Effects of Umba River Sedimentation on the Distribution and Root Morphology of Mangroves of Vanga, Kenya

Abstract

The study examined the influence of Umba River, a trans-boundary resource draining approximately 16 million m3 of freshwater into the Indian Ocean, on the distribution and root morphology of mangrove of Vanga. Mangroves of Vanga, situated a few kilometers from the border that transverse the Kenya-Tanzania border, south coast of Kenya, harbor countless marine and coastal biodiversity and contributes to the socioeconomics wellbeing of the adjacent communities. This ecological survey aimed at understanding the influence of the river on sediment surface elevation change, physicochemical parameters, and the resultant effect on mangrove distribution and root morphology. Sixty-three plots were sampled along twenty-two belt transects laid perpendicular to the river within three forest blocks (A, B and C) representing landward, midstream and seaward sites respectively. Highest mangrove stem density (3268±325 stem/ha) was recorded seaward at relatively lower burial levels (2.69±0.49 cm) and relatively high salinity (30.09±13.85). Nutrient concentration was relatively low across all study blocks but mainly dominated by ammonia (70%), signifying hypoxia in sediment. Among the three blocks, Avicennia marina recorded the largest number (242±45/m2) and longest (>15 cm) pneumatophores in the landward block. The difference in the height of Rhizophora mucronata prop roots was however not significant within the blocks. These findings suggest that sediment elevation change had the most influence on mangrove. Increase in sediment deposition influenced mangrove distribution, species zonation and root morphology, with high stem density being recorded at lower burial levels and species demonstrating a specific range of tolerance to related environmental variables. In addition, mangrove complex root system, depending on species, may adjust to cope with the increasing sedimentation. It is therefore important to use these findings to inform the development and management of the proposed Kenya-Tanzania Transboundary Conservation Area (TBCA).