Water, sediment, red swamp crayfish (Procambarus clarkii) and black bass (Micropterus salmoides) ... more Water, sediment, red swamp crayfish (Procambarus clarkii) and black bass (Micropterus salmoides) from Lake Naivasha were analyzed for selected organochlorine and organophosphorus pesticide residues. The mean p,p'-DDT, o,p'-DDT and p,p'-DDE residue levels recorded in black bass (28.3 (± 30.0), 34.2 (±54.0) and 16.1 (±16.1) µg kg −1 , respectively) and crayfish (4.6 (±5.1), 3.2 (±2.8), and 1.4 (±1.1) µg kg −1 , respectively), were higher than previously recorded. This indicated recent usage of technical DDT in the lake's catchment. Levels of p,p'-DDT, higher than those of p,p'-DDE further emphasized this. Mean lindane, dieldrin, β-endosulfan and aldrin concentrations in black bass were 100.5, 34.6, 21.6 and 16.7 µg kg −1 , respectively. The same residues were detected at lower concentrations in crayfish at 2.0, 2.0, 2.0 and 1.9 µg kg −1 , respectively. The higher fat content (3.7 ± 2.7% SD) in black bass (compared to 0.6 ± 0.3% in crayfish) accounted for the significantly higher residue concentrations in black bass. Organophosphate pesticides were the most commonly used pesticides in the lake's catchment, but none was detected in any of the samples. The results indicate that there is need for further work to identify sources and fate of pesticide contaminants, as well as to improve monitoring of pesticide use throughout the catchment.
Acacia (s.l.) spp. are the dominant natural woody plants in the Kenyan Rift Valley, the exact spe... more Acacia (s.l.) spp. are the dominant natural woody plants in the Kenyan Rift Valley, the exact species depending upon the altitude and water table. Lakes, in particular, support relatively thick fringing woodland, from which Lepidoptera have been collected, reared and identified. The significance of Acacia woodlands for biodiversity is discussed.
Between January 1993 and January 1995, the number of Great Cormorants (Phalacrocorax carbo) using... more Between January 1993 and January 1995, the number of Great Cormorants (Phalacrocorax carbo) using Lake Naivasha, Kenya (00 • 45 S, 36 • 20 E) for foraging and resting increased 56%, while the number of sympatric Long-tailed Cormorants (Phalacrocorax africanus) decreased 64%. In 1995 and 1996, we documented habitat changes and conducted monthly population and resource-use surveys of the two species in an attempt to discover the most likely reasons for these changes. The increase in Great Cormorants was probably the result of immigration from nearby Lake Nakuru due to extreme water level reductions there. Lake Naivasha also experienced falling water levels and transparency during this period, but these changes were not as severe and are not considered likely reasons for the decline in Long-tailed Cormorant numbers. Despite some probable dietary overlap, the two species were well separated in terms of foraging locations, foraging methods, resting habitats and breeding timing. The decline in Long-tailed Cormorant numbers may be connected with increased disturbance by fishermen along the lake littoral, this species' primary feeding location.
Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from c... more Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from continental geothermal systems, with only a few reports available from the Yellowstone US National Park. In this study, we explored the chemodiversity of DOM in water samples collected from two geothermal hot springs from the Kenyan East African Rift Valley, a region extremely rich in fumaroles, geysers, and spouting springs, located in close proximity to volcanic lakes. The DOM characterization included in-depth assessments performed by negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Reduced, saturated and little aromatic DOM compounds were dominant in the hot spring waters collected from either the Ol Njorowa gorge (ON) or the south shore of the soda-saline Lake Elementaita (ELM). Oxygen-poor and sulfur-bearing DOM molecules prevailed in ON, probably reflecting abiotic sulfurization from sulfide-rich geofluids. Nitrogen-bearing a...
Lake Naivasha, a shallow tropical lake in Kenya’s Rift Valley, has an unstable water column and i... more Lake Naivasha, a shallow tropical lake in Kenya’s Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) con-trol. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.
Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and i... more Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.
