Kagera River Research Articles

Future Changes in Climate Extremes: Insights from CMIP6 Model Simulations for the Kagera River Sub-basin, Tanzania

Future Changes in Climate Extremes: Insights from CMIP6 Model Simulations for the Kagera River Sub-basin, Tanzania

Managing Transboundary Water Resources At the Edge of Human Interface: A Case of Kagera River Basin, Tanzania

Globally, water concerns stemming from human factors are evident, and the Kagera River Basin exemplifies this challenge. This study investigated the influence of human elements on transboundary water resource management in the Kagera River Basin. Utilizing focus group discussions, household surveys, key informant interviews, and field observations, the study analysed land use changes via remote sensing; and processed quantitative data using SPSS 20 and ArcGIS 10.4. Results highlighted economic, political, and social factors as key factors influencing transboundary water resource management. The institutional analysis identified major players to include the Ministry of Water, Lake Victoria Basin Water Board, NELSAP, Ministry of Agriculture, village governments, and CONCERN/OXFAM. Non-climatic factors such as water infrastructure management, socio-economic factors, conflicting water uses, agricultural practices causing soil erosion, and land use/cover changes were identified as central to transboundary water resource challenges. Agricultural expansion and land use shifts in the basin in 1986, 2000 and 2021 were pinpointed as contributors to water resource challenges, impacting availability, access, and quality. Anthropogenic-driven land use changes emerged as significant contributors to pollution, habitat loss, and alterations in water quality. While human factors substantially influence water resources, their impact is exacerbated by anthropogenic activities. Thus, fostering collaborative planning and stewardship initiatives at the local level is crucial for sustaining transboundary water resources. This involves engaging community members and leaders in developing resilience plans and initiating international cooperation, and innovative participatory approaches for effective transboundary water resources management.

Assessing the potential of Rainwater Harvesting (RWH) for sustaining small-scale irrigated coffee farming in the Bigasha watershed, Uganda

A study was carried out during 2022 to assess the potential of rainwater harvesting for sustaining small-scale irrigated coffee farming in the Bigasha watershed of Isingiro District, south-western Uganda. In this study, remote sensing, Geographical Information System (GIS), the Quantum GIS Soil and Water Assessment Tool (QSWAT+) and Crop Water and Irrigation Requirements Program (CROPWAT) models were utilised to assess the potential of rainwater harvesting (RWH) that can be used for irrigation. The criteria used to identify suitable RWH sites were based on four factors namely; topography, soils, land use land cover and rainfall. It was found out that seventy per cent of the watershed is hilly with slopes greater than 10 per cent and half of the area is covered by Leptosols. Three temporal land use land cover (LULC) maps (of 1999, 2010 and 2022), were derived using the K-nearest neighbor (KNN) algorithm. These LULC databases were used to assess the impacts of land use land cover changes on the runoff hence potential for RWH. The annual average rainfall received in the study area over the 21-year period studied (2000 – 2020) was 954 mm which is adequate for irrigation if harvested. The QSWAT+ model simulated runoff depths from 62, 125 and 114 Hydrologic Response Units (HRUs) of the 1999, 2010 and 2022 LULC maps respectively, were used to estimate potential RWH sites. The 114 HRUs from the 2022 LULC map could generate an annual average surface runoff volume of 1.39 million cubic meters (MCM) which could potentially irrigate 101.8 per cent of the current coffee production areas in the Bigasha watershed. The prediction accuracy of the Kagera river basin model was very good with NSE (R2) of 0.81(0.82) for calibration and 0.87(0.88) for validation. The Kagera watershed model fitted parameters were further used to calibrate the Bigasha watershed model. This was done because the Bigasha is a sub-basin of Kagera and it does not have its own gauged outlet.

