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Effective use of sensor nodes’ batteries in wireless sensor networks is critical since the batteries are difficult to recharge or replace. This is closely connected to the networks’ lifespan since once the battery is used up, the node is no longer useful. The entire network will not function if 60 to 80% of the nodes in it have completely depleted their energy. In order to minimize energy usage and sustain the network for a long time, many cluster head selection algorithms have been developed. However, the existing cluster head selection algorithms such as K-Means, particle swarm selection optimization (PSO), Density-Based Spatial Clustering of Applications with Noise (DBSCAN) and Fuzzy C-Means (FCM) cluster head election algorithm have not fully reduced the issue of energy usage in WSN. The objective of this paper was to develop an extended K Mean Cluster Head selection(CHS) algorithm that uses remaining energy, distance between node and base station, distance between nodes and neighbour nodes, node density, node degree Maximum Cluster size, received signal strength indicator (RSSI) and Signal to Noise Ratio. The algorithm developed was used to enhance the lifespan of WSNs. The performance of the simulated variants of LEACH routing protocols is measured and evaluated using the quantitative research methodology. Utilizing residual node energy, packet delivery ratio, throughput, network longevity, average energy usage, and the number of live and dead node, the suggested approach is contrasted to previous approaches. From the study we observed that the proposed approach outperforms existing actual LEACH, Mod-LEACH and TSILEACH approaches.

The extension of the sensor node's life span is an essential requirement in a Wireless Sensor Network. Cluster head selection algorithms undertake the task of cluster head election and rotation among nodes, and this has significant effects on the network's energy consumption. The objective of this paper is to analyze existing cluster head selection algorithms and the parameters they implement to enhance energy efficiency. To achieve this objective, systematic literature review methodology was used. Relevant papers were extracted from major academic databases Elsevier, Springer, Wiley, IEEE, ACM Digital Library, Citeseer Library, and preprints posted on arXiv. The results show that there are many existing Cluster Head Selection Algorithms and they are categorized as deterministic, adaptive and hybrid. These algorithms use different parameters to elect Cluster Heads. In future the researchers should derive more parameters that can be used to elect cluster heads to improve on energy consumption.

Open-source enterprise resource planning (ERP) software has become a preferred alternative for modern organizations due to its affordable cost, availability and ease of access. Open-source software allows access to customizable code which in most instances may have security loop holes due to the nature of its releases. The study is motivated by need for accountability and security assurance by stakeholders and the need for justification of investments towards information security. The objective was to analyse security indicators for open-source resource planning software. Papers and journals published between 2017 and 2021 from IEEE, ACM, Springer, arXiv, Wiley online library and EBSCO were reviewed. Out of the publications generated through the Google search, 62 publications were selected by reading the title, abstract, introduction and full text. Results indicate un-updated software, full access rights, inadequate training, failure to comply, single authentication and unauthorized software as some of the factors that indicate open-source enterprise resource planning software security. In conclusion effectiveness of mitigation measures to address these factors shows security or insecurity. Notably, there is need to institute security control measures and metrics for the identified factors to help assess security posture of enterprises during ERP software implementation. We recommend the design of security a measurement framework and definition of a metrics suite for assessing open-source ERP software security.

Modern organizations are adopting new ways of measuring their level of security for compliance and justification of security investments. The highly interconnected environment has seen organizations generate lots of personal information and sensitive organizational data. Easiness in automation provided by open-source enterprise resource planning (ERP) software has accelerated its acceptability. The study aimed at developing a security measurement framework for open-source ERP software. The motivation was twofold: paradigm shift towards open-source ERP software and the need for justified investment on information security. Product quality evaluation method based on ISO 25010 framework guided the selection of attributes and factors. A security measurement framework with security posture at the highest level, attributes and factors was developed presenting a mechanism for assessing organization’s level of security. Security posture promotes customers’ confidence and gives management means to leverage resources for information security investment. The future work includes definition of metrics based on the framework.

