Limnological study of Malilangwe reservoir in the South-eastern lowveld of Zimbabwe

Abstract

The study provides a nine month record of the limnological investigation of the Malilangwe reservoir between February to October 2011. Malilangwe reservoir is large (211 ha), and shallow (mean depth 4.54) reservoir situated in the south-eastern lowveld of Zimbabwe. The reservoir has not spilled in nearly 11 years which makes it a unique system as most reservoirs of comparable size spill annually. This is the first limnological study of the reservoir, where the physicochemical quality of the water body was examined. The reservoir was not strongly stratified during the hot-wet and –dry season with oxygen depletion being observed in the bottom layers (< 6m depth) of <2 mgl-1 DO concentrations. Nutrients concentrations varied throughout the seasons. The reservoir exhibits marked seasonal fluctuations in water level and water level decreased by over 149 cm (February – October). The N: P ratio ranged from 0 – 10.9 and generally reflect higher levels of phosphorus in the reservoir. There were significant differences (p < 0.05) between the study sites and secchi depth transparency. Differences observed in the water quality were due to water level fluctuations with low water quality conditions being experienced during the hot-dry season and the cool-dry season, when water levels were low. The reservoir was classified as being mesotrophic. Current loadings indicate that nitrogen and phosphorus concentration in both water and sediment will continue to increase using the 1-Layer PAMOLARE model. Therefore the risk of eutrophication is a possibility since the reservoir is now just a sink for nutrients. Seasonal variations of plankton expressed in terms of species composition, cell density and biomass in relation to a selected physicochemical water parameters, were investigated. A total of 98 phytoplankton species; 2 Dinophyta, 4 Euglenophyta, 14 Bacillariophyta, 50 Chlorophyta, 13 Desmids and 15 Cyanophyta and 68 zooplankton species; 2 others (Chaoborus sp. and Hydracarina), 13 Cladocerans, 7 Calanoids/Cyclopoids and 46 Rotifers were recorded. The Chlorophyta, Dinophyta and Cyanophyceae comprised the bulk of the phytoplankton, while the Rotifera and Cladocera comprised the bulk of the zooplankton. Seasonal variations in plankton species composition, density and biomass were high. A typical seasonal succession of plankton species occurred from February to October. Algal blooms were observed during May to July dominated by Anabeana sp., Nostoc sp., Anabeana circinalis, Zygenema sp., Anabeana sporiodes, Ceratium hirudinella and Perinidium sp. Redundancy Analysis (RDA) showed that water level, conductivity, pH, dissolved oxygen, temperature, reactive phosphorus and macrophyte cover accounted for most of the variations in the plankton species. The major conclusion is that the plankton community of Malilangwe reservoir was not dominated by Cyanophyta algae and cladocerans during the entire study period but showed a typical successional pattern. However, Malilangwe reservoir is an example of a poorly-flushed, nutrient-rich reactor which could perpetuate the dominance by Cyanophyta algae and cladocerans in the future. Variation in species composition, distribution and abundance of macrophytes was investigated at 4 sites on three occasions; March (hot-wet season), June (cool-dry season) and September (hot-dry season) over a 9 month period. Thirteen macrophyte species representing eight families were recorded during the study period. Submerged macrophytes had a patchy distribution and mean transect cover per species was below 3% with the exception of Ceratophyllum demersum and Potamogeton pusillus. Canonical Correspondence Analysis (CCA) identified four distinct macrophyte groups closely associated with the three seasons; hot-wet, cool-dry and hot-dry and environmental factors; pH, dissolved oxygen, phosphorus and water level. Change of seasons and subsequent fluctuations in water levels resulted in successional changes in macrophyte community structure from the dominant emergent macrophytes (Cyperus sp., Panicum repens, Ludwigia stolonifera, Phragmites mauritianus and Schoenoplectus corymbosus) in the hot-wet season to submerged macrophytes (Najas sp., Potamogeton crispus, Potamogeton pusillus, Potamogeton tricarinatus and Persicaria decipiens) in the hot-dry season. Although changes in water level variations seemed to influence successional macrophyte structure and composition changes, further research is needed to evaluate to what extent water level fluctuations interact with other seasonal factors acting independently. A Macroinvertebrates community assessment was carried out at five sites in Malilangwe reservoir. The main aim was to investigate macroinvertebrate communities so as to understand factors and processes structuring communities in the reservoir. Forty-two macroinvertebrate families were identified. Thiaridae and Physidae (Mollusca) were the dominant and most abundant taxa with Mollusca constituting 57.71 % of the total sample and Hemiptera (27.31 %). Redundancy Analysis revealed that environmental factors water level, conductivity and macrophyte cover had a strong influence on macroinvertebrate distribution. It was concluded that macroinvertebrates in the