Inference of the spatio-temporal variability and storage potential of groundwater in data-deficient regions through groundwater models and inversion of impact factors on groundwater, as exemplified by the Lake Victoria Basin

Groundwater is an important resource for supporting domestic water use for people's livelihoods and for maintaining ecosystems. Borehole observations provide the first-hand data that characterise the fluctuation, depth, and aquifer conditions of the groundwater. Unfortunately, such observations are not available or are insufficient for scientific use in many regions. Taking the Lake Victoria Basin (LVB) as an example of data-deficient regions, this study proposes a simple knowledge-based approach that uses the Global Land Data Assimilation System (GLDAS) Catchment Land Surface Model (CLSM) for the main data, with rainfall, hydrological, topographical and geological datasets as supports, by which to infer the spatio-temporal variability and storage potential of groundwater. The method is based on analysis and inversion of impact factors on groundwater, and the feasibility of such a method is proven by showing that the groundwater results from GLDAS CLSM can correctly indicate the seasonality, as well as the link to topographical and geological features. For example, both results from the water balance equation (WBE) and GLDAS CLSM indicate that there are two groundwater recharge seasons in the basin, e.g., March to May and September to November. Compared to the eastern side of the LVB, the western side has mountains blocking surface runoff, and thus, reasonably, has larger storage potential estimates in GLDAS CLSM. Due to the low degree of weathering of the basement rocks, it is expected that there is only small storage potential and variation of groundwater in the southeastern parts of the LVB. GLDAS CLSM also correctly reflects this behaviour. Additionally, the largest groundwater storage potential over the LVB is found in regions near the Kagera River and the western shoreline, since it associates with unconsolidated rocks and behaviours of large groundwater recharge from GLDAS CSLM during the wet year of 2006. The major limitation of this knowledge-based method is that the uncertainty in terms of magnitude on GLDAS CLSM groundwater changes cannot be assessed, in addition to the fact that the reliability of the results cannot be quantified in terms of specific numbers. Therefore, the results and interpretation of groundwater behaviours using such methods can only be a guide for ‘where’ and ‘when’ to find groundwater.

Hydro-meteorological processes driving solute transport in Lake Victoria

ABSTRACT This study explores by a vertically integrated tracer transport model, hydro-meteorological event characteristics and their influence on solute transport. Changes in Hydro-meteorological processes and increasing frequency of extreme weather events are responsible for changing the lake water balance, influencing streamflow variations, and lake tracer transport. We compare historical data over a long time with model data from a vertically integrated model in COMSOL Multiphysics. We consider water balance, sources of data uncertainty, correlations, extreme rain and inflow years, and seasonal variations. The lake transport model has estimated soluble loading and transportation. The results showed there are strong correlations between tributary inflows and precipitation, and between lake outflow and water level. It was found that “events” influence lake level fluctuations. The solute transport was shown to vary more in wet periods. Modeled transportations were higher in Kenya and Uganda lake zones than in Tanzanian zones. The major inflow, from the Kagera river, appears to strongly influence lake solute transportation, so the composition of this river must be considered.

An Investigation of Kagera River Basin Land Cover Dynamics for Sustainability of Lake Victoria Water Regime

This paper is reporting on an investigation done to assess land cover dynamics in KRB(Kagera River Basin) (KRB) over a period of 21 years i.e.1990-2011 using Remote Sensing, Hydrology and GIS techniques as a basis for initiating policy interventions for sustainability of Lake Victoria water regime.The extent of land cover changes in the KRB have not been quantified with the view of assessing the extent of exploitation of the natural resources.The assessment is important firstly for identification of causative factors and secondly as a mechanism for formulating necessary policies and remedial measures to that effect.Specifically, Landsat 5, Landsat 7 and ETM+ (2000) satellite images were used.Eight Land Cover classes were identified namely, Forests, Woodland, Bushland, Cultivated Land, Swamps, Grassland, Bareland and Urban areas.Hydrographical data were used to assess changes in quantity of water flowing from the KRB into Lake Victoria and relating it with land cover changes in KRB at in the period of study. Spatial data processing, analysis and interpretation were carried out in ERDAS Imagine and ArcGIS software environments; while water quantity data and other non spatial data were processed and analyzed in Microsoft Excel application 2007. 
 