The paper presents feature extraction methods and classification algorithms used to classify maize leaf disease images. From maize disease images, features are extracted and passed to the machine learning classification algorithm to identify the possible disease based on the features detected using the feature extraction method. The maize disease images used include images of common rust, leaf spot, and northern leaf blight and healthy images. An evaluation was done for the feature extraction method to see which feature extraction method performs best with image classification algorithms. Based on the evaluation, the outcomes revealed Histogram of Oriented Gradients performed best with classifiers compared to KAZE and Oriented FAST and rotated BRIEF. The random forest classifier emerged the best in terms of image classification, based on four performance metrics which are accuracy, precision, recall, and F1-score. The experimental outcome indicated that the random forest had 0.74 accuracy, 0.77 precision, 0.77 recall, and 0.75 F1-score.

Several researchers have proposed the various classes of software attributes to guide in the derivation of metrics for software products. These existing classifications have targeted traditional software paradigms such as procedural and object-oriented software. Sassy cascading style sheets (SCSS) has unique features since it combines Cascading style sheets (CSS) features with traditional software features such as variables, functions and control flows. Due to this uniqueness, there arises a need to develop a new classification scheme that can be effectively used to classify all the possible structural attributes for Sassy cascading style sheets. The aim of this paper, therefore, is to develop and validate a comprehensive software complexity attributes classification framework for SCSS. The new framework was validated through an online expert opinion survey, where thirteen SCSS experts were involved. Results show that the proposed framework is complete and effective to guide metrics researchers in defining new metrics for SCSS

Cascading style sheets (CSS) is a Web-based style sheet language that is used for the presentation of Web documents. CSS has advanced from CSS1 to CSS3.and extensions to CSS known as CSS pre-processors have also emerged in the last few years. As is the case with regular software, CSS have inherent complexity that keeps on increasing with age which is undesirable, and metrics are needed to measure with the aim of controlling it. Although several Web metrics have been proposed in the literature, the area of stylesheets is still lagging. Findings show that few CSS-related metrics exist, and there is no evidence of proof for their mathematical soundness through the popularly known frameworks such as Briand framework and Weyuker’s properties. In addition, they have not been empirically validated. In order to address this gap, future studies should focus on defining and validating new metrics for CSS and its pre-processors.

Riverine transport and discharge are the primary pathways for terrigenous organic matter (OM) input to lacustrine environment. Understanding the influence of catchment land use on the source, composition and fate of terrigenous particulate OM in a lake is therefore vital. We used C/N, δ13C, δ15N and lipid biomarkers in the suspended particulate OM (SPOM) at the seven river discharge sites in Lake Victoria, in an effort to evaluate the potential of upstream catchment land use on the biogeochemistry of SPOM. The total fatty acids (FAs), alcohols and sterols in SPOM ranged from 4.61 to 7.42 μg/g, 1.52 to 3.32 μg/g and 3.01 to 5.81 μg/g respectively. At sites receiving discharge from industrial effluent the FAs were dominated by short chain FAs (SCFAs), polyunsaturated FAs (PUFAs), monounsaturated FA (MUFAs) and branched FAs (brFAs); the alcohols were dominated by short chain alcohols (SCOHs) and phytol; sterols were dominated by 27Δ5, 27Δ0, 28Δ5,22, 28Δ5, which are associated either with phytoplankton or zooplankton. Sites receiving discharge from substantial agricultural activity in the catchment were dominated by long chain FAs (LCFAs), long chain alcohols (LCOHs) and 29Δ5,22 and 29Δ5 sterols, reflecting vascular plant terrigenous input. There was also evidence of OM from mixed autochthonous and allochthonous sources at the sites regardless of the upstream catchment land use activities. The findings suggest that human land use in upstream watersheds affect the source, composition and distribution of SPOM discharged by rivers, which may not only impact nutrient cycling in the lacustrine environment, but may also change the quality of SPOM.

Batch experiments were carried out to determine the capacity of E. crasippes biomass to adsorb Cd (II) ions from aqueous solutions with respect to contact time and initial concentration of Cd (II) ions, adsorbent dosage, pH and temperature. The experimental data were modeled by Langmuir and Freundlich isotherm models. The data fitted well with the Freundlich model which implies that the adsorption was multilayer. The data was also subjected to Kinetic models and the pseudo-second-order model found to fit the data. The macrophyte biomass used in this study did not undergo any chemical modification or pretreatment, which when added to its abundance and its low cost make it a green alternative for the removal of Cd (II) ions from water and waste water.

