Abstract
Intermittent, dire droughts and water abstraction pressures impact shallow man-made reservoirs with multiple designated water uses, often leading to water quality deterioration, and loss of biological integrity and utility value of a lake, threatening the livelihoods of lake shore communities. Thus, water quality information is crucial in setting up guidelines for freshwater resources management. In this study we investigated the water quality, determined the trophic state and assessed the influence of lake zones on the physical-chemical parameters of the Manjirenji Dam, Zimbabwe. Furthermore, we tested the applicability of two customary temperate water quality indices, the Canadian Council of Ministers of the Environment (CCME) Water Quality Index and the Carlson Trophic State Index, for a tropical lake system. Ten littoral and seven pelagic sites were sampled monthly over 9 months for the following water parameters: pH, conductivity, turbidity, total dissolved solids, suspended solids, chlorophyll a , temperature, dissolved oxygen, water transparency, ammonia, nitrogen, nitrates, total and reactive phosphorus. Despite slight fluctuations/variations, water quality in the Manjirenji Dam was generally fair, with a CCME value averaging 78.1, and the Carlson Trophic State Index reflecting oligotrophy. Non-significant differences in water quality parameters between pelagic and littoral sites in the Manjirenji Dam reflect the high connectivity of different spatial zones in a shallow lentic system. Index scores of the adapted temperate water indices detect similar water quality conditions for the Manjirenji Dam, thus perhaps indicating their potential applicability. The current water quality data set for the Manjirenji Dam is vital for formulating prudent management strategies to formulate/ensure adequate multi-purpose water usage and service for this aquatic ecosystem. Keywords: ecotone, lake zones, water quality, trophic state, Manjirenji Dam
Highlights
In large water bodies there are two distinct zones, littoral and pelagic, which respond differently to changes in lake levels (Wetzel, 2001)
In order to address this knowledge gap, this study aimed to investigate the water quality and determine the trophic state of the Manjirenji Dam using two adapted customary temperate water quality indices, the Canadian Council of Ministers of the Environment (CCME) Water Quality Index (CCME, 2001) and the Carlson Trophic State Index (Carlson, 1977), for a tropical lake system
Two measures were used for trophic categorisation of the reservoir – the Canadian Council of Ministers of the Environment (CCME) Water Quality Index (CCME, 2001) with the scoring criteria indicated in Appendix 1 (Table A1), and the Carlson Trophic State classification criteria by Carlson (1977)
Summary
In large water bodies there are two distinct zones, littoral and pelagic, which respond differently to changes in lake levels (Wetzel, 2001). The littoral zone is defined as the zone of shallow water around the edges of lakes to the maximum depth at which light still penetrates to the bottom segments to allow macrophytic growth (Zohary and Ostrovsky, 2011). In deeper lakes, which are defined as water bodies deep enough to stratify (Zohary and Ostrovsky, 2011), effects of excessive water level fluctuations may not be as noticeable as in shallow lakes, with visible changes only evident in the littoral zone (Nowlin et al, 2004). Littoral zone changes can noticeably affect the pelagic zone (Zohary and Ostrovsky, 2011)
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