On steep, unvegetated slopes, sands (particle sizes 0.063 to 2 mm) and gravels (2-64 mm) erode fr... more On steep, unvegetated slopes, sands (particle sizes 0.063 to 2 mm) and gravels (2-64 mm) erode from the shoreline of Lake Naivasha (Kenya) and enter the lake basin. This occurred freely where fringing papyrus (Cyperus papyrus) swamp had been cleared in favour of landing jetties or agriculture. Gravel-sized particles have been recovered up to 60 m offshore and sands to 80 m. In an area where papyrus was undisturbed and the swamp margin was 52 m wide, gravel did not enter the lake and sands penetrated to around 35 m. Large particles are much easier to trace to source and to manage than the finer silts and clays (<0.63 µm) that form the bulk of Lake Naivasha's sediment. The pattern of mass specific magnetic susceptibilities for the <63 µm fraction of lake sediment around the southern shoreline of the lake suggests that fine particles enter open water directly from the shoreline and are not transported from the lake's only perennial inflow, the River Malewa in the north. Such particles originate from a hinterland that supports high intensity horticulture and are therefore a potential source of contamination. Mean ± SD susceptibility (χ If) immediately offshore papyrus fringe was 0.49 ± 0.08 × 10 −6 m 3 kg −1 compared with higher values of 1.33 ± 0.14 × 10 −6 m 3 kg −1 where there was no papyrus barrier (P < 0.0001). The value for five sites in the middle parts of the lake was 0.45 ± 0.02 × 10 −6 m 3 kg −1 with 1.38 ± 0.10 × 10 −6 m 3 kg −1 near the mouth of the River Malewa. The results of this study are evidence, therefore, that conservation of a continuous papyrus margin of about 50 m width is a priority for intercepting particulate material.
The floating water fem Salvinia molesta has occurred in Lake Naivasha since the early 1960s and d... more The floating water fem Salvinia molesta has occurred in Lake Naivasha since the early 1960s and during this period has obstructed fishing activities and navigation. In recent years the extent of Salvinia has declined marked1y. Since 1988, a second floating weed, Eichhomia crassipes (water hyacinth), has colonised the lake. Salvinia formed large mats around the edges, bays and lagoons and had very few plant or invertebrate species associated with it. Mats of E. crassipes, however, support a greater abundance and variety of animal and plant species, which shows a relationship with mat size. This seems to have faci1itated plant succession but without a zonation typical of the classic hydroseral sequence for the lake.
The Impact of Species Changes in African Lakes, 1995
Unlike most of the small lakes in the Rift Valley, Lake Naivasha is fresh despite the absence of ... more Unlike most of the small lakes in the Rift Valley, Lake Naivasha is fresh despite the absence of an exit river. The most likely reason.is that the dissolved substances in the input are dilute, biochemical and chemical processes are in place to remove ions such as sulphates and carbonates and there is seepage from the lake bottom {Harper et al., 1990). Because the lake is fresh, there might be an expectation of a rich fish fauna, in line with the diversity found in other lakes such as Victoria and Malawi. It comes as a surprise to many that the fish fauna is sparse and contains mostly introduced species. In the past, these have died out after only a few years. A clue to the low fish diversity, and to the frequent extinctions, may lie in the variability that is characteristic of the water level, the abundance of the aquatic macrophytes and the limnological conditions. In the face of this variability, three fish species and a crayfish form the basis of a commercial fishery that has lasted for 30 years.
Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological fun... more Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological functioning have had to wait until enough was known about its structure. The energetics of the lake has only been studied to date at primary producer level. Following the identification of the invertebrate components of the littoral and profundal benthos, this study was initiated. The absence of native fish species in Lake Naivasha, combined with a fishery based only on three, introduced species, added an applied dimension to the work. The introduction of additional fish species which will utilize unexploited ecological niches has been suggested. The benthic invertebrates form one such niche. Two oligochaetes dominated the community, Branchiura sowerbyi Beddard and Limnodrilus hoffmeisteri Claparade. The former had a productivity of 7.4 g m −2 ann −1 (as dry weight), the latter 0.6. These figures are not particularly high and do not support the introduction of a new fish species on their own.
