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Rufiji delta table 4. He found the mean of 286 m
3
ha. For the size class 5cm=DBH=10 cm diameter class 1 the volume was 15 m
3
ha
-1
and for 10=DBH=20 cm diameter class 2 it was 64 m
3
ha
-1
and for DBH 20 cm diameter class 3 it was 189 m
3
ha
-1
. Rhizophora mucronata contributed 42.3 of the total volume followed by Xylocarpus granatum
with 21.1 , Ceriops tagal 11.0, Sonneratia alba 9.1, Avicennia marina, 8.8, Bruguiera gymnorrhiza
4.5, and Heritiera littoralis 3.2. Rhizophora mucronata and Bruguiera gymnorrhiza had the largest bark percent, 12 and 11, respectively. This is why these two species were previously exploited
commercially for tannin Grant 1938. No wonder R. mucronata is referred to as mkoko in Kiwahili because it was the main source of mangrove bark barks are called makoko in Kiswahili. Avicennia marina has the least bark, only
3, Xylocarpus granatum has 6 but Ceriops tagal and Heritiera littoralis has around 8.
Table 5.4: Proportion of wood volume and number of trees per ha of each mangrove species in the Rufiji delta and the percentage of bark Source: Mattia 1997 and the tannin content in the bark Source: Grant 1938.
Species Proportions
of wood volume
Proportions of number of
trees per ha Bark
percent Tannin content
in the bark
Avicennia marina 8.8
12.0 3.3
- Bruguiera gymnorrhiza
4.5 13.1
11.2 35.8
Ceriops tagal 11.0
18.2 8.2
25.8 Heritiera littoralis
3.2 6.6
8.6 -
Rhizophora mucronata 42.3
33.6 12.0
36.5 Sonneratia alba
9.1 2.3
8.3 -
Xylocarpus granatum 21.1
14.3 6.0
29.8
5.5.3 Genetics, evolution and palaeontology
There are no studies on the genetics, evolution and palaeontology of mangroves of Tanzania.
5.5.4 Physiology, biochemistry, biophysics, nutrient dynamics
Physiology studies of mangroves in Tanzania are very few and these are mainly linked to salt tolerance. Walter and Steiner 1936 estimated the tolerance levels of salinity of the common mangrove tree species of Tanga and McCusker
1971 carried some work on the physiology of the mangrove trees at Kunduchi. Walter and Steiner 1936 found out that the chloride of Rhizophora mucronata, Ceriops tagal and Avicennia marina seedlings still attached to the
parent tree, were much lower than normal seawater. The upper limit of tolerance of mangrove trees in terms of soil osmotic potential atm for the Kunduchi mangroves are: S. alba 38.8 atm, B. gymnorrhiza 40.7 atm, X. granatum
44.0 atm, R. mucronata 46.7 atm , C. tagal 50.1 atm and A. marina 97.8 atm McCusker 1977.
Nutrient dynamic studies are linked with those done on mangrove sediments, microbial and crab activities, litter decomposition and litter movement in and out of the mangroves. The studies on sediments and nutrient dynamics
are quite recent Shunula 1996, Mohamed 1998, Julius, 1998, Lyimo 199, Lyimo et al 2000a, Machiwa 1999, Hedman and Strandberg 1999 and have mainly been part of M.Sc. and Ph.D. studies. The studies show that
mangrove sediments are often waterlogged and have pH values that range from 3.5 to 8.3 Mohamed 1998, Machiwa 1999, Lyimo 1999, Julius 1998. This is due to their limited buffer capacity and intense acidifying
processes such as aerobic degradation of organic matter, oxidation of reduced components, ammonium uptake by roots and root respiration. The oxygen diffusion is greatly reduced but crab holes form interconnected tubes that
help in draining and flushing salts. Near the roots there is higher level of oxygen due to aeration by the roots Lyimo 1999, Julius 1998. Similarly benthic communities dominated by crabs and litter feeding snails effect the
removal of fallen leaf litter in mangrove forests. Rates of litter decomposition vary with the season and also depend on the species. For example decomposition rates are higher for Sonneratia alba leaves followed by
Avicennia marina
than R. mucronata and least for C. Tagal Chale, 1992, Shunula 1996, Julius 1998 and the rate
148
are highest during the rainy season. Rapid weight loss occurs in the early days of incubation but slow down with time.