 The study revealed a significant decline of vegetation cover and water quantity discharged from Kagera River to Lake Victoria over the study period. This contravenes the global efforts of ecological sustainability, which urge avoidance of depletion of natural resources for maintenance for ecological balance. These results are in early warning to the Management of Lake Victoria for its potential failure towards achieving the Millennium Development Goal on responsible consumption and protection of natural resources in proximity with Lake Victoria. The Management of Lake Victoria in collaboration with other relevant authorities should thus, take remedial measures to control further depletion of vegetation cover in KRB for sustainability of Lake Victoria and its ecosystem.

An integrated social-ecological assessment of ecosystem service benefits in the Kagera River Basin in Eastern Africa

Sustaining multiple ecosystem service benefits in transboundary river basins is a complex and challenging task in the developing world. This can be attributed to conflicting conservation and human development needs and exacerbated by climate change impacts, especially episodic drought and flooding events. We use a case study from Rwanda in the Kagera River Basin in Eastern Africa to contextualize and examine how land use cover change, water access, and agro-ecosystems are vulnerable to myriad human and natural drivers of change. An integrated framework is employed for a nested social-ecological assessment of ecosystem service benefits drawing upon landscape and vulnerability mapping, agro-commodity value chains, and institutional analyses. The conceptual framework and case study provide leverage points for vertical and horizontal linkages that include cross-sectoral partnerships, multi-level governing networks, integrated water resource management, and livelihood security. Moreover, synergy between development and conservation outcomes can be achieved through joint adaptation planning and stewardship initiatives at the local district level with full participation of resource users and community leaders. These lessons from Rwanda and the Kagera River Basin provide opportunities for mainstreaming adaptation and development planning and building resilience towards regional environmental change in Eastern Africa.

Description of three new species of Potamonautes MacLeay, 1838 from the Lake Victoria region in southern Uganda, East Africa (Brachyura: Potamoidea: Potamonautidae)

Three new species of potamonautid freshwater crabs are described from the Lake Victoria region in southern Uganda, East Africa. Two of the new species (Potamonautes busungwe sp. nov. and P. entebbe sp. nov.) are from the shores of Lake Victoria, while the third (P. kantsyore sp. nov.) is from an inland locality on the Kagera River that flows into the lake. In addition, two of the new taxa (P. busungwe sp. nov. and P. kantsyore sp. nov.) are among the smallest species of freshwater crabs so far known from Africa. Diagnoses, illustrations and distribution maps are provided for these taxa, which are compared to similar species from Uganda and elsewhere in East Africa.

Water hyacinth hotspots in the Ugandan waters of Lake Victoria in 1994–2012: implications for management

Water hyacinth invaded Lake Victoria in the 1980s and, by 1998, had attained peak coverage of approximately 2 000 ha in the Ugandan waters of the lake. Control interventions, especially via biological means, significantly reduced the weed's coverage to non-nuisance levels (<10 ha) by 1999. Although resurgence was noticed in 2001, total coverage never reached the infestation levels attained between 1994 and 1998. By March 2012, infestations still occurred in hotspot bays, including MacDonald (52.1 ha, SE 3.0), Fielding (38.0 ha, SE 1.4), Bunjako (33.7 ha, SE 1.9), Murchison (17.1 ha, SE 0.9), Lwera (8.5 ha, SE 0.3), Napoleon Gulf (2.9 ha, SE 0.3) and Berkeley (2.1 ha, SE 0.1), in addition to some bays of the Ssesse Islands (47.8 ha, SE 0.5) and on the Kagera River. These hotspots were characterised by shelter from offshore winds and ample macronutrients (NO3-N > 13.8 µg l−1, SE 1.6; SRP > 17.2 µg l−1, SE 3.2) in the water column. Control efforts, mainly the use of biological control agents on a large scale, manual and mechanical removal at strategic sites, plus a reduction in nutrient loading, should therefore target the identified hotspot bays of the lake.