Intermediate host snails of schistosomiasis were surveyed in this study to determine their abundance and distribution in the lake and land aquatic habitats of Lake Victoria basin of Kenya. Several sites were sampled at eight locations, both in the lake and on the land. The habitat and/or vegetation type (i.e. open water, hippo grass, hyacinth, ambatch trees, other vegetation, stream, swamp, pond, dam) of the sampled aquatic sites within the locations were also differentiated, water physicochemical parameters were determined, and the abundance of different species or taxa of phytoplankton and zooplankton were enumerated and correlated with the abundance of schistosomiasis snails in the sites. The results indicated significantly more Biomphalaria sudanica snails than Bulinus africanus snails in different physical habitats on land (Student’s t-test, P < 0.05), as well as in different locations on land (Student’s t-test, P = 0.026). Regression analyses revealed that several physicochemical parameters, including dissolved oxygen (R 2 = � 0.659; n = 8; P = 0.014), pH (R 2 = 0.728; n = 8; P = 0.007) and turbulence (R 2 = � 0.616; n = 8; P = 0.02), were predictive of Biomphalaria spp. abundance, while pH (R 2 = 0.610; n = 8; P = 0.02) and turbulence (R 2 = � 0.578; n = 8; P = 0.028) were predictive of Bulinus spp. abundance in different locations in the lake. Cyanobacteria (R 2 = 0.638; n = 8; P = 0.02) and chlorophyceae (R 2 = � 0.50; n = 8; P = 0.05) were shown to be predictive of both Biomphalaria spp. and Bulinus spp. abundance in different locations in the lake. Zooplankton abundance varied significantly between different locations in the lake (One-way ANOVA, P < 0.001). Bosmina spp. were found to be predictive of both Biomphalaria spp. (R 2 = � 0.627; n = 8; P = 0.01) and Bulinus spp. (R 2 = � 0.50; n = 8; P = 0.05) in different locations in the lake. The results from this study will help inform policy regarding control measures for schistosomias and intermediate snail hosts in Lake Victoria waters, as well as in adjacent terrestrial aquatic habitats and even beyond.

Aspects of the biology of Labeo cylindricus from Lake Baringo were investigated, based on experimental beach seining and gillnetting between August and October 2007. The length‐weight relationship indicated the species exhibited positive allometric growth (b = 3.7083), with a condition factor (K) of 0.84 ± 0.0298 S.D. Males dominated fish catches, with an adult sex ratio of males ⁄females of 1:1.7. The fish length-at-50% maturity (Lm50) was 127.7 and 126.1 mm total length (TL) for males and females, respectively. Fish age and growth were determined from sagittal otoliths considered to be from the young-of-the-year. Clear circuli observed in the otoliths were used to determine fish age in days. Validation by oxytetracycline marking indicated that a single growth increment (1.1 ± 0.1 SE) is formed daily on fish otoliths. Fish length (TL mm) was correlated significantly with age (in days), TL = 1.398 Age + 26.523 (r 2 = 0.91). Fish growth was subsequently estimated to be 1.398 mm day )1 . The weights and ages (in days) of fish exhibited a significant (P < 0.05) power relationship defined by the equation: W = 0.0003 Age 2.5804 (r 2 = 0.88). The results of this study provide parameters that, if appropriately monitored, can be used to predict responses of fish populations in Lake Baringo and elsewhere to human interventions (exploitation) and natural environmental change.