reservoir are unevenly distributed in space and time, and that they respond strongly to hydrologically linked parameters such as water level and macrophyte cover and less to water quality variables. Modelling the sedimentation rates with the aid of remote sensing to assess land degradation was carried out. Using NDVI, the catchment showed progressive decline of the vegetation over the years as shown by the decrease in cover. An analysis of NDVI 3 values using ANOVA identified significant differences between the years and sites (p < 0.05). The reservoir capacity to inflow ratio was estimated at 0.8 with a sedimentation rate of 120.1 tkm-2yr-1. Calculated probability of the dam filling is 26.8%. The reservoir is expected to lose 16% of its storage capacity in 100 years at current sedimentation rates according to the Wallingford method. With such a high capacity-inflow ratio for the dam, the reservoir is expected to have a short economic life mainly because it has a sediment trap efficiency of 100%. While acknowledging the limitations of techniques used, this study demonstrates in part the effectiveness of sedimentation modelling and remote sensing as a tool for the production of baseline data for assessment and monitoring of levels of land degradation in the Malilangwe reservoir catchment. A fish diversity fauna survey of the Malilangwe reservoir was conducted taking into account fish surveys done in Malilangwe reservoir, Save, Chiredzi and Runde Rivers, Hippo Valley swamp and irrigation channels between February 2009 - 2011. Gill, seine and fyke nets were used to catch fish within the Malilangwe reservoir. Thirtysix fish species belonging to 11 families were recorded in the Malilangwe reservoir, Save, Chiredzi and Runde Rivers, Hippo Valley swamp and irrigation channels with 8 species from 5 families being recorded from the reservoir. Ten species recorded were introduced species including four exotics and with six species; Glossogobius giuris, Hydrocyanus vittatus, Micropterus salmoides, Oreochromis macrochir, Oreochromis placidus and Labeo altivelis being found in the reservoir. Tilapia rendalli species was of conservation significance as its habitat was being severely affected by water level fluctuations. Fish diversity differed between the water systems. An assessment of the impact of Lernaea cyprinacea on fish populations ten years after its first outbreak was also carried out. Eight fish species from the reservoir were examined for ectoparasite prevalence and intensity. Two parasite species, L. cyprinacea in Oreochromis mossambiccus, Oreochromis placidus, Oreochromis macrochir, Labeo altivelis and Tilapia rendalli and trematode cysts (Clinostomoides brieni) in Clarias gariepinus were found. Lernaea cyprinacea prevalence was 100% amongst all cichlids but varied for L. altivelis. Parasite intensity increased during the cool-dry season (May – July) with greatest mean intensity being observed amongst the cichlids. There was a significant relationship between parasite intensity and environmental factors; dissolved oxygen (r > 0.5, P < 0.05), temperature (r < 0.5, P < 0.001) and pH (r > 0.5, P < 0.001). The continuous spread of L. cyprinacea in the reservoir has potential adverse implications on fish biodiversity and has the potential to wipe out host populations resulting in loss of biodiversity and causing an imbalance to the ecosystem. Length-weight relationships and condition factors (K) for the eight fish species was calculated. The length-weight relationship had a significant positive correlation (r > 0.5) for the eight species. The growth exponent (b) indicated a negative allometric growth for seven species (b range = 1.52 – 6.7) with Tilapia rendalli showing positive allometric growth (b = 6.7). Condition factor (K) values were greater than one (1.34 – 9.29) for O. macrochir, O. mossambicus, C. gariepinus, O. placidus, L. altivelis and G. giuris while it was less than one for some H. vittatus (0.82 – 3.09) and T. rendalli (0.36 – 4.44) fishes. The value of K varied with seasons. The Lake Habitat Survey method was developed to assess the ecological integrity of the physical habitat around lake and reservoir ecosystems and can be used to determine the magnitude of human pressure on a lake system. The LHS method has not been applied to tropical lakes but could potentially be a useful tool. The LHS approach was applied on a tropical African lake, Malilangwe reservoir, in March 2011. The LHS methods that include Lake Habitat Metric Survey (LHMS) and Lake Habitat Quality Assessment (LHQA) were used to assess the habitat quality and the magnitude of human impact. Results show that although Malilangwe reservoir is coming under increasing human pressure, it does not appear to suffer from a major invasion of alien plants. The LHQA score (76 out of 112) and LHMS score (16 out of 42) are indicative of relatively few human pressures (e.g. water pumping structures and residential areas); hence the system can be considered natural. We conclude that the use of LHS can directly enhance quality and reliability of lake assessments and can lead to better lake conservation and rehabilitation. It is clear that for conservation management, a holistic assessment of naturalness, representativeness and species rarity needs to be made in conjunction with scoring systems.