An ecological and palynological study of Manguo Wetland in Kiambu County, Kenya

Abstract

Wetlands provide vital natural products and environmental services that support human development. The ecological character of wetlands are closely linked with the surrounding terrestrial ecosystems hence land use activities on the catchment area have far-reaching effects on the ecological character of the wetland. The overall goal of this study was to investigate the past and contemporary dynamics of vegetation communities in response to environmental changes at Manguo wetland, Kiambu County Kenya. This was achieved through determining present vegetation structure and distribution, assessing water and soil characteristics and reconstructing the historical vegetation changes through palynology. The study area measured 7.5 ha, sampling for vegetation and soils was carried out at regular intervals along 14 intersecting transects. Vegetation sampling was carried out both during the dry period and the wet period so as to capture changes between seasons (October/November and January/February). Top and sub soil samples were collected from selected quadrats in the flooded grassland and dry ground zones while water samples were obtained using a standard sampling procedure from the existing open water body. The soil and water samples were transported for physical and chemical analysis at the Kenya Agricultural Research Institute (KARI). A Russian soil corer was used to extract a 2.25m deep soil core in sections of 0.5m. The core sections were lithologically described then wrapped in PVC and aluminium foil for transportation to the Palynology Laboratory at the National Museums of Kenya. The core was sub sampled at 0.1m intervals for processing and analysis of pollen content. The modern vegetation sampled was from 20 species from 8 orders and 12 families identified in both the wet and dry period samples. The vegetation abundance was higher during the wet period sampling (6387 individuals) compared to the dry period sampling (4272 individuals) with Pennisetum clandestinum having the highest abundance in both periods. The wet period diversity was also higher at HI = 1.05 while the dry period diversity was HI = 0.64. The species richness calculated using the Margalef index was higher during the dry period at 4.13 than the wet period at 3.94. The plant species composition changed along soil moisture and nutrient gradients but not significantly. However seasonality and zonation also did not have a significant impact on the composition and distribution of the wetland vegetation. XIV The soils though water logged had adequate levels of nutrients as per the Kenya Agricultural Research Institute (KARI) standards. The Ph levels were found to have significantly dropped from the 2007 level of 7.03 to 5.12. The water was also determined to be of irrigation quality but not direct consumption due to solid waste pollution. The canonical correspondence analysis (CCA) showed that the soil characteristics with the highest influence on vegetation distribution were variations in pH, copper and potassium content. The sediment core revealed the water sheds historical succession from high forest species abundance to open grassland in the recent past. There was high presence of Juniperus, Olea and Podocarpus which increased up the core. Acacia, Apodytes, Cordia and Croton present at the lower levels gradually but consistently reduced up the core and completely disappeared by the 60cm level. The presence of Amaranthaceae/Chenopodiaceae and Asteraceae is consistent and increased up the core, they are indicators of human impact mainly cultivation and degradation respectively. Poaceae and Cyperaceae are present throughout the core at different abundance levels and are driven by moisture levels changes (climate). These vegetation changes observed across the wetland temporally and spatially are evidence of both climatic and human induced pressures on upland wetlands in Kenya. The information can be used in the development of conservation measures for Manguo wetland but this can be replicated and tailored to specific ecosystems in the development of conservation policies and also in ecological models.