Papyrus wetlands a lunar-modulated refuge for aquatic fauna

A field study was undertaken of the use of papyrus (Cyperus papyrus) wetlands fringing Rubondo Island, Lake Victoria, Tanzania, as a refuge from predators by freshwater shrimps (Caridina nilotica) and tilapia (Oreochromis niloticus) larvae. During the full moon nights, fish larvae took refuge more than 5 m in the papyrus and did not venture out into the lake, while they ventured into fringing lake waters during moonless nights. The freshwater shrimps and lake flies (Chaoborus sp.) did not migrate with the moon. This fauna took refuge in the papyrus wetlands even though the dissolved oxygen concentration was as low as 0.4 ppm (<5% of saturation).     

Papyrus wetlands creation, a solution to improve food security and save Lake Victoria

A demonstration project was set up to create two small papyrus wetlands in villages on the shores of Lake Victoria near Rubondo Island National Park, aimed at helping the community to replenish the fish stock in the lake and to improve socio-economics. The wetlands were constructed by using locally available means and they are owned and successfully managed by the villages to support community-based activities. We describe the approach, methodology and design of these plots. 2 years after the wetlands were created, the above-ground papyrus biomass was found to be comparable with that of pristine papyrus wetlands at Mlaga Bay in Rubondo Island National Park. Light trap data shows increased fish around the area. This correlated well with the results of questionnaire survey from the community around the created wetland. Our study shows that the degraded wetlands around Lake Victoria can be recreated by using locally available means, to restore most of the vital functions of those wetlands as they were before destruction, and improve the socio-economics of the local communities.     

Relationships between wetland ecotones and inshore water quality in the Ugandan coast of Lake Victoria

Much of the lake shore in Lake Victoria is covered by extensive wetlands, often dominated by dense papyrus stands that extend out over the lake waters. These wetlands, their extension and management play a role in the physical, chemical and biological conditions of the inshore waters. Continuous transects along 180 km of shoreline together with spatial grids of sampling sites in eight bays were performed in the Ugandan inshore waters in order to analyze the relationships between the wetland characteristics and water quality. Measurements of extension of the wetland ecotones, water temperature (T), pH, Secchi disk depth (SD), dissolved oxygen (DO), total nitrogen (TN), total phosphorous (TP), dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and chlorophyll-a (CHL) were made in each sampling area. Data of T, pH and DO collected during the transects showed that the water characteristics of the bays differ from the open shoreline. Moreover, the magnitude of these physical–chemical differences is strongly conditioned by the dimension of the bordering wetlands. Bays with extensive wetlands ecotones were characterized by cooler, more acidic and poorly oxygenated waters. TN : TP ratios and especially DIN : SRP ratios decreased with the wetland presence along the coastline, showing a higher probability of N limitation in the inshore waters where large wetlands are present. Results point to denitrification processes in the wetland ecotones as the cause of this trend. The distribution of CHL was found to be highest in the presence of two significant point loading sources: a river (in Katonga Bay) and a major population centre (Kampala, in Murchison Bay). The reduction of external P loading is shown as an important step in the management of the eutrophication process of Lake Victoria inshore waters.     

A simulation model for nitrogen retention in a papyrus wetland near Lake Victoria,Uganda (East Africa)

