Eutrophication of a Tropical African Impoundment (Lake McIlwaine,Rhodesia)

Lake McIlwaine is an artificially eutrophic impoundment in tropical Africa, which has received sewage effluent since 1952, and has developed troublesome algal blooms. Rapid chemical changes have taken place since about 1960, and the lake is very much richer than other impoundments in the area. Chemical composition of the inflowing rivers shows that sewage effluent is responsible for this eutrophication, and rural runoff is not considered significant. The presence of an anaerobic hypolimnion is a doubtful index of eutrophication as this occurs in tropical lakes, regardless of trophic status. By contrast, the epilimnion of Lake McIlwaine shows permanent oxygen super-saturation, probably due to the high phytoplankton productivity. The lake has a high seasonal inflow and rapid replacement time, factors which could enhance the success of measures to reduce the inflow of sewage effluent.     

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.     

Aspects of the ecology of benthic fauna in Lake McIlwaine, Rhodesia

SUMMARY. An investigation into the ecology of benthic fauna in the eutrophic Lake McIlwaine (Rhodesia) was carried out in 1968–69. The profundal zone was only colonized in winter when the lake was isothermal and oxygen was present. The littoral zone was greatly affected by lake level fluctuation during the study period, and the population of lamellibranchs was destroyed by a drop in water level. While the water level was low, numbers of oligochaetes were high, but they declined as the water level rose. Chironomid larvae colonized the newly-flooded land and their numbers increased considerably. The effects of eutrophication were less clear but it is suggested that the presence of Chironomus in the profundal in winter, and an increase in Limnodrilus hoffmeisteri , was linked with eutrophication.     

Phytoplankton and its dynamics in two tropical lakes: a tropical and temperate zone comparison

Temporal patterns of phytoplankton biomass and community structure are described for two Kenyan lakes and subsequently compared with patterns reported in other tropical and temperate lakes. Lake Naivasha had a lower and more seasonally variable (10×) biomass, with a seasonal shift between diatoms and blue-greens, while the L. Oloidien biomass was less variable (3.7×) and dominated by blue-greens. Biomass and chlorophyll a were strongly correlated and in turn were coupled to the level of total phosphorus. A total of 143 and 94 taxa were described for L. Naivasha and L. Oloidien, respectively.The comparative analysis showed: a) a paucity of exclusively tropical species; b) that more than 30 percent of the species in two highly saline Kenyan lakes were also present in the two freshwater lakes; c) no evidence for a postulated decline of phytoplankton species abundance with latitude from the temperate zone to the tropics; d) that the low fraction of chrysophyte biomass in tropical lakes is a function of trophy rather than of latitude; e) that the fraction of chlorophyte biomass in tropical lakes is generally higher than in temperate lakes; f) that the proportion of nannoplankton in the two Kenyan freshwater lakes is not different from that in temperate lakes of the same trophy; g) that seasonal or annual biomass oscillations in the tropics are not systematically lower than in the temperate zone; h) evidence for large inter-year difference in the max.:min. biomass ratio in the only tropical lake (L. Naivasha) for which such data are available; i) that an average biomass ratio appears predictable for tropical lakes from the proportion of the sediment surface in contact with epilimnetic water. Overall, no evidence was found that the freshwater tropical phytoplankton composition or dynamics differ in any fundamental fashion from that observed in the temperate lakes during the summer.Contribution number 147 of the Limnology Research Centre, McGill University.     

Seasonal Dynamics of Periphyton in a Large Tropical Lake

Tropical aquatic systems are generally assumed to have little seasonality in productivity patterns. However, this study indicated that there was substantial seasonal variation in epilithic productivity and biomass in tropical Lake Tanganyika, due primarily to seasonal patterns in lake hydrodynamics that influence nutrient availability. Although they support much of the lake’s biological diversity, epilithic algae made a minor contribution to the total energy budget in Lake Tanganyika. A comparison among large, oligotrophic lakes revealed no significant latitudinal trends in periphyton productivity or biomass. However, Lake Tanganyika has relatively low benthic algal biomass and is therefore more efficient at photosynthesis than the temperate lakes. The influence of wave action and consumer density and diversity may be important in moderating productivity of the epilithic community.     