Nutrients and salinity in the sediments have been found to vary with season, with the location in the mangroves and tidal water movements Mohamed, 1998, Julius 1998, Shunula, 1996, McCusker, 1971. There is thus large
variation of nutrients and salinities between replicate samples. The sediments have usually high CN ratio. Mohammed 1998 in his PhD studies did measurements on nitrate, nitrite, ammonium and soluble phosphates in
the water column and sediments in the Mapopwe creek and Chwaka Bay in the East Coast of Zanzibar. Investigations were also made to determine the exchange between sediments and water column in the forest as
well as between mangrove and in the adjacent bay and the influence of mangrove produced nutrients on the productivity of adjacent communities in the Chwaka bay was also investigated. His conclusions were that nutrient
distribution patterns in the water column and sediment as well as the exchange across sedimentwater interface is regulated with sediment properties and the activities of microalgal populations present in the sediments. There is
no significant flux of nutrients between sediments and the water column in mangroves of Chwaka. Similarly, there was little export of the nutrient from the system. Chwaka mangroves export particulate organic matter to the
seagrass. Large variations in the data were noted because of the striking heterogeneity of mangrove sediments.
Studies done elsewhere show that ground water inflow to mangrove areas induces stratification of water column, limits salinity in dry season, supplies nutrients and is an important factor governing plant zonation. Groundwater
outflow sustains the mangroves during periods of severe salinity stress and nutrients deficiency in dry seasons Wolanski et al. 1992, Kitheka et al. 1999. The freshwater influx via rivers and direct rainfall also is responsible
for lower salinities in riverine mangroves. In the mud there is little water movement except near the creeks. Therefore better growth of the mangroves takes place near the creeks.
Mangrove sediments are nitrogen-rich compared to mangrove litter, as a result of microbial nitrogen retention, uptake and fixation, and import of nitrogen-rich material Middelburg et al. 1996. In mangroves at Gazi in Kenya
which is very similar to many mangroves of Tanzania, Woitchik et al. 1997 showed the maximum rates of nitrogen fixation in a C. tagal zone to be 380 nmol N
2
h
-1
g
-1
dw in the rainy season but 78 nmol N
2
h
-1
g
-1
dw in the dry season. It was estimated that biological nitrogen fixation could account for between 13 to 21 of the
maximum nitrogen immobilised in the decaying mangrove leaves. Cutting mangroves on the other hand alters benthic nitrogen dynamic because the abundance of burrowing animals decreases Stromberg et al. 1998.
Machiwa 1999 found in a Maruhubi mangrove in Zanzibar that there is a significant export of mangrove litter to the adjacent marine environment during spring tides. Net organic carbon export from the entire forest was 79 ×
10
6
g C y
-1
, dissolved organic carbon accounted for about 78 of the total export. Rates of import and export of particulate matter in the forest were not statistically different. A relatively high export of macrodetritus was
recorded at the marine fringe mainly colonized by Sonneratia alba. There was low export of litter in the terrestrial fringe zone, a mono-specific stand occupied by Avicennia marina. While Machiwa 1999 showed a
mangrove stand in Zanzibar was a net exporter of carbon, Middelburg, et al. 1996 on the other hand showed that mangrove sediments in Gazi Bay act as a nutrient and carbon sink rather than as a source for adjacent seagrass and
reef ecosystems. Wolanski 1989 reported that the brackish and turbid water plume in the mangrove creeks is often trapped in the mangrove swamp and does not reach the coral reef. Other studies show that tidal asymmetry,
characterised by stronger ebb flows than flood flows in the mangroves partly promote the net export of organic matter to the seagrass beds.
Microorganisms play important role in the carbon, sulphur and nitrogen cycles in the mangroves. These are involved in the reduction and oxidation of the carbon, nitrogen and sulphur compounds Lyimo et al. 2000a and b,
Julius, 1998. These authors have shown that in mangroves there is predominance of sulphate reducing bacteria over methanogens. Mangrove sediments have been found to produce limited quantities of methane and large amounts
of hydrogen sulphide and that the production levels vary within the mangrove areas. The rate of sulphate reduction decreases with sediment depth and was 10 to 200 times higher than the methane production rate in Mtoni
mangrove, Dar es Salaam Lyimo 1999. In situ values of methane production were found to range from 0 to 2.09 ml m
-2
day
-1
Julius 1998. During the rain periods the values of methane were found to be higher than during the dry
149
period. Methane production measured in the laboratory using the mangrove sediment with organic carbon of 2 and 15 showed that sediment 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
when leaf litter was used as substrates 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. Apparently there are no studies on the biopysics and biochemistry of the mangrove trees of Tanzania
5.5.5 Biodiversity of mangrove ecosystems
