Asian aquatic plant moth Parapoynx diminutalis,accidentally introduced earlier,contributes to control of an aquatic weed Hydrilla verticillata in South Africa

The Shire River drains from Lake Malawi to the Lower Zambezi River. Annual flow is dependent mainly on lake level, partially controlled by the operation of a barrage at Liwonde to regulate flows for hydroelectricity generation in the escarpment reaches of the river. Downstream of the escarpment, the floodplains of the Lower Shire River support important subsistence fisheries that yielded on average 5 100 t y^?1 between 1977 and 1993. Catches from the major fishery area, the Elephant Marsh, were positively correlated with mean annual flow for the same year. Since the target fish species (mainly clariid catfishes) were mostly caught in their second and third year, improved recruitment through higher breeding success at higher flow levels is less likely to explain interannual variation in catches than is greater upstream migration of fish from the Lower Zambezi River in high-flow years. In 1978 a sudden closure of Liwonde Barrage resulted in rapid drainage of stagnant water from the Elephant Marsh floodplain vegetation into the river, causing a mass fish mortality and thus a marked decline in catches from 1979, followed by gradual recovery by 1981. The delay in recovery contrasts with the annual direct catch/flow relationship, and raises questions about the most important influences on Lower Shire catch rates.     

Genetic diversity of two Southern African cichlids (Oreochromis andersonii and O. macrochir) in the Zambezi and Congo River basins

Oreochromis andersonii and O. macrochir are two important cichlid species native to Southern Africa. We describe in this paper their genetic population structure in the Upper Zambezi River, Kafue River, and Lake Bangweulu representing part of the Congo and Zambezi River Basins. Microsatellite genetic markers were employed to analyse the genetic population structure of the two species using 177 tissue samples. The average allele richness of O. andersonii was higher in the Zambezi River (10.500) than in the Kafue River (9.583) though not statistically different. For O. macrochir, it was highest in the Zambezi River (11.170) followed by the Kafue River (9.781) and least in Lake Bangweulu (7.067) and their differences were significant. The gene diversity indices; gene diversity (hs), observed heterozygosity (H_O), expected heterozygosity (H_E) were high and similar in O. andersonii populations. However, in O. macrochir H_E was significantly lower in Lake Bangweulu (0.678) compared to the Kafue River (0.799) and Zambezi River (0.802) populations. Population differentiation estimated by R_ST and D_EST revealed high differentiation in both species (R_ST = 0.598, D_EST = 0.777 for O. andersonii; R_ST = 0.379, D_EST = 0.710 for O. macrochir). The highest source of variation was among populations (84.71%) for O. andersonii and within populations (67.09%) for O. macrochir. Comparisons of population pairs revealed a close genetic similarity between the Zambezi River and Lake Bangweulu populations of O. macrochir. Bottlenecks were observed in both species using the Two-Phase Model (T.P.M.) indicative of a recent genetic loss or reduction in effective population size. Though our results indicate that the populations of both species still maintain sufficiently high levels of genetic diversity in the sampled areas, the bottlenecks observed are a source of concern. We recommend a more robust study of genetic diversity of these species in all sections of these river systems and that some key conservation sites should be identified to protect the gene pool of these native species.     

Phylogeography and speciation in the <Emphasis Type="Italic">Pseudocrenilabrus philander</Emphasis> species complex in Zambian Rivers

