Mangrove dieback due to massive sedimentation and its impact on associated biodiversity

Abstract

Emerging scientific evidence strongly suggests that the 1997/8 and 2000 rains in the Western Indian Ocean (WIO) region previously attributed to El-Niño were mainly caused by the Indian Ocean Dipole (IOD), a phenomenon whose frequency and intensity have increased over the years due to climate change. The abnormally high rainfall experienced during this time combined with poor land use managment, caused intense sedimentation due to erosion of terrigenous sediments, leading to extensive mangrove die-back in several areas in the WIO region. The extent and impact of this die-back on mangrove-associated biodiversity has not been assessed to date. The objective of this study was to assess the impact of the die-back on mangrove associated biodiversity at Mwache Creek, Mombasa, Kenya, where about 200 ha of mangroves were decimated. Biodiversity in impacted sites was compared to reference sites (natural forests) in order to assess the impact of IOD-related massive sedimentation to mangrove-associated biodiversity namely crabs and molluscs. Transects (sea-landward transect) were laid 10 m apart to ensure independence of sampling units in both impacted and natural sites. Along each transect, 5 m² quadrats were made at 100 m intervals. Within the 5 m² quadrats, one 2 m × 2 m sub-quadrat was randomly placed for actual sampling where relevant physico-chemical variables were measured and mangrove biodiversity determined as an indicator of ecosystem change. Environmental factors showed a strong variability among transects whereas faunal assemblages significantly differed among the treatments (impacted and non-impacted). Salinity, temperature and total dissolved oxygen (TDSO) were the environmental factors that contributed to the changes in biotic composition among the treatments, whereas Uca inversa, Uca annulipes, Cerithidea decollata and Perisesarma guttatum contributed more to the differences in faunal composition between treatments. The degradation seems to have significantly reduced crab and mollusc species richness and densities and led to loss of other mangrove-associated faunal species in the impacted site. These results emphasize the effects of climate change-related impacts on mangrove-associated biodiversity and by implication, the ecosystem functions they support