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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
The occurrence of individual mangrove species within the forest is reliant on environmental factors such as salinity, nutrient availability, and oxygen level in the soil and wave energy. Normally, it is the extreme factors that
limit the distribution of a species. As mangrove species differ in their tolerance of these factors, a pattern of species distribution known as zonation occurs Walter and Steiner 1936, Chapman 1977, Semesi 1986. Commonly
Sonneratia alba
occurs in areas where the salinity is almost constant, close to that of seawater where tidal water reaches daily. Many Sonneratia trees in various stands are dying and little regeneration of this species is taking place.
Avicennia marina is the most widely distributed because it can tolerate high ranges of salinity, varied flooding
regimes, compacted substrate, sand flats and newly deposited sediments. On the seaward side the species attains large sizes but on the landward margin is present only as bushes and it does poorly on muddy soils. When mixed
with other species, it is commonly associated with Ceriops and Xylocarpus. Xylocarpus is most often found mixed with Avicennia, and it grows on raised portions where flooding takes place only for a few days a month and where
there is fresh water influence. Xylocarpus is an important element of the riverine mangroves but does not form pure stands in Tanzania.
Ceriops is largely found on the landward side of the Rhizophora zone. It becomes dominant more frequently in areas
where mud is thin and on relatively higher ground than the Rhizophora zone. Forests with mixed vegetation of Ceriops
and Avicennia are found on slightly raised ground where flooding occurs only during spring tides. The substrate is usually firm during low tides. Rhizophora mucronata forests are dominant on muddy soils and often
form extensive pure stands. On sandy soils, however, the species fails to compete with others. Bruguiera gymnorrhiza is relatively less abundant in the mangroves of Tanzania and is often found as a narrow zone
between Rhizophora and Ceriops zones or mixed with them. However, unlike most sites in the country, Mnazi Bay, Ruvura, Mana Hawanja and Mongo islands in Mtwara region are dominated by Bruguiera gymnorrhiza at the edge of
forest followed by Ceriops tagal.
Heritiera littoralis, a riverine mangrove species, grows only in habitats with low salinity and thus it is restricted to
areas in the vicinity of river mouths or where there is ground freshwater seepage. Such sites are usually only flooded by spring high tides, and usually the substrate is firmer compared to those on which other mangrove species are
found.
Therefore different mangrove stands have varying dominant tree species and the total number also varies. This information is well documented in the 30 map sheets produced in 1991 for the mangroves of mainland Tanzania
Semesi 1991b-h. These maps are found in Catchment Forest section in the Forest and Beekeeping Division.
Mangrove leaves represent a major source of organic carbon to the mangrove sediments Machiwa, 1998, Shunula, 1996, Julius, 1998, Rao et al. 1994. As mangrove leaves drop into tidal waters they are colonized within
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a few hours by marine fungi and bacteria that convert difficult to digest carbon compounds into nitrogen rich detritus material. The decomposing litter covered with Microorganisms become food for the smallest animals such
as worms, snails, shrimp, molluscs, mussels, barnacles, clams, and oysters. These detritus eaters are food for carnivores including crabs and fish, subsequently birds and game fish follow the food chain, culminating with
man. In the Rufiji delta for example, Villagers report that “Kima” monkeys eat crabs and fruits of Avicennia marina
and Xylocarpus granatum. Both inshore and offshore fisheries, including the highly profitable offshore shrimp fishery, depend on inshore nursery areas, some of which are associated with mangrove.
The benefit of mangroves on marine ecology is summarised as follows. Basis of a complex marine food chain; creation of breeding habitat. Establishment of restrictive impounds that offer protection for maturing offspring;
filtering and assimilating pollutants from upland run-off; stabilization of bottom sediments; water quality improvements and protection of shorelines from erosion.
5.5.7 Distribution of mangroves in Tanzania
