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Sediment Mobilization

Further Information on Mangroves

Mangroves are thought to possess the ability to control coastal water quality. The complexity of the mangrove forest habitat increases the residence time of water which assists in the assimilation of inorganic nutrients and traps particulate matter. They function as a flood control barrier and binder of sediments leading to reduced erosion. The benefits are of special import to coral reefs which require waters of low turbidity.

Mangroves, seagrass beds, unvegetated shallows and coral reefs often exist as integrated ecosystems of high productivity. The detritus exported from mangroves is considered to be important in enhancing production in other coastal ecosystems.

The higher the water turnover within the mangrove forest, the higher the productivity of the system. Higher turnover tends to lead to:

1) increase in nutrient supply;

2) increase soil aeration;

3) reduce the accumulation of toxic substances; and,

4) inhibit formation of saline soils.

Mangrove forests cover an area of 181000k km2 spread over more than 100 countries. Causes of mangrove loss are:

establishment of shrimp aquaculture ponds;
over exploitation of forest resources by local communities;
conversion into large scale development such as agriculture;
forestry, salt extraction; and,
urban development and infrastructure.

One of the main underlying reasons of mangrove removal is undervaluation of mangrove ecosystems. The consequence of this is dismissal of mangroves as a habitat of economic importance. However, 80% of all marine species of commercial or recreational value in Florida, USA, are considered to be dependent upon mangrove habitats for at least one stage of their life cycle. This is of special import in the case of the juveniles and larvae of many fish and crustacean families since this is a critical stage in their development.

Most extensive areas of mangrove swamp tend to lie along sedimentary shorelines where large rivers discharge onto low gradients. Excess input of sediment can, however, cause death of trees due to root smothering. Indirect degradation of mangroves has been attributed to upstream diversion of freshwaters and pollution caused by oil spills, heavy metals, pesticides and nutrients.

It is clear that mangroves play a large role as a nursery for larvae; most species, fish and crustacean spawn at sea and the larvae arrive at the estuary through active swimming and/or passive tidal transport. There are 3 main factors to which the nursery role of the mangrove habitat can be attributed to:

high levels of water turbidity which increases the survival rate of larvae due to the reduced perception distance of predators;

nutrient inflow is high from freshwaters and mixing efficient resulting in a abundant food base for zooplankton, mysids and shrimp to feed on and in turn are fed upon by commercially important post-larvae and juvenile species; and,

The physical and structural complexity of the habitat itself provides a wide variety of niches favourable to juveniles.

Fish and shrimp catches have been found to be highly correlated to the presence of mangroves. It is understood that a certain amount of mangrove is necessary before the correlation takes effect (i.e., the degree of dependence is unknown). This could lead to smaller areas of mangroves being more vulnerable to disturbances. It is not clear, however, as to the causal factor of this correlation which might be due to a variety of factors relating to the presence of mangroves, such as extensive shallow seas, tidal creeks, intertidal area, organic matter or coastline length.

Mangroves are known to be important in the role of fishery nurseries. Major factors in the role of mangroves as nurseries are described in the table below:

Factors	Descriptors
1. Trophic resources	Tidal mixing, nutrient trapping and freshwater inflow result in high primary productivity and provide the base of a food web from zooplankton to post-larvae fish and juveniles.
2. Water turbidity	Turbidity increases the escape rate by reducing the perception distance of predators, thus enhancing the survival of resident organisms.
3. Structural diversity	Structural complexity and shallow estuarine habitats provide a variety of spatial and trophic niches favourable to the nursery species.

There are 3 possible impacts of mangrove rehabilitation:

Impacts	Descriptors
1. Mangrove reforestation	Reforestation will lead to an increase in organic matter, a higher primary productivity and an increase in nursery habitat.
2. Modification of river hydrology	a) Removal of dams would lead to a larger annual outflow causing an increase in nutrients and hence primary production of the mangrove ecosystem. Another impact of this action would be the increased sedimentation and a reduction in coastal erosion. b) Increased hydrological variability (e.g., removal of concrete embankments and channels) of a river would amplify the effects of a seasonal flushing of the system. Estuarine organisms would gain improved access to any adjacent wetlands for use as nurseries.
3. Decrease of water turbidity (by stopping dredging , for example)	Some turbidity is required by resident organisms as a smokescreen to protect them from predators but too high a rate of sedimentation is detrimental to the health of mangrove tree species.
4. Expand the mouth of the relevant estuary (by removal of harbour dykes, for example)	Studies in West Africa have established that the larger the mouth of the estuary, the larger the numbers of marine and coastal species present including larger predator species of higher economic value.

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