Haplochromine cichlids form the most species-rich lineage of cichlid fishes that both colonized almost all river systems in Africa and radiated to species flocks in several East African lakes. The enormous diversity of lakes is contrasted by a relatively poor albeit biogeographically clearly structured species diversity in rivers. The present study analyzed the genetic structure and phylogeographic history of species and populations of the genus Pseudocrenilabrus in Zambian rivers that span two major African drainage systems, the Congo- and the Zambezi-system. The mtDNA phylogeny identifies four major lineages, three of which occur in the Congo-system and one in the Zambezi system. Two of the Congo-clades (Lake Mweru and Lunzua River) comprise distinct albeit yet undescribed species, while the fish of the third Congo-drainage clade (Chambeshi River and Bangweulu swamps), together with the fish of the Zambezi clade (Zambezi and Kafue River) are assigned to Pseudocrenilabrus philander. Concerning the intraspecific genetic diversity observed in the sampled rivers, most populations are highly uniform in comparison to lacustrine haplochromines, suggesting severe founder effects and/or bottlenecking during their history. Two bursts of diversification are reflected in the structure of the linearized tree. The first locates at about 3.9% mean sequence divergence and points to an almost simultaneous colonization of the sampled river systems. Subsequent regional diversification (with about 1% mean sequence divergence) occurred contemporaneously within the Kafue River and the Zambezi River. The clear-cut genetic biogeographic structure points to the dominance of geographic speciation in this lineage of riverine cichlid fishes, contrasting the importance of in situ diversification observed in lake cichlids.     

The influence of river flow on pelagic sardine catches in Lake Kariba

The nutrient level of Lake Kariba is governed by the Zambezi river which provides about 70% of its water supply. Other tributaries are richer in nutrients and have an appreciable local effect. This is shown by the catches of Limnothrissa miodon which increase after strong flows in the Sanyati river. This is in contrast to natural lakes such as Lake Tanganyika and may be an important management factor in Lake Kariba and other man-made lakes. Possible periods of poor rainfall could thus reduce the Limnothrissa yield in Lake Kariba.     

PRODUCTION OF THE AFRICAN CATFISH (CLARIAS GARIEPINUS) I. GROWTH,MORTALITY AND YIELD SOUTH OF THE ZAMBEZI

SUMMARY

Clarias gariepinus were sampled from eight water bodies in southern Africa between 1973 and 1975. Aging results indicate a similar growth pattern for most areas with a near linear age-length relationship. Between year growth compensation was found to be present in Lakes Kyle and Mcllwaine in Zimbabwe, and to a lesser extent in Hardap Dam, South West Africa, but not present in Lake Kariba, Zimbabwe.

The fishable biomass of catfish from Lake Kyle is estimated between 0,61 to 1,14 kg ha^?1. Fishable biomass for all species is in the range 12,1 to 22,7 kg ha^?1.

The instantaneous growth rate was found to be between 0,20 and 0,25 for fully recruited age groups. The fishing mortality appears to be in the range of 0,4 for Lakes Kyle and Kariba and 0,5 for Lake McIlwaine. All of these levels are above the yield per recruit F₀, 1 levels of 0,27, 0,28 and 0,24 respectively which would normally indicate that each of these stocks is being overexploited. However, this does not appear to be the case in these tropical impoundments.     

The limnology of Cabora Bassa, Moçambique, during its first year

SUMMARY. Cabora Bassa, a new impoundment in the Middle Zambezi River, was closed in December 1974, filling rapidly to 12m below full supply by May 1975.
The lake, with five basins, lies along a west–east axis almost parallel to prevailing south-easterly winds. Limnological records, from several stations along the lake axis, indicated homoiothermal conditions until September, the hot dry season. From then until records ceased in December, thermoclines and oxyclines were present in the deeper eastern basins. Hydrogen sulphide was recorded in hypolimnial waters towards the end of the year. Open water of the shallow, wind- and wave-swept eastern basins remained oxygenated throughout the year.
Water transparency increased fivefold from April to December, increasing too from east to west. pH was alkaline becoming more acid in cool winter months and in bottom waters of stratified basins. Conductivity ranged from 95 to 119 °S cm⁻¹ with higher values in shallow lake margins. Chemical content of surface waters showed little seasonal or spatial variation and was, in general, comparable with pre-impoundment levels.
Duration of the initial 'productive phase' will probably be shorter than Kariba due to more rapid lake water exchange but indications are that productivity will be higher in the maturation phase.     