Water Resources Conflict Management of Nyabarongo River and Kagera River Watershed in Africa

In the process of exploiting and using water resources of river basin, the benefit conflict problems among upper and lower river districts and among different departments restrict to sustainable exploiting and using water resources of river basin. In this paper, the water resources conflict management of Nyabarongo River and Kagera River watershed is studied. The Nyabarongo is a major river in Rwanda, begins in Nyungwe Forest, and flows up to the north-western part of the country, then down through the center to the south-east, eventually forming the main tributary of the Kagera River watershed, the main affluent of Lake Victoria, which drains into the Nile River. The basin is shared among 11 riparian states. This trans-boundary character of the Nile presents a great challenge of water conflicts; national interests have historically been promoted at the expense of regional interests. The framework of this paper is as follows: the water resources bulletin is firstly described, and then the cooperation and regional conflicts are discussed; finally a sustainable framework for governing the water resources is proposed to meet water management in riparian states.

Land use and land cover change and its implications in Kagera river basin, East Africa

The Kagera basin has experienced major land use/cover changes in tropical forests, woodlands, and savannas due to the incessant conversion of land to agricultural and settlement use. While the land use/cover change has accelerated land degradation, biodiversity loss, and climate change in the region, few studies exist on the drivers and implications of land use/cover dynamics in the basin. The study quantified the historical land use/cover changes beginning from 1984 to 2011 and predicted future changes using multi-level data-sets. We also examined population growth and government policies as they relate to land cover/use change. Data-sets from disparate sources consisting of multi-temporal satellite images, digital elevation model, population, and relevant ancillary data were used in the study. A combination of post-classification change detection method, intensity analysis, and Markov chain models were used to analyze and evaluate historical land use/cover changes and predict future change scenarios. We observed a major expansion of agriculture land at the loss of woodland savanna. Overall, change is more pronounced and fast during the period 1984–1994 and relatively slow during 1994–2011. Category and transition intensities were more pronounced in 1984–1994 than 1994–2011. These changes suggest policy intervention from government and individual response during the period 1994–2011. In addition, the study identified population growth, settlement expansion, and local policies as key drivers of land use/cover change. Future scenarios indicate: (1) increase in agriculture land use, (2) loss in woodland savanna and forest cover, and (3) significant wetland loss to agriculture.

Upstream surveillance - Scanning for zoonoses in wildlife

a rapid situation analysis was carried out to characterize existing disease surveillance systems in animal and public health sectors in Burundi, Democratic Republic of Congo (DRC) and Tanzania and it was found that therewas no formal, routine collaboration between the two sectors. It was further observed that in the two sectors, disease surveillance and reporting starts with the lowest level where officials residing within community are engaged. The new system designed combines the use of participatory and OH approaches with district-based passive(DPS) and community-based active (CAS) surveillance system components promoted to participate in detection of disease events at source. The CAS system is based on use of community-based health reporters (CHRs) to search for and report disease events occurring in animal and human populations in the study areas. Mobile technologies are being employed as the technology enablers. We selected two sites, Ngorongoro (an ecosystem with maximum human-wildlife and domestic animal interactions) and Kagera River Basin (a cross-border ecosystem with twodistricts across theborderbetweenBurundi andTanzania) to test performance of the system.We have piloted and introduced android mobile phones running the EpiCollect and Open data kit (ODK) applications to aid geo-spatial and clinical data capture and transmission from the field to the remote IT servers at the research hubs for storage, analysis, feedback and reporting. Phase I results have proved that the participatory disease surveillance approach in combination with use of mobile technologies are capable of improving early detection and response to disease events particularly in remote hard-to-reach areas. A total of 387 disease events were reported by CHRs between 2011 and 2013. In some cases, reports of disease events submitted by CHRs triggered immediate response from official system making use of them in public health relevant. Phase II of thiswork focuses on integrated disease surveillance combining voice recording, hand-writing recognition,mobile spatial and participatory epidemiology revolutionalizing collection and reporting of disease event data. We also plan to set up a one health disease surveillance resource center that will strengthen regional capacities in designing ‘fit-for-purpose’ technologies to support disease surveillance and response and develop appropriate technology transfer platform to expedite use of mobile phone technologies in disease surveillance.