SOME ASPECTS OF THE DISTRIBUTION OF REACTIVE PHOSPHORUS IN LAKE McILWAINE,RHODESIA: PHOSPHORUS LOADING AND ABIOTIC RESPONSES

SUMMARY

This study was designed to investigate the apparent loss from the water column in Lake McIlwaine This study of significant quantities of reactive phosphorus. The Total Reactive Phosphorus mass-balance for the lake for 1977/78 was calculated, and both in situ and laboratory experiments were carried out to determine the effect of the lake sediments on the phosphorus loading of the lake. The experimental results showed that both uptake and release of phosphorus occurs in the lake, but that uptake of phosphorus by the sediments was by far the dominant process, thus accounting for the observed loss of phosphorus from the water column. The availability of the bound phosphorus for algal growth was also studied and it is suggested that algal uptake of bound phosphorus is possible. Various factors affecting phosphorus uptake by the sediments are discussed.     

Vertical diffusion and nutrient transport in a tropical lake (Lake McIlwaine,Rhodesia)

Measurements of the vertical temperature in tropical Lake McIlwaine were used to calculate the time-averaged ( 6 months) vertical diffusivity coefficient (K_z) in the metalimnion and hypolimnion. The mean value of K_z (0.21 cm² s^–1) was correlated with the lake surface area. The mass transport rates of PO₄-P and NH₄-N, upward from the hypolimnion to the metalimnion, were calculated using K_z and measured values of the nutrient concentration gradients. During a period of 4.5 months when the water was stably stratified, PO₄-P was transported upward at a mean rate of 42 kg day^–1 and NH₄-N at a mean rate of 162 kg day^–1 over the entire lake.     

Patterns of temporal variation in Lake Titicaca. A high altitude tropical lake. I. Background,physical and chemical processes,and primary production

A statistical analysis is presented of patterns of variation in some physical, chemical, and biological variables for a 6 year series of data from the tropical, high altitude Lake Titicaca (Peru-Bolivia). ANOVA techniques and autocorrelation analyses were used to partition the variance in Titicaca, and in some comparison tropical and temperate series, into components with repeatable annual cycles and components attributable to other kinds of patterns.In Titicaca, insolation and stratification are highly seasonal in pattern of variation, although the amount of variance relative to means is small compared to temperate lakes. However, the seasonal pattern of physical variation is only weakly imposed on chemical and biological processes, to judge from analyses of silicate, oxygen, and primary production series. Comparable temperate series of primary production and chlorophyll a are much more seasonal.     

High productivity by Fish Eagles on a polluted dam near Harare

Mundy, P.J. & Couto, J.T. 2000. High productivity by Fish Eagles on a polluted dam near Harare. Ostrich 71 (1 & 2): 11–14.

Lake Chivero, in the Robert McIlwaine Recreational Park west of Harare, is the capital city's main reservoir of drinking water. It has been the subject of five surveys for pesticides and/or heavy metals in the period 1974–1995. The (geometric) average DDE residue level in seven Fish Eagle eggs collected in 1980 was 53 ppm dry weight. By 1995, DDT levels had considerably declined. There are now five pairs of Fish Eagles on Lake Chivero (1996), at a very low density of 14.8 km shoreline per pair. In the 14-year period, 1984–1997, 40 pair-occupations have been found and inspected, which produced 64 fledglings. One nest had fledglings in 11 years, and on three occasions broods of three were produced from it. A fourth brood of three was produced at another nest. The dam receives treated sewage effluent from the city and is now highly eutrophic; this has contributed to the eagles' breeding performance.     