Papyrus wetlands around Lake Victoria, East Africa play an important role in the nutrient flows from the catchment to the lake. A dynamic model for nitrogen cycling was constructed to understand the processes contributing to nitrogen retention in the wetland and to evaluate the effects of papyrus harvesting on the nitrogen absorption capacity of the wetlands. The model had four layers: papyrus mat, water, sludge and sediment. Papyrus growth was modelled as the difference between nitrogen uptake and loss. Nitrogen uptake was modelled with a logistic equation combined with a Monod-type nitrogen limitation. Nitrogen compartments were papyrus plants, organic material in the floating mat; and total ammonia, nitrate and organic nitrogen in the water, sludge and sediment. Apart from the uptake and decay rates of the papyrus, the model included sloughing and settling of mat material into the water, mineralization of organic matter, and nitrification and diffusion of dissolved inorganic nitrogen. Literature data and field measurements were used for parameterization. The model was calibrated with data from Kirinya wetland in Jinja, Uganda which receives effluent from a municipal wastewater treatment plant. The model simulated realistic concentrations of dissolved nitrogen with a stable biomass density of papyrus and predicted accumulation of organic sludge in the wetland. Assuming that this sludge is not washed out of the wetland, the overall nitrogen retention of the wetland over a three-year period was 21.5 g N m⁻² year⁻¹ or about 25% of input. Harvesting 10, 20 and 30% of the papyrus biomass per year increased nitrogen retention capacity of the wetland to 32.3, 36.8 and 38.1 g m⁻² year⁻¹, respectively. Although the nutrient flows estimated by the model are within the ranges found in other papyrus wetlands, the model could be improved with regard to the dynamics of detrital nitrogen. Actual net retention of nitrogen in the sludge is likely to be lower than 21.5 g N m⁻² year⁻¹ because of flushing out of the sludge to the lake during the rainy season.     

Functioning and dynamics of wetland vegetation of Lake Victoria: an overview

The aquatic macrophytic vegetation constituting the wetlands situated along the coast of Lake Victoria provides valuable services to both local and regional communities as well as an important ecological function through the transition between terrestrial and aquatic ecosystems. The wetland vegetation is typically rooted in the substrate on the landward side of the lake, but forms a floating mat towards the middle of the wetland and at the wetland/lake interface. Cyperus papyrus and Miscanthidium violaceum vegetation typically dominate the permanently inundated wetland areas along most of the shores of Lake Victoria. Due to the prevailing climatic and hydrological catchment conditions, these macrophytic plants (papyrus in particular) tend to exhibit high net productivity and nutrient uptake which strongly influences both wetland status and lake water quality. In addition, these wetlands provide important economic livelihoods for the local populations. The integrity and physical structure of these wetlands strongly influences their associated mass transport mechanisms (water, nutrients and carbon) and ecosystem processes. Wetland degradation in Africa is an increasing problem, as these ecosystems are relied upon to attenuate industrial, urban and agricultural pollution and supply numerous services and resources. In an integrated project focused on the wetlands of Lake Victoria, the ecological and economic aspects of littoral wetlands were examined and new instruments developed for their sustainable management.     

Nutrient status and retention in pristine and disturbed wetlands in Uganda: management implications

Wetlands in Uganda experience different forms of human pressure ranging from drainage for agriculture and industrial development to over harvesting of wetland products. In order to develop sustainable management tools for wetland ecosystems in Uganda and the Lake Victoria Region, water quality analyses were carried out in a rural undisturbed (pristine) wetland (Nabugabo wetland in Masaka) and two urban wetlands that are experiencing human and urban development pressure (the Nakivubo wetland in Kampala and Kirinya wetland in Jinja). The former wetland forms the main inflow into Lake Nabugabo while the other two border the northern shore of Lake Victoria, Uganda. Nabugabo wetland buffers Lake Nabugabo against surface runoff from the catchment, while Nakivubo and Kirinya wetlands provides a water treatment function for wastewater from Kampala City and Jinja town respectively, in addition to buffering Lake Victoria against surface runoff. Water quality was assessed in all the wetland sites, and in addition nutrient content and storage was investigated in the main plant species (papyrus, Phragmites, Miscanthidium and cocoyam) in Nakivubo and Kirinya wetlands. A pilot experiment was also carried out to assess the wastewater treatment potential of both the papyrus vegetation and an important agricultural crop Colocasia esculenta (cocoyam). Low electrical conductivity, ammonium–nitrogen and ortho-phosphate concentrations were recorded at the inflow into Nabugabo wetland (41.5 μS/cm; 0.91 mg/l and 0.42 mg/l respectively) compared to the Nakivubo and Kirinya wetlands (335 μS/cm; 31.68 mg/l and 2.83 mg/l and 502 μS/cm; 10 mg/l and 1.87 mg/l respectively). The papyrus vegetation had higher biomass in Nakivubo and Kirinya wetlands (6.7 kg DW m⁻²; 7.2 kg DW m⁻² respectively), followed by Phragmites (6.5, 6.7), cocoyams (6.4, 6.6) and Miscanthidium (4.0, 4.2). The papyrus vegetation also exhibited a higher wastewater treatment potential than the agricultural crop (cocoyam) during the pilot experiment (maximum removal degree of ammonium–nitrogen being 95% and 67% for papyrus and yams). It was concluded that urbanisation pressure reduces natural wetland functioning either through the discharge of wastewater effluent or the degradation of natural wetland vegetation. It is recommended that wetland vegetation be restored to enhance wetland ecosystem functioning and for wetlands that are not yet under agricultural pressure, efforts should be made to halt any future encroachment.     