Participatory surveillance of zoonotic disease: Putting the public back in public health

The Southern African Centre for Disease Surveillance (SACIDS) in collaboration with the East African Integrated Disease Surveillance Network (EAIDSNet) has designed and piloted One Health (OH) disease surveillance system in East and Southern Africa. Initially, a rapid situation analysis was carried out to characterize existing disease surveillance systems in animal and public health sectors in Burundi, Democratic Republic of Congo (DRC) and Tanzania and it was found that there was no formal, routine collaboration between the two sectors. It was further observed that in the two sectors, disease surveillance and reporting starts with the lowest level where officials residing within community are engaged. The new system designed combines the use of participatory and OH approaches with district-based passive- (DPS) and community-based active (CAS) surveillance system components promoted to participate in detection of disease events at source. The CAS system is based on use of community-based health reporters (CHRs) to search for and report disease events occurring in animal and human populations in the study areas. Mobile technologies are being employed as the technology enablers. We selected two sites, Ngorongoro (an ecosystem with maximum human-wildlife and domestic animal interactions) and Kagera River Basin (a cross-border ecosystem with two districts across the border between Burundi and Tanzania) to test performance of the system. We have piloted and introduced android mobile phones running the EpiCollect and Open data kit (ODK) applications to aid geo-spatial and clinical data capture and transmission from the field to the remote IT servers at the research hubs for storage, analysis, feedback and reporting. Phase I results have proved that the participatory disease surveillance approach in combination with use of mobile technologies are capable of improving early detection and response to disease events particularly in remote hard-to-reach areas. A total of 387 disease events were reported by CHRs between 2011 and 2013. In some cases, reports of disease events submitted by CHRs triggered immediate response from official system making use of them in public health relevant. Phase II of this work focuses on integrated disease surveillance combining voice recording, hand-writing recognition, mobile spatial and participatory epidemiology revolutionalizing collection and reporting of disease event data. We also plan to set up a one health disease surveillance resource center that will strengthen regional capacities in designing ‘fit-for-purpose’ technologies to support disease surveillance and response and develop appropriate technology transfer platform to expedite use of mobile phone technologies in disease surveillance. This work was funded by the Rockefeller Foundation Grant (2009 DSN 305) and the IDRC Grant No: [107030-001] to Sokoine University of Agriculture for its Southern African Centre for Infectious Disease Surveillance (SACIDS)www.sacids.org

Towards One Health disease surveillance: The Southern African Centre for Infectious Disease Surveillance approach

Africa has the highest burden of infectious diseases in the world and yet the least capacity for its risk management. It has therefore become increasingly important to search for 'fit-for- purpose' approaches to infectious disease surveillance and thereby targeted disease control. The fact that the majority of human infectious diseases are originally of animal origin means we have to consider One Health (OH) approaches which require inter-sectoral collaboration for custom-made infectious disease surveillance in the endemic settings of Africa. A baseline survey was conducted to assess the current status and performance of human and animal health surveillance systems and subsequently a strategy towards OH surveillance system was developed. The strategy focused on assessing the combination of participatory epidemiological approaches and the deployment of mobile technologies to enhance the effectiveness of disease alerts and surveillance at the point of occurrence, which often lies in remote areas. We selected three study sites, namely the Ngorongoro, Kagera River basin and Zambezi River basin ecosystems. We have piloted and introduced the next-generation Android mobile phones running the EpiCollect application developed by Imperial College to aid geo-spatial and clinical data capture and transmission of this data from the field to the remote Information Technology (IT) servers at the research hubs for storage, analysis, feedback and reporting. We expect that the combination of participatory epidemiology and technology will significantly improve OH disease surveillance in southern Africa.