Lake Naivasha is unique in the chain of East African Rift Valley lakes in being the only freshwat... more Lake Naivasha is unique in the chain of East African Rift Valley lakes in being the only freshwater ecosystem in an otherwise soda/saline lake series. Catchment rivers have never previously been surveyed. Valley topography, precipitation patterns and geology create a diversity of river systems draining into Lake Naivasha. Semi-quantitative assessment using the River Habitat Survey technique demonstrates a diversity of habitats and erosion/sedimentation patterns, reflecting the tectonic origin of the area. The survey descriptions form a basis for identification of areas where efforts will need to be directed for physical and biological protection.
The loss of phosphorus from the catchment of Lake Naivasha, Kenya, was 0.2 kg ha −1 ann −1 , 76% ... more The loss of phosphorus from the catchment of Lake Naivasha, Kenya, was 0.2 kg ha −1 ann −1 , 76% of it particulate in a 'normal' year of wet and dry periods. It rose to a mean of 1.8 kg ha −1 ann −1 , 90% of which was particulate, in the months following the extreme rainfall which followed the 1997-1998 El Niño event in the Southern Atlantic. Total and particulate phosphorus were positively correlated with suspended solids and with discharge, and conductivity was negatively correlated with discharge. The magnitude of losses pose both threats to the water quality of Lake Naivasha and to the sustainable soils resources of the catchment.
Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five specie... more Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five species of fish are present, all of which have been introduced. They are Oreochromis leucostictus, Tilapia zillii, Micropterus salmoides (largemouth bass), Barbus amphigramma and Poecilia reticulata (guppy). The first three of these form the basis of an important gill net fishery and bass are also taken by rod and line for sport. Barbus are occasionally caught by dip net. Actual and potential yields for the Lake Naivasha fishery are discussed and the fishery is shown to be under-performing. The feeding regimes of the commercially exploited fish were examined in the context of available food supply, in particular the benthic fauna. Small bass depend heavily on Micronecta and large bass mostly take crayfish. Detritus predominates in the diet of Oreochromis leucostictus and Tilapia zillii but the former also eats algae and the latter, Micronecta and macrophyte. Various food resources, especially the benthos, appear to be under-utilized and so it is possible that further species could be introduced to enhance the commercial fishery.
The implementation of Ecohydrology (EH)-a transdisciplinary problem-solving sciencehas to be base... more The implementation of Ecohydrology (EH)-a transdisciplinary problem-solving sciencehas to be based upon the development of system solutions for river basins which have to harmonise hydrological processes with ecosystems dynamics and societies' needs. The network of the UNESCO International Hydrological Programme EH Demonstration Projects was organised for scientific advancement, knowledge transfer, calibration of methods, education and capacity building. From the methodological point of view, the goal has been to accelerate transition from the stage of information collection and knowledge development into wisdom generation, which in practical dimension means a shift from understanding of relationships and patterns to understanding of processes and finally, formulation of principles for action. The selected Demonstration Projects from different continents cover the broad scope of water, ecosystem and society problems in such ecological systems as basin landscapes, wetlands/grasslands and floodplains, cities, lakes & reservoirs and estuaries & costal zones. This unique set of projects of large scale, long term testing and development by adaptive assessment and management, has become not only a fundamental lesson for costefficient implementation of the ecohydrology principles for IWRM, but also a starting point for "engineering harmony" between society and environment.
Isotopes in Environmental and Health Studies, 2002
The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the inge... more The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the ingested material must therefore be defecated directly into the aquatic system and can thus be considered an allochthonous resource available to aquatic consumers. The utility of stable isotope analyses of carbon and nitrogen to distinguish hippo faecal matter from other potential basal resources was tested at Lake Naivasha, Kenya. Hippopotami proved faithful to a short grass diet although supplementary grazing of aquatic macrophytes was observed. The typical isotopic ratios of C4 grasses ingested were not altered substantially by gut processes, and were clearly distinct from algal and aquatic macrophyte isotopic ratios. However, marginal plants such as Cyperus papyrus exhibit C4 ratios, and so the technique is suitable only for use in localities where &#39;contamination&#39; from such sources is negligible.