Enhancing the fish production potential of Lake Victoria papyrus wetlands,Kenya, using seasonal flood-dependent ponds

We present the findings of a participatory experiment on integrated aquaculture-agriculture systems (Fingerponds) at the Lake Victoria wetlands, Kenya. Fingerponds are flood-based lacustrine or floodplain wetland fishponds. The aim of the study was to explore, within a wetland/floodplain interface environment, the potential of semi-intensive fish production to enhance the wetland fishery and protect the natural ecosystem from wide-scale, destructive encroachment. The ponds were stocked naturally by flood water. After flood recession, livestock manure was added to the ponds and the effects of manuring on water and sediment quality and fish yields were studied. Manuring had positive effects on the nitrogen, phosphorous and chlorophyll a concentrations of the pond water. Regression analysis results indicated that site, manuring and environmental and climatic variables explained 58–70% of the variation in dissolved nitrogen and phosphorous, and 71% of the variation in chorophyll a. Manuring enhanced the total phosphorus concentration in the sediment but it only had marginal effects on total nitrogen. Although the net fish yields were highly variable between sites and seasons, ranging from 402 to 1069 kg ha⁻¹, the data showed that manuring was advantageous. The duration of the culture period, site variability and manuring explained 82% of the variation in fish yields. We conclude that Fingerponds fertilized with livestock manure from abutting riparian subsistence agriculture can improve fish production, enhance food diversity and security and contribute to more efficient use of papyrus wetlands for food production. This in turn may reduce large-scale conversion of wetlands to agriculture.     

Papyrus wetlands, nutrients balance, fisheries collapse, food security, and Lake Victoria level decline in 2000–2006

The future of Lake Victoria and its people is highly related to the future of its papyrus wetlands. This appears to be threatened by the overdrawing of water at two dams at the outlet of Lake Victoria in Uganda, which can lead to wetland loss, tilapia fisheries collapse, enhanced eutrophication of the lake, loss of food security for the empoverished population, and a measurable contribution to global warming.     

Primary production in papyrus (<Emphasis Type="Italic">Cyperus papyrus</Emphasis> L.) of Rubondo Island,Lake Victoria,Tanzania

Above- and below-ground production and morphological characteristics of papyrus wetlands were measured at monthly intervals from July 2005 to June 2006 at Rubondo Island, Lake Victoria, Tanzania. The average value of live culm biomass (5,789 ± 435 g DW m⁻²) was higher than that of umbel biomass (2,902 ± 327 g DW m⁻²) by 50%. Root to rhizome means biomass value ratio was 1:1.8, rhizome biomass (4,144 ± 452 g DW m⁻²) being higher than roots biomass (2,254 ± 314 g DW m⁻²) by 45%. Direct proportion was observed between shoot density and culm–unit (culms and umbels) biomass. The average value of detritus/litter biomass (1,306 ± 315 g DW m⁻²) was less than total aerial biomass by 86%. The values of biomass are average of 12 sampling months from July 2005 to June 2006.     

