Background
Invertebrates are one of the major biota in streams and are composed mainly of aquatic insects, crustaceans, and mollusks among others. These organisms are the link between primary food sources (algae, microorg-anisms, and detritus) and their predators (fish) in a stream food web (Cummins, 1983). In lotic systems, invertebrates are ubiquitous, energetically essential primary consumers that serve as conduits to higher and lower trophic levels (Wallace and Webster, 1996). They form an important component of the trophic structure of freshwater ecosystems since they play an important role in the food webs (Bryne and Dates, 1997) and stimulate nutrient cycling by reducing the size of organic particles. These aquatic organisms are affected by various water quality parameters that can be organized hierarchically according to the spatial scale of the river network within landscapes (Poff a¬¬rs (temperature, pH, conductivity, and redox) have been considered to be the primary drivers of riverine ecosystems, and have become a fundamental part of ecological information for riverine ecosystems (Allan, 1995). The influence of these physico-chemical attributes on the structure and composition of invertebrate communities has been a dominant theme in aquatic ecology (Poff and Allan, 1995). Because water quality is such an important factor influencing the distribution and abundance of invertebrates, ecologists all over the world are using invertebrates to give an early warning to possible harm of the water resources (Chapman, 1996).

The Mwekera stream is one of the most important perennial streams on the Copperbelt province. The stream supplies water to the National Aquaculture Research Development Centre (NARDC), Zambia Forestry College (ZFC) and to the local people in Mwekera. However, due to high deforestation activities in Mwekera national forest, the stream has been exposed to land erosion. Most of the eroded materials from the upper land are deposited into the Mwekera stream and dam causing siltation. As a result of this, the water quality has been affected in the stream and the dam which is the only water source for the named institutions. This basically, has contributed to a high degree of spatial and temporal variability in the aquatic fauna. The Mwekera stream has a diverse array of invertebrate habitat elements (such as littoral grass, aquatic macrophytes, algae, overhanging and submerged vegetation, woody debris, leaf litter and root masses) and the substrate composition is comprised mostly of mud, sand, cobbles and gravel. Registered air temperatures in the study area vary between 24.0⁰C and 36⁰C (Nyambe and Feilberg, 2009).

Freshwater ecosystems in Zambia are relatively isolated and physically fragmented within a largely terrestrial landscape. They are vulnerable to many physical, chemical and biological parameters introduced by natural forces and human (anthropogenic) activities. A study by Tesfaye (1988) indicated that invertebrate composition and abundance varied along streams as pollution gradient increased. This clearly indicates that invertebrates in Zambia could be good evidence for developing bio-assessment methodology especially using invertebrate as indicators. Furthermore, most of the European researchers use non-systematic units such as fish, macrophytes, phytoplankton and diatoms for regular observations and determination of ecological statuses of the streams (De Pauw and Vanhooren, 1983). Among which the most frequently used community to determine the water quality in the streams are the invertebrates. However, such study is lacking in Zambia. Thus, this study was conducted to assess influence of water quality on the diversity and distribution of invertebrates in the Mwekera stream.

The relationship between physio-chemical parameters and invertebrates
According to the study conducted by Uyanik et al., (2005) on the relationship between invertebrates and water quality parameters, such as conductivity, pH, and dissolved oxygen (DO), in a river in Turkey. This study was carried out in order to assess the ecological impact of wastewater discharge into rivers. The kick sampling method was used in collecting benthic macro-invertebrates. The researchers used the Biological Monitoring Working Party (BMWP), Trent Biotic Index (TBI), and Chandler Score to quantify the conditions of the biota at the study sites. The results of their study supported its hypothesis that the use of invertebrates as indicators in a water body is an ideal methodology for the assessment of the water quality. Physico-chemical parameter and invertebrate indicator were both effective in assessing  the quality of the water. However, the use of both of these indicators together may have significant relationship and have more accurate results.

George et al., (2009) studied the benthic invertebrate fauna and physico-chemical parameters in Okpoka creek sediments for a period of one year. They found out that the distribution pattern of the invertebrates in all the stations of the creek did not show major differences. Their results also showed strong relationship between the physico-chemical quality and the distribution of organisms along the creek they have studied. Another study was conducted by Sharma and Chowdhary (2011) using both abiotic and biotic indicators. The relative diversity, species richness, dominance, evenness indices, physico-chemical parameters and percentage of Annelida + Arthropoda + Mollusca (AAM) individuals were determined. Significant relationships were recorded between physico-chemical parameters and the occurrence of specific genera. They concluded that the changes in invertebrate assemblages were primarily due to changes in water quality.