Biological invasions are a significant driver of human-induced global change and many ecosystems ... more Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between c...
Marginal increment ratio analysis of scales collected from the exploited common carp Cyprinus car... more Marginal increment ratio analysis of scales collected from the exploited common carp Cyprinus carpio (Linnaeus, 1758) population of Lake Naivasha between June 2008 and November 2009 revealed they were valid for ageing purposes, with an annual growth check formed. Individuals were fast growing and only present to the age of 4 years. Growth was sexually dimorphic (females being faster growing) and the temporal pattern in the gonadosomatic index suggested reproduction was asynchronous and occurred throughout the sampling period. A meta-analysis of traits of carp from across their range revealed that temperature was a major determinant of their growth parameters (L ∞ and the K of the von Bertalanffy growth model). Populations in more seasonal climates (as described by increased differences between the minimum and maximum monthly mean temperature in a year) were slower growing but had increased potential for attaining larger sizes. This helped explain the expression of their traits in Lake Naivasha where the mean monthly temperatures of between 20 and 23 • C were aseasonal. The life history traits of carp in Lake Naivasha provided their population with resilience to fishery exploitation with increased catches being independent of fishing effort. The influence of temperature on this globally invasive fish is in line with the findings for other invasive fishes and provides insights into their invasion patterns and processes.
The biodiversity of developing countries is increasingly threatened by introductions of invasive ... more The biodiversity of developing countries is increasingly threatened by introductions of invasive alien species. This study on the allodiversity in Lake Naivasha, Kenya reviews the pathways, establishment rates and outcomes of introduced species, and provides the basis for determining conservation actions that, if implemented, could prevent potentially harmful effects of similar events in other East African lakes. Introductions into Naivasha commenced in the 1920s with the release of a sport fish and have since produced an allodiversity of 23 species. This includes species that are no longer present (e.g., some tilapia species), presumed no longer present (e.g., the Nile perch Lates niloticus) or whose distribution is highly localised and ecologically neutral (e.g., the coypu Myocastor coypus). It also includes species that established successfully and invoked major changes in lake ecology (e.g., the red swamp crayfish Procambarus clarkii) and a species that is producing apparent economic benefits to the local population (i.e., the common carp Cyprinus carpio). The most frequent donor continents were the Americas and most species were the result of secondary introductions. The main introduction vector was active release that aimed to enhance fishery production. Alien species now dominate each main level of the lake's food web and produce impacts that are rarely restricted to a single ecosystem service. With a few exceptions, the majority of introductions translate into socioeconomic costs that contribute to rising social conflicts and exacerbating poverty. Development of appropriate conservation management tools within a regulatory framework could help protect Naivasha from further damage and could be used elsewhere in East African lakes to ensure that subsequent introductions enhance ecosystem services without affecting biodiversity.
The present ecological and hydrological state of Lake Naivasha, a tropical freshwater Ramsar site... more The present ecological and hydrological state of Lake Naivasha, a tropical freshwater Ramsar site, is reviewed from existing research. The ecology of the lake was formerly regulated by ecohydrological control exerted on the inflowing hydrochemistry by the continuous fringing papyrus, with a full swamp on the inflow rivers' delta. The ecology of the lake since that time has been severely disrupted by alien (or exotic) species invasions, particularly the Louisiana crayfish Procambarus clarkii. Lake-wide papyrus degradation has also been occurring since about 1980, following a lake level decline of up to 3m through agri-industrial abstraction. The river Malewa, which previously ran through a swamp, now runs directly into the lake, bringing high silt and nutrient loads in wet seasons, making the lake eutrophic. It is suggested that sustainable management of the lake should focus upon three ecohydrological objectives-control of abstractions to achieve hydrological balance, physical restoration of the former North Swamp and the control of illegal fishing. The latter would enable the commercial fishery returns to maximum sustainable yield so that large M. salmoides (large mouthed bass) individuals once again impose a 'topdown' control upon P. clarkii, and the crayfish itself be copmmercially exploited.
