An Assessment of plankton diversity as an water quality indicator in small man-made reservoirs in the Mzingwane catchment, Limpopo basin, Zimbabwe

I. Introduction

1.1 Background

Water quality is at present a global issue, especially when considering its implications to humanity in terms of water borne diseases (Ongley, 1996). The deterioration of water quality has led to the destruction of ecosystem balance, contamination and pollution of ground and surface water resources (Ongley, 1996). Very soon quality will become the principal limitation for sustainable development in many countries. The crisis of freshwater quality is predicted to have the following global dimensions according to Ongley (1996): decline in sustainable food resources due to pollution; cumulative effect of poor water resources management decisions because of inadequate water quality data in many countries; many countries can no longer manage pollution, leading to high level of aquatic pollution, escalating cost of remediation and potential loss of «creditworthiness». It is widely accepted nowadays that land use changes have a huge impact on the water quality of reservoirs. In the Great Lakes of America for example, rapid population growth, intensive industrial and agricultural activities, and sprawling urban development have resulted in significant stress to the near-shore ecosystem (Thorp et al., 1996). Anthropogenic development of Lake Kivu's shoreline in the Bukavu basin has resulted in a change of littoral aquatic biota of the area, many of the species having disappeared (Basima et al., 2005).

Water quality can be defined as an ensemble of physical, chemical and biological (including bacteriological) characteristics of the given water (Straskraba and Tundisi, 1999). Which characteristics are considered important depend on the intended use of the corresponding water; safe drinking water has to fulfil many restrictions, for example. Water quality investigations are carried out to provide information on the health of water bodies and for developing strategies that help in better management of catchment and water resources. In particular, they may assist in preparing an impact assessment, forecasting `what if' scenarios, and assessing the state of the ambient water environment and trends. The investigations may be a single study to tackle a particular water quality

related issue, or they may be an ongoing program to monitor water quality and understand long-term impacts of land uses and other activities in the catchment (Brainwood et al. 2004).

A number of techniques are available, which use the response of different biological species to assess water quality. These techniques include the South African Scoring System (SASS) developed by Dr Mark Chutter (Davis and Day, 2002), The British River Invertebrate Prediction Classification (RIVPACS), The Australian National River Health Programme (AusRivAS) (Muthimkulu, 2004) and The Nepalese Biotic Score (NEPBIOS) (Sharma, 2003). The biological assessment of surface waters started more than a century ago and consisted mainly of the analysis of the differences of organisms living in clean waters from organisms living in polluted waters (Rosenberg and Resh, 1993, Sharma and Moog, 1996 in Sharma, 2003). It is known that ecosystem functioning reflects the collective life of plants, animals, and microbes and the effect their activities-, such as feeding, growing, moving, and excreting waste, -have on the physical and chemical conditions of their environment (Naeem et al. 1999).

Plankton constitute the foundation of the food web in aquatic ecosystems and represent one of the most direct and profound responses to pollution entering reservoirs¹. Plankton is regarded as the community of plants and animals adapted to suspension in the sea or in freshwater, which is liable to passive movement by wind and current (Reynolds, 1984). These microscopic plants and animals are conveniently segregated into the terms «phytoplankton» and «zooplankton» respectively, though there are differences in opinion where the dividing line is drawn (Cander-Lund and Lund, 1995). The plankton proliferation is greatly affected by the water quality (Schindler, 1978) and the predatorprey relationships in the reservoir (Arcifa et al., 1986). Water quality is in turn affected by land use and water sources. Brainwood et al. (2004) presented evidence to suggest that within reservoirs, chemical trends are strongly linked with the differing water sources. Trends were evidenced by quite distinct patterns of water chemistry (Brainwood et al. 2004), which are related to nutrient inputs.

¹ http://www.dnr.state.md.us/bay/monitoring/mon_mngmt_actions/chapter5.html

The great majority of Zimbabwean rural population lives in areas where the mean annual rainfall is below 80 mm and extremely erratic. Soils in this region tend to be infertile sands to sandy loams (Moyo, 1995; Grant, 1981). This unfortunate status makes small, man-made water reservoirs important in Zimbabwe. Their importance is indicated by the fact that 7,000 such reservoirs have been constructed since independence in response to recurrent droughts (Senzanje and Chimbari, 2002). Reservoirs are classified in Zimbabwe as small, medium, large or major in consideration of the maximum height above cleared foundation level and gross capacity (Kabell, 1986). Kabell (1986) defines small reservoirs as one below 106 m³ of capacity and a dam wall height below 8 m. Straskraba and Tundisi (1999) classify reservoirs according to their size capacities. A reservoir was considered small if it had an area of 1 to 100 km² and a capacity of 1 million to 100 million m³ (10⁶-10⁸ m³). Straskraba and Tundisi (1999) call very small reservoirs those of less than one million m³ of capacity and an area of less than 1 km². Small reservoirs in Zimbabwe are often neglected though they support a number of activities in many parts of the country. This is shown by the fact that very little information exist on the capacities, year of construction, water storage capacity, depth of water, sedimentation and most of all about their ecosystems health. Water quality is not monitored and it is difficult to state whether the quality is acceptable or not with regard to the intended uses.

