Sedimentation in response to sea level rise in the mangroves of Mwache Creek, Mombasa-Kenya: a field and modeling study

Abstract

The stability of mangrove ecosystems in the face of rising sea level highly depends on their ability to maintain their surface elevations relative to the rising sea level through the accumulation of mineral and biotic sediments. To gain an understanding regarding the magnitude and interaction of sedimentation processes and other environmental factors with mangrove surface elevation, a study was carried out in Mwache Creek, Mombasa-Kenya, considered an appropriate site for this objective. Suspended sediment concentrations (SSC), sediment accretion rates, soil vertical elevation change and variation of water levels were measured at two sites with varying degree of degradation due to human and natural impacts. Short term sediment accretion rates averaged between 51.78 – 144.48 g m-2 to 190.88 – 344.33 g m-2 in the highly degraded and the less degraded site respectively per 14-day spring-neap cycle. SSC varied on average between 0.076 g L-1 to 0.128 g L-1 in highly degraded site and the less degraded site respectively, while the average elevation change rates (mm yr-1) varied between 1.65 mm yr-1 to 3.95 mm yr-1 in the highly degraded and the less degraded site respectively. The sea level in Mombasa was found to be rising at a rate of 3.1 mm yr-1 and this correlated well with IPCC projected global rates of 3.0 mm yr-1. Using the data collected as part of this project, we applied a marsh sedimentation (MARSED) model to forecast the response of mangrove surface elevation to the effects of different environmental conditions by varying the model input variables. The model simulations showed that the growth of mangrove surface elevation is influenced by its elevation relative to sea level (which controls inundation and hence sediment supply by the tides), suspended sediment concentrations, mineral sediment accretion and compaction rates but the effect of organic sedimentation and low settling velocities were found to have a minimal effect. From the observed accretion rates and elevation changes the mangroves of Mwache Creek act as sediment traps which is in agreement with previous research in both temperate and tropical coastal wetlands. However in Kenya, more short-term measurements and quantification of these sedimentation processes would provide further understanding of the integrity and sustainability of mangrove ecosystems in the face of global threats including relative sea level rise.