Assessment of Wetland Dynamics and Ecological Condition in Mathioya Watershed, Murang’a County, Kenya

Abstract

Wetland degradation in Murang’a County, particularly due to anthropogenic activities in the area, is a major problem. The effects of anthropogenic activities on the ecosystem service of wetlands and the wetland health are not well documented. This study determined the major anthropogenic stressors affecting ecosystem service of wetlands in Mathioya watershed, and evaluated the ecosystem health of wetlands in Mathioya watershed. Analysis of land use and land cover (LULC) changes in Mathioya watershed, alongside interviews with the key informants and the local residents identified the anthropogenic activities impacting wetlands in the watershed. The analysis of Landsat images of 1987, 1997, 2007 and 2020 was used to establish the trend of wetland loss. Further, the study used plant metrics and water quality parameters to develop a Plant Index of Biotic Integrity (PIBI), which we used to evaluate the health of the wetlands in response to anthropogenic disturbance. Accordingly, 22 plant metrics were evaluated based on their response to anthropogenic disturbance and Mann-Whitney U test. Of the 22 metrics, only four (number of tolerant species, sensitive species, exotic species and shrub species) were selected for inclusion in the final PIBI. A trisect interquartile method was used to assign values to the core metrics. The sum of the scores of the selected four metrics gave the final PIBI, which we validated using the measured water quality parameters. Analysis of Landsat images revealed that between 1987 and 2020, human activity led to decrease in the area covered by wetlands, forestland, water bodies, and barren land. Area under these land cover classes decreased by 45%, 34%, 50% and 27%, respectively. Agricultural land and built-up areas increased by 43% and 85%, respectively. Expansion of agricultural and built-up area were the major contributors to wetland loss. As a result, the ecosystem service value attributable to wetlands decreased by 45% between 1987 and 2020. The metric scores for the wetlands ranged between 4 and 14. Only two wetlands were of moderate health, six were of poor health, and one wetland was of very poor health. The average score shows that wetlands in Mathioya watershed are generally of poor health. This study concludes that agricultural activities and expansion of built-up areas are the main anthropogenic stressors leading to wetland loss. Furthermore, while the majority of wetlands (6 out of 9) were of poor health, wetlands in areas with relatively low human activities were of better health. It is therefore recommended that the identified causes of degradation be addressed, and a restoration project initiated to restore the ecological integrity of the wetlands.