Cyanobacteria and cyanotoxins in the source water from Lake Chivero,Harare, Zimbabwe,and the presence of cyanotoxins in drinking water

The phytoplankton community and cyanotoxins in Lake Chivero (formerly Lake McIlwaine) and the presence of cyanotoxins in treated drinking water were investigated between 2003 and 2004. A typical seasonal succession of Cyanobacteria species occurred from January to April, Bacillariophyta from May to July, and Cryptophyta and Chlorophyta from August to December. Microcystis aeruginosa and M. wesenbergii, known producers of the toxin microcystin, and the non-toxic cyanobacterium M. novacekii dominated during summer. The highest concentrations of microcystins and lipopolysaccharide endotoxins occurred when cyanobacterial biomass was highest. Lipopolysaccharide endotoxin concentrations in the lake ranged between 8 and 3 200 Endotoxin Units (EU) ml^?1. Microcystin concentrations in treated water were below the recommended safe limit for drinking water. Lipopolysaccharide endotoxin concentrations in treated water ranged from 0.15 to 11 EU ml^?1. The phytoplankton community comprised non-microcystin-producing species for the greater part of the study period.     

The role of the littoral zone in the functioning of a shallow tropical lake ecosystem

Lake Chilwa (Malawi) is a shallow tropical lake surrounded by an extensive zone of littoral swamp mainly composed of Typha domingensis. The data produced show that the physical and chemical conditions are different and more varied in the littoral region when compared with the open lake. Primary production in the littoral is confined almost entirely to Typha domingensis, which also acts as a nutrient pump by absorbing nutrients from the soils and eventually releasing them into the lake waters. Long term and short term exchanges in organic and inorganic materials between the littoral swamps and the open lake are described. These, as well as available evidence on the distribution, reproductive habits and diet of certain faunal groups, indicate that the littoral region of Lake Chilwa has an important regulating influence on the functioning of the whole lake.     

Vegetation Controls on Weathering Intensity during the Last Deglacial Transition in Southeast Africa

Tropical climate is rapidly changing, but the effects of these changes on the geosphere are unknown, despite a likelihood of climatically-induced changes on weathering and erosion. The lack of long, continuous paleo-records prevents an examination of terrestrial responses to climate change with sufficient detail to answer questions about how systems behaved in the past and may alter in the future. We use high-resolution records of pollen, clay mineralogy, and particle size from a drill core from Lake Malawi, southeast Africa, to examine atmosphere-biosphere-geosphere interactions during the last deglaciation (∼18–9 ka), a period of dramatic temperature and hydrologic changes. The results demonstrate that climatic controls on Lake Malawi vegetation are critically important to weathering processes and erosion patterns during the deglaciation. At 18 ka, afromontane forests dominated but were progressively replaced by tropical seasonal forest, as summer rainfall increased. Despite indication of decreased rainfall, drought-intolerant forest persisted through the Younger Dryas (YD) resulting from a shorter dry season. Following the YD, an intensified summer monsoon and increased rainfall seasonality were coeval with forest decline and expansion of drought-tolerant miombo woodland. Clay minerals closely track the vegetation record, with high ratios of kaolinite to smectite (K/S) indicating heavy leaching when forest predominates, despite variable rainfall. In the early Holocene, when rainfall and temperature increased (effective moisture remained low), open woodlands expansion resulted in decreased K/S, suggesting a reduction in chemical weathering intensity. Terrigenous sediment mass accumulation rates also increased, suggesting critical linkages among open vegetation and erosion during intervals of enhanced summer rainfall. This study shows a strong, direct influence of vegetation composition on weathering intensity in the tropics. As climate change will likely impact this interplay between the biosphere and geosphere, tropical landscape change could lead to deleterious effects on soil and water quality in regions with little infrastructure for mitigation.     

Mallomonas fenestrata sp. nov. and M. perforata sp. nov. (Chrysophyceae, Mallomonadaceae) from tropical lakes

Two new species of Mallomonas (Chrysophyceae, Mallomonadaceae) are described from tropical lakes. M. fenestrata sp. nov. was found in Lago Tupé (Brazil) and M. perforata sp. nov. in Lake Danau Bratan (Indonesia), Negril ponds (Jamaica) and Marondera ponds (Zimbabwe).     

