Tamuka Nhiwatiwa

The impact of water hyacinth ( Eichhornia crassipes) in a eutrophic subtropical impoundment (Lake Chivero, Zimbabwe). II. Species diversity

We compared abundance and diversity of phytoplankton, zooplankton, and fishes among limnetic (P: always without macrophytes) and littoral habitats with (L +) and without (L- ) hyacinths in Lake Chivero, a man-made hypertrophic reservoir near Harare (Zimbabwe). In addition, the littoral macrophyte community, and macro-inver- tebrates associated with hyacinth mats were inventoried. The phytoplankton commu- nity was dominated by blue-green algae (mainly Microcystis aeruginosa ), typical for a hyper-eutrophic lake. Total absolute densities were about 10 to 30 times higher at the L+ sites than at the unvegetated L- and P sites. On the basis of relative species abun- dances the L- zones were more similar to the P than to the L + zones. There was an in- creasing importance of chlorophytes ( Staurastrum sp. and Pandorina morum ) and di- atoms (Cyclotella meneghiniana and pennales) and a decreasing dominance of Mycro- cystis along the discriminant axis from L +, L- to P. The zooplankton community was most dense in the unvegetated zones. Daphnids and bosminids were more abundant in the pelagic than in both littoral zones. Calanoids and Diaphanosoma were dominantly represented in the unvegetated zones. The two littoral zones were characterised by higher densities of chydorids, while they could be discriminated by the dominance of cyclopoids in the vegetated site. Seventeen different fish species were captured by at least one of the different fishing methods. Apparent habitat preferences differed ac-

The distribution and relative abundance of stream fishes in the upper Manyame River, Zimbabwe, in relation to land use, pollution and exotic predators

A survey of the upper Manyame River catchment, middle Zambezi system, Zimbabwe, yielded a total of 22 fish species from 48 stations. The most widespread species (present at >20 stations) were Marcusenius macrolepidotus, Barbus paludinosus, Labeo cylindricus, Clarias gariepinus and Tilapia sparrmanii. The most numerous species (>10% of the total) were B. paludinosus, B. lineomaculatus and T. sparrmanii. A number of species that occurred, or formerly occurred, in the catchment were not collected and the possible reasons for their absence are discussed. Some distinctive patterns of distribution could be discerned, with the presence of exotic predators (Micropterus salmoides and Serranochromis robustus) and pollution being the most important determinants. Although the species richness of indigenous fishes was relatively high when the predators were present, their relative abundance was low. Pollution from sewage, in contrast, brought about a reduction in both species richness and biomass. Land use also influenced both species richness and abundance, which were lower on commercial farms where most streams were regulated by dams and where exotic predators were present. In contrast, both variables were highest on communal lands where subsistence farmers had not built dams, leaving the streams unregulated, and where exotic predators were generally absent.

Temporal variation of the plankton communities in a small tropical reservoir (Malilangwe, Zimbabwe)

ABSTRACT Seasonal variations of plankton, expressed in terms of species composition and cell density in relation to selected physicochemical water parameters, were investigated in Malilangwe reservoir, Zimbabwe, between February and October 2011. In total, 98 phytoplankton species (2 Dinophyta, 4 Euglenophyta, 14 Bacillariophyta, 50 Chlorophyta, 13 Desmids and 15 Cyanophyta), 66 zooplankton species (13 Cladocerans, 7 Calanoids/Cyclopoids, 46 Rotifers) and two others (Chaoborus sp. and Hydracarina) were recorded. Chlorophyta, Dinophyta and Cyanophyceae comprised the bulk of the phytoplankton, while Rotifera and Cladocera dominated the zooplankton community. 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 distribution of the plankton species.

Water quality and plankton dynamics in two small dams in Zimbabwe

Two small dams on the Munwahuku River was investigated in relation to the influence of hydrology on their water chemistry over the period January 2000–August 2001. The water level of the upper dam fluctuated more than that of the lower dam and their theoretical water retention times were nine days and 3.6 days, respectively. Seasonal fluctuations in conductivity, TDS, TSS and transparency were due to effects of the first rains, while increases in BOD and COD suggested increased organic matter at low water levels. There were no dramatic seasonal variations in pH, alkalinity or nutrients, although total nitrogen was higher in the upper dam. The N:P ratios ranged from 0.57–3.75 in the upper dam and 0.92–14 in the lower dam and generally reflect the high levels of phosphorus in the dams. This could make them susceptible to colonisation by Cyanophyta. There were significant differences (p < 0.05) between the dams in total phosphorus, reactive phosphorus, nitrites and chlorophyll a. Small differences observed in the water quality of the two dams could be due to short water residence times. Extremes in water quality conditions were experienced during the dry season, when water levels were low, and during the rainy season when levels were higher. Peaks in phytoplankton and zooplankton abundance occurred during summer in both dams, and differences observed between the plankton community of the period January–October 2000 and that of November 2000–August 2001 were unexpected. The changes in the plankton communities, especially in the lower dam, were remarkable. There were no significant differences in plankton abundances between the dams. Phytoplankton dynamics were weakly associated with environmental factors in both dams and, apart from predation by fish and grazing by zooplankton, washouts during the rainy season and unstable stratification regimes were considered to be contributory factors. Canonical Correspondence Analysis showed that temperature and water transparency (also linked with suspended solids) were important variables in zooplankton dynamics in both dams. Hydrological factors, such as water retention time, floods and water levels, are key determinants of plankton communities in small dams.