Agricultural encroachment: implications for carbon sequestration in tropical African wetlands

Tropical wetlands have been shown to exhibit high rates of net primary productivity and may therefore play an important role in global climate change mitigation through carbon assimilation and sequestration. Many permanently flooded areas of tropical East Africa are dominated by the highly productive C₄ emergent macrophyte sedge, Cyperus papyrus L. (papyrus). However, increasing population densities around wetland margins in East Africa are reducing the extent of papyrus coverage due to the planting of subsistence crops such as Colocasia esculenta (cocoyam). In this paper, we assess the impact of this land use change on the carbon cycle and in particular the impacts of land conversion on net ecosystem carbon dioxide exchange. Eddy covariance techniques were used, on a campaign basis, to measure fluxes of carbon dioxide over both papyrus and cocoyam dominated wetlands located on the Ugandan shore of Lake Victoria. Peak rates of net photosynthetic CO₂ assimilation, derived from monthly diurnal averages of net ecosystem exchange, of 28–35 μmol CO₂ m^?2 s^?1 and 15–20 μmol CO₂ m^?2 s^?1 were recorded in the papyrus and cocoyam wetlands, respectively, whereas night‐time respiratory losses ranged between 10 and 15 μmol CO₂ m^?2 s^?1 at the papyrus wetland and 5–10 μmol CO₂ m^?2 s^?1 at the cocoyam site. The integration of the flux data suggests that papyrus wetlands have the potential to act as a sink for significant amounts of carbon, in the region of 10 t C ha^?1 yr^?1. The cocoyam vegetation assimilated ~7 t C ha^?1 yr^?1 but when carbon exports from crop biomass removal were accounted for these wetlands represent a significant net loss of carbon of similar magnitude. The development of sustainable wetland management strategies are therefore required to promote the dual wetland function of crop production and the mitigation of greenhouse gas emissions especially under future climate change scenarios.     

A LIMNOLOGICAL INVESTIGATION OF LAKE LIAMBEZI,CAPRIVI

SUMMARY

A general investigation was carried out on Lake Liambezi to characterize the limnological features of the lake. The lake, 101 km² in area and situated within the tropics, was found to be shallow and bordered by reed swamps, particularly on the south-western shore. It was polymictic and consequently oxygen levels were high at all depths. Transparency was such that photosynthesis occurred to the bottom. Conductivity ranged from 17,3 to 41,2 mSm^?1 and the waters were moderately alkaline. Transfer of organic detritus from the reed swamp into the open water has resulted in the presence of high levels of organic nitrogen and phosphorus whilst ambient levels of inorganic forms were low. Planktonic chlorophyll a concentrations ranged between 1 and 104 μg l^?1 and Microcystis was the dominant phytoplankter. The dominant zooplankter was Bosmina. The important species of phytoplankton and zooplankton are listed and include two new species of blue-green algae which are described. The lake was found to be generally similar to most shallow tropical African lakes with a wide diversity of food sources for fish. Progressive encroachment by Salvinia appears to hamper access to fishing areas and the fishery potential is threatened.     