Infestation of fish by endoparasites may potentially influence metal uptake and elimination by the host. We quantified the metal uptake rate constant (k(u)) and efflux rate constants (k(e)) of radiolabeled Cd and Co in the cyprinid fish Rastrineobola argentea experimentally infected with the parasite Ligula intestinalis. During 24h, the accumulation of Cd and Co increased linearly with no evident steady state in uninfected fish, infected fish and in the parasite. Following aqueous exposures, the k(u) for Cd in parasites was about 3× higher than that of infected fish and 6× higher than for the uninfected fish. The k(u) for Co was up to 15× higher in the parasites than that of infected fish and 7.5× higher than for the uninfected fish. The k(e) for excretion of Cd were consistently higher for the uninfected fish than for the infected fish and also higher for uninfected fish than the parasite. The k(e) for Co for the uninfected fish was 1.4-2.0× lower than in the infected fish, but higher for parasites compared to uninfected fish (1.3-2.3×). Pulse-chase feeding experiments with radiolabeled copepods showed that Cd assimilation efficiency from food was higher in infected fish, while Co was assimilated more effectively by uninfected fish. The observed differences in metal dynamics between infected and uninfected R. argentea in the laboratory concord with differences in metal concentrations measured in natural populations in Lake Victoria. Our findings provide evidence that L. intestinalis infection enhances Cd accumulation, but depletes the essential Co in the cyprinid fish R. argentea. We conclude that the combined stress of parasites and pollution changes metal risks to fish hosts in a metal specific manner.

Infestation of fish by parasites may influence metal accumulation patterns in the host. However, the subcellular mechanisms of these processes have rarely been studied. Therefore, this study determined how a cyprinid fish (Rastrineobola argentea) partitioned four metals (Cd, Cr, Zn and Cu) in the subcellular fractions of the gut in presence of an endoparasite (Ligula intestinalis). The fish were sampled along four sites in Lake Victoria, Kenya differing in metal contamination. Accumulation of Cd, Cr and Zn was higher in the whole body and in the gut of parasitized fish compared to non-parasitized fish, while Cu was depleted in parasitized fish. Generally, for both non-parasitized and parasitized fish, Cd, Cr and Zn partitioned in the cytosolic fractions and Cu in the particulate fraction. Metal concentrations in organelles within the particulate fractions of the non-parasitized fish were statistically similar except for Cd in the lysosome, while in the parasitized fish, Cd, Cr and Zn were accumulated more by the lysosome and microsomes. In the cytosolic fractions, the non-parasitized fish accumulated Cd, Cr and Zn in the heat stable proteins (HSP), while in the parasitized fish the metals were accumulated in the heat denatured proteins (HDP). On the contrary, Cu accumulated in the HSP in parasitized fish. The present study revealed specific binding of metals to potentially sensitive sub-cellular fractions in fish in the presence of parasites, suggesting interference with metal detoxification, and potentially affecting the health status of fish hosts in Lake Victoria.

The objective of this study was to survey and document pesticide residue levels in the lower Nyando/Sondu-Miriu catchment areas of Lake Victoria, Kenya, during the dry and rainy seasons of 2009. Water and sediment samples from the Nyando/Sondu-Miriu Basin were analysed for selected pesticide residues, using gas chromatography equipped with Ni63 and CP-SIL 8CB-15m and TSD detectors for organochlorine and organophosphorus pesticide residues, respectively. The findings indicated that banned organochlorines are still being used in the catchment. Dieldrin and p,p′-DDD were notably higher (P

This study was conducted from September to December 2008 to investigate the relative abundance of malaria vectors and schistosomiasis host snails associated with aquatic weeds in Nyanza Gulf (Lake Victoria). Larval and adult’s stages of mosquitoes, lakeflies and snails were collected and identified with standard entomological and malacological techniques. The relative species composition and abundance of fish associated with macrophytes were also determined. Physicochemical parameters were determined with standard analytical methods. Community-based surveys were also conducted, using standard questionnaires, focused group discussions and direct observations. The results of this study indicated that the abundance of malaria-causing mosquitoes was low, accounting for only 0.4% of the total number of mosquitoes and lake flies collected from the gulf. Lake flies (Chaoborus and Chironomus spp.) were the most abundant flying insects associated with aquatic macrophytes (84.2%), followed by Culicines Culex spp. (12.2%) and Aedes spp. mosquitoes (3.2%). Biomphalaria sudanica and Bulinus africanus, the two most common hosts for schistosomiasis in the gulf, were detected in both types of macrophytes, but were most significantly attached to water hyacinth (P < 0.0001) and hippo grass (P = 0.0003). There were significantly fewer snails attached to the hippo grass, compared with those unattached in the open water (P < 0.05, GENMOD). Different habitats exhibited low Secchi disc transparency values, but elevated total phosphorous (TP), total nitrogen (TN), chlorophyll-a concentrations, as well as algal cell counts. Furthermore, Oreochromis niloticus and Haplochromine fishes were more abundant in water hyacinth mats compared with hippo grass mats and open-water habitats. The low mosquito abundance indicated that the sampled habitats were unsuitable for mosquito breeding, likely attributable to water turbulence and ⁄ or predation by larvivorous fish. The strong association between B. sudanica and B. africanus and aquatic macrophytes, and the observation that local communities perform many lakeshore-related activities that bring them into contact with water, can potentially lead to a higher prevalence of schistosomiasis in the Nyanza Gulf region.