Metal concentrations in water, sediment and sharptooth catfish Clarias gariepinus from three peri-urban rivers in the upper Manyame catchment, Zimbabwe

Concentrations of zinc, cadmium, chromium, nickel, lead, copper and iron were measured in flowing water, riverbed sediments and tissues of sharptooth catfish Clarias gariepinus from three rivers in the upper Manyame catchment over seven months in 2008–2009. The Manyame and Mukuvisi rivers are severely polluted by industrial and domestic effluent, whilst the Gwebi River is not influenced by urban effluent. Key water quality parameters, including dissolved oxygen and conductivity, clearly showed a pollution gradient in the Mukuvisi and Manyame rivers, but water quality in the Gwebi River was good. Levels of zinc, iron, copper, nickel and lead in fish tissues from the three rivers sampled were unusually high, with zinc and iron concentrations being the highest in all the tissues. This was also positively correlated with the concentrations of these metals in water and sediments. Notable differences existed between the water (zinc and copper) and sediments (iron and zinc) of each river. The relatively high metal concentrations in the Gwebi River, as well as conductivity and dissolved ions, were explained by the geological influence of the Great Dyke in its subcatchment. Metals are bound in the sediment but these can be rapidly mobilised into water if environmental changes occur, therefore efforts to monitor and prevent further water quality deterioration are required. The results of this study may have significant negative implications for aquatic organisms and for human health through fish consumption and therefore risk assessment investigations are imperative.

Impact of irrigation based sugarcane cultivation on the Chiredzi and Runde Rivers quality, Zimbabwe

Agriculture is vital in sustaining human livelihoods. However, agriculture as it is currently practiced, is contributing to the degradation of freshwater ecosystems globally. We investigated impacts of irrigation return flows from sugarcane farming on water quality and health status of the Chiredzi and Runde Rivers, a biodiversity hotspot region in south-eastern Lowveld of Zimbabwe. The water quality at inlets from the crop field into the wetland system; wetland outlets into the river systems; and river sites upstream and downstream of wetland outlets were monitored during the dry and wet seasons. The wetland system formed naturally from excessive drainage from the cane fields but its purifying capacity was unknown to date. An assessment of the water physical-chemical variables (at all sites) and macroinvertebrate communities (at river sites only) was carried out. Results showed that the wetland was deficient in its purifying capacity as it was already saturated by salts and nutrients from high irrigation return flow loads. A significant seasonal variation was observed for conductivity, reactive phosphorus (RP), pH and total phosphorus (TP) concentrations among the inlets to the wetland whereas among the river sites significant seasonal differences were observed for ammonium, dissolved oxygen, nitrate, RP, pH, TP and turbidity concentrations during the dry season. From the macroinvertebrate community data the impact of the irrigation return flows on the river system was apparent, as the good water quality sites were characterised by a high diversity of pollution sensitive macroinvertebrate taxa, while the irrigation impacted sites were characterised and dominated by pollution tolerant taxa. High ion concentration (conductivity and salinity) and pH were found to be important in structuring macroinvertebrate communities as determined using multivariate analysis in the river system. In conclusion, the river water quality was significantly impacted by irrigation return flows and this greatly reduced the ecological health as highlighted by low macroinvertebrate diversity and increases in physical-chemical variables.

Aquatic macrophytes in a tropical African reservoir : diversity, communities and the impact of reservoir-level fluctuations

ABSTRACT Variation in species composition, distribution and abundance of macrophytes in the Malilangwe reservoir, a tropical African reservoir, was investigated at four sites in three seasons; March (hot-wet season), June (cool-dry season) and September (hot-dry season) over a nine-month period. Thirteen macrophyte species, representing nine families, were recorded during the study period. Submerged macrophytes had a patchy distribution and the 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 changes in macrophyte community structure from the dominant emergent macrophytes (Cyperus involucratus, Panicum repens, Ludwigia stolonifera, Phragmites mauritianus and Schoenoplectus corymbosus) in the hot-wet season to submerged macrophytes (Najas horrida, Potamogeton crispus, Potamogeton pusillus, Potamogeton tricarinatus and Persicaria decipiens) in the hot-dry season. Although changes in water level variations seemed to influence macrophyte structure and composition changes, further research is needed to evaluate to what extent water level fluctuations interact with other seasonal factors acting independently.