Water, sediment, red swamp crayfish (Procambarus clarkii) and black bass (Micropterus salmoides) ... more Water, sediment, red swamp crayfish (Procambarus clarkii) and black bass (Micropterus salmoides) from Lake Naivasha were analyzed for selected organochlorine and organophosphorus pesticide residues. The mean p,p'-DDT, o,p'-DDT and p,p'-DDE residue levels recorded in black bass (28.3 (± 30.0), 34.2 (±54.0) and 16.1 (±16.1) µg kg −1 , respectively) and crayfish (4.6 (±5.1), 3.2 (±2.8), and 1.4 (±1.1) µg kg −1 , respectively), were higher than previously recorded. This indicated recent usage of technical DDT in the lake's catchment. Levels of p,p'-DDT, higher than those of p,p'-DDE further emphasized this. Mean lindane, dieldrin, β-endosulfan and aldrin concentrations in black bass were 100.5, 34.6, 21.6 and 16.7 µg kg −1 , respectively. The same residues were detected at lower concentrations in crayfish at 2.0, 2.0, 2.0 and 1.9 µg kg −1 , respectively. The higher fat content (3.7 ± 2.7% SD) in black bass (compared to 0.6 ± 0.3% in crayfish) accounted for the significantly higher residue concentrations in black bass. Organophosphate pesticides were the most commonly used pesticides in the lake's catchment, but none was detected in any of the samples. The results indicate that there is need for further work to identify sources and fate of pesticide contaminants, as well as to improve monitoring of pesticide use throughout the catchment.
Acacia (s.l.) spp. are the dominant natural woody plants in the Kenyan Rift Valley, the exact spe... more Acacia (s.l.) spp. are the dominant natural woody plants in the Kenyan Rift Valley, the exact species depending upon the altitude and water table. Lakes, in particular, support relatively thick fringing woodland, from which Lepidoptera have been collected, reared and identified. The significance of Acacia woodlands for biodiversity is discussed.
Between January 1993 and January 1995, the number of Great Cormorants (Phalacrocorax carbo) using... more Between January 1993 and January 1995, the number of Great Cormorants (Phalacrocorax carbo) using Lake Naivasha, Kenya (00 • 45 S, 36 • 20 E) for foraging and resting increased 56%, while the number of sympatric Long-tailed Cormorants (Phalacrocorax africanus) decreased 64%. In 1995 and 1996, we documented habitat changes and conducted monthly population and resource-use surveys of the two species in an attempt to discover the most likely reasons for these changes. The increase in Great Cormorants was probably the result of immigration from nearby Lake Nakuru due to extreme water level reductions there. Lake Naivasha also experienced falling water levels and transparency during this period, but these changes were not as severe and are not considered likely reasons for the decline in Long-tailed Cormorant numbers. Despite some probable dietary overlap, the two species were well separated in terms of foraging locations, foraging methods, resting habitats and breeding timing. The decline in Long-tailed Cormorant numbers may be connected with increased disturbance by fishermen along the lake littoral, this species' primary feeding location.
Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from c... more Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from continental geothermal systems, with only a few reports available from the Yellowstone US National Park. In this study, we explored the chemodiversity of DOM in water samples collected from two geothermal hot springs from the Kenyan East African Rift Valley, a region extremely rich in fumaroles, geysers, and spouting springs, located in close proximity to volcanic lakes. The DOM characterization included in-depth assessments performed by negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Reduced, saturated and little aromatic DOM compounds were dominant in the hot spring waters collected from either the Ol Njorowa gorge (ON) or the south shore of the soda-saline Lake Elementaita (ELM). Oxygen-poor and sulfur-bearing DOM molecules prevailed in ON, probably reflecting abiotic sulfurization from sulfide-rich geofluids. Nitrogen-bearing a...
Lake Naivasha, a shallow tropical lake in Kenya’s Rift Valley, has an unstable water column and i... more Lake Naivasha, a shallow tropical lake in Kenya’s Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) con-trol. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.
Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and i... more Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.
