Rivers are fast becoming one of the most endangered precious natural resources on earth [1] . The sustainable existence of these freshwater ecosystems continues to be threatened by different anthropogenic activities like agriculture, industrialization and urban development, all of which have the potential to cause deterioration in river water quality [2] . The discharge of wastewaters from industrial, domestic and agricultural sources into river systems introduce different pollutants, which present various detrimental effects, particularly if the physical or chemical properties of the discharges differ from that of the natural river system [3] . While discharges influence the quantity and quality of the water, it can also impact on the ecological integrity and biodiversity of the system [4] . Even though many different organisms contribute to the biodiversity of river water ecosystems, aquatic macro-invertebrates are among the most ubiquitous and diverse group of organisms in these systems because they play a central ecological role in nutrient cycling in these ecosystems [5] [6] . It is on this basis that macro-invertebrates find wide application as ideal organisms in freshwater bio- monitoring studies [7] .

Macroinvertebrates are simply defined as organisms that lack a spine and are large enough to be seen with the naked eye [8] . Examples of macroinvertebrates include flatworms, crayfish, snails, clams and insects, such as dragonflies [9] . Many aquatic insects live as juveniles, called nymphs or larvae, in the water, and become flying insects as adults [10] . They may be aquatic living organisms and other aquatic invertebrates associated with the substrates of water bodies [11] . Each macroinvertebrate has a unique characteristic. Some are pollution tolerant, some are somewhat tolerant to pollution and others are pollution sensitive [12] . The use of biological organisms to monitor the health of ecosystems is not new with earliest man undoubtedly choosing “healthy” water resources over polluted ones [13] . Benthic aquatic macroinvertebrates are ideal for use in biological monitoring of rivers and is a basic tool that can be used in maintaining the status of the environment, also regarded as the eco status [14] .

The traditional means of assessing the impacts of pollution on water bodies are done through the measurement of biological, physical and chemical parameters [15] . However, such measurements only reflect the concentration of pollutants at the time of sampling and measurement, but not episodic or intermittent pollution events [2] . As such, these methods fail to provide enough ecological information because the synergistic effects of pollution on aquatic biotic community may not be fully and easily accessed through chemical and physical measurements [16] . Since physical and chemical assessments cannot on their own form the basis for biodiversity conservation, this leaves biomonitoring of stream conditions as one of the most credible sources of freshwater ecologicalstatus since it provides an integrated and comprehensive assessment of the health of a water body over time [17] . In South Africa for example, the Dragonfly Biotic Index (DBI) has been developed for prioritizing and assessing site conditions for conservation purposes [3] . The shortcomings of physical and chemical water assessments have therefore necessitated the adoption of biological organisms to assess the impacts of anthropogenic activities on water quality in aquatic ecosystems and have given rise to a branch of ecology called biological monitoring (biomonitoring) [18] .

Biomonitoring is based on the principle that organisms are the ultimate indicators of the health of the environment they are within [19] . 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 reasons: they are not very mobile and therefore are representative of the area from which they are collected, they have relatively short life cycles and therefore can reflect environmental changes quickly through changes in their community composition and they respond to pollutants in both water column and sediments [4] [20] .

Benthic macro invertebrate fauna are considered to be an excellent water quality indicator because of several reasons [21] [22] [23] . Begum et al. (2009) noted that benthic macro invertebrates often respond to heavy metal contamination and different pollution in different ways, with the quantity and type of the heavy metal in water, sediment, or food determining the degree of pollution [24] . Apart from the fact that benthic macro invertebrates live in intimate contact with the sediments, they are also widespread and often the dominant component of the aquatic fauna, they are relatively easy to collect, identify and enumerate unlike plankton and microorganisms [23] . Contrary to fish and other aquatic animal life, benthic macro invertebrates cannot move around as much, so they are less able to escape the effects of sediment and other pollutants that reduce water quality [25] . Since benthic macro invertebrates portrays a wide variation in sensitivity to heavy metal contaminants and all forms of pollution, the presence or absence of sensitive or tolerant taxa within communities make them excellent bio-indicators of heavy metal pollution. For instance, decreased number in mayfly population indicate severe water quality degradation, while caddisflies can tolerate high exposures to heavy metals even when coated in oxides of manganese (Mn) and iron (Fe) precipitate, and calcium (Ca) minerals [26] .

Biomonitoring is therefore a product of the assumption that the response or “health” of biota is a reflection of the “health” of the environment in which they live [27] . It uses resident biota such plants, animals and microorganisms to evaluate effects caused by anthropogenic stress on aquatic ecosystems [28] . Stressed water bodies are often dominated by tolerant organisms with corresponding reduction in the number of sensitive ones [29] . Biomonitoring uses the health or responses of biological organisms to depict changes in the environment that could provide indications of environmental stress and hence the need for remedial actions in stressed environment [16] .