The small reservoirs that are the object in this study are of a size less than three million cubic meters (3 x 106 m³⁾ of capacity and a maximum height of eight (8) m. These reservoirs are multipurpose in use and the main functions include: water supply for rural people and their livestock, brick making; small-scale irrigation, gardening, recreation and tourism, focal point for rural community, small-scale fisheries, wildlife, aquatic life, and microclimate control (Senzanje and Chimbari, 2002). The long-term effects of land use dynamics on water quality, and their potential harm to ecosystem integrity has often been neglected during the planning of small reservoirs. There is also limited information on the small reservoirs in terms of biological diversity and responses of organisms to land and water use factors. Nhiwatiwa (2004) concluded there is a historical bias in limnological investigations towards larger water bodies like Lake Kariba that resulted in small reservoirs not being studied. Studies on these water bodies in Zimbabwe are mainly

focused on improving water availability for sustainable food production and rural development and are mainly concerned by sedimentation and siltation problems (Lawrence and Hasnip, 2004; Lawrence and Lo Cascio, 2004a, Lawrence and Lo Cascio, 2004b, Lawrence and Lo Cascio, 2004c, HR Wallingford, 2004), and increasing productive water use (Mugabe et al. 2003). An inventory of small reservoirs in Zimbabwe has been done by Senzanje and Chimbari (2002) and was mainly concerned with ways to rehabilitate or build new reservoirs, or to assist communities in developing sustainable management plans/strategies for the catchment area of their reservoirs. However, the limnology and ecology of these small reservoirs was not addressed. Historical profiles of small reservoirs in communal areas have been presented by Zirebwa and Twomlow (1999). A comparison of organochlorine pesticide residues in Upper Ncema and Lower Umguza Dams (Zimbabwe) was done by Siwela et al. (1996). DDT and other pesticides originating from farms were found in fish species. These pesticides might have detrimental consequences to aquatic organisms and to a certain extent to humans who feed on them. On the few works done on plankton in Zimbabwe, Thornton and cotterill (1978) provided a list of phytoplankton and zooplankton species occurring in five small reservoirs in the Eastern Highlands, while Green (1990) made a list of zooplankton in eighteen small reservoirs in Marondera and Nyanga. Nhiwatiwa (2004) is among the few authors who studied the limnology and ecology of two small reservoirs in Zimbabwe. The two small reservoirs were built in series on the Munyahuku River. He concluded that very little is known about the ecology of such small water bodies, and that further investigation into this topic is warranted.

Because of this lack of basic ecological knowledge, and the potential for water quality to change as a result of anthropogenic activities, it is important to ecologically assess small reservoirs that exist in rural Zimbabwe. Certain knowledge of the responses of biota to changes in water quality could constitute an important tool to be used by water managers in Zimbabwe to continually and rapidly asses s the quality of waters that they are managing. The present work will focus on an assessment of zooplankton and phytoplankton diversity as a well as some water quality parameters in relation to land and water use on eight selected small reservoirs in the semi arid region of Zimbabwe.

The variability in consideration of reservoirs is highly dependent on the many purposes for which they have been built, as there is some correspondence between their features and their uses (Straskraba and Tundisi, 1999). Small reservoirs differ from large ones in that there are usually no strict rules and regulations guiding access and usage of the water resource. Also they have a semi permanent character in terms of water content and tend to dry out during part of the year (Wotro, 2004).

The importance of the ecological status of surface water systems have been recognized and have contributed to our understanding of the health of our systems². The biological state of health of small reservoirs is to a large extent a function of their use and the land use in the reservoir catchment area. Small reservoirs are known to cause some negative environmental and health impacts; they create or enhance ecological environments that are favourable to the proliferation of vector borne diseases like malaria and schistosomiasis (Hunter et al., 1982; Oomen et al., 1994; Bolton, 1994) and water related diseases. It is believed that different nutrient inputs are flowing to small reservoirs in such areas. Since nutrients are the major support of the proliferation of the phytoplankton community, it is interesting to know how the discharges from different activities affect the plankton community structure. National Parks are considered to have conditions near to pristine and have been selected as control sites while doing this investigation. This study may have important implications on the integrity of ecosystems and therefore on the integrity of the catchment as a whole. The major activities that take place in the study area will be identified for the characterization of the reservoirs and for effective comparisons. The results of the study may show some water resources management implications and might provide guidance for better management of water resources with due respect to land use activities. Findings of this investigation will enable policy and decision makers to find a way forward in the management plans and actions in areas that are having similar characteristics.

Water quality management is important to water managers and decision makers who
often need an understanding of the interactions in small water bodies ecosystems as well

² See http://www.csir.co.za/rhp/provinces/wcape capementro.html

as short term corrective solutions. The short-term solutions aim at diminishing difficulties and long-term solutions that aim to prevent problem creation. The most important management items are summarized in Straskraba and Tundisi (1999). These management items include: - negligence of interrelations of problems might create new unexpected ones; - planning ahead is a step toward success; - responsibility to our children and grand-children mandate fundamental attention to sustainable development; - consideration of biogeophysical, economic and social aspects is fundamental; - environmental impact assessment procedures are useful decision-making tools; - monitoring is an important decision-making tool; - most reservoirs are or eventually become multipurpose that is the basis for conflicts, resolution of these conflicts is enabled by joint participation of the respective parties. The reservoir water quality management publication (Straskraba and Tundisi, 1999) shows managers the importance of a whole system approach and respect to water quality problems. It further shows water quality engineers the need for more advanced theories and inclusion of biological considerations in management decisions. More, it shows limnologists the strength of the whole system approach and the need to include catchments and considerations of human activities including socio-economic and political aspects.

It is important to note that very few investigations have been done on small reservoirs in Zimbabwe, particularly on biological assessment of the integrity of these fragile ecosystems (Twomlow, pers.comm.). This investigation will be therefore a contribution to the understanding of the biology and health of small reservoirs as well and will provide a starting point to future researchers in their management.