On steep, unvegetated slopes, sands (particle sizes 0.063 to 2 mm) and gravels (2-64 mm) erode fr... more On steep, unvegetated slopes, sands (particle sizes 0.063 to 2 mm) and gravels (2-64 mm) erode from the shoreline of Lake Naivasha (Kenya) and enter the lake basin. This occurred freely where fringing papyrus (Cyperus papyrus) swamp had been cleared in favour of landing jetties or agriculture. Gravel-sized particles have been recovered up to 60 m offshore and sands to 80 m. In an area where papyrus was undisturbed and the swamp margin was 52 m wide, gravel did not enter the lake and sands penetrated to around 35 m. Large particles are much easier to trace to source and to manage than the finer silts and clays (<0.63 µm) that form the bulk of Lake Naivasha's sediment. The pattern of mass specific magnetic susceptibilities for the <63 µm fraction of lake sediment around the southern shoreline of the lake suggests that fine particles enter open water directly from the shoreline and are not transported from the lake's only perennial inflow, the River Malewa in the north. Such particles originate from a hinterland that supports high intensity horticulture and are therefore a potential source of contamination. Mean ± SD susceptibility (χ If) immediately offshore papyrus fringe was 0.49 ± 0.08 × 10 −6 m 3 kg −1 compared with higher values of 1.33 ± 0.14 × 10 −6 m 3 kg −1 where there was no papyrus barrier (P < 0.0001). The value for five sites in the middle parts of the lake was 0.45 ± 0.02 × 10 −6 m 3 kg −1 with 1.38 ± 0.10 × 10 −6 m 3 kg −1 near the mouth of the River Malewa. The results of this study are evidence, therefore, that conservation of a continuous papyrus margin of about 50 m width is a priority for intercepting particulate material.
The floating water fem Salvinia molesta has occurred in Lake Naivasha since the early 1960s and d... more The floating water fem Salvinia molesta has occurred in Lake Naivasha since the early 1960s and during this period has obstructed fishing activities and navigation. In recent years the extent of Salvinia has declined marked1y. Since 1988, a second floating weed, Eichhomia crassipes (water hyacinth), has colonised the lake. Salvinia formed large mats around the edges, bays and lagoons and had very few plant or invertebrate species associated with it. Mats of E. crassipes, however, support a greater abundance and variety of animal and plant species, which shows a relationship with mat size. This seems to have faci1itated plant succession but without a zonation typical of the classic hydroseral sequence for the lake.
The Impact of Species Changes in African Lakes, 1995
Unlike most of the small lakes in the Rift Valley, Lake Naivasha is fresh despite the absence of ... more Unlike most of the small lakes in the Rift Valley, Lake Naivasha is fresh despite the absence of an exit river. The most likely reason.is that the dissolved substances in the input are dilute, biochemical and chemical processes are in place to remove ions such as sulphates and carbonates and there is seepage from the lake bottom {Harper et al., 1990). Because the lake is fresh, there might be an expectation of a rich fish fauna, in line with the diversity found in other lakes such as Victoria and Malawi. It comes as a surprise to many that the fish fauna is sparse and contains mostly introduced species. In the past, these have died out after only a few years. A clue to the low fish diversity, and to the frequent extinctions, may lie in the variability that is characteristic of the water level, the abundance of the aquatic macrophytes and the limnological conditions. In the face of this variability, three fish species and a crayfish form the basis of a commercial fishery that has lasted for 30 years.
Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological fun... more Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological functioning have had to wait until enough was known about its structure. The energetics of the lake has only been studied to date at primary producer level. Following the identification of the invertebrate components of the littoral and profundal benthos, this study was initiated. The absence of native fish species in Lake Naivasha, combined with a fishery based only on three, introduced species, added an applied dimension to the work. The introduction of additional fish species which will utilize unexploited ecological niches has been suggested. The benthic invertebrates form one such niche. Two oligochaetes dominated the community, Branchiura sowerbyi Beddard and Limnodrilus hoffmeisteri Claparade. The former had a productivity of 7.4 g m −2 ann −1 (as dry weight), the latter 0.6. These figures are not particularly high and do not support the introduction of a new fish species on their own.
