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Walter and Steiner 1936, Grant 1938, McCusker 1971, Semesi 1991a-h and Shunula 1996, Von Mitzllaf 1990 etc., are therefore quite useful. The most extensive baseline information on the distribution, coverage, uses,
and status is found in the mangrove management plans of mainland Tanzania Semesi 1991 b-l. For example the mangrove maps show the different forest vegetations, their area, stand density and stand height. Also the area
covered by saltpans, water channels, clear felled and bare-saline areas are given. The status of the mangroves was assessed using aerial photographs and intensive fieldwork Semesi 1991b-l, 1992. A socio-economic survey of the
communities dependent on the mangroves was also carried out von Mitzllaf 1990. The aerial photography included a total of 854 photos, all of which were clear and cloud-free, which reduced uncertainty in the photo interpretation
process. These aerial photos which also show the extent of the mud flats can now be used to assess the changes that might have taken place since 1989.
The baseline information on mangroves of Kunduchi by McCusker 1971 for example has been useful in estimating the rate of loss of mangroves in the area Banyikwa and Semesi 1986.
5.2.2 Observational studies
Observational studies are many but not all of them are found in published forms. Some can be found in the files of the Forest and Beekeeping Division on the mainland, in the Ministry of Agriculture and livestock or Natural
resources commission in Zanzibar, the archives and field reports of travellers and researchers.
5.2.3 Experimental studies
Experimental studies on mangroves are rare and recent and they include both field experiments and laboratory work. Experimental studies on mangrove sediments of Mtoni, Dar es Salaam by Lyimo 1999 revealed that the
rate of sulphate reduction decreases with sediment depth and was 10 to 200 times higher than the methane produc- tion rate. In situ values of methane production in the same sediments ranged from 0 to 2.09 ml m
-2
day
-1
Julius, 1998. Higher values were measured in the middle part of the mangrove dominated with Avicennia marina and
decreased landward and also towards the sea. It was also found that values of methane were higher during the rain periods than in the dry period. Methane production measured in the laboratory using the mangrove sediment with
organic carbon of 2 and 15 showed that sediments with higher percentage organic carbon gave higher values of methane and that the highest peaks of methane were observed in the batch cultures with salinity of 18
00
Julius 1998. Besides methanogens were found to be the most important dimethlysulphide DMS utilizer in mangrove
sediments as compared to sulphate reducing bacteria Lyimo et al. 2000a. In order to assess how the mangrove sediments react when exposed to an increased nutrient load from sewage
water, Hedman and Strandberg 1999 conducted a microcosm experiment using sediments at Maruhubi and Chwaka mangroves in Zanzibar. A sediment characterization was conducted which revealed that sediment show
great variations in terms of physical grain size distribution, porewater content, chemical pH, oxygen level, organic content, ammonium, nitrate, nitrite, soluble reactive Phosphorus and biological properties chlorophyll a,
carotenoids and phaeopigments. On the other hand Ólafsson and Ndaro 1997 designed a laboratory experiment microcosm to assess the impact of the mangrove crabs Uca annulipes and Dotilla fenestrata on meiobenthos.
Mohamed 1998 studied nutrient dynamics and also estimated the levels of chlorophyll in sediments. He found out Chlorophyll a on sediments vary from 3.9 to 27
µ g chl. a per gram of sediment in the dry season and from 2.7
to 5.6 µ
g chl a per gram of sediment in the wet season. Lowest levels were in sites with good mangrove trees. While Chlorophyll a in Mtoni mangrove ranged from 0.27-1.33 mgg fresh wet of sediment Julius 1998.
Machiwa 1999 using core incubation techniques measured remineralisation rates of organic carbon in a partly polluted mangrove stand in Maruhubi mangrove stand in Zanzibar.
Other experimental studies are those on leaf litter production and litter decomposition Shunula, 1996, Chale 1992 and Julius 1998.
5.2.4 Applied studies