Biomass Allocation in Cyperus papyrus in a Tropical Wetland,Lake Naivasha,Kenya1

The nutrient status of Lake Naivasha, a freshwater lake in southeastern Kenya, has been rising since at least 1982. A potential effect of increases in nutrient supply to the lake's floating papyrus is increasing of the plants’ investment in above–water material and reduction of the amount of energy invested in uptake and storage. Biomass and its allocation between culms, panicles, roots, and rhizomes was measured in 17 sites around the 150–km² lake. Although above–water biomass was greatest in sites closest to the lake's major nutrient inflow, the River Malewa, there was little evidence of corresponding decreases in the biomass of uptake and storage tissues. In August 1995, the average ± SD biomass of papyrus in the lake was 11,540 ± 3020 g/m², with the papyrus containing about 4500 ± 1900 g total carbon/m² and about 100 ± 70 g total nitrogen/m². Plant nitrogen contents did not vary with distance from the main external nutrient supply. Together with low nitrogen concentrations in the plants (0.60 ± 0.26 in culms and 0.99 ± 0.50% in rhizomes), very high carbon to nitrogen ratios (49 ± 20:1) and nitrogen fixation in the rhizosphere explaining only about half of the plants’ nitrogen, papyrus is a likely net sink for nitrogen supplied from the lake's increasingly cultivated watershed. Despite this role, clearance of papyrus in favor of agriculture partly explains the reductions in the area of papyrus within the lake basin from 48 km² in the late 1960s to 14 km² in 1995.     

The macroinvertebrate fauna of water hyacinth fringes in the Sudd swamps (River Nile,southern Sudan)

The water hyacinth now forms an almost ubiquitous fringe to open-water habitats in the permanent wetland of the Sudd. In a typical river-lake the fringe varied between 9 and 16 m in breadth with highest plant biomasses in the centre. Largest quantities of macroinvertebrates in hyacinth root-mats were obtained in drop-trap samples within 6 m of the open water. They were dominated by coleopterans, odonatans, gastropods andCaridina nilotica. Outer edges of the fringe offer easy access, good dissolved oxygen concentrations, and a variety of potential food resources for aquatic invertebrates. By contrast the less hospitable landward zone contained a reduced fauna in which coleopterans, hydracarines and gastropods were prominent. A marked reduction in the numbers of invertebrates in lift-net samples of detached hyacinth rafts was attributed to browsing by fish. Hyacinth root-mats appear to have replaced the niches formerly provided by those of the Nile cabbage for aquatic invertebrates in the Sudd.     

Stable isotope analysis of larval mosquito diets in agricultural wetlands in the coastal plain of Georgia,U.S.A.

Previous studies have used C and N isotope ratios to investigate the use of different food resources such as plant and animal detritus by container‐breeding mosquitoes. This study is the first to report on the potential food resources assimilated by larval mosquitoes in agricultural and reference wetlands. Larval mosquitoes (Diptera: Culcidae) were sampled, along with their potential food resources, from agricultural and reference wetland habitats throughout a seasonal hydroperiod. IsoSource mixing model results indicated that food resources had greater δ¹⁵N isotope values in agricultural wetlands compared with cypress‐gum swamps. In February, Aedes vexans (Meigen) and Culex territans Walker larvae fed primarily on lower quality food resources (coarse particulate organic matter and sediment) based on C:N. In contrast, higher quality food resources (fine particulate organic matter) were utilized by Anopheles spp. throughout the study and by Psorophora columbiae (Dyer and Knab) in May. This research contributes to a more comprehensive understanding of the food resources available and assimilated by larval mosquitoes in agricultural wetlands.     

Carbon and water cycles in tropical papyrus wetlands

Highly productive papyrus (Cyperus papyrus L.) wetlands dominate many permanently flooded areas of tropical East Africa; however, the cycling of carbon and water within these ecosystems is poorly understood. The objective of this study was to utilise Eddy Covariance (EC) techniques to measure the fluxes of carbon dioxide and water vapour between papyrus vegetation and the atmosphere in a wetland located near Jinja, Uganda on the Northern shore of Lake Victoria. Peak, midday rates of photosynthetic CO₂ net assimilation were approximately 40 μmol CO₂ m⁻² s⁻¹, while night time losses through respiration ranged between 10 and 20 μmol CO₂ m⁻² s⁻¹. Numerical integration of the flux data suggests that papyrus wetlands have the potential to sequester approximately 0.48 kg C m⁻² y⁻¹. The average daily water vapour flux from the papyrus vegetation through canopy evapotranspiration was approximately 4.75 kg H₂O m⁻² d⁻¹, which is approximately 25% higher than water loss through evaporation from open water.     