CHLOROPHYLL-a CONCENTRATION IN THE LOWER KAFUE RIVER AND CHONGWE RIVER BASINS

SUMMARY

The concentration of chlorophyll a was measured in the warm monomictic impoundment, Itezhi-tezhi Lake ([xbar] = 2,00 mg m^?3); in the small, slightly polluted Chongwe Dam ([xbar] = 11,0 mg m^?3); and in the temporary Mungasiya River ([xbar] = 53,4 mg m^?3). The average standing crop of the periphyton of green filamentous algae in the Chongwe River reached 28,3 mg m^?2. The highest concentrations of chlorophyll ([xbar] = 69,6 mg m^?2) were found in the Cyanophyta community of the extreme biotope of Longola Hot Springs.     

Synopsis of scientific results of the sarec sponsored project on the ecology of Lake Kariba

A summary of current research on Lake Kariba is given. Lake Kariba is now a phosphorus limited oligotrophic lake, dependent on annual nutrient input for the maintenance of production. Nitrogen fixation by cyanobacteria has become an important source of nitrogen in the dry season and that fish harvesting is an important phosphorus sink. An up to date plankton composition list has been produced and plankton biomass determined. Diving studies indicate large biomass of bivalves. The aquatic vegetation displays both longitudinal as well as depth gradients, related to light regimes. Only preliminary data are available on mud/water interactions, but the available data indicate considerable fluxes in the river mouth stations and shallow protected littoral areas. The role of predatory birds and crocodiles in the fishery economy of the lake is evaluated and indicates no major conflict between these predators and human fishing interests. The research also draws attention to pesticides in Lake Kariba as indicated by work on fish eating birds and crocodile research. Since the publication of ‘Lake Kariba’ by Balon & Coche (1975) the research project, THE ECOLOGY OF LAKE KARIBA, being undertaken by the University Lake Kariba Research Station constitutes the first multidisciplinary study of the lake, 30 years after the Zambezi river was impounded at Kariba Gorge. It could have offered a unique opportunity of comparing the lake now with what it was soon after impoundment.     

Life history and growth of Asellus aquaticus (L.) in relation to environment in a eutrophic lake in Norway

Eichhornia crassipes and Salvinia molesta, both notorious weeds, are present in the catchment of Cabora Bassa, a new man-made lake on the Zambezi River, Moçambique. Weed dispersal, controlled by wind and current (and indirectly by rate of lake filling and lake morphology) culminated in the distribution of mats in the eastern and western extremities with very small cover in central lake areas. Eichhornia offset production was initially very rapid later diminishing markedly with many plants showing symptoms o: nutrient deficiency. Eichhornia completely dominated mat composition at the end of the year whilst Salvinia cover was negligible. Heavy drawdown in the middle of the year lead to destruction of nearly 50% of the weed mats. Whilst pre-drawdown levels were attained by the end of the year, there was no evidence for expected explosive population growth and extensive weed colonization in the filling phase.     

Comparing the fish assemblages and food‐web structures of large floodplain rivers

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Fish population changes in the Volta lake in Ghana during its first sixteen months

Summary A study has been made on changes which have occurred in the fish fauna as a result of the formation of the Volta Lake in Ghana. The data on the relative abundance of fish in the Lake were compared with those obtained from fish landings on the Black Volta and from the Niger river survey, as no observations of fish abundance were made on the Volta River before the Akosombo dam was closed. General changes in the relative abundance of the most important families of fish are described and the changes in the relative abundance of six feeding types in the Volta Lake and Black Volta, based on more than 700 stomach and gut analyses of 50 species of fish, are compared with those of the fish found in the Zambezi River and Lake Kariba in Central Africa.The most characteristic features of the changes in fish fauna are an increase in plankton feeders, so that these are becoming dominant in some areas; a great decrease in mormyrids in the Lake as compared with the river, and a similar drop in the characid Alestes, while Tilapia has increased. These changes are not uniform throughout the lake, and differences are most marked along the North-South axis.Department of Zoology and Volta Basin Research Project, University of Ghana     

Biology of the African clupeid Limnothrissa miodon with reference to its small size in artificial lakes