Beginning in the mid-1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake-wide surveys carried out 1999–2001 and 2005–2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi-enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lake Victoria is African's most important source of inland fishery production, exhibiting an annual catch of ≈500 000 mt. The lake moderates local climate, serves as a means of transport, and a source of human food and raw material for the fishmeal industry. The introduction of Nile perch transformed the fisheries from a locally based artisanal fishery to a national and international capital investment industry. The fisheries produce an annual income of $US 600 million, providing employment opportunities for over 3 million people. Although the benefits of transforming the fishery of Lake Victoria appear to be quite remarkable, the negative impacts of this development are even more dramatic. Several challenges face the lake fishery, including environmental degradation, the introduction of exotic species, overfishing, and a fish export ban. Nutrients inputs into the lake have resulted in eutrophication and increased algal blooms. Deteriorated water quality and increased anoxia have caused the displacement and decline of some fish species. Exotic tilapiines have hybridized, displacing the indigenous fish species. Nile perch predation has caused the disappearance of once abundant native fish species. The once complex food web is now dominated by Nile perch. Unhygienic fishing methods have led to several fish export bans, while water hyacinth invasion in the lake has affected water quality, fishing and transportation possibilities. The current top-down management and use of different regulations by the riparian states have been a major drawback to sustainable management of the lake. There is a need for a paradigm shift, in which the lake is considered to be one ecosystem and relevant stakeholders are involved in the decision-making process.

The Eichhornia crassipes roots for the removal of Pb (II) ions from aqueous solutions has been investigated. The adsorption of Pb (II) ions was found to be affected by solution pH, contact time, adsorbent dosage, initial metal ion concentration, and temperature. The equilibrium was analysed using Langmuir and Freundlich isotherm models. The data was found to have a closer correlation with the Freundlich isotherm as evidenced by a higher correlation coefficient (R 2 ). The biosorption capacity for E. crasippes was found to be 16.350 mg g-1. The Kinetics data was also subjected to pseudo-first- order and the pseudo-second-order kinetic models. The data could be explained better using the pseudo-second-order kinetic model.

Aquatic macrophytes and sediments from two contrasting wetland ecosystems of Lake Victoria, Kenya (Lower Sondu Miriu and Kibos systems) were analyzed for their stable carbon isotopic composition in order to observe patterns in system functioning in these two ecosystems. The aquatic macrophytes had carbon isotope ratios ranging from -8.92 to -29.18 per mil (parts per thousand difference from the reference). For the dominant macrophytes, we observed most I3C enriched valuesfor Cyperuspapyrus and most 13Cdepleted values for Eichhomia crassipes. On transects from the river to the lake, Kibos sediments maintained lower carbon isotope signatures compared to the Lower Sondu Miriu sediments. The possible causes of the observed variation in the ~I3C signatures from sediments and aquatic macrophytes in the two-wetland ecosystems are outlined.