River pH
Rivers show regional differences in pH due to differences in geology and hydrology of the catchment area, input of acidifying substance, and productivity of the system, but the pH in the majority of streams on earth is between 6 and 9 (Bronmark and Hasson,  1998). One of the most significant environmental impacts of pH is involvement on synergistic effects. For example, very acidic water can cause heavy metals, such as copper and aluminum to be released into the water. The pH value of a stream mainly depends on the relative quantities of calcium, carbonate and bicarbonate ions in the water (Sivakumar and Karuppasamy, 2008). 

Redox
Redox potential is a measure for the presence of oxygen. Its values are used much like pH values to determine water quality. Just as pH values indicate a system’s relative state for receiving or donating hydrogen ions, redox values characterize a system’s relative state for gaining or losing electrons. it is one of the most important abiotic factor affecting invertebrate abundance and diversity (Davies and Day, 1998). Redox of natural lakes and streams is relatively insensitive to changes in dissolved oxygen concentration except under very low oxygen concentrations. The more oxygen levels, the higher the redox values.

Conductivity
Conductivity is a measure of the water’s ability to conduct an electric current. It is also useful for estimating the concentration of total dissolved solids (TDS) in the water. Because the measurement is made using two electrodes placed one centimeter apart, conductivity is generally reported as microsiemen’s per centimeter (μS/cm). The streams with high alkalinity often have high conductivity (Bronmark and Hansson, 1998).

Temperature
Water temperature is a critical physical property of rivers and streams. Temperature has a major influence on the biological productivity and development of freshwater organisms. It defines suitable habitat ranges, and controls chemical characteristics and processes of stream ecosystems (Brown and Krygier, 1967). Stream temperature has been studied by many researchers due to its essential role in defining stream ecosystems. Stream temperatures can be affected by environmental factors including atmospheric and climatic conditions, physical characteristics of the watershed and stream, and hydrologic inputs (Brown and Krygier, 1967). In addition, human activity has an increasingly important effect on stream ecosystems and on stream temperature Stream temperature changes as a result of heat fluxes between the stream and surrounding environment. Changes in stream temperature are dependent on net heat fluxes and stream discharge, and are directly proportional to the stream surface area and inversely proportional to discharge (Webb, 1996). The exchange of heat between the environment and the stream occurs primarily across the air-water boundary and the streambed-stream water interface through short- and long-wave radiation inputs, evaporation, convective heat transfer between the stream and atmosphere, conductive transfer between the steam water and bed, and advective energy transfer between water sources (Brown and Krygier, 1967). The thermal regime of small streams varies widely depending on atmospheric and physical conditions. For example, shallow streams with low flows react to heat flux changes more dramatically than do larger rivers (Brown, 1969). Water in headwater streams is generally close to a baseline temperature, which can be the temperature of groundwater, and increases as the water flows downstream towards equilibrium with atmospheric temperature. Atmospheric conditions can include air temperature, vapor pressure, solar radiation, wind speed, cloud cover, and relative humidity (Erickson and Stefan, 2000). Most aquatic organisms have adapted to survive within a range of water temperature. Organisms like stoneflies and mayflies prefer cooler water, while others like dragonflies need warmer condition. As the temperature of water increases, cool water species will be replaced by warm water organisms. Temperature also affects aquatic life sensitivity to toxic wastes and disease, either due to rising water temperature or the resulting decrease in dissolved oxygen. Water temperature influences aquatic weeds, algal blooms and surrounding air temperature (Kalyoncu et al., 2009). The metabolic and physiological activity and life process such as feeding, reproduction, movements and distribution of aquatic organisms are greatly influenced by water temperature.

The overall objective of the study was to assess the impact of water quality on invertebrate species abundance and richness in Mwekera stream with specific objectives, which were; to assess the effect of water quality (temperature, conductivity, pH, and redox) on invertebrate species abundance and richness and to assess whether invertebrates can be used to monitor changes in water quality in the Mwekera stream using min-SASS.