Changing Trends of Natural Resources Degradation in Kagera Basin: Case Study of Kagera Sub-Basin, Uganda

In many respects, river basins are extremely convenient natural resources management units and hence calls for an integrated approach in case of transboundary nature. Environmental resources in Kagera basin are under great threat due to demographic factors leading to wide spread environmental degradation. Land degradation and biodiversity loss are central issues in the basin, but the extent and severity of the degradation pressures are not yet clearly illustrated and their implications largely unknown. To date, natural resource mapping in Kagera basin has been based on isolated case studies for specific purposes and not much has been done in mapping resources and classification of resources degradation by remote sensing applications considering the whole basin. In this study, basin-wide mapping approach was adopted and hot spot areas associated with natural resources use in the basin identified and trends over time established. However, this paper presents results from Kagera River sub-basin, Uganda. Mapping exercise was done by using landsat images and aerial photos of Kagera basin covering the years 1984-2002. Overall, bushland in Kagera sub-basin, Uganda increased by 78% and woodland cover showed mere 6% gain; but a 53% decrease in open woodland sub-type and 29% decrease in closed woodland. Significant shift occurred in cultivation with herbaceous crops (mainly banana) from year 1984-2002 moving from east to west of Kagera sub-basin, Uganda representing 167% increase. Area occupied by permanent swamp decreased 31%. Over the same period, land cover change detection matrix indicates main land cover changes include conversion to bushland (59.34%) followed by conversion to grassland (7.29%) and cultivated land (7.16%), with only 24.19% of the land cover remaining unchanged. It is concluded that the observed changes are, a result of human-induced factors and show unsustainable utilization of natural resources as most of the changes make the land susceptible to degradation.

Impact of interspecific competition by compatriot aquatic weeds on water hyacinth Eichhornia crassipes (Martius) Solms growth and development in the Kagera River

The phenotype of a plant is strongly influenced by the presence of neighbouring plants often resulting into change in growth and development. We have demonstrated to our knowledge for the first time, the influence of three aquatic weed species (Commelina sp., Justicia sp. and Vossia cupsidata) on growth and development of water hyacinth (Eichhornia crassipes). We found that Commelina sp., Justicia sp. and V. cupsidata significantly (P < 0.001) suppressed water hyacinth total fresh weight when growing together both in the Kagera River and greenhouse. Justicia sp. gave the highest (51%) water hyacinth fresh weight reduction when compared to fresh weight in pure stand probably due to competition effect. In the Kagera River but not greenhouse, water hyacinth plants were significantly taller (F (4,1371) = 150.53, P < 0.001) in pure stands than when growing in mixtures. V. cupsidata strongly suppressed water hyacinth plant height although water hyacinth compensated by producing elevated number of ramets (daughter plants). In greenhouse environment, the number of ramets per plant was higher in mixtures than in pure water hyacinth treatment. Correlation analysis revealed a strong significant negative relationship between plant height and number of ramets. Competition for space and resources was suggested to be the major influential factor as water hyacinth in pure stands tended to have higher density and total biomass per unit area than when grown in mixtures. Analysis of water hyacinth ratios to other aquatic weeds showed a significant effect on water hyacinth growth and development in terms of fresh weight, plant height, leaves per plant and ramets. These data show that neighbouring aquatic weeds are important component in the regulation of water hyacinth growth and development in the aquatic ecosystem. Key words: Commelina sp., Justicia sp., Kagera River, water hyacinth.

Phosphorus sorption in relation to soil grain size and geochemical composition in the Simiyu and Kagera river basins, Tanzania

The phosphorus sorption characteristics of eight surface soil samples representative of the land use and landscape in the Simiyu and Kagera River basins were determined using the Langmuir equation. The Langmuir P adsorption maximum (Cmax) ranged from 109 to 802 mg kg-1, while Langmuir coefficient (K) ranged from 59 to 736 mg L-1. Cmax and were positively correlated to clay content, aluminium (Al), and iron (Fe) and negatively correlated to calcium (Ca). By using stepwise regression, the combination of Al, Fe, clay and Ca predicted more than 94% of the variation in the P sorption capacity of soils samples from Simiyu and Kagera basins. These four soil properties, which are strongly related to P sorption, could therefore be used as quick tests for predicting the P sorption capacity of soils in the two basins. Tanzania Journal of Science Vol. 31 (2) 2005: pp. 31-40