Table 2 shows the identified macroinvertebrates on the selected sites (A1, A2, B1

and B2). The results directly indicate that the pollution levels at site A1 and B1 was moderate as compared to site A2 and B2. This is in agreement with the observations of Tables 3-6.

Tables 3-6 show the macroinvertebrate scores and tested water quality parameters for sites A1, A2, B1 and B2 and their interpretations. Assessment of the physico-chemical and biological water quality characteristics was done through the use of WHO (2008) guidelines, which are considered to be universal and accepted worldwide [43] . As shown in Table 3, at site A1 on Muchekeriver, according to SASS5 benchmark, the measured score indicates moderatepollution on the water quality levels due to the presence of macroinvertebrates that are considered moderately tolerant to pollution. The identified macroinvertebrates included Anodontasp., Caenidae sp., Psephenidae sp., Leptoceridae sp., Psychomyiidae sp., Hydroptilidae sp. and Hexatomasp. that indicate good or moderate water quality (Table 2). The score was also validated by the tested coliforms, pH, DO and conductivity parameters that all indicated that the water was within the accepted limits of pollution. These observations resonate well with the situation on the ground, because site A1 is situated upstream where there are less anthropogenic or industrial operations that directly dispose their waste water into the river. The likely source of pollution at this site comes from agricultural run-off from communities living upstream and a quarry stone plant’s effluent that can easily flow into the river thus compromising the quality of the river water.

Table 4 shows the macroinvertebrate scores and tested water quality for site A2. At this sampling site which is downstream of Muchekeriver, the recorded SASS5 score is 3.4 which indicates that the water is highly polluteddue to the presence of mainly macroinvertebrates that are pollution tolerant. Macroinvertebrates identified at this site include Glossiphonidae sp. and Erpobdellidae sp.

(Table 2) that are common indicator species of poor quality water levels. The SASS5 score was also confirmed by the pH levels of that above the maximum acceptable limit for good quality water. Despite the DO and conductivity being within their accepted ranges, their threshold steeply increased from the previous sampling point, site A1, which is located upstream of the same river. This indicates the effect of anthropogenic pollutants introduced into the river as it flows past Masvingo city’s residential and industrial areas, as well as the city’s central business district (CBD).

Table 5 shows the macroinvertebrate scores and tested water quality for sampling site B1, which is found upstream of the Shagasheriver. The SASS5 score of 10.5 shows that the water is moderately polluted. This scenario is strongly supported by the coliform bacteria counts that confirm that the water quality is good. The macroinvertebrates found on this site included, Antocha sp., Leptophlebiidae, Perlidae, Siphlonuridae, Hydrophilidae, Dytiscidae, Ephemeridae, Tipula sp., and Tabanidae and Arctopsychidae sp., which indicate good water quality.

Table 6 shows the results from sample site B2, which is downstream of Shagasheriver. The site recoded a SASS5 score of 4.5 which indicates the presence of mainly pollution tolerant macroinvertebrates implying that the water is polluted. The observed macroinvertebrates were Chironomidae sp., Ceratopogonidae sp., and Lymnaeidae sp., all of which survive in polluted waters, thus indicating extremely poor water quality. These dominate in polluted water with high concentration of organic materials and nutrients, but other species cannot survive [44] . ThisSASS5 score was also validated by the pH and DO values that were both out of the accepted range for good quality water and a very high coliform count that indicated a very poor water quality rating, unfit for human use [43] . To a large extent the levels of pollution on this part of the river can also be confirmed by the high concentration of the water hyacinth plants, which are also a signature mark of the presence of high water nutrient content [41] . The mainreason to justify the high levels of water pollution observed on this site is the presence of city’s domestic and industrial wastewater effluent treatment plant upstream that discharges its treated effluent into the river. There is high likelihood that the discharged effluent from the treatment plant may be of poor quality due to persistent power-cuts and also due to poorly serviced and dilapidated wastewater treatment infrastructure that is no longer fit for purpose. In addition, most of the waste and pollutants that are discharged from the city of Masvingo eventually end up on this part of the river including all the polluted water from Mucheke river, which joins the Shagasheriver upstream.