This study aimed at assessing the effect of water quality which is driven by many physical, chemical and biological parameters introduced by natural forces and human (anthropogenic) activities on invertebrate species diversity in the Mwekera stream. More importantly, the study provided the only best way to monitor the health of the stream and measure the general quality of the water flowing into the National Aquaculture Research and Development Center (NARDC) at Mwekera using mini-SASS. Also it provided a good understanding of the factors that affect water quality status of the stream and finally the fish farm, for effective management as well as for conducting environmental impact assessment.

4.0 Results
During the study period, a total of 106 individuals were sampled and identified, mainly in twelve invertebrate orders in the Mwekera stream. Decapoda, Tricoptera, Plecoptera, Coleoptera, Diptera, Gastropoda, Oligochaeta, Turbellaria, Hemipetera, Emphemeroptera, Hirudinea and Odonata. Generally, cross sampling sites pH and conductivity differed significantly, while temperature and redox were similar (Appendix 2).

Effect of conductivity on invertebrate species abundance, richness and diversity
The mean invertebrate species abundance, richness and diversity were 12.5±2.43; 2.9±0.41; 0.79±0.11 respectively. Generally, across sampling sites significant differences occurred in abundance (F=18.16; P=0.028);
richness (F=3.18; P=0.01) and diversity (F=4.91; P=0.05). However, conductivity was significantly related to diversity and abundance (Figure 2; Table 1). But species richness remained unaffected (Table 1).

Anova table 1

Figure 2

Effect of pH on invertebrate species abundance, richness and diversity
Across sampling sites, the mean invertebrate species abundance, richness and diversity varied significantly. However, the results show that pH was significantly related to abundance and diversity (Figure 3; Table 1). but with imaginary significance to richness (Table 1). This basically suggests that abundance and diversity increased with increasing alkalinity levels.

Figure 3

Effect of temperature on invertebrate species abundance, richness and diversity
The mean invertebrate species abundance, richness and diversity were 12.5±2.43; 2.9±0.41; 0.79±0.11 respectively. Across sampling sites they all differed significantly: abundance (F=4.91; P=0.028); richness (F=3.18; P=0.01); diversity (F=4.91; P=0.05). However, the results show that temperature had a significant effect on the abundance of invertebrates (Figure 4; Table 1). but with an imaginary significant effect on diversity (Figure 4; Table 1)

Figure 4

Effect of redox on invertebrate species abundance, richness and diversity
The mean invertebrate species abundance, richness and diversity were 12.5±2.43; 2.9±0.41; 0.79±0.11 respectively. Generally, across sampling sites significant differences occurred in abundance (F=18.16; P=0.028); richness (F=3.18; P=0.01) and diversity (F=4.91; P=0.05). However, the results show that there was no significant relationship between redox and the response variables (Table 1). Generally, this suggests that increasing redox levels in the stream had no effect on the distribution of invertebrates along the stream.(Figure 5).

Figure 5

Discussion
The effect of physico-chemical parameters on invertebrate species abundance and diversity According to the results, the diversity and distribution of aquatic invertebrates in the Mwekera stream were influenced by severally physico-chemical parameters. These parameters basically did explain the diversity of invertebrates along the Mwekera stream.

Conductivity varied significantly across sampling sites, and decreased successively from sit S1 to S10. Sit S1 and S2 had high conductivity because they were exposed to land erosion. Furthermore, it is argued elsewhere that conductivity can be influenced largely by geology since it is highly influenced by mineral salts (Kalyoncu et al., 2009). However, an increase in conductivity possibly occurs when additional wastes containing ions enter the stream section (Kalyoncu et al., 2009). Thus, it is highly probable that the increase in conductivity in the stream from sampling site 1 up to sampling site 2 is due to the additional waste from charcoal burners as well as other anthropogenic activities. Conductivity indicated a positive significant effect on both invertebrate species abundance and diversity (Table 1).  albeit it showed an imaginary significant effect on species richness. This shows that high levels of conductivity favoured both invertebrate species abundance and diversity (Figure 2).

The pH results indicate that the waters from the Mwekera stream during the sampling period were almost natural (Appendix 2). According to Davies and Day (1998) most natural waters’ pH values range from 6 to 9. This implies that the Mwekera stream has the capacity to buffer it’s self as indicated by Li et al., (2007) that most rivers and streams have a buffering capacity which affects the rate of change of pH in aquatic ecosystems as they tend to resist rapid change of pH especially when the flow is high. However, the results indicate that pH positively correlated with both invertebrate species abundance and diversity (Table 1). The slight decrease in alkalinity from site S1 to S10, made species abundance and diversity to decrease as well. According to Bronmark and Hasson (1998) aquatic insects are extremely sensitive to pH values below 6. The Gastropods, mayflies, stoneflies and caddis flies are some of invertebrate groups that prefer pH levels from 7-9.5, and these were found on site S1, S2, and S3 which had slightly alkaline conditions.

