Analysis of macrophyte biomass productivity, utilization and its impact on various eco-types of Yala Swamp, Lake Victoria Basin, Kenya.
The overall aim of this study was to assess the sustainability of the current utilization of the wetland resources and the impact of human activities on the Yala swamp ecosystem in West Kenya. The socio-economic results indicate that this swampland provides a wide range of support and products to the local communities. These include direct benefits like thatch material and fish and other benefits such as land for small-scale farming and grazing. Approximately 70% of the wetland products are used at the domestic level with the rest being used to generate modest incomes. Marketing of wetland products is ineffective, resulting in low profit margins, which again discourage sustainable wetland use. Nevertheless, farming is an important activity, which engages 90% of the holdings in the swampland and supplies about 70% of the domestic food requirements. Post-harvest growth of the macrophyte that are commonly used by the local communities was high in the first four weeks ranging from 5-300% of the initial biomass. This was followed by a lower growth rate in the next 10 weeks averaging 1-30% with the less disturbed eco-types achieving higher values (10-30%) than the highly disturbed eco-types (1- 15%). The growth rate after the 14th week was highly diminished in all species. During the dry season, fast growth was also restricted to the first 14 weeks, but with an overall reduction in average height gain, growth rate and biomass in all the ecotypes. This variability was attributed to seasonal ecological dynamics and not to the effect of repeated harvesting. The average biomass was about 1,050g dry wt m2, which is within the values for other tropical papyrus wetlands. The plant nutrient N:P ratio ranged from 6-3.5, which was above ecological limiting levels of phosphorus. These results indicate that the macrophyte can be sustainably harvested at intervals of 14 weeks if the natural ecological setup is maintained. Ecological conditions were more favourable for macrophyte growth during the wet season (as compared to the dry season) and in the less disturbed ecotypes (as compared to the highly disturbed ecotypes). Soil parameters were more influenced by eco-type than by season. In contrast, water chemistry was more influenced by the seasons. Both soil total N (0.25 -0.3%) and P (0.07- 0.06%) as well as water P (0.03 - 0.14 mg/l) and N (3.72- 2.01mg/l) were above ecological limiting levels. Land-cover analysis was done using Landsat satellite images taken in the dry season (February 5, 1973, MSS and February 2, 2001 ETM). The most prominent change was a more than three-fold increase in agricultural land from 1,564 ha in 1973 (7 % of the total wetland) to 5,939 ha in 2001 (28 % of the total wetland). However, this excluded temporary land use during other seasons. This conversion of natural vegetation was mainly located along the swamp edges, in particular on the northern and eastern side of the swamp. The satellite images also allowed identification of the siltation areas, which have increased along the Lake Victoria shoreline. The overall classification accuracy was high at 75% with Kappa statistics at 70%. The Normalized Different Vegetation Index recorded a high reduction of the positives values from +0.909 in 1973 to +0.405 in 2001, mainly due to a reduction in the vegetation cover of the swamp. This was attributed to anthropogenic activities, mainly farming. The main driving factors for land-use changes in the Yala swamp were identified as (i) household numbers, (ii) household and population densities, and (iii) wetland accessibility (combining swamp coverage and terrain suitability). These drivers act as proxy for a whole range of factors, in particular the demand for farming land and high dependence of the local community on the swamp resources for their livelihoods. The statistically computed land-use change using the conversion index (11,696 ha) show a high co-validation with the land-cover changes derived from the satellite images (11,735.44 ha). In conclusion, it can be expected that under the current utilization scenario, swamp conversion is expected to increase as a function of household densities. The big challenge is to balance between increasing swamp farming and sustainable ecosystem utilization, e.g., macrophyte-based water filtering, while maintaining the benefit flow to the local communities.
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| Format: | Report biblioteca |
| Language: | English |
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Center for Development Research, University of Bonn
2006
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| Subjects: | Biomass, Macrophytes, Aquatic plants, Freshwater plants, Fishery resources, Ecotypes, Habitats, Wetlands, Inland waters, Man-induced effects, |
| Online Access: | http://hdl.handle.net/1834/6837 |
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