Climate change may threaten the fisheries of Lake Victoria by increasing density differentials in the water column, thereby strengthening stratification and increasing the intensity and duration of deoxygenation in the deeper waters. Between 1927 and 2008 the lake’s temperature increased by 0.99 °C at the surface and by 1.34 °C at depths >50 m, with the rate of warming increasing most rapidly between 2000 and 2008. In February 2000 there were marked thermal discontinuities in the water column at a number of deep stations, with marked oxyclines at depths ranging from 30–50 m, and with all stations being anoxic from 50 m downwards. In contrast, in February 2007 the lake’s temperature had risen, especially at the bottom, and both the thermal discontinuities and oxyclines were much reduced, only one station recording a dissolved oxygen concentration of

This study, conducted in Nyanza Gulf of Lake Victoria, assessed ecological succession and dynamic status of water hyacinth. Results show that water hyacinth is the genesis of macrophyte succession. On establishment, water hyacinth mats are first invaded by native emergent macrophytes,Ipomoea aquaticaForsk., andEnydra fluctuansLour., during early stages of succession. This is followed by hippo grassVossia cuspidata(Roxb.) Griff. in mid- and late stages whose population peaks during climax stages of succession with concomitant decrease in water hyacinth biomass. Hippo grass depends on water hyacinth for buoyancy, anchorage, and nutrients. The study concludes that macrophyte succession alters aquatic biodiversity and that, since water hyacinth infestation and attendant succession are a symptom of broader watershed management and pollution problems, aquatic macrophyte control should include reduction of nutrient loads and implementing multifaceted approach that incorporates biological agents, mechanical/manual control with utilization of harvested weed for cottage industry by local communities.

Murithi Njiru1, Chrisphine Nyamweya2, John Gichuki2, Rose Mugidde3, Oliva Mkumbo4 and Frans Witte5 1Department of Fisheries and Aquatic Sciences, Moi University, Eldoret, 2Kenya Marine and Fisheries Research Institute, Kisumu, Kenya 3COWI, Uganda Ltd, Plot No. 3, Portal Avenue, Kampala, 4Lake Victoria Fisheries Organisation (LVFO), Jinja, 5Institute of Biology Leiden, Leiden University, Leiden, 1,2Kenya 3,4Uganda 5Netherlands

Cholera outbreaks continue to occur regularly in Africa. Cholera has been associated with proximity to lakes in East Africa, and Vibrio cholerae has been found experimentally to concentrate on the floating aquatic plant, water hyacinth, which is periodically widespread in East African lakes since the late 1980s. From 1994 to 2008, Nyanza Province, which is the Kenyan province bordering Lake Victoria, accounted for a larger proportion of cholera cases than expected by its population size (38.7% of cholera cases versus 15.3% of national population). Yearly water-hyacinth coverage on the Kenyan section of Lake Victoria was positively associated with the number of cholera cases reported in Nyanza Province (r = 0.83; P = 0.0010). Water hyacinth on freshwater lakes might play a role in initiating cholera outbreaks and causing sporadic disease in East Africa.

This chapter discusses the environmental conditions in Lake Victoria and how they impacts on the fish stocks. Results show that in last 4 decades Secchi disc visibility decreased by about 75%. Oxycline depth decreased by 50% indicating that a large body of the lake water in the deeper waters cannot support life. Chlorophyll a has increased three times as compared to historical values. Results show that the redfield ratio has decreased to 8.2:1 (N: P). Low oxygen conditions in the deep water causes rapid denitrification with subsequent loss of nitrogen. Primary productivity has doubled over the period and algal biomass increased by 8-10 folds. The algal biomass is currently dominated by Cyanophyta. Zooplankton communities have changed to smaller sized species and a dominance of rotifers while Caridina nilotica has a higher abundance in inshore waters compared to offshore waters. Environmental changes have influenced changes in herbivorous and Zooplanktovore fish species resulting to increase in “Dagaa” Rastrineobola argentea, and decline of carnivore species. Changes in ecological interactions due to species introduction, predation accelerated by the environmental changes and increased fishing pressure have further complicated the ecosystem dynamics of Lake Victoria and pose serious uncertainties on the lakes future stability and sustainability of the fisheries resources. Lake Victoria’s future sustainability requires effective management of fishing effort and phosphorous loading.