In this study, water temperatures varied slightly significantly across sampling sites (appendix 2). Sampling site S1 recorded the highest temperature (19.9 ± 0.45℃), while site S6 located in the up reaches of the stream recorded the lowest temperature (15.5 ± 0.28℃). High temperatures at site S1 was caused by the exposure of the stream to the sun, complete lack of vegetation canopy and heat exchange with the atmospheric air. In contrast low temperature recorded at site S6 may, however, be attributed to the cooling effects of the dense forest canopy. This observation concurs with study findings by Shivoga (2001) who found forests to influence the temperature regime of rivers and their invertebrate communities. However, the results indicate that water temperatures did significantly affect invertebrate species abundance (Table 1). However, no significant relationship was found between water temperature and diversity, meaning that water temperature could not explain diversity patterns in the Mwekera stream. These results show that as the water temperature increases, even the number of invertebrates per site increases. This conclusion is in perfect agreement with the results found by Davies and Day (1998). Their results showed that high temperatures within the range of 20-25℃ favored invertebrate metabolic and physiological activity and life processes such as feeding, reproduction, movements and abundance.

Redox potential which is used to indicate the presence of oxygen, showed no significant variation across sites (Appendix 2). However, the results revealed that redox correlated negatively with both invertebrate species abundance and diversity. These results suggest that the redox levels were therefore not a limiting factor to the diversity of invertebrates along the Mwekera stream at the time of sampling. This is supported by Chapman (1996) who observed that it is at levels below 100mV that the survival of biological communities will be affected, thus changing diversity structure. So, the lack of significant relationship in this study may be due to the oxygen levels, as their significance to aquatic biota depends on the frequency, timing and duration of depletion.

South Africa scoring system index (Mini-SASS)
An indication of the quality of each sampling site was obtained using the Mini-SASS scoring system version 5 (Chutter, 1998). Many of the invertebrate taxa collected from the Mwekera stream were sensitive to water quality change. Using SASS5 score, the range was between 5 and 27 which were from site S5 to site S2 respectively (Appendix 3). ASPT (average Score per taxa) scores ranged from 2.5 to 7.3 at site S5 and S1 respectively as shown in Appendix 3. The water qualities for site S2, S3, S4, S6, S6, S7, S8, S9, and S10 were classified to be fair, except for site S1 and S5. The water quality for site S1 was classified to be excellent, suggesting a number of sensitive invertebrate species were present. Site S5 was classified to be poor, meaning that it had the most deteriorated ecosystem health than all the sites, and the invertebrate species present there were tolerated. These findings from the mini-SASS results have generally offered a more holistic approach, based on the recognition that monitoring of physico-chemical water variables only is not sufficient to achieve integrated ecosystem monitoring.

Mini-SASS was the only available tool at the time of carrying out this study; however Lang et al., (2013) and Lowe et al., (2013) have since developed a Zambian water monitoring protocol (ZISS) for use in stream water quality assessment built on the min-SASS framework. The Zambian Invertebrates Scoring System (ZISS) uses the identified macro invertebrates to score for the water quality of any given fresh water locality in Zambia.

Materials and Methods
Study site
This study was performed in Mwekera stream, a first order perennial stream (12º 52'N and 28º 16' 0"E) located in the south-east part of Kitwe on the Copperbelt province. The study area is a tributary of the Kafue River and is at an elevation of 1,158 meters above sea level. From its origin the stream flows downstream through a natural forest dominated mainly by indigenous trees. In the lower reaches it passes through the National Aquaculture Research Development Centre before it eventually coalesces with the Kafue River. Along its water course especially in its up reaches, the stream is dominated by indigenous trees and other riparian vegetation that include the following: Brachystegia, Julbernadia and Isorberlinia.

The National Aquaculture Research Development Centre (NARDC) was established in 1994 for fish propagation that included developing techniques for the mass production of fingerings, fish feed formulation and assessing the pond environment. The quality of the water upstream is therefore of great importance to the Centre as the immediate end user before it gets to the Kafue river further downstream.