Water hyacinth decline across Lake Victoria—Was it caused by climatic perturbation or biological control? A reply

Several authors have identified that a combination of interacting factors involving the El Niño of 1997/1998 and biocontrol by weevils resulted in the rapid reduction of water hyacinth across Lake Victoria [Albright, T.P., Moorhouse, T.G., McNabb, J., 2004. The rise and fall of water hyacinth in Lake Victoria and the Kagera River Basin 1989–2001. J. Aquat. Plant Manage. 42, 73–84; Williams, A.E., Duthie, H.C., Hecky, R.E., 2005. Water hyacinth in Lake Victoria: why did it vanish so quickly and will it return? Aquat. Bot. 81, 300–314; Wilson, J.R.U., Ajuonu, O., Center, T.D., Hill, M.P., Julien, M.H., Katagira, F.F., Neuenschwander, P., Njoka, S.W., Ogwang, J., Reeder, R.H., Van, T., 2007. The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp. Aquat. Bot. 87, 90–93]. It would appear to us that any disagreement between these papers centres on the order and magnitude of the contributory factors. In this reply we reiterate that whilst weevils almost certainly played their part, the synchronous lake wide reduction of water hyacinth during the second quarter of 1998 was the result of the 1997/1998 El Niño.

Phosphorus sorption characteristics of sediment in the Simiyu and Kagera River basins: implications for phosphorus loading into Lake Victoria

As part of a larger study to assess the influence of land use on riverine and atmospheric phosphorus (P) loading to Lake Victoria, P sorption characteristics of eight composite bottom sediment samples from the Simiyu and Kagera rivers were determined using the Langmuir equation. The samples had low to medium Langmuir adsorption maxima (Γm), ranging from 107 to 201μg g−1. Langmuir binding energy co-efficient (K) ranged from 60 to 181μg l−1 and the equilibrium P concentration at zero sorption (EPC0) from 0.1 to 2.75μg g−1. By using Langmuir co-efficients derived from P sorption experiments and soluble reactive phosphorus (SRP) concentrations measured in rivers as well as the in-shore waters of Lake Victoria, it was possible to determine the potential release of SRP into the lake by sediment from the two catchments. For the 2000 water-year, it was estimated that about 28.65 ± 0.89 (mean ± SD) and 66 ± 6.76 tons of SRP were released into Lake Victoria by sediment deposited by the Simiyu and Kagera rivers, respectively. The implications of these results to future management of cultural eutrophication in Lake Victoria are also discussed.

A histological description of ovarian recrudescence in two Labeo victorianus populations

The ovaries of Labeo victorianus are paired organs situated in the peritoneal cavity and suspended on either side of the midline by a mesovarium. A capsule, composed of dense, regularly-arranged collagen and elastic fibres mixed with a few smooth muscle cells, enclosed the ovaries and gave off connective tissue septa, forming the ovigerous lamellae, which contained germ and follicle cells. Eight discrete stages of recrudescence were identified: oogonia, chromatin nucleolar oocytes, perinucleolar oocytes, primary yolk vesicle oocytes, secondary yolk vesicle oocytes, tertiary yolk vesicle oocytes, post-ovulatory follicles and atretic oocytes. Ovulation seemed to be synchronised with the onset of rainfall, with some deviations in the Sio River population. Gonadosomatic index variation followed a bimodal pattern, with maxima between January–February and between September–October for both populations. The same pattern was exhibited for both rainfall and water levels at the two study sites. Successful ovulation was followed by the formation of post-ovulatory follicles and Type I atresia, while failed spawning was characterised by Type II atresia. Clearance of post-ovulatory follicles was by phagocytosis and formation of melanomacrophage centres. There were variations in post-ovulatory changes between the two populations. Reproductive patterns in the Kagera River population conformed to the 'norm' in African labeines of the synchronisation of spawning with rainfall. Slight deviations from this pattern were, however, observed in the Sio River population where spawning occurred prior to the onset of rainfall.