The underwater light conditions in the African Great Lakes depend on the complex dynamics of ecological and hydrological forces, and are strongly influenced by local environmental conditions and global biogeochemical cycles. Changes in the optical conditions in these lakes have direct impacts on ecosystem productivity, carbon dynamics and nutrient availability. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. The chromophoric fraction of these compounds can mediate ecosystem change through its influence on the attenuation of ultraviolet and PAR radiation, microbial carbon cycling and radiative transfer. In the African Great Lakes, little information is available regarding the dynamics of dissolved organic matter and those sources and sinks which control its presence in the water column. We present an extensive spatial analysis of three major bays on the Kenyan and Ugandan shores of Lake Victoria. We use these data to examine the dynamics of chromophoric dissolved organic matter in different bays and we develop a model to estimate its flow from these bays to the Lake, considering both conservative mixing and photodegradation processes. While some bays release chromophoric dissolved organic matter practically unmodified into the Lake, increased residence time and exposure to solar ultraviolet radiation create conditions where chromophores are lost before entering the open lake.

The impacts of global warming have been reported from several deep lakes in the African Rift Valley and in each thermal gradients within the water column have increased thus strengthening already existing oxyclines, below which the water is permanently anoxic^1,2,3,4^. The temperature of Lake Victoria rose by 0.9°C between 1960 and 1990 raising fears that thermal stability would increase resulting in more extensive and severe anoxia in the deeper waters^5^. This is of concern because of the eutrophication of the lake, which began in the 1960s,^6,7^ and led to dense blooms of sometimes toxic cyanobacteria, increased deoxygenation of the bottom waters, and fish kills in some parts of the lake^8,9,10^. Here we show that thermal gradients in the water column have weakened over the last decade and that deoxygenation of deeper waters is less pronounced than expected. Since 1927 the temperature of the deeper waters has risen by 1.3°C compared to only 1.0°C in the surface layers, thereby decreasing thermal and density differentials in the water column. This contradicts the view that eutrophication would increase deoxygenation of the water column perhaps to the point where fish production could not be sustained^11^. Our results suggest that the impacts of global warming on tropical lakes are likely to highly variable and may not, in the short term at least, be uniformly detrimental.

Papyrus swamps usually form at the interface between river inlet and open lake. From one such wetland ecosystem (the Kibos system located in the Nyanza Gulf, Lake Victoria, Kenya), three sediment cores were recovered using piston corer in order to determine the fate of organic matter derived from papyrus and possible nutrient pathways in this system. The coring represented a transect from the river through the floating papyrus mat to the lake. Two short cores were retrieved from the lake and river. One long core (2 m) was recovered on a floating papyrus mat. The C:N ratio showed similar trends down core from the three locations. This may possibly be due to diagenic processes such as autolysis, dissolution and microbial mineralisation occurring in the sediments. Statistical analysis through one-way ANOVA revealed no significant differences in the C:N ratios between stations. Results of the stable carbon isotope ratios revealed that the delta(13)C of the river and lake samples were persistently more negative than -20 per thousand over the whole profile indicating possible contribution from terrestrial derived carbon. Regarding the floating mat core, the delta(13)C values ranged from -18.99 per thousand on the top of the floating mat but gradually increased to -16.82 per thousand towards the bottom of the core indicating possible contribution of carbon from Cyperus papyrus that has a delta(13)C value of -13.45+/-0.62 per thousand. Statistical analysis through one-way ANOVA revealed significant differences in the delta(13)C values between stations. The stable nitrogen isotope values were highly positive both in the river and in the lake station (delta(15)N10 per thousand), indicating possible contamination from sewage wastes. Values in the swamp were less positive suggesting first, the formation of ammonium depleted in (15)N from intense organic matter mineralisation, secondly indicating the delta(15)N signal of papyrus and, finally that nitrogen fixation processes were possibly occurring in the swamp. Statistical analysis through one-way ANOVA revealed significant differences in the delta(15)N values between stations. The stable isotope findings suggested that carbon derived from papyrus is retained in the swamp. Impoverished oxygen concentration in the swamp suggests high mineralisation of organic matter in the swamp indicating that the retained papyrus-derived carbon is largely respired. We conclude that further studies should be undertaken to determine the respiration rates in the wetland.