The optical qualities of shallow wetland lined bays in Lake Victoria

In tropical lakes, the characteristics and dimension of the coastal wetlands can have a strong influence on the quality of the inshore waters. The interaction between littoral wetlands and the open water environment is complex and requires an understanding of the material and energy exchanges between these compartments. In the present analysis, we examine the impact of wetlands on the optical properties (underwater light environment) of two adjacent bays on the Ugandan side of Lake Victoria. We use both irradiance profiles within the water column as well as measurements of the dissolved and particulate fractions of the water column. By introducing a new term to identify that part of the solar spectrum most affected by wetland released dissolved organic matter, it is possible to determine the area of each bay that is influenced by the wetland. Depending on the period of analysis, wetland released chromophoric dissolved organic matter can play a dominating role in the attenuation of UV and visible radiation in the underwater environment.     

The food, feeding habits and distribution of a siluroid catfish, Bagrus docmac (Forsskal), in the Kenya waters of Lake Victoria

Adult Bagrus docmac in the Nyanza Gulf of Lake Victoria feeds mostly on fish–Haplochromis spp. and Engraulicypris argenteus (Pelligrin). Juveniles prefer aquatic benthic invertebrates with Phyllogomphus aethiop.?(K.elys), Phanostoma senegalense(Brauer), Chironomus spp. and Caridina nilotica (Roux) dominating, followed by Chaoborus spp., Povilla adusla (Navas), Valvata spp. and Neoperla spio (Newman) in order of importance. B. docmac exhibits diurnal changes in feeding habits. It feeds actively during dusk and dawn, taking advantage of the twilight environment for successful feeding mission–a crespuscular feeding behaviour. Selectivity of food seems dependent on occurrence of prey items. The species has an ubiquitous distribution in the Gulf but occurs more frequently with increasing water depth and decreasing water temperature. The biomass of the predator shows little correlation with water conductivity. Bagrus docmac and the other important piscivorous species in the lake, Lates niloticus (L.), are mutually exclusive in their bathymetric distribution within the Gulf. They also have different preferences for water temperature and conductivity.     

Massive fish kills within the Nyanza Gulf of Lake Victoria,Kenya

The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes.The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.     

鄱阳湖湿地水位变化的景观响应

根据《湿地公约》和《全国湿地资源调查技术规程(试行)》关于湿地内涵和界定条件,利用现代遥感技术和地理信息技术,提取鄱阳湖湿地面积为3886km²。在此基础上,利用多时段遥感影像,分析了湿地在相似水位条件和不同水位条件下的景观变化。结果表明,在相似水位条件下,鄱阳湖湿地景观变化主要表现为居民地和裸地面积增加,水域面积变化不明显;在不同水位条件下,鄱阳湖湿地景观变化比较显著。景观变化驱动力分析表明,湿地水位变化是湿地景观变化的主要因素。     

Three new interstitial dorvilleids (Annelida: Polychaeta) from the Cymodocea nodosa meadows of the Canary Islands

Resource–consumer relationships in Lake Victoria were investigated by use of stable isotope data. ¹³C and ¹⁵N signatures were determined for organisms at a deep (22 m) and a littoral (5 m) site in the Napoleon Gulf near Jinja, Uganda. Results suggest that two food chains operate at the deep site, one leading from a shrimp (Caridina nilotica) to juvenile Nile perch (Lates niloticus), the second leading from zooplankton (copepods and cladocerans) to a cyprinid (Rastrineobola argentea) and lake flies (Chaoborus). Isotopic evidence suggests that shrimp eat suspended particulates and benthos, not crustacean zooplankton or water hyacinth (Eichhornia crassipes). Resource–consumer relationships revealed in this study have implications for understanding future yields of the economically important Nile perch fishery.