Biological monitoring tools for water quality assessment are largely lacking for many developing countries including Zimbabwe, resulting in adoption of tools developed from other countries. In many instances, their applicability to a new environment has not been explicitly evaluated in Zimbabwe, hence the objective of this study. The results obtained generally show that the SASS5 system can be easily adopted and used for the general rapid assessment of water quality of rivers found in Masvingo, which is a semi-arid region in Zimbabwe. This is in agreement to studies done to test the applicability of this system in other streams in Zimbabwe [45] . This also confirmed the occurrence of ubiquitous macro- invertebrate taxa in Zimbabwe sharing similar environmental tolerances with those recorded for South African systems [29] [46] .

During the validation of the observed macroinvertebrates results, it was noted that the other water quality parameters such as pH, DO and coliform counts were more consistent with the obtained SASS5 scores further confirming the suitability of this system. In their study of the applicability of the SASS5, Bere and Nyakupingidza (2014) applied it to test some urban streams in Zimbabwe [45] . The study assessed the correlation between physical and chemical water quality variables and SASS5 indices. A high degree of agreement between water chemistry parameters and SASS5 metrics was observed, thus indicating the suitability of using SASS scores in detecting water changes in Zimbabwe. Despite the above agreement, the applicability of SASS5 metrics still needs to be tested in different geographical and climatic regions of the country and any observed disparities among the different regions noted in order to adopt the index for the entire country [45] .

Zimbabwe is also not the only country in Africa to test the applicability of the SASS5 scores. Munyika et al. (2014) carried out a study along the Orange River in Namibia aimed at assessing the current water quality and overall health status of the Orange River in Namibia using the South African Scoring System 5 (SASS5) [47] . Gerhardt, (2000) notes that land use activities that have an effect on water quality and river health are believed to increase the pollution of water in most rivers [10] . Namibia most practiced their agriculture along the Orange river and this increased the pollution tolerant macroinvertebrates along the Orange river with other anthropogenic activities including urban development, weir construction and population increase along the river has resulted in proliferation of unplanned settlements with no proper sanitation facilities and increases in pollution levels along the river [47] .

The fact that sampling sites A1 and B1, both located upper stream of the two studied river systems have relatively moderate-good quality water according to SASS5 scores can be explained by the fact that there are less anthropogenic or industrial operations that directly dispose their wastewater effluent into the rivers at these two sites. The SASS5 scores were in agreement with the recorded values for pH, DO, conductivity and the coliform bacteria. The obtained pH values gave an indication that the water was not too acidic or too alkaline (basic) and DO values showed an abundance of oxygen in the water that adequately supports the growth and proliferation of most aquatic organisms [43] . The coliform counts indicated good water quality levels fit for bathing or swimming at site A1and within the accepted limit for drinking water at site B1. The likely source of pollution at sites A1 and B1 comes from agricultural run-off from communities living upstream. However, because sampling was done in April, at the end of the rain season, it is safe to assume that most agricultural contaminants and runoff were already washed away and no longer in the river system due to the end of the rain-fed farming season and also the high rainfall levels that were received in 2017.

The observed significant deterioration in the river water quality at sample sites A2 and B2 downstream, according to SASS5 scores and validated by high coliform counts and pH can be explained by the fact that the rivers have both passed through industrial and residential areas where there is thriving anthropogenic activities upstream. In addition, the city’s wastewater treatment plant also disposes its treated waste effluent, which has often been regarded as of poor quality, into Shagashe river upstream of site B2, thus likely contributing to the high coliform counts recorded at this site [48] . Furthermore, it has been reported in various studies that there is massivestreambank cultivation within the residential suburbs of Mucheke and Rujeko upstream of sites A2 and B2, as well as reported direct discharge of untreated sewage, industrial effluent, abattoirs wastewater and agricultural effluent into the rivers, thus likely impacting on the quality of the water [48] [49] . The observed results also resonate with the existence of the extensive growth of water hyacinth on site B2, an invasive plant species that commonly grows and colonises polluted water bodies. The existence of this plant is thought to reduce the available oxygen (DO) required for the survival of most aquatic organisms [43] .

The research indicates that macroinvertbrates are indeed good indicators of water pollution, despite not being commonly used in water quality studies as compared to the other microbiological, physical and chemical water quality testing parameters. Therefore, there is need to adopt this biomonitoring method widely in Zimbabwe to complement the other commonly used water quality parameters in order to yield effective results. Going forward, there is a need for continuous biomonitoring of various water resources across the country for pollution control and also to establish and optimize Zimbabwean biomonitoring standards of scoring rather than relying on standards adopted from foreign countries.

Chikodzi, D., Mabhegedhe, M. and Tunha, T.K. (2017) Biomonitoring of Mucheke and Shagashe Rivers in Masvingo, Zimbabwe Using Macro-Invertebrates as Indicators of Water Quality. Journal of Geoscience and Environment Protection, 5, 221-237. https://doi.org/10.4236/gep.2017.59016