Lake Naivasha is unique in the chain of East African Rift Valley lakes in being the only freshwat... more Lake Naivasha is unique in the chain of East African Rift Valley lakes in being the only freshwater ecosystem in an otherwise soda/saline lake series. Catchment rivers have never previously been surveyed. Valley topography, precipitation patterns and geology create a diversity of river systems draining into Lake Naivasha. Semi-quantitative assessment using the River Habitat Survey technique demonstrates a diversity of habitats and erosion/sedimentation patterns, reflecting the tectonic origin of the area. The survey descriptions form a basis for identification of areas where efforts will need to be directed for physical and biological protection.
The loss of phosphorus from the catchment of Lake Naivasha, Kenya, was 0.2 kg ha −1 ann −1 , 76% ... more The loss of phosphorus from the catchment of Lake Naivasha, Kenya, was 0.2 kg ha −1 ann −1 , 76% of it particulate in a 'normal' year of wet and dry periods. It rose to a mean of 1.8 kg ha −1 ann −1 , 90% of which was particulate, in the months following the extreme rainfall which followed the 1997-1998 El Niño event in the Southern Atlantic. Total and particulate phosphorus were positively correlated with suspended solids and with discharge, and conductivity was negatively correlated with discharge. The magnitude of losses pose both threats to the water quality of Lake Naivasha and to the sustainable soils resources of the catchment.
Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five specie... more Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five species of fish are present, all of which have been introduced. They are Oreochromis leucostictus, Tilapia zillii, Micropterus salmoides (largemouth bass), Barbus amphigramma and Poecilia reticulata (guppy). The first three of these form the basis of an important gill net fishery and bass are also taken by rod and line for sport. Barbus are occasionally caught by dip net. Actual and potential yields for the Lake Naivasha fishery are discussed and the fishery is shown to be under-performing. The feeding regimes of the commercially exploited fish were examined in the context of available food supply, in particular the benthic fauna. Small bass depend heavily on Micronecta and large bass mostly take crayfish. Detritus predominates in the diet of Oreochromis leucostictus and Tilapia zillii but the former also eats algae and the latter, Micronecta and macrophyte. Various food resources, especially the benthos, appear to be under-utilized and so it is possible that further species could be introduced to enhance the commercial fishery.
The implementation of Ecohydrology (EH)-a transdisciplinary problem-solving sciencehas to be base... more The implementation of Ecohydrology (EH)-a transdisciplinary problem-solving sciencehas to be based upon the development of system solutions for river basins which have to harmonise hydrological processes with ecosystems dynamics and societies' needs. The network of the UNESCO International Hydrological Programme EH Demonstration Projects was organised for scientific advancement, knowledge transfer, calibration of methods, education and capacity building. From the methodological point of view, the goal has been to accelerate transition from the stage of information collection and knowledge development into wisdom generation, which in practical dimension means a shift from understanding of relationships and patterns to understanding of processes and finally, formulation of principles for action. The selected Demonstration Projects from different continents cover the broad scope of water, ecosystem and society problems in such ecological systems as basin landscapes, wetlands/grasslands and floodplains, cities, lakes & reservoirs and estuaries & costal zones. This unique set of projects of large scale, long term testing and development by adaptive assessment and management, has become not only a fundamental lesson for costefficient implementation of the ecohydrology principles for IWRM, but also a starting point for "engineering harmony" between society and environment.
Isotopes in Environmental and Health Studies, 2002
The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the inge... more The hippopotamus grazes nocturnally on land and resides in water during the day. Much of the ingested material must therefore be defecated directly into the aquatic system and can thus be considered an allochthonous resource available to aquatic consumers. The utility of stable isotope analyses of carbon and nitrogen to distinguish hippo faecal matter from other potential basal resources was tested at Lake Naivasha, Kenya. Hippopotami proved faithful to a short grass diet although supplementary grazing of aquatic macrophytes was observed. The typical isotopic ratios of C4 grasses ingested were not altered substantially by gut processes, and were clearly distinct from algal and aquatic macrophyte isotopic ratios. However, marginal plants such as Cyperus papyrus exhibit C4 ratios, and so the technique is suitable only for use in localities where &#39;contamination&#39; from such sources is negligible.
