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
Tree biodiversity in Mangroves is low Table2 because only few tree species can withstand the high salinity, anaerobic sediments, acidic soils and unstable substrates. However, if the total biodiversity including all groups of
living organisms such as microorganisms bacteria, fungi, cyanobacteria, microlagae, protozoa and animals both resident and visitors are considered, the mangroves are found to be quite rich in biodiversity. For example the
surface layer 0 - 10 cm of the mangrove sediment contain high bacterial numbers Lyimo et al., 2000b. The most probable numbers of methanogenic Archaea in mangrove sediments of Mtoni, Dar es Salaam were in the
order of 10
5
- 10
6
cellsg fresh weight while those of sulphate reducing bacteria were in the order of 10
6
- 10
7
bacteriag fresh weight. The number of bacteria on the leaf litter of Avicennia and Rhizophora when decomposition was studied below their tree stands was found to be 6.7 x 10
10
and 1.20 x 10
11
bacteriag of detritus, respectively Julius 1998. Recently a new species of an obligatory methlylotrophic methanogenic Archaea, Metahnosarcina
semesii MDI
T
was isolated in mangroves of Mtoni, Tanzania Lyimo et al. 2000a. Julius et. al. 1996 found a total of 23 species of benthic microalgae in Mtoni mangrove sediments. Most of the
species encountered were pennate diatoms such as Navicula spp, Pleurosigma spp, Hantzschia spp, Nitzschia spp and Amphora spp.
The mean average number of microalgae was found to be 91,856 cells per gram of soil sample while an average of 77,920 and 9,262 cells were diatoms and cyanobacteria, respectively. The biomass of these microalgae
ranges from 0.27 to 1.33 mgg of soil sample. Species in the genera Oscillatoria, Spirulina, Lyngbya, Richelia, Nostoc
, Pleurosigma, Gyrosigma, Hantzschia, Nitzschia are found in the channels and as algal mats in the Mangroves Lugumela et al. 1999. Cyanobacteria such as Oscillatoria spp, Spirulina spp and Nostoc spp were also
abundant. On mangrove roots, Julius 1998 reported that the epiphytic algae Caloglossa leprieurii and Bostrychia murayella
were dominant on Avicennia marina roots. While Rhizoclonium grande was dominant on the Rhizophora mucronata
roots and Bostrychia radicans on the roots of S. alba. Mohamed 1998 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 for wet season in the same sites
it varied from 2.7 to 5.6 µ
g chl a per gram of sediment The most conspicuous animals in the mangrove are crabs and the most common crab species are Sersama marsh
crab and Uca fiddler crabs which have an enlarged claw MacNae 1968, Mgaya et al. 1999, Shunula 1996, Stromberg et al. 1998. Stromberg et al. 1998 found out the Ocyponinae and Serminae comprise a large
component of the intertidal macrofauna in Kisakasaka mangroves, Zanzibar. These authors also showed that there is higher diversity of species in healthy mangroves than in degraded ones. These show characteristic zonation.
Sersama crabs inhabit the upper zone when it is sandy. The Uca crabs are found mainly in areas dominated by the Ceriops
and Bruguiera mangrove species. Mangrove snails like Telebralia spp, Cerithidea spp and Nerita spp crawl over the mud surface sometimes climbing the trees and clustering on the shaded side of trunks Mainoya et al. 1984,
MacNae 1968. Barnacles are common on stems of the mangroves especially Sonneratia trees. On the leaves of mangroves especially those of Bruguiera, Sonneratia and Ceriops species of Littorina are seen. Mangrove oysters
Crassostrea cuculata are very prolific on pop roots and lower branches of Rhizophora mucronata and of Sonneratia alba
Mgaya et al. 1999. A survey by Olafsson 1995 within 5 mangrove areas on the west and east coast of Zanzibar showed that
Meiofauna densities in surface sediments 0-5 cm ranged from 205 to 5263 individuals per 10 cm
2
, being on average 1493 individuals per 10 cm
2
. Of the 17 major taxa recorded, nematodes dominated 64-99 in all samples while harpacticoid copepods were usually second most abundant. Within all areas the numbers of
meiofauna were very variable and significant differences among areas were only detected for oligochaetes and turbellarians. Densities of nematodes, harpacticoids, polychaetes and turbellarians were, however, significantly
150
P0.001 higher at low water stations compared with mid and high water stations. Harpacticoids were negatively correlated with the numbers of fiddler crab Uca spp. burrows. Other correlation between environmental factors
grain size, temperature, salinity, oxygen tension, prop root density, fiddler crab burrows and major meiofaunal taxa were non-significant. A total of 94 nematode genera were recorded from four mangrove areas. The most
abundant and frequent genera were Microlaimus and Spirinia, followed by Desmodora and Metachromadora. In a hypersaline area diversity was much reduced and where salinity was over 100‰ the fauna was restricted to 3
nematode genera, Microlaimus, Theristus and Bathylaimus. Ólafsson and Ndaro 1997 found that ocypodid crabs do not regulate resident nematode assembalges, but may inhibit settlement of colonisers e.g. harpacticoid
copepods that have not adapted to the intense surface disturbance created by these crabs.
The density of Ologochaetes in uncut mangrove area was found to be 3105 individuals m
-2
but only 40 individuals m
-2
in cut areas Stromberg et al. 1998. In general the macrofauna decreased in the cut areas except the crab density was similar in both the cut and uncut areas because they can move around.
5.5.6 Ecology of mangroves and mangrove forest ecosystems
