Sediment dynamics in a transboundary mangrove habitat: a perspective of sediment sources and sedimentation in the Vanga estuary, Kenya

Abstract

Mangroves are essential providers of environmental, economic, and ecological ecosystem services. Mangroves of Vanga occur in the most southerly coastal area of Kenya. They transcend the Kenya-Tanzania border and receive fresh water and particulate materials from the transboundary Umba River. Consequently, they provide a unique opportunity to study system dynamics borne of both anthropogenic and natural disturbances. Cognizant of the universal threat that mangroves face with respect to sea-level rise (SLR), the need to understand sediments dynamics within the Vanga estuary is vital for their conservation and management. This is because sedimentation has been identified as one of the factors that will allow mangroves to mitigate and modulate the effects of SLR. However, our knowledge and understanding of the sediment dynamics in the Vanga Estuary are limited. Consequently, it jeopardizes plans on mitigation and adaptive measures to be undertaken in the face of SLR and other resource management issues. We hypothesized that most of the Vanga Estuary sediments are sourced from the Umba River catchment, with minimal contribution from tidal and in-situ production. Therefore, the overall aim of this doctoral study is to investigate the influence of the transboundary Umba River on the connectivity of downstream coastal habitats and the terrestrial catchment. To achieve the objectives of this study, we evaluated 1) the geochemical and mineralogical composition of the Umba River sediments 2) analyzed and modeled elemental carbon (C), nitrogen (N), and their stable isotopes to determine the possible sources of sediment organic matter delivered to the Vanga estuary and 3) measured current sedimentation rates in the Vanga estuary to evaluate the possible response of Vanga mangroves to current and future rates of SLR. First, the Umba River sediments showed similar source geology along the river’s course from the source to its mouth in the Vanga Estuary. This is reflected by the similarity in the mineralogy and the chemical composition of sediments evident by the dominant mafic to felsic minerals discernible in both upstream and downstream sediments. The Umba River sediments also exhibit a moderate to high Chemical Index of Alteration (CIA) attributed to the variation from the dry to cold climate upstream (near the source) and humid climate further downstream. Secondly, based on the combined evaluation of elemental carbon (C) and nitrogen (N), C/N ratio, carbon, and nitrogen isotopes, together with the results of the applied Stable Isotope Mixing Model, a mixed source of sediment organic matter in the Vanga Estuary was observed. The mixing is attributed to the daily hydrodynamic flushing and bioturbation under the mangrove canopy. The model results also indicated that riverine organic matter was the dominant source with ~60% contribution to the total sediment organic matter. It validated our hypothesis on the influence of the Umba River on the Vanga Estuary. With sediment organic matter as a proxy, we conclude that the terrestrial catchment and Vanga Estuary are connected, with Umba River supplying both freshwater and sediments. Finally, having determined the connectivity of the Umba River catchment, downstream mangrove habitat in Vanga, and the consequent sediment input, we further evaluated the fate of the sediments in terms of deposition and mangrove surface elevation. Our data indicate that mangrove surface elevation is experiencing shallow subsidence of up to -15 mm/yr. At the same time, sediments also accreted by a rate of up to 9 mm/yr on the mangrove surface. This accretion rate exceeds the current rates of local SLR (3.8 mm/yr) and global SLR (3.1 mm/yr). The mangroves of Vanga would therefore be able to keep pace and persist under the current rates of SLR. However, accelerated SLR would marginally outpace the net mangrove surface elevation in Vanga. Our study implies that adaptive measures on mangrove conservation and management in Vanga need to capture the complex intra- and interhabitat sediment dynamics and interactions. This would include policy interventions through comprehensive and integrated cross-border resource management and governance.