Ecological studies on selected mangrove swamps in Zanzibar.

A description of the species composition, forest extent and general status of the mangroves of Zanzibar is presented, together with their uses and exploitation. Two sites at Maruhubi and Chwaka Bay, both located on Unguja Island, have been studied in detail. Nine species of mangroves, 33 species of algae, six species of lichens and six species of non-woody angiosperms were found to occur in Zanzibar. The areas of many of the mangrove stands have decreased over the last 40 years while others have shown oscillations in size. Exploitation of mangroves is much greater on Unguja Island than on Pemba Island. A study of the mangrove soil pH, organic matter content (during the dry season) and salinity (during both the rainy and the dry season) was undertaken at Maruhubi. The more landward soils were the most saline during the dry season (35- 50 ppt.) and had low salinities during the wet season (20-25 ppt). Acidity was also high (pH 4-5),as well as percentage content of organic matter (2.7-4.6.6%). Demographic studies based on height classes were carried out on Avicennia marina and Sonneratia alba at the Maruhubi mangrove stand, to show the size structure of the populations and to determine if regeneration was occurring. A. marina showed considerable regeneration with a mean of 234 seedlings per plot of 15x15 m. Regeneration in S. alba was poor, with a mean of 24 seedlings per plot. The reproductive phenology of A. marina, Bruguiera gymnorrhiza, Ceriops tagal, S. alba and Rhizophora mucronata in terms of flowering, fruit and propagule production and shedding, was studied over a two year period. Phenology was strongly seasonal in A. marina, occurring between November and May each year. Seasonality was less obvious in the other species. However, litter trap data showed a seasonal trend in propagule shedding in the species R. mucronata, occurring between December and March; B. gymnorrhiza, occurring between May and August. A two year study was also conducted to determine the rate of production of leaf litter, flower litter and propagule litter for these species. Leaf litter was found to constitute 50-80% of the total litter production and while it occurred year round there was a trend towards greatest production in the period of September to March. Leaf decomposition rates were shown to be higher in S. alba (with 50% weight loss in one week) and A. marina(with 50% weight loss in 21 days), than in R. mucronata (with 50% weight loss in 40 days), and C. tagal (with SO% weight loss in 60 days). The initial C:N ratios were lower in A. marina (43:l) and S. alba (57:l) than in C. tagal (76:1) and B. gymnorrhiza (82:). These C:N ratio differences were maintained during decomposition. Salinity between 10 and 25 ppt. was found to have a positive effect on both root elongation and number in the seedlings of A. marina. These findings are presented in details together with recommendations for future investigations