Climate
Zambia being divided into three ecological zones or regions namely I, II and III. The climatic condition of the study area falls under climate zone III. The area experiences average temperatures ranging from 15ºC to as high as 36ºC and receives rainfall ranging from 800mm to over 1 000 mm.

Experimental design
In this study, a systematic sampling design was employed. The initial sampling site was selected randomly. Sampling sites were spaced at uniform intervals throughout the 1 km section of the stream sampled. Each sampling site of 10 m x 10 m was established every after 100 m from the initial sampling site.

Sampling
This study was conducted in the dry season of 2014. Samples for both physico-chemical parameters and invertebrates were collected in July and August 2014. Ten sampling sites along the longitudinal stream gradient were selected and the sites were designated as S1-S10.

Invertebrates
Sampling took place from 31 July 2014 to 4 August 2014. Invertebrate samples were always collected by the same operator with a standard hand net consisting of a metal frame holding a conical net (20 cm×30 cm, 150µm mesh size). In the field, the collected material was sieved through 500 mm and 250 mm mesh sieves and put into collection bottles. The sampling effort at each site was 30 min. Within a site two riffles and two pools were sampled. All samples were preserved with 4% formalin until counting. All the organisms in the sample were counted and identified to the lowest possible taxonomic level (family level) using a mini-SASS identification key (Appendix 4). There are no current keys available to the Zambian fauna, but the mini-SASS key used made it possible to classify the fauna specimens to the family level without loss of accuracy.

Physicals-chemical parameters
Samples for physico-chemical parameters were taken at the same location and almost simultaneously with the samples for invertebrates. Water temperature, pH, conductivity and redox were measured in situ using electronic measure equipment.

Data analysis
Descriptive statistics were used to analyze physico-chemical data. For the invertebrate communities one index was calculated for each site. The Shannon- Wiener Diversity Index (Hʹ) a diversity index that incorporates richness and evenness was used to calculate diversity. Hʹ was calculated as follows:Hʹ = - ∑ (P¡ In [P¡])Where P¡ is the relative abundance (n¡/N) of family ¡, n¡ = number of individuals in family ¡ and N = total number of individuals in all families. H¡ ranges from 0 for a community with a single family, to over 7 for a very diverse community.  The statistical software package (Analyse it) was used for statistical analysis. Simple regression was used to determine the level of significance of the relationship between invertebrates and physico-chemical parameters.

The water quality of the Mwekera stream was assessed using the South African Scoring System version 5 (SASS5) (Dickens and Graham, 2002), in which invertebrates are identified to family level and each family is assigned a tolerance level, from 1-10 to indicate their resistance to pollution. Tolerant taxa are given low scores and sensitive ones high scores. The total score for each site was calculated by summing the individual taxon scores. The Average Score per Taxon (ASPT) was calculated by dividing the total SASS5 score by the number of taxa in the sample. The higher the SASS5 score and/or the ASPT value, the better the water quality is deemed to be (assuming that habitat availability is not limiting).

From the results, the water quality of Mwekera stream varied from excellent to fair in the upstream direction due to increase in land erosion as evidenced by low species richness, abundance and diversity of the invertebrate fauna. This was as a result of natural forces and an increase in charcoal burning activities. The abundance of invertebrates were highest at sit S1 and lowest at S5. Sampling site S1 had an excellent water quality while site S5 had poor water quality, and the rest of the sites had fair water quality. The results indicated the existence of a significant relationship between water quality parameters of Mwekera stream with the invertebrate community. Therefore, this basically rejects the null hypothesis in favour of the alternative hypothesis that water quality parameters have effect on invertebrate species abundance and richness. The results obtained from this study were precise and reliable. Therefore, the National Aquaculture Research and Development Centre (NARDC) can use invertebrates as bio-indicators in monitoring the quality of the water flowing in the fish farm and environmental degradations the pond effluents cause to the ecosystem in the stream as they are released. The findings of the water quality assessment of Mwekera stream by use of the invertebrate abundance and richness were in consistence with other similar study findings in other parts of the world. Pollution has always impacted negatively on water quality status of rivers and streams as well as on invertebrate’s richness, composition and diversity.

Acknowledgements
The authors would like to acknowledge the support and assistance from the Copperbelt University for the space, facilities and the time to be able to carry out this work and to be able to work with final year students on projects. The staff at the Mwekera Aquaculture centre in Mwekera for the co operation and support while doing the field work.

The staff at the School of Natural Resources of the University for their support and useful comments on the draft.

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