the lowest possible taxonomic level (family level) using a dissecting microscope and standard keys (Edmondson, 1959; Merrit and Cummins 1988; Jessup et al., 1999; Gooderham andTysrlin, 2002;Bouchard, 2004). There were no keys available to the Ethiopian fauna, but the standard keys used made it possible to classify the macro-fauna specimens to the family level without loss of accuracy.

2.3. Data Analysis

Descriptive statistics were used to analyze physico-chemical data. For the macro-invertebrate communitiestwo indices were calculated for each site and each sampling date. The Shannon-Wiener Diversity Index (H′) is a diversity index thatincorporatesrichnessandevenness. A high H′ indicates a good water quality. H′ was calculated as follows:

H′ = - ∑ (Pi ln [Pi]) Eqn 1

Where:Piis the relative abundance(ni/N) of family i, ni = number of individuals in familyi and N = total number of individuals in all families. H′is ranging from 0 for a community with a single family, to over 7 for a very diverse community.An H’ value of less than 1 indicates highly polluted, 1-3 moderately polluted, and greater than 4 unpolluted water bodies (Wilhm and Dorris, 1968).

The Hilsenhoff Family-level Biotic Index (HFBI was not designed for tropical rivers, but there are no limitations for use in Ethiopian tropical streams because the same families are present in tropical and temperate rivers. It is calculated by multiplying the number of individuals of each family by an assigned tolerance value for that family. Assigned tolerance values range from 0 to 10 for families and increase as water quality decreases (Hilsenhoff, 1988; Bode et al., 1996). This Index was calculated as follows:

HFBI = Σ [(TVi) (ni)] ⁄ N …………………………………..Eqn 2

Where:TVi is tolerance value for family i, niis the number of individuals in family i and N is the total number of individuals in the sample collection.High HFBI community values are an indication of organic pollution, while low values indicate good water quality.

Excel spreadsheets and statistical software (SPSS version 16) were used for the statistical analysis. Kruskal-Wallis analysis by ranks was used to evaluate differences in physico-chemical data and macro-invertebrate metrics among the sampling sites, whereas differences between seasons were assessed with the Mann-Whitney U test. Spearman’s rankcorrelation coefficients were used to determine the relationships between physico-chemical parameters and diversity and biotic indices and macro-invertebrate metrics.

3. Results

3.1. Physico-chemical Parameters

The mean values (mean + SE) of dissolved oxygen (DO) ranged from 3.27+ 0.23 mg l-1 at the mouth of the river (E4) to 6.08+ 0.14mg l-1in the headwaters (E1) (Table II).The mean value of dissolved oxygen showed significant variation among sampling sites (p<0.05), the value at E1 being significantly higher than at the other sites. The mean values of dissolved oxygen in wet and dry season weresimilar and not significantly different ( p>0.05). The percent oxygen saturation values at ambient temperatures showed the same trend, high at the headwaters (67%) and low at the mouth of the river (37%). All saturation values were well below 100%.

Temperature did not differ significantly among sampling sites (p> 0.05, range: 18.1-29.1 oC) and seasons (p>0.05, range for wet season:18.1-23.4oC, range for dry season:18.9-29.1oC).

The mean value of total dissolved solids (TDS) along the study sites ranged from 82.78 + 10.51 ppm at E2 to 146.5 + 20.04 ppm at E4, while its value in wet and dry season was 92.45 + 11.41 ppm and 117.9 + 12.62 ppm, respectively (Table II). Conductivity followed the same trend. There was significant variation among sampling sites (p<0.05). TDS values at E4 were significantly higher than the values at E2 and E3. However, this was not the case for conductivity. Although conductivity values at E4 were significant higher than at E2, differences between E4 and E3 were not significant. In both cases, differences among seasons were not significant (p>0.05).The grand mean value of pH was 7.1 (Table II). Values did not differ significantly among sampling sites (p>0.05) and seasons (p>0.05).