This paper documents ambient concentrations of nutrients in the Belgian coastal waters of the North Sea during the spring of 1996 and 1997. The paper elaborates the differences of uptake rates of oxidised nitrogen (NO3-) and reduced nitrogen (NH4 and urea) by Phaeocystis and diatoms. The nitrogen concentrations were dominated by NO3- with a maximum concentration of 30 µM (January 1997) and 40 µM (March 1996). In 1996, Phaeocystis dominated the spring biomass with a maximum of 521 µg C l-1, while maximum diatom biomass was 174 µg C l-1. In 1997, the maximum Phaeocystis spring biomass was 1600 µg C l-1 and diatom maximum biomass was below 100 µg C l-1. A maximum bacteria biomass of about 55 µg C l-1 was observed in mid-May 1996. The maximum nitrogen uptake rates were recorded during spring and were dominated by NO3- (0.005 h-1 in 1996 and 0.032 h-1 in 1997). Maximum specific NH4 uptake rates were between 0.005 h-1 in May 1996 and 0.006 h-1 in April 1997. The NO3- uptake rates displayed exponential decrease versus increasing ambient reduced nitrogen concentrations (ammonium and urea), whereas the reduced nitrogen uptake increased but never compensated the decreased nitrate uptake. The NH4 uptake kinetics of diatoms displayed lower vmax compared to Phaeocystis. Consequently, Phaeocystis showed ability to increase their NH4 uptake capacity when more NH4 became available while diatoms failed to do so, after ammonium had exceeded their saturation concentration (>1 µM). Although reduced nitrogen has a negative effect on the uptake of NO3-, Phaeocystis have more advantage than diatoms on the uptake of ammonium. This might be contributing to the biomass domination shown by Phaeocystis over extended periods in spring.

Carbon stable isotopes in papyrus swamps indicate different functioning of small and large wetlands (Lake Victoria, Kenya)

Aquatic macrophytes and sediments from two contrasting wetland ecosystems of Lake Victoria, Kenya (Lower Sondu Miriu and Kibos systems) were analyzed for their stable carbon isotopic composition in order to observe patterns in system functioning in these two ecosystems. The aquatic macrophytes had carbon isotope ratios ranging from –8.92 to –29.18 per mil (parts per thousand difference from the reference). For the dominant macrophytes, we observed most 13C enriched values for Cyperus papyrus and most 13C depleted values for Eichhornia crassipes. On transects from the river to the lake, Kibos sediments maintained lower carbon isotope signatures compared to the Lower Sondu Miriu sediments. The possible causes of the observed variation in the 13C signatures from sediments and aquatic macrophytes in the two-wetland ecosystems are outlined.

International audience; 1. Phytoplankton species composition, numerical abundance, spatial distribution and total biomass measured as chlorophyll a concentration were studied in relation to environmental factors in September 1994 (dry season) and March 1995 (rainy season), respectively, in the Kenyan waters of Lake Victoria; 103 species were recorded. 2. Blue-green algae (Cyanophyceae) were most diverse, followed by diatoms (Bacillariophyceae), green algae (Chlorophyceae) and dinoflagellates (Dinophyceae). 3. Twinspan separated the phytoplankton communities in the Nyanza Gulf and those in the open lake during both seasons. During the dry season, the Nyanza Gulf was strongly dominated by blue-greens, while diatoms dominated in the open lake. During the rainy season, blue-greens remained dominant in the Nyanza Gulf although the number of species found was lower than during the dry season; in the open lake, blue-greens replaced diatoms as the dominant group and there were more species than in the dry season. 4. Canonical correspondence analysis indicated that the phytoplankton species distribution was significantly correlated with turbidity during the dry season and with SiO2 during the rainy season. Chlorophyll a concentrations ranging from 2.0 to 71.5 mg m-3 in the dry season and 2.0-17.2 mg m-3 in the rainy season confirm earlier reports of increasing phytoplankton biomass in Lake Victoria since the 1960s.

The fate of organic matter in a papyrus (Cyperus papyrus L.) dominated tropical wetland ecosystem in Nyanza Gulf (Lake Victoria, Kenya) inferred from d13C and d15N analysis