Summary Limnothrissa miodon is a clupeid from Lake Tanganyika which has been introduced to Lakes Kivu and Kariba and which invaded Lake Cahora Bassa. These lakes differ considerably from one another but the biology of Limnothrissa is similar in many respects in all of them. Similarities include its feeding and breeding biology, whilst its populations fluctuate, on both an annual and a seasonal basis, in relation to environmental changes. The major differences between the lakes concern the size to which Limnothrissa grows. Their average length in the two natural lakes, Lakes Tanganyika and Kivu, is about twice the average length in the two artificial lakes, Lakes Kariba and Cahora Bassa. This in turn affects their age of maturity and they breed during their first year in the reservoirs but during their second in the natural lakes. Their growth rates and performance are comparable up to about 6 months of age and the difference in their size seems to be due to high mortality in the artificial lakes where few Limnothrissa survive for more than a year. Possible explanations for their small size include the genetic composition of the original introduction and the unpredictability of the environment. Neither of these explanations is supported by evidence at present. Deficiencies in the available food and the effects of intense predation are probably the most likely causes. In both natural lakes, Limnothrissa are predatory or cannibalistic from 100 mm in length and 12 months in age, and fish in their diet may enable them to survive into their second year. The fish in Lakes Kariba and Cahora Bassa, in contrast, are neither predatory nor cannibalistic to any degree. There are no predatory fish species in Lake Kivu but there is a diverse community of pelagic piscivores in Lake Tanganyika which tends to contradict the predation hypothesis. However, the position of Limnothrissa in the reservoirs probably resembles that occupied in Lake Tanganyika by Stolothrissa which is also a small, annual species. The significance of this phenomenon is that Limnothrissa can maintain a high biomass and productivity in the face of intense predation. This trait may be of importance to fisheries management because it means that their yield can be greatly increased.     

Phytoplankton primary productivity seasonality and changes in a small African lake,Lake Hora-Kilole,Ethiopia

The seasonality of primary productivity by phytoplankton in relation to physico-chemical and biological variables was studied in Lake Hora-Kilole from August 2007 to May 2008. In 1989, the Mojo River was temporarily diverted to flow into the lake, which substantially changed its physico-chemical conditions and the composition of the phytoplankton. Primary productivity was controlled primarily by soluble reactive phosphorus (SRP), ammonia (NH₃), temperature and euphotic depth (Z_eu). The light-saturated rate of photosynthesis (A_max) varied from 370 to 3 843?mg O₂ m^?3 h^?1 with the maximum value corresponding to the seasonal maximum of phytoplankton biomass. Compared to the period before the diversion of the river, A_max was reduced by more than ninety-fold in early 1990s and by less than five-fold in 2007 and 2008. Similarly, average phytoplankton chlorophyll a was reduced by more than 2.5 × in the early 1990s and to less than 50% in 2007 and 2008. This highlights the importance of the diversion river water on the physico-chemical and biological environment of the lake.     

洱海入湖河流弥苴河下游氮磷季节性变化特征及主要影响因素

入湖河流携带污染物对洱海水环境的影响日益明显,对洱海入湖水量最大的河流——弥苴河下游水体氮磷进行了连续采样分析,以期为河口湿地建设和水质改善提供基础数据.结果表明:1)弥苴河水质介于地表水Ⅲ-Ⅴ类之间,主要污染物为氮和磷,其中总氮平均浓度为1.17 mg/L,最高浓度达到2.00 mg/L;总磷平均浓度为0.06 mg/L;2)弥苴河下游总氮、总磷浓度丰水期高于枯水期,并呈现出季节性变化规律;3)弥苴河下游水体总氮、总磷年均浓度远高于洱海水体总氮、总磷年均浓度,其中总氮高出2.10倍,总磷高出2.90倍;4)弥苴河下游河段非点源污染占据主导地位.     