3.2.Macro-invertebrates

Taxa

A total of 15,286 macro-invertebrate individuals belonging to 35 families and 2 higher taxa were collected from 4 sites during the survey work (Fig. 2, Table III). The total number of individuals present at each site ranged from 2,690atE1 to 6,473 at E4, and 6,512 and 8,774 during wet and dry seasons, respectively. Libellulidae was the most abundant family (2,540 individuals), followed by Chironomidae (1,747 individuals), Belostomatidae (1,135 individuals), Coenagrionidae (1,106 individuals), Culicidae (678 individuals), then Corixidae (675 individuals).

We reviewed how many families (taxa) were represented in each of four sampling sites.At consecutive sampling sites (E1, E2, E3, E4) 30, 28, 22 and 21 taxa were represented, but changes resulted not only from loss of individual taxa. At E2 two ephemeropteran families, 4 trichopteran families and 1 family belonging to Odonata were lacking as compared to E1, but 1 hemipteran family, 3 molluscan families and 1 arachnides family appeared. At E3 one trichopteran family, 1 Odonata family, 1 Coleoptera family, 3 molluscan families, 1 arachnides family and 1 Hydracarina familywere lacking as compared to E2, but 1 molluscan family, 1 arachnides family and 1 Hirudinea family appeared. At E4 one ephemeropteran family, 2 hemipteran families and1 Coleoptera family were lacking as compared to E3, but 1 molluscan family, 1 arachnides family and 1 Hydracarina family appeared.

The mean proportions of ephemeropterans varied between 0.12% and 23.59%. The lowest value was observed at E4 and the highest value at E1, differences between downstream stations (E3, E4) and stations upstream (E1, E2) were large (Table IV). Differences among sampling sites were significant (p<0.05). The proportions at E4 were significantly lower than those at sites 1, 2 and 3. In contrast, the difference between seasons were small and not significant (p>0.05).

The mean proportions of trichopterans ranged from 0.00% at E4 and E3 to 12.59% at E1 (Table IV). Differences between downstream stations (E3, E4) and stations upstream (E1, E2) were large.Differences among sampling sites were significant (p<0.05). We did not observe any trichopterans at E3 and E4.The difference between seasons was relatively large (Table IV) and significant (p<0.05).The mean proportions of dipterans varied from 4.29 % at E1 to 29.77% at E3 (Table IV). Difference between downstream stations (E3, E4) and stations upstream (E1, E2) was large.Differences among sampling sites were significant (p<0.001).Values at E3and E4 were significantly higher than those at E1 and E2. The differencebetween seasons wassmall and not significant (p>0.05).

Diversity and biotic indices

The mean values of H’ at sampling sites ranged from 2.39 at E4 to 3.02 at E2 (Table IV). The H′ value showed significant variation among sampling sites (p<0.05); the value being significantly higher at E2 than E1, E3 and E4.Difference between seasons was not significant (p>0.05).,,, The mean values of HFBI ranged from 5.38 to 8.19.The lowest value was at E1 and the highest value was at E4 (Table IV). Differences among sampling sites were significant (p<0.05). Mean values for wet and dry season were 6.86 and 7.17, respectively (Table IV); difference was significant (p<0.05).

3.3. Correlations between physico-chemical parameters and macro-invertebrate metrics

Spearman’s correlation coefficients between physico-chemical parameters and macro-invertebrate metricsare presented in Table V.Macro-invertebrate metrics were significantly correlated to some of the physico-chemical parameters. Dissolved oxygen was the only one which was significantly correlated with all macro-invertebrate metrics (p < 0.05). In contrast, neither temperature nor pH showedany significant correlations. Shannon-WienerDiversity Index, percent Ephemeroptera and percent Trichoptera were positively correlated to dissolved oxygen, whereas HFBI and percent dipterans showed negative correlations. Furthermore, percent Ephemeroptera was negatively correlated with conductivity (p<0.05)and diversity was negatively related to TDS and conductivity.

4. Discussion

The water quality of Enfranz River downstream was rather poor compared with the upstream region. Most likely this was mainly caused by the farmers in the downstream wetlands. Kidan (2010) concluded that the high values of dissolved solids and conductivity in the river mouth were primarily the result of surface run-off from agricultural lands and riverbank erosion caused by cattle grazing and watering along the river shores.