Biological invasions are a significant driver of human-induced global change and many ecosystems ... more Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between c...
Marginal increment ratio analysis of scales collected from the exploited common carp Cyprinus car... more Marginal increment ratio analysis of scales collected from the exploited common carp Cyprinus carpio (Linnaeus, 1758) population of Lake Naivasha between June 2008 and November 2009 revealed they were valid for ageing purposes, with an annual growth check formed. Individuals were fast growing and only present to the age of 4 years. Growth was sexually dimorphic (females being faster growing) and the temporal pattern in the gonadosomatic index suggested reproduction was asynchronous and occurred throughout the sampling period. A meta-analysis of traits of carp from across their range revealed that temperature was a major determinant of their growth parameters (L ∞ and the K of the von Bertalanffy growth model). Populations in more seasonal climates (as described by increased differences between the minimum and maximum monthly mean temperature in a year) were slower growing but had increased potential for attaining larger sizes. This helped explain the expression of their traits in Lake Naivasha where the mean monthly temperatures of between 20 and 23 • C were aseasonal. The life history traits of carp in Lake Naivasha provided their population with resilience to fishery exploitation with increased catches being independent of fishing effort. The influence of temperature on this globally invasive fish is in line with the findings for other invasive fishes and provides insights into their invasion patterns and processes.
The biodiversity of developing countries is increasingly threatened by introductions of invasive ... more The biodiversity of developing countries is increasingly threatened by introductions of invasive alien species. This study on the allodiversity in Lake Naivasha, Kenya reviews the pathways, establishment rates and outcomes of introduced species, and provides the basis for determining conservation actions that, if implemented, could prevent potentially harmful effects of similar events in other East African lakes. Introductions into Naivasha commenced in the 1920s with the release of a sport fish and have since produced an allodiversity of 23 species. This includes species that are no longer present (e.g., some tilapia species), presumed no longer present (e.g., the Nile perch Lates niloticus) or whose distribution is highly localised and ecologically neutral (e.g., the coypu Myocastor coypus). It also includes species that established successfully and invoked major changes in lake ecology (e.g., the red swamp crayfish Procambarus clarkii) and a species that is producing apparent economic benefits to the local population (i.e., the common carp Cyprinus carpio). The most frequent donor continents were the Americas and most species were the result of secondary introductions. The main introduction vector was active release that aimed to enhance fishery production. Alien species now dominate each main level of the lake's food web and produce impacts that are rarely restricted to a single ecosystem service. With a few exceptions, the majority of introductions translate into socioeconomic costs that contribute to rising social conflicts and exacerbating poverty. Development of appropriate conservation management tools within a regulatory framework could help protect Naivasha from further damage and could be used elsewhere in East African lakes to ensure that subsequent introductions enhance ecosystem services without affecting biodiversity.
The present ecological and hydrological state of Lake Naivasha, a tropical freshwater Ramsar site... more The present ecological and hydrological state of Lake Naivasha, a tropical freshwater Ramsar site, is reviewed from existing research. The ecology of the lake was formerly regulated by ecohydrological control exerted on the inflowing hydrochemistry by the continuous fringing papyrus, with a full swamp on the inflow rivers' delta. The ecology of the lake since that time has been severely disrupted by alien (or exotic) species invasions, particularly the Louisiana crayfish Procambarus clarkii. Lake-wide papyrus degradation has also been occurring since about 1980, following a lake level decline of up to 3m through agri-industrial abstraction. The river Malewa, which previously ran through a swamp, now runs directly into the lake, bringing high silt and nutrient loads in wet seasons, making the lake eutrophic. It is suggested that sustainable management of the lake should focus upon three ecohydrological objectives-control of abstractions to achieve hydrological balance, physical restoration of the former North Swamp and the control of illegal fishing. The latter would enable the commercial fishery returns to maximum sustainable yield so that large M. salmoides (large mouthed bass) individuals once again impose a 'topdown' control upon P. clarkii, and the crayfish itself be copmmercially exploited.
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