Inhibition of <Emphasis Type="Italic">Zizania latifolia</Emphasis> growth by <Emphasis Type="Italic">Phragmites australis</Emphasis>: an experimental study

The influence of Phragmites australis on the growth of Zizania latifolia, and the morphological characteristics of both species, were investigated at two nutrient levels. The experiment was carried out over three growing seasons between May 2003 and December 2005. The experimental plot was longitudinally divided into two equal halves, one of which was planted with Zizania latifolia (Zizania zone) and the other one with Phragmites australis (Phragmites zone). Four weeks after transplantation, the plot was again divided horizontally, perpendicular to the previous division, and subject to low (LN) and high nutrient (HN) treatments. Measured growth indices of the two species were more developed in the HN than in the LN treatments. Both species were observed to invade each other’s zone, but Phragmites latifolia seemed to out-compete Zizania latifolia individuals. This was evident from the decrease in below ground biomass of Zizania latifolia in the third growing season and deterioration in above ground organs with time. It was concluded that Phragmites australis out-competes Zizania latifolia due to better developed root and rhizome system.     

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.     

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.     

Water circulation and fish larvae recruitment in papyrus wetlands,Rubondo Island,Lake Victoria

A field study was undertaken of the hydrodynamics, water quality and adult and larval fish abundance in papyrus wetlands and surrounding coastal waters at Rubondo Island, Lake Victoria. Because they were exposed to the prevailing wind, the two bays facing east had small (0.01 km²) wetlands and were well flushed, with minimal accumulation of organic detritus and little oxygen depletion. Because it faced west, the third bay, Mlaga Bay, was sheltered and poorly flushed; organic matter accumulated in a large (1.41 km²) papyrus wetland. The rises and falls of the lake level at decadal time scales, may, by drowning or drying out wetlands, contribute significantly to storing organic matter as detritus. Vegetation decay in the wetlands, as well as primary production, resulted in large diurnal fluctuations of pH and dissolved oxygen concentration. Shading in daytime and cooling at night, cooled the wetlands water sufficiently to generate a baroclinic circulation whereby cold, wetland water sank and moved offshore in the lake while warm, lake water intruded in the wetlands near the surface. This flushing prevented the occurrence of anoxic conditions in the wetlands which were used by adult and larval fish, mainly tilapia, and freshwater shrimps (Caridina nilotica). No fish larvae were found at more than 150 m from the shore in the three bays. Both larval fish and Caridina nilotica appeared to move in and out of the wetlands at night, possibly as a result of low dissolved oxygen as well as sheltering from predation. Their abundance varied with the lunar phase, with maximum near new moon and first quarter. Being the only protected wetlands in Lake Victoria, Rubondo Island may increasingly become an important source of replenishment for fisheries in the lake which elsewhere appears overfished.     

The influence of wetlands, decaying organic matter, and stirring by wildlife on the dissolved oxygen concentration in eutrophicated water holes in the Seronera River, Serengeti National Park, Tanzania

The dissolved oxygen concentration (DO) was sampled during a diurnal cycle in three water holes heavily used by wildlife and with distinctive biological features along the Seronera River. The DO fluctuated widely (by up to 11.5 mg l⁻¹) as a function of time, mechanical stirring and aeration by animals, and the presence of fringing wetlands. The DO cycle was successfully modeled (within 0.3 mg l⁻¹) by assuming that the four dominant processes were photosynthesis and respiration by algae near the surface, trapping by wetlands, decomposition of dead organic matter on the bottom, and stirring/aeration by hippos. The rate of DO decline from the decay of dead organic matter was equal to the rate of DO removal by algal respiration at night.     

The role of vegetation in the water budget of the Usangu wetlands, Tanzania

The Usangu wetlands were severely degraded over the last twenty years by cattle and the shortage of water due to rice irrigation upstream. The eastern Usangu wetlands that were previously perennial dried out in 2000 and 2002 in the dry season. Following the removal of cattle in 2006 from the eastern Usangu wetlands, perennial wetlands has re-established itself and in 2011 the vegetation had recovered and covered about 95?% of the wetted surface mainly as floating vegetation. These wetlands are the source of water of the Great Ruaha River and the volume of water entering the river has nearly doubled after cattle removal. We suggest that this may be due to the shading effect of the floating vegetation reducing the loss of water through net evaporation to about 0.5?cm?day^?1 as opposed to about 1?cm?day^?1 for open water evaporation in this tropical climate. This suggests the important role of the biology in controlling the water budget. By contrast cattle and rice farms have not been removed from the western Usangu wetlands, located upstream, where the wetlands are now reduced to small areas fringing the rivers. We suggest that the western Usangu wetlands should also be restored in order to further increase flows in the Great Ruaha River. At the same time water governance in the catchments and irrigation areas upstream of Usangu wetlands is also urgently required because present water yields are insufficient to meet the hydroelectric needs of Tanzania, the water users all along the river, as well as the important coastal wetlands associated with the Rufiji Delta during a drought year.     

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).