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6. Physical, chemical and ecological processes

The Sierra Leone continental shelf is located in a unique position on the West African coast where it comes under the influence of the Guinea Current, southern off-spins of the Canary Current, the Equatorial Counter Current and the coastal drift occurring in the shallower waters. The resultant effect of these forces is crucial to the ecology of the shelf by creating a vacuum in the northern parts of the shelf and generating some amount of upwelling so necessary for primary production.

The physical and chemical processes of a marine environment determine the quality of its biodiversity and habitats. The overall health of marine ecosystems can therefore be assessed by examining the status and trends of the major physical and chemical processes taking place in it. Such processes as erosion of rock and sediments, volcanic activity, gas exchange with the atmosphere, the metabolic and breakdown products of organisms and precipitation, affecting the coastal morphology, marine biota, biological productivity and the structure of marine assemblages.

The continental shelf of Sierra Leone is very broad in the north and tapers towards the south. Wave action and tidal activity play a significant role in the shallow areas. Besides regulating the diurnal life of aquatic organisms they are involved in the transport of pollutants and the disposal of waste especially along the beaches.

Sixteen physical and chemical processes have been assessed. Most of them were found to be in good condition (Fig. 6.1), according to the assessment at the workshop (Fig. 6.2). Sediment transportation is a problem (assessed to be in poor condition) due to large amounts of sediments of agricultural and mining activities entering some of the rivers, estuaries and lagoons. 

In addition to the physical and chemical processes that have been assessed, two ecological processes have been assessed as well (Fig. 6.3). These are related to disturbances in the physical environment. In general the processes are still in good condition and functioning due to the fact that the coasts of Sierra Leone remain largely undeveloped.

The impacts of spatial or physical disjunctions are high around harbours and seem to have increased over the past 5 years. Some wetlands have been affected by dredging and development, with an impact on their function as stopover site for migrating birds.


Physical, chemical and ecological processes assessment

Composante Info Niveau d'évaluation Confiance
Très pauvre Pauvre Bon Très bon À niveau En tendence
Guinea Current structure and dynamics Some changes (worst places) due to climate change and Guinea Current North Atlantic 'El Nino' Worst 10% trend: Stable Most trend: Stable Best 10% trend: Stable High High
Severe storms, wind patterns Increase in severe storms and wind patterns is climate change related. Areas in the north and south of Sierra Leone are more affected Worst 10% trend: Declining Most trend: Declining Best 10% trend: Declining High High
Sediment inputs Examples of least impacted coastal areas are found in the Scarcies river region. The worst places are influenced by agriculture, damming of rivers, deforestation, diamond mining, coastal development (most extreme around Freetown) Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Inshore water turbidity, transparency and colour The best locations are found near the Scarcies river region. Examples of the worst impacted areas are around mining areas north and south, the Freetown coastal development in bays and estuaries Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Sea surface temperature The global rise in SST has not caused any significant local impacts, no ENSO effects are being recorded Most trend: Stable Medium Medium
Sea level Places in Sierra Leone where there is no significant impact include rocky coasts and natural mangrove coasts. This phenomenon is associated with global sea level rise, coastal erosion and local subsidence (eg. at Bonthe Island) Worst 10% trend: Declining Most trend: Stable Best 10% trend: Stable High Medium
Land-sourced nutrient supply and cycling This includes river and ground-water input plus surface runoff. The best examples are found around the Scarcies river. The worst places are associated with fertilizers, sewage etc. Examples include the Addax area (Rokel River, Malen River) with sugarcane and palm oil plantations for biofuel, pineaple plantations Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Ocean-sourced nutrient supply and cycling Includes upwellings, mangroves and anthropogenic inputs (eg. fish farms) Worst 10% trend: Not available Most trend: Not available Best 10% trend: Not available Medium Enter confidence trend
toxins, pesticides, herbicides The best places include the Scarcies river region Worst 10% trend: Stable Most trend: Stable Best 10% trend: Stable High High
Dumped waste Domestic waste and trash are widely dumped into the sea. Worst examples include the Freetown bays, Kingtom Granville dump site, Fish kills reported in the 1980's were caused by suspected dumping, dredge spoil dumped in Freetown estuary and sewage discharge from Freetown Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Marine debris, wracks Worst places include the Freetown beaches, Mahera Beach in Lungi Worst 10% trend: Declining Most trend: Stable Best 10% trend: Stable High High
Sediment transportation Examples of least impacted coastal areas are found around the Scarcies river. Examples of places most disturbed by humans causing sediment mobilisation (mining, coastal development, sand mining, deforestation, etc). Examples of these areas are found around Freetown and mining areas in the north and south Worst 10% trend: Declining Most trend: Declining Best 10% trend: Declining High Medium
Coastal/shoreline erosion Examples of locations where erosion is not an issue include the southern region around Sulima and Bonthe island. Examples of areas severely affected by erosion include the area around Freetown, the Freetown peninsula and islands Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Freshwater inflow, surface and groundwater runoff Best examples: many rivers in Sierra Leone which are free flowing into the sea: Scarcies river region. Worst example: Rokel river and Moa river which have been dammed but freshwater still reaches the coast Worst 10% trend: Stable Most trend: Stable Best 10% trend: Stable High High
Ocean land salinity/acidity Worst places are found in zones affected by the acid sulphate soils disturbed in rice farming - Kambia region Worst 10% trend: Declining Most trend: Stable Best 10% trend: Stable High High
Nutrient supply and cycling: ocean based Best places: open coasts and well mixed and flushed by the ocean: Southern coast, Turners peninsula. Worst places: bays where ocean sourced nutrient supply and cycling have been disturbed by human impacts: Kroo Bay, Susana Bay, Man-of-War Bay, Jul Port (Bunce River) Worst 10% trend: Unclear Most trend: Unclear Best 10% trend: Not available Medium Enter confidence trend
Spatial/physical disjunctions Harbour authorities are reporting impacts around harbours due to dredging (Sierra Leone River estuary). Other causes: aggregate mining, removal of mangroves, beach mining, coastal development (Aberdeen marine creek) Worst 10% trend: Declining Most trend: Declining Best 10% trend: Stable High High
Biological migration, flyways The Aberdeen creek flyway stop-over site is threatened. The Sierra Leone River estuary is affected by dredging and development Worst 10% trend: Declining Most trend: Stable Best 10% trend: Improving High High
Legend
 Très pauvre  Pauvre  Bon  Très bon
Tendances récentes  Amélioration  Stable Confiance  Confiance élevée basée sur des données de haute qualité
 En déclin  Peu clair  Confiance modérée basée sur quelques preuves limitées
 Pires/meilleurs 10% des places  Faible confiance basée sur le jugement d'experts avec peu ou pas de données