Marine habitats are a valuable resource for food value, recreation, shore protection and intrinsic value. These habitats tend to be susceptible to disturbances such as dredging, construction as well as changes in water quality from land runoff and direct discharges. Increasing rates of sedimentation are a direct consequence of anthropogenic disturbances. Deleterious effects of excess sedimentation in the freshwater and marine environment are:

(i) Possible reduction in oxygen levels in surface waters, resulting in eutrophication, increased acidity and toxicity;

(ii) Burial of sessile aquatic fauna and flora at greater than recovery rates and reduction of suitable habitats;

(iii) Loss of open water, decreased water depth, increased muddiness of sediments;

(iv) Increased turbidity of surface waters leading to a decrease in light availability.

CONTENTS

1. Coral Reefs

2. Seagrasses

3. Lagoons

4. Mangroves

High rates of sedimentation in coastal areas are known to cause to high levels of turbidity, which are detrimental to the proliferation of corals. Corals are composed of marine invertebrates (polyps) which exude a calcareous skeleton which forms reefs. Corals' zooxanthellae are symbiotic algae, residing within the polyps, and require sunlight or photosynthesis for growth. As the algae photosynthesize they give off oxygen as a byproduct which is vital to the polyps. Thus if they are unable to photosynthesize (e.g., in turbid waters which contain suspended sediment and removes light), their demise brings about the demise of the polyps, too. There are three major reef types:

1. Fringing reefs which possess the growth form of a submerged platform and grow close to the shore.
2. Barrier reefs which are grow further away from the shore and follow the coastline
3. Atolls which form a low-lying island, forming a ring of reefs around a lagoon.

Fringing reefs are the most prone to the effects of excess sedimentation. All types of reefs, however, are susceptible to high levels of turbidity, thereby reducing light availability for photosynthesis.

2. Seagrasses

Back to Contents List

Sea grass beds are important ecosystems in the tropical and temperate areas of the world due to their function in stabilizing sediment and providing nurseries for commercial fishery species such as tiger prawns. Seagrasses are of particular ecological importance as primary food sources for threatened or endangered species such as manatees and green turtles. These plants are angiosperms and obtain their nutrients from sediments, as opposed to phytoplankton and benthic algae which obtain nourishment directly from dissolved nutrients in the water column. Seagrasses are also susceptible to water turbidity as well as smothering of the entire plants causing large scale mortality.

3. Lagoons

Back to Contents List

A lagoon is a broad, shallow estuarine system separated from the ocean by a barrier island. Exchange with the sea is limited through inlets. The resulting brackish water is moved more by the wind than by the tide and does not flow from headwaters to a mouth like a river. Lagoons are delicate ecosystems whose balance is easily upset through excessive inputs of either fresh or marine waters. Being close to land they tend to be highly polluted and sediments discharged directly into them. They are of high value to fisheries, since the quiet waters are ideal for juvenile and larval forms of fish and crustaceans.

Back to Contents List

Mangroves are defined as areas of wooded vegetation found in littoral tropical zones. They are characterize by the alternation of tidal floods (intertidal areas) and the presence of freshwater, at least temporarily. Combined with a minimal temperature of 16° C, these factors delineate the classical tropical mangrove belt.

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.

Tree species in mangrove habitats possess aerial roots which allow root respiration in anaerobic, water-logged soils. Roots of some species of tree significantly reduce the velocity of tidal water and have a sediment-binding capability higher than that of sea grasses and algal mats.

The natural rate of sedimentation (reviewed from over 8 locations) seems to be at <0.5 cmy-1 and a maximum of 1 cmy-1. Sedimentation in mangroves greater than these rates cause a disturbance leading to death of the trees. Some species can respond to gradual burial of several centimetres a year due to physiological responses such as developing higher root arches and secondary thickening at the top of buttress roots. The basic premise here is that sedimentation be gradual, allowing the trees time to develop these responses.

Certain species of trees such as Nypa spp. do not have aerial roots but grow vegetatively from underground rhizomes, allowing them to survive and thrive in conditions of rapid sedimentation. These species would be more suitable for rehabilitating mangroves as opposed to replanting species which have died and are probably more sensitive to higher rates of sedimentation.

Also related to mangrove survival is the texture of sediment when trees are buried. Sand does not appear to cause death of trees to the same extent that silt or clay does. This could be due to the better soil aeration through sand.

See Further Information on Mangroves for more details

Back to Contents List