PRESIDENTIAL ADDRESS, 1955

Tomlinson, D. N. S. 1974. Studies of the Purple Heron, Part 1: Heronry structure, nesting habits and reproductive success. Ostrich 45: 175–181.

At Lake McIlwaine, Rhodesia, Ardea purpurea nests in bulrush Typha latifolia stands or on island termite mounds covered with reeds Phragmites mauritianus, which are connected to the mainland by Polygonum senegalense weed. Nest construction was different in thickness, diameter, height and materials used between the two types of nesting habitat. Clumping of nests in pairs and trios was apparent andMayhave some social significance. The Marsh Harrier Circus ranivorus preyed on the eggs and small chicks of the heron, whilst the Clawless Otter Aonyx capensis probably preyed on large chicks destroying the nest completely in the process. Low lake levels appear to be a major limiting factor to nesting success.     

热带森林植物功能群及其动态研究进展

热带森林极高的物种多样性和结构复杂性给生态学研究带来了很多挑战。植物功能群是对特定环境响应相似或对主要的生态过程具有相似作用的物种组合。应用植物功能群的方法是有效减少热带森林群落复杂性,并揭示其格局和过程的良好途径。在介绍植物功能群概念和划分途径的基础上,分析了热带森林植物功能群的时空动态规律。一般来讲,划分植物功能群通常有3种途径,并可通过5个步骤来完成。热带森林植物功能群的空间分布常受景观格局的制约,而环境异质性往往是影响不同植物功能群组配比例变化的直接原因。不同类型的植物功能群随演替过程发生显著的替代,而干扰体系和全球气候变化对功能群的动态过程具有重要的驱动作用。以功能群为基础的动态模型在模拟热带林群落动态和预测植被潜在分布等方面具有广阔的发展前景。探索有效的植物功能分类方法、构建完善的植物功能性状数据库、开发更为精确的功能群动态模型以及加强基于景观水平的植物功能群动态机制的认识等是未来热带森林植物功能群及其动态研究的重要方向。     

Nutrient limitation in a tropical saline lake: a microcosm experiment

There is increasing evidence that nitrogen limitation is of widespread occurrence in tropical lakes. Nonetheless, data on the deep tropical Lake Alchichica (Mexico) show that dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratio fluctuates widely. To elucidate further the role of nitrogen and phosphorus limitation on the phytoplankton growth in tropical saline lakes, we present the results of a series of nutrient enrichment experiments with natural assemblages of Lake Alchichica phytoplankton conducted monthly for a year. Our assays indicate that phosphorus and nitrogen alternate in limiting Lake Alchichica phytoplankton biomass. Phosphorous limited phytoplankton growth most (41.7%) of the time, followed by nitrogen (33.3% of the time), and both nutrients for the rest of the time (25.0%). This alternation in nitrogen and phosphorus responsible for phytoplankton growth limitation in Lake Alchichica is attributed to the combination of natural conditions (e.g., young volcanic terrain rich in phosphorus) that would favor nitrogen limitation and anthropogenic impacts (e.g., agricultural nitrogen fertilization) which would cause phosphorus limitation. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

The reproductive biology of a tropical cyprinid, Hampala macrolepidota (Van Hasselt), from Zoo Negara Lake, Kuala Lumpur, Malaysia

From a study on some aspects of the reproductive biology of the tropical cyprinid Hampala macrolepidota (Van Hasselt) in Zoo Negara Lake, Kuala Lumpur, Malaysia, analyses of the maturity stages, gonadosomatic indices and oocyte diameter patterns show that this fish is able to spawn throughout the year. However, physicochemical and environmental factors determine the actual spawning time of this fish. Spawning activity is associted with decreasing temperatures and daylengths, rising water levels and increasing turbidities. Spawning is protracted and coincides with the rainy season which extends from November to March the following year.     

