Response of benthic fauna to mangrove degradation and restoration in Gazi bay - Kenya

Abstract

The recovery towards a natural state of a restored Rhizophora mucronata mangrove ecosystem was investigated by assessing the sediment physical characteristics, densities, community composition and diversity of benthic macro-endofauna and meio-endofauna from a natural, a 10 years reforested, a 5 years reforested and a degraded (clear felled) mangrove ecosystem. The natural forest was used as a reference (baseline state) while the degraded site was to provide information on the effects of mangrove degradation on macro-endofauna. Samples for sediment physical characteristics and macro-endofauna were taken using a 6.4 cm diameter corer while meiofauna and nematode samples were taken using a 3.2 cm diameter corer. Nematode extraction was done by centrifuging using Magnesium Sulphate (MgSO4) solution. There were significant differences (ANOVA, p < 0.05) between the study sites in Total Organic Matter (TOM), with the natural site recording the highest TOM levels (53.6 %). The 10 years reforested site was characterised by a significantly higher (72.9 %; ANOVA, p < 0.05) silt/clay fraction than the other sites. The natural site recorded significantly higher macro-endofauna densities (27,469 ± 11,189 Ind. /m2) than all the other sites (ANOVA, p < 0.05). Oligochaeta was the dominant macrofauna taxon in the natural and the 10 years reforested sites, while Polychaeta and Nemertina dominated the 5 years reforested and the degraded sites respectively. The natural and the 10 years reforested sites recorded significantly higher (ANOVA, p < 0.05) meiofauna and nematode densities than the 5 years reforested and the degraded sites. Nematoda was the dominant meiofauna taxon in all the study sites. Both the natural and the 10 years reforested sites were characterised by high densities of the nematode genera Terschellingia and Pierickia, respectively, while the degraded site was dominated by the genera Metachromadora. The index of Trophic Diversity (ITD) was low in all sites indicating that all nematode trophic groups were represented in almost similar proportions in all the sites. PCA and nMDS analysis together with ANOSIM using sediment physical characteristics and macro-endofauna taxa composition, respectively, gave a clear separation of all the sites. However, no separation of the natural and the 10 years reforested sites was observed based on meiofauna and nematofauna community assemblages. This shows that macro-endofauna is more sensitive to habitat modifications, and therefore, a better indicator of ecosystem recovery since the densities and community composition are not yet fully established to the natural state even after 10 years of reforestation. Inorder to understand the main source of organic matter (detritus) supporting meiobenthos re-colonisation of the reforested sites, a field experiment was done utilising mangrove leaves, sea grass leaves and diatoms as different food types. ANOSIM on meiofauna and nematode community composition gave a clear and significant separation (R > 0.5) of mangrove leaf litter from all the other food types, showing that mangrove leaf litter is the preferred source of detritus compared to sea grass and diatoms, for meiofauna within the studied mangrove ecosystems. This study shows that mangrove degradation leads to alterations in sediment physical characteristics, drastic declines in benthic-endofauna densities and changes in community composition. It is also evident that mangrove derived organic matter is the preferred source of detritus and greatly influences recolonisation of restored mangroves by benthic– endofauna. It further shows that the reforested mangrove ecosystems are evolving slowly towards ecosystems that are ecologically similar to the natural forests. However, the recovery may take more than 10 years before being fully realised as evidenced by the differences in TOM, macro-endofauna densities and community composition between the natural and the 10 years reforested sites. The results of this study have clearly shown that artificial mangrove reforestation programmes should be initiated, encouraged and increased since they lead to recovery of the forest as well as the benthic community. This will lead to sustainability of the economic goods, ecological services and ultimately biodiversity conservation. From the results of this study, it is recommended that alternative building materials and energy sources like establishment of Casuarina plantations should be explored to reduce pressure (clear felling) on mangroves, which ultimately leads to deleterious effects in the benthic community. There is also need to analyse which aspect of the benthic community (density, community structure or diversity) is the best indicator of the recovery of the once degraded mangrove ecosystem.