An assessment of chlorophyll-a concentration spatio-temporal variation using Landsat satellite data, in a small tropical reservoir

Traditional approaches to monitoring aquatic systems are often limited by the need for data collection which often is time-consuming, expensive and non-continuous. The aim of the study was to map the spatio-temporal chlorophyll-a concentration changes in Malilangwe Reservoir, Zimbabwe as an indicator of phytoplankton biomass and trophic state when the reservoir was full (year 2000) and at its lowest capacity (year 2011), using readily available Landsat multispectral images. Medium-spatial resolution (30 m) Landsat multispectral Thematic Mapper TM 5 and ETM+ images for May to December 1999–2000 and 2010–2011 were used to derive chlorophyll-a concentrations. In situ measured chlorophyll-a and total suspended solids (TSS) concentrations for 2011 were employed to validate the Landsat chlorophyll-a and TSS estimates. The study results indicate that Landsat-derived chlorophyll-a and TSS estimates were comparable with field measurements. There was a considerable wet vs. dry season differences in total chlorophyll-a concentration, Secchi disc depth, TSS and turbidity within the reservoir. Using Permutational multivariate analyses of variance (PERMANOVA) analysis, there were significant differences (p < 0.0001) for chlorophyll-a concentration among sites, months and years whereas TSS was significant during the study months (p < 0.05). A strong positive significant correlation among both predicted TSS vs. chlorophyll-a and measured vs. predicted chlorophyll-a and TSS concentrations as well as an inverse relationship between reservoir chlorophyll-a concentrations and water level were found (p < 0.001 in all cases). In conclusion, total chlorophyll-a concentration in Malilangwe Reservoir was successfully derived from Landsat remote sensing data suggesting that the Landsat sensor is suitable for real-time monitoring over relatively short timescales and for small reservoirs. Satellite data can allow for surveying of chlorophyll-a concentration in aquatic ecosystems, thus, providing invaluable data in data scarce (limited on site ground measurements) environments.

A hotspot of large branchiopod diversity in south-eastern Zimbabwe

Large branchiopods are considered threatened across much of their global range. However, because several regions, including Zimbabwe in general and its south-eastern lowveld in particular, remain largely unstudied, interpretations of species distribution patterns are often based on limited data. A detailed study of large branchiopods was carried out, with repeated sampling of 36 seasonal pans, in 2004-2008 in the Save Valley Conservancy, south-eastern lowveld. Sixteen large branchiopod species were collected, comprising eight Anostraca, one Notostraca, two Laevicaudata, four Spinicaudata and one cyclestherid species, representing 33% of all species known in southern Africa. Most species were new records for Zimbabwe. The enigmatic morphotype, Streptocephalus cf. bidentatus, was encountered for the first time in Zimbabwe, presenting an opportunity to resolve its taxonomic status. Streptocephalus wirminghausi, a species endemic to Zimbabwe, was also recorded. Large endorheic pans had higher species richness (an average of eight species) than small endorheic pans (average of six species), whilst floodplain pans housed fewest species (average of four species), probably due to predation by fish after floods. Since regional diversity and coexistence levels in the Save Conservancy were high, compared to other regions, this area should be considered a hotspot of large branchiopod diversity.

Seasonal and diurnal stratification in two small Zimbabwean reservoirs

Seasonal and diurnal stratification patterns were studied in two small Zimbabwe reservoirs. The water level of the upper reservoir fluctuated more than that of the lower one and theoretical water retention times were 9 days and 3.6 days, respectively. Rainfall is seasonal but water demand for irrigation and potable water persist throughout the year. There was a distinct cycle of seasonal thermal stratification with associated deoxygenation, but this was not as stable or pronounced as in some larger Zimbabwean reservoirs. Weaker thermal stratification was established during the winter. Seasonal trends were related to changes in incoming solar radiation and the corresponding changes in air temperatures. Diurnal cycles of thermal stratification and oxygen occurred in both reservoirs throughout the year, but were less established during the winter months. The frequent mixing recorded is a significant finding because, if there is mature sediment in such reservoirs, there may be a rapid exchange of nutrients between sediments and the water, leading to high overall productivity. The shallow nature of small reservoirs and the considerable fluctuations in their water levels makes them more vulnerable to external fluxes such as daily changes in incoming short-wave solar radiation and wind runs, and hence the prevalence of diel stratification regimes. Consequently, small reservoirs are characterised by low thermal stability as compared to larger, deeper reservoirs. Diurnal heat fluxes were observed in both reservoirs accounting for their weak thermal stability. Birgean energy budgets for these reservoirs were small (11.6 and 54.3 MJ m−2 yr−1 for the upper and lower reservoirs, respectively). Reasons for this are that their small size severely limits their capacity to capture and retain heat energy and, although not quantified in this study, high evaporation rates contribute to low energy budgets in tropical waterbodies as compared to temperate ones.