An Ant Mosaic Revisited: Dominant Ant Species Disassemble Arboreal Ant Communities but Co-Occur Randomly

The spatial distributions of many tropical arboreal ant species are often arranged in a mosaic such that dominant species have mutually exclusive distributions among trees. These dominant species can also mediate the structure of the rest of the arboreal ant community. Little attention has been paid to how diet might shape the effects of dominant species on one another and the rest of the ant community. Here, we take advantage of new information on the diets of many tropical arboreal ant species to examine the intra- and inter-guild effects of dominant species on the spatial distribution of one another and the rest of the tropical arboreal ant community in a cocoa farm in Bahia, Brazil. Using null model analyses, we found that all ant species, regardless of dominance status or guild membership, co-occur much less than expected by chance. Surprisingly, the suite of five dominant species showed random co-occurrence patterns, suggesting that interspecific competition did not shape their distribution among cocoa trees. Across all species, there was no evidence that competition shaped co-occurrence patterns within guilds. Co-occurrence patterns of subordinant species were random on trees with dominant species, but highly nonrandom on trees without dominant species, suggesting that dominant species disassemble tropical arboreal ant communities. Taken together, our results highlight the often complex nature of interactions that structure species-rich tropical arboreal ant assemblages.     

Founding events influence genetic population structure of sockeye salmon (Oncorhynchus nerka) in Lake Clark, Alaska

Bottlenecks can have lasting effects on genetic population structure that obscure patterns of contemporary gene flow and drift. Sockeye salmon are vulnerable to bottleneck effects because they are a highly structured species with excellent colonizing abilities and often occupy geologically young habitats. We describe genetic divergence among and genetic variation within spawning populations of sockeye salmon throughout the Lake Clark area of Alaska. Fin tissue was collected from sockeye salmon representing 15 spawning populations of Lake Clark, Six-mile Lake, and Lake Iliamna. Allele frequencies differed significantly at 11 microsatellite loci in 96 of 105 pairwise population comparisons. Pairwise estimates of FST ranged from zero to 0.089. Six-mile Lake and Lake Clark populations have historically been grouped together for management purposes and are geographically proximate. However, Six-mile Lake populations are genetically similar to Lake Iliamna populations and are divergent from Lake Clark populations. The reduced allelic diversity and strong divergence of Lake Clark populations relative to Six-mile Lake and Lake Iliamna populations suggest a bottleneck associated with the colonization of Lake Clark by sockeye salmon. Geographic distance and spawning habitat differences apparently do not contribute to isolation and divergence among populations. However, temporal isolation based on spawning time and founder effects associated with ongoing glacial retreat and colonization of new spawning habitats contribute to the genetic population structure of Lake Clark sockeye salmon. Nonequilibrium conditions and the strong influence of genetic drift caution against using estimates of divergence to estimate gene flow among populations of Lake Clark sockeye salmon.     

Comparative sequence stratigraphy of low-latitude versus high-latitude lacustrine rift basins: seismic data examples from the East African and Baikal rifts

Lakes Baikal, Malawi and Tanganyika are the world's three largest rift valley lakes and are the classic modern examples of lacustrine rift basins. All the rift lakes are segmented into half-graben basins, and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time. In the early stages of rifting, basins are fed primarily by flexural margin and axial margin drainage systems. At the climax of syn-rift sedimentation, however, when the basins are deeply subsided, almost all the margins are walled off by rift shoulder uplifts, and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas. Flexural margin unconformities are commonplace in the tropical lakes but less so in high-latitude Lake Baikal. Lake levels are extremely dynamic in the tropical lakes and in low-latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems. Evaporation is minimized in relation to inflow in the high-latitude Lake Baikal and in most high-latitude systems, and consequently, major sequence boundaries tend to be tectonically controlled in that type of system. The acoustic stratigraphies of the tropical lakes are dominated by high-frequency and high-amplitude lake level shifts, whereas in high-latitude Lake Baikal, stratigraphic cycles are dominated by tectonism and sediment-supply variations.