A quantitative diving survey of benthic vegetation and fauna in Lake Kariba, a tropical man-made lake

SUMMARY. 1. The biomass distribution of submerged vascular vegetation and benthic fauna were investigated by diving in Lake Kariba. The vegetation was well correlated with transparency of the water. Maximum biomass (1400 g dryweight m^?2) and a depth penetration of 6 m were found in areas little influenced by river inflow, while these were only 110 g m^?2 and 2 m, respectively, in the basin receiving water from the Zambezi river. 2. The lake is mesotrophic-oligotrophic. The total biomass for the lake was 101,000 tons dryweight of rooted vegetation composed of Lagarosiphon ilicifolius Oberm. (52%), Najas pectinata (Parl) Magnus (33%), Vatlisneria aethiopica Frenzl (11%), Ceratophyllum demersum L. (3%) and Potamogeton octandrus L. (0.5%). Average plant biomass for the potentially colonizable depth zone of 0-12 m and for the total lake amounted to 79.9 g m^?2 and 18.8 g m^?2, respectively. 3. The distribution of the benthic fauna generally followed that of the vegetation. The total animal biomass of 118,840 tons dryweight. including shells, consisted of mussels (95.8%), snails (4.1%) and insect larvae (0.1%). Four species of mussels were found: Caelatura mossambicensis (von Martens) (81% of mussel biomass), Corbicula africana (Krauss) (9%). Mutela dubia (Gmelin) (5%) and Aspatharia wahlbergii (Krauss) (5%). Among the snails Melnoides tuberculuta (Müller), Cleopatra spp. and Bellamya capillata (Frauenfeld) dominated. 4. The average animal biomass was high compared to most other lakes perhaps due to lack of predators. For the colonizable 0-12 m depth interval and the total lake it was 96.2 g m^?2 including shells (15.0 g m^?2 shell-free dryweight) and 22.6 g m^?2 including shells (3.4 g m^?2 shell-free dryweight). respectively. Biomass of plants and animals was even higher prior to the recent lowering of the water level by 7 m, which was estimated to have stranded 84,000 tons of mussels on the shore.     

Effect of Electron Donor to Sulfate Ratio on Mercury Methylation in Floodplain Sediments under Saturated Flow Conditions

A column transport experiment was conducted to examine the release and methylation of Hg using Hg contaminated sediment from the floodplain of the South River near Waynesboro, Virginia. Three input solutions were sequentially introduced into the column. Input 1 was unamended South River water, Input 2 was river water amended with 100 mg L^?1 SO₄ and 3600 mg L^?1 lactate, and Input 3 was river water amended with 500 mg L^?1 SO₄ and 340 mg L^?1 lactate. During the first stage of the experiment (Input 1) the effluent Hg concentration was initially 4 µg L^?1 and peaked at 21 µg L^?1 and after 21 pore volumes stabilized at 13 µg L^?1. During the second stage, at high lactate to SO₄ ratios, elevated concentrations of acetic and propionic acids were detected, indicating that fermentative bacteria were dominant. During the third stage, at high SO₄ to lactate ratios, a decrease in SO₄ and an increase in H₂S concentrations were detected in the column effluent indicating that SO₄ reduction was occurring. Concentrations of methyl Hg (MeHg) in the effluent were variable over the duration of the experiment. During the first phase, concentrations of MeHg remained <3.3 ng L^?1. During the fermentative stage, concentrations of MeHg increased to a maximum value of 32 ng L^?1, and during the sulfate-reducing stage to a maximum value of 266 ng L^?1. When the column was deconstructed both molecular and cultural techniques indicated that sulfate reducing bacteria were most dominant near the influent port. These results indicate that the formation of MeHg in the sediment is not limited by the availability of Hg and that the bacterial community that contributes to mercury methylation can respond quickly to changes in the abundances of electron donors and acceptors.     

Hydrological variability, organic matter supply and denitrification in the Garonne River ecosystem

1. Groundwater nitrate contamination has become a worldwide problem as increasing amounts of nitrogen fertilisers are used in agriculture. Alluvial groundwater is uniquely juxtaposed between soils and streams. Hydrological connections among these subsystems regulate nutrient cycling. 2. We measured denitrification using an in situ acetylene‐block assay in a nitrate‐contaminated portion of the Garonne River catchment along a gradient of surface water–ground water mixing during high (snowmelt) and low flow. 3. During high flow (mid‐April to early June) the water table rose an average of 35 cm and river water penetrated the subsurface to a great extent in monitoring wells. Denitrification rates averaged 5.40 μgN₂O L^?1 min^?1 during the high flow period, nearly double the average rate (2.91 μgN₂O L^?1 min^?1) measured during base flow. This was driven by a strong increase in denitrification in groundwater under native riparian vegetation. Nitrate concentrations were significantly lower during high flow compared with base flow. Riparian patches had higher dissolved organic carbon concentrations that were more aromatic compared with the gravel bar patch closest to the river. 4. Multiple linear regression showed that the rate of denitrification was best predicted by the concentration of low molecular weight organic acids. These molecules are probably derived from decomposition of soil organic matter and are an important energy source for anaerobic respiratory processes like denitrification. The second best predictor was per cent surface water, reflecting higher denitrification rates during spring when hydrological connection between surface water and ground water was greatest. 5. Our results indicate that, while denitrification rates in Garonne River alluvium were spatially and temporally variable, denitrification was a significant NO₃ sink during transport from the NO₃‐contaminated floodplain to the river. DOC availability and river–floodplain connectivity were important factors influencing observed spatial and temporal patterns.     

A silicon budget for an Alaskan arctic lake

The cycling of dissolved reactive silicate (DRS) and sedimentary biogenic SiO₂ has been examined in ultraoligotrophic Toolik Lake, Alaska. Watershed output of DRS (∼ 7 mmol m ^{− 2} yr ⁻¹) is similar to other arctic study sites, but a short water residence time results in the poor retention of DRS (∼17%) within the lake. Biogenic SiO₂ concentrations in surficial sediments average 123 mg g⁻¹ with the highest concentrations arising from the production of benthic diatoms in the littoral zone. Lake water DRS concentrations are highest prior to spring flow and photic zone concentrations exceed 23 μmol 1⁻¹ at the time of greatest primary production. Wintertime increases in watercolumn DRS concentrations and spring and summer pore water DRS flux estimates indicate that internal cycling within the lake may supply an amount of DRS equivalent to that which biologically utilized.     

Dissolved organic carbon, seston and macroinvertebrate drift in an acidified and limed humic stream

SUMMARY. 1. Total seston, and invertebrate drift were studied before and after lime addition to Fyllean River, a stream-iake system in Halland county, southwest Sweden, with poorly buffered waters undergoing acidification. 2. The largest effect of liming was on the chemistry of the water. Following liming with 23 mg CaCO₃ l^?1 the pH of the water changed from 5.8 to 6.8 and alkalinity from 0.04 to 0.13 meq l^?1.Turbidity increased from 3.4 to 4.7 JTU with no change in colour. 3. Dissolved organic carbon (DOC) concentration of all samples was in the range 10.7–13.3 mg C l^?1 with no significant change occurring due to liming. 4. Total seston increased from 4.35 mg DM 1^?1 in unlimed conditions to 6.25 mg DM l^?1 after lime addition. All significant changes in seston occurred in the smaller size fraction (0.45–25 μm). 5. Liming reduced the organic content of the partieulate material from an average of 61% to 39% immediately downstream of a lime silo (within 1 km) but had little effect when the river course was interrupted by lakes and impoundments. 6. The lakes in the river system had a larger effect on seston concentration than any effect of the lime addition by itself. Particle concentrations were reduced by 50–55% and DOC by about 1 mg C l^?1as the water passed through the lakes. 7. Macroinvertebrate drift density was low in all samples before and after liming and typical of oligotrophic streams. Drift was significantly lower at limed (0.024 ind. m^?3) than at unlimed (0.083 ind. m^?3) locations. The decrease was only in total drift density with no significant change in the relative abundance of functional groups or in densities of single taxa, except for a reduction in drift of predators in the limed condition.