Comparison of sediment meiofauna in cleared, restored and natural mangrove areas in Gazi bay, Kenya.

Abstract

There are a number of ecological services provided by mangroves. The nursery role of mangroves for fish and crustaceans is one of them and has been well documented. Studies have also revealed a positive correlation between mangrove cover and fish and penaeid shrimp catches. Due to the degradation of mangrove ecosystems a world-wide movement has started to plant new areas of mangroves. This study was conducted in Kenya. south of Mombasa in Gazi bay in the mangrove forest of Sonneratia alba. The aim of this study was to look at the composition of higher meiofauna taxa in cleared, restored and natural mangrove areas and relate this to their role in diets of primarily juvenile fish sampled at the same sites. It is the role of meiofauna as fish food that is the main focus in this Study and how this function might differ between the sites. To extract the meiofauna the classical method of decantation by hand using a 63 ~km and a 1000 ~km sieve respectively was used. For extraction of the meiofauna from the fine grained sediment a Ludox solution with density 1.1560 glcm3 was used. The major meiofaunal taxa found in this study were, Nematoda, Harpacticoida, Oligochaeta. Polychaeta and Turbellaria. Total meiofauna numbers ranged between III individuals per 10 cm-2 and 2131 individuals per 10 cm-2 with a dominance of Nematodes 63-84%. Positive correlations were found between: Harp:lcticoida and organic content: Polychaeta and organic content; Harpacticoida and prop roots; Polychaeta and prop roots; Polychaeta and crab holes; Harpacticoida and grain size. A positive correlation was also found between chlorophyll a and Nematoda; Polychaeta; Oligochaeta; prop roots and crab holes. A significant positive correlation between total fauna and median sediment grain size and a significantly negative between Polychaeta and median grain size were found. It seems Nematodes are less affected by the sediment grain size (and all other abiotic factors) which is the primary factor affecting the abundance and species composition of meiofauna. The result of the present study is that all meiofauna groups more or less prefer finer sediment, which is found in the natural mangroves and the integrated plantation. Higher meiofauna densities were found in the natural stand and integrated plantation than in the clear-cut area. The meiofauna taxa contributing to the difference in total meiofauna abundance were, Harpacticoida in the natural area and in the integrated plantation. Polychaeta, Since no significant differences were found between the integrated plantation and the natural mangrove the study shows that the meiofauna community was quickly restored after mangroves were replanted.