Spatial variations in macrobenthic fauna recolonisation in a tropical mangrove bay

Abstract

Abstract. Recolonisation by crab species and sediment-infauna taxa (at class level) in artificially regenerated mangrove stands of Avicennia marina, Rhizophora mucronata and Sonneratia alba (5 yr old) were studied using respective bare sites (open without mangroves or denuded) and natural sites (relatively undisturbed) as controls. The controls were chosen based on site history, physical proximity and tidal inundation class in reference to the particular reforested mangrove stand and samples randomly taken. A number of environmental variables were measured; interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover, with the natural sites having the highest content. The bare sites were generally sandier, whereas the areas with mangrove cover had higher proportions of clay and silt. Generally, there was a higher crab density in the reforested sites than in the bare sites, whereas crab species diversity (Shannon diversity index) did not vary from one site to another for any of the mangrove species. In terms of crab species composition, the reforested sites were more similar (Sørensen similarity coefficient) to the natural sites and less to the bare controls. For sediment-infauna, the reforested sites had a significantly higher density than the respective bare controls, while the natural sites had the highest density. The number of sediment-infauna taxa in both the reforested and natural sites of all the mangrove species was similar and higher than in the comparable bare sites. The results suggest that the reforested sites are supporting more faunal recolonisation, and therefore becoming more akin to the natural mangrove sites in terms of the investigated functional indicators. The findings seem to support the use of artificial mangrove regeneration (in areas where natural regeneration has been impeded by physical conditions or otherwise) as an effective management tool in the restoration and conservation of the functional integrity of degraded mangrove habitats.

Litter degradation and CN dynamics in reforested mangrove plantations at Gazi Bay, Kenya

“…No amphipods were however observed in the R. mucronata bare treatment litter bags, which had the lowest decay rate. This corresponds to the findings of Bosire et al (2004) who found this bare treatment to be the most impoverished in terms of faunal colonisation.…”

“…Bosire et al (2004) found sediment-infauna densities in these treatments (Table 4) to follow a similar trend (natural > reforested > bare), and recorded more sesarmid crab species in the natural and reforested treatments. Crab species composition was also similar among these treatments, while Uca spp.…”

“…The findings of this study suggest that other than direct litter input, mangrove reforestation modifies sites conditions in such a way that enhances the systemÕs litter degradation and nutrient remineralisation efficiency. Coupled with this is recovery of the systemÕs inherent ecological functions, e.g., faunal colonisation and natural revegetation, which have been found to be either impoverished or completely impaired in the bare treatments used in this study (Holmgren, 2002;Bosire et al, 2003Bosire et al, , 2004. The findings seem to support a need for the retention of standards through selective logging to not only ensure seed availability for restocking the affected sites, but also to provide for continuity of inherent ecological functions, albeit at a potentially reduced scale.…”

“…Litter in R. mucronata treatments was thus exposed to occasional drying during neap tides, which when compounded with less input of nutrients (especially N) from seawater (Gulis and Suberkropp, 2003), slowed down leaching and saprophytic decay more effective when leaves are wet Mfilinge et al, 2002). Water soaking causes leaching of labile materials and promotes microbial activity both of which Table 4 The density of sediment infauna (macrobenthos) in the S. alba and R. mucronata treatments (Extracted from Bosire et al, 2004) increase decomposition rates (Tam et al, 1990;Ashton et al, 1999). Other than specific mangrove stand physical characteristics, differences in litter quality have also been found to be critical in influencing decay rates (Robertson, 1988;Steinke et al, 1990;Twilley et al, 1997).…”

The spatial variation of macrobenthic community in the Negombo estuary in relation to physico-chemical parameters

“…This endorsed the finding of Bosire et al (2004) where the species diversity is higher in sediments that are associated with mangroves and sea grasses. As described earlier, these sites are undisturbed by man and possess a thick growth of sea grasses, and directly